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So, assuming that the technology is readily available and that we could build these mecha as easily as we build modern tanks or planes.

What would be a good advantage in using a legged (not necessarily bipedal) combat mecha over using conventional tanks and planes? I don't need scientific reasoning (though if you have some that would be amazing), simply logistical or tactical reasoning would be enough.

Some notes:

  1. The mecha should use legs to maneuver rather than tracks.
  2. The mecha should still have a human pilot inside the mech.
  3. Soft science is good enough, but the harder the better.
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    $\begingroup$ Just a thought, not a full answer... if mecha can fill a similar role to tanks (and can thus be compared to them), then I would think that mecha would be much more of a challenge for the enemy to hit: after all, a tank can't dodge and weave or duck to avoid fire. $\endgroup$ – 2012rcampion Feb 16 '15 at 3:06
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    $\begingroup$ Some thoughts: is the pilot inside or remote (drone)? What's the terrain? (e.g. in a forest tall/narrow units are better than wide and flat; they may also be able to climb cliffs) $\endgroup$ – Chris H Feb 16 '15 at 10:54
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    $\begingroup$ Good question. also...now I have to go watch Pacific Rim tonight... $\endgroup$ – James Feb 16 '15 at 19:43
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    $\begingroup$ @2012rcampion : a mecha could also not dodge an artillery shell. $\endgroup$ – vsz Feb 17 '15 at 7:22
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    $\begingroup$ @2012rcampion : A tank presents a flat profile, while a mecha of the same weight must be much taller. This makes a mecha both easier to hit and having a thinner armor: the same weight of armor must cover a larger area. A tank can have an armor shape which is angled to deflect shots, while the mecha will have a lot of parts where incoming shots will hit in 90 degrees. A mecha will have a lot of joints, easily damageable even by indirect fire. Erratic movement doesn't gain you much benefits if by the time the shell comes in, you only moved a few meters and are still in the area of explosion. $\endgroup$ – vsz Feb 17 '15 at 20:30

28 Answers 28

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The benefits and merits of using mecha are highly dependent on how big these mecha are.

Under 9' Tall

At the lowest end of the size scale, you have what is probably better coined powered armour. But mecha this big should be considered, if for no other reason than to illustrate where larger mecha sizes potentially don't have much use. A 90' tall isn't going to fit inside a building, but a 9' tall is... providing they smash a 9' tall door frame upon entry.

I chose 9' because I pictured a person and scaled them up until they had to duck uncomfortably in my office. Then I replaced this person with a robot. This mecha is tiny by mecha standards, but is by no means a lightweight. The average person displaces 66 litres of liquid when fully submersed, and is 5'4" tall (which strikes me as unusually small). Scaling this up to 9', my mecha displaces 332 litres. If we assume the mecha is predominantly made of Iron (7.87 kg/L), it weighs-in at 2.5 tonnes (sans room for the pilot).

Modern militaries still use infantry for a great number of reasons, but the two I think are relevant to you are; they can go where vehicles can't, and they can make high-level decisions where-as a machine cannot (yet).

Mecha this sized should be able to withstand small-arms fire much better than infantry can. If you need shock-troopers to lead an assault against a defended position, this guy is your best friend.

Over 9' Tall

For the moment, I'm going to assume that mecha weigh the same as an M1A2 Abrams Tank (~62 tonnes). And has a similarly sized gun, similarly rated armour, and a single pilot, and is vaguely human-shaped. The M1A2 has a crew of four(4), so a single pilot is a massive improvement with regards to minimising loss of life.

Applying my scaling rules from the 9' tall case, this mecha should be in the order of ~25' tall. Taller than a two-story building, but only slightly.

In urban environments, the mecha has a surprise advantage over M1A2. It can poke-out from cover faster than a tank can. It can then retreat to cover quickly too. However this is a pretty moot point because I don't think city buildings withstand cannon fire very well. The M1A2 simply has to aim where the mecha is hiding, and shoot through the cover and will either hit the target, or shower it with chunks of the (now missing) concrete wall.

Since this mecha is vaguely human-shaped, it might be capable of replacing construction equipment. By construction, I mean demolition, and equipment, I mean lets knock this wall down and block traffic. Pile those cars up in places that obscure enemy line of sight and give friendly troops a place to hide.

This doesn't need fine motor control. Just hydraulic grippers.

Where problems occur

If you want to follow some degree of scientific hardness, then the square-cube relationship is what stops you from scaling up mecha in size. If you make something twice as tall (and wide and thick) it becomes eight times heavier. But it only becomes four times stronger. Eventually, it reach a point where it weighs so much that it can't support itself (scaling down, you get proportionally stronger - which is why ants carry many times their body weight with ease).

I won't say where this point lies. I that kind of maths is some heavy mechanical engineering, and it depends on what you build your mecha out of. 25' might be attainable with today's metallurgical knowledge. Then again; it might not.

Speed. This is where I think tanks trump mecha.

Scaling up human running speeds to 25' tall people gives silly results. Think, 200km/h sprinting. Which is more than three(3) times the M1A2's [reported] top speed. This is because I didn't factor in the square-cube relationship.

I expect tanks to have a higher top speed than mecha. On an open plain, you will want tanks. That said, using tanks in an open plain today is probably a nigh-suicidal tactical decision. Heat-seeking missiles have speed and ranges far greater than what a tank can react to. The best defence is not presenting a target to shoot at in the first place. Land mines exist, as do people with rocket launchers and rocket-propelled grenades.

Moving parts. Put simply; the more moving parts something has, the more parts that will need maintenance, and the more parts that can (and will) get damaged during use. The fewer moving parts your mecha has, the more robust it will be. Mecha that have as many degrees-of-freedom as a human are probably too fragile to be practical and will require too many man-hours of maintenance. Mecha should be treated as sledgehammers, not scalpels.

Plausible role for mecha

I think the most plausible place for mecha in a near-future military would be in situations where you need mobile firepower that's more than infantry can comfortably carry, but comes in a package that's smaller than a tank.

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    $\begingroup$ All that stuff that I don't have to write now. Nice answer. $\endgroup$ – zxq9 Feb 16 '15 at 7:05
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    $\begingroup$ You also missed the track displacement vs. feet (larger is better). Tanks can go places a mecha will sink in up to its knees. Also, low-profile is better (hide in gullies/wide ditches/behind low-rises). Multilimbs may be better for tougher terrain. Bipedal balance is tough (CoM issues). Etc, etc. But yeah, echo @zxq9 $\endgroup$ – user3082 Feb 17 '15 at 7:44
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    $\begingroup$ I think your density estimate is way high. The Abrams has a density more like 1600 kg/m^3. If mecha, using lighter armor and advanced materials, are more similar to aircraft, they may have a density as low as 150 kg/m^3 (F-35). However, you may want to account for the fact that mecha as typically depicted are significantly 'chunkier' than a bare human due to armor and mechanisms. $\endgroup$ – 2012rcampion Feb 17 '15 at 18:29
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    $\begingroup$ Wait, you claim a mecha can poke out from cover faster tyan a tank. What is the basis for this? You're still moving 62 tonnes, but now you're moving 62 tonnes with a MUCH higher centre of gravity. My gut says you'd be hard pressed to make the same times as a vehicle with the far lower centre of gravity and much larger traction surfaces. $\endgroup$ – Smithers Jul 17 '15 at 13:31
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    $\begingroup$ Additional problem I haven't seen anyone mention: if a tank suffers a malfunction to its movement system, either from mechanical failure or enemy action, even when moving at speed it's not likely going to suddenly topple over on its side. For all the people touting "agility" and "quickness" of bipedal mecha, imagine the result of taking a hit to one leg that causes the hydraulics (or whatever) to stop working in mid stride. $\endgroup$ – Keith Morrison Nov 29 '17 at 23:43
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Barring some form of setting specific contrivance, none.

The goal in building an armoured fighting vehicle like a tank is to give it the best armour for its mass, to spread the weight out over the largest area you can so it has grip and won't sink into the ground, to give it the lowest profile you can so it is a difficult target and can hide behind terrain features, and in the particular case of tanks, to pack in the biggest gun you can manage so that you can beat the armour of enemy AFVs, and to provide stability for that weapon so you can fire it accurately, even when moving.

A flat, low to the ground box with enormous treads and a single big gun mounted in a rotating turret in the middle does this very well.

A giant humanoid is very hard to armour, It has a large surface area for its size and complex, multi-axis joints that are near impossible to armour effectively. This is also the major difficulty with human body armour. A tank is a compact box with a turret that has two single axis rotating joints making it fairly easy to armour. The treads are more difficult, but still easier than limbs, especially at the front.

Bipeds also have very high ground pressure compared to a tank. Even if given snow-shoe-like feet, a biped is going to find a much larger range of terrain to be "soft" than a similarly massive tank is.

Because of its low slung design and top mounted turret, a tank is able support a gun that is a significant proportion of its own mass. A mech the size of a tank would not be able to use a gun anywhere near the size of the tank's gun.

A tank is always laying prone compared to a mech and still able to move at full speed. A giant humanoid is an easy target. It can drop prone, but only if it knows in advance that it is about to be attacked, and it will take much longer to do so that a human would (it takes time to fall a greater distance) and will take more damage from doing so (Square cube law plus longer fall.) Afterwards it will be unable to move effectively until it stands back up.

The agility and familiarity of movement often suggested as benefits of mecha is also going to be hampered by the square-cube scaling issues. They just wouldn't be able to move like we can, any more than an elephant could move like a cheetah. Even with a full on neural interface that makes it feel as if the vehicle is your body, the limitations and change in scale would feel extraordinarily alien. It would be something like adapting to having osteoporosis, with ordinary ground feeling soft and slippery like mud, in low gravity.

With fewer degrees of freedom to worry about, it would probably be much easier to adapt existing training with other ground vehicles to tanks than body sense to mecha. Without neural interfaces or something similar, there's absolutely no benefit to a humanoid shape, and a massive increase in complexity in terms of controls.

Besides overestimating the agility a giant robot would have, many people underestimate the agility of tanks. Considering they weigh 40-50 tonnes or so, modern tanks are phenomenally manoeuvrable. They are also extremely stable while maneuvering, allowing them to attack effectively while on the move, and they are extremely resistant to being flipped.

Some have suggested that height gives an advantage in seeing the enemy, but there are tanklike vehicles already have telescoping periscopes that they can deployed without having to expose the rest of the vehicle. This gives the benefits of height without the drawbacks of raising the entire vehicle.

A tank is also just a lot simpler to build as it has fewer things that need to be moved around. It doesn't need a complex dynamic stability system just to keep it from falling over, and it doesn't need a neural interface or motion capture system. So you can build more tanks with the same resources.

So that leaves contrived reasons:

We have giant humanoid chassis from some outside source that are just better than any tank we can build for whatever reason: Robots built by aliens for some inscrutable reason, or maybe giant alien carcasses. We don't have the capability to make the same materials from scratch or to re-shape them into more practical forms.

Magic in some form is used to operate them and for whatever reason, magic vehicles work better if human shaped. The same goes if magic is called something else like 'psionics'.

They aren't meant to be practical. Like much of the Goa'uld technology in Stargate, there are psychological/social reasons to have a design that's much less effective than could be produced otherwise. The imperial walkers in Star Wars are probably best explained as terror weapons.

They are just better by authorial fiat because the author wants giant war robots. This explains them in most settings that have them including Gundam, Macross, and Battletech.

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    $\begingroup$ "Even if given snow shoe like feet" - do you mean feet which resemble snow shoe or snow shoe which resembles feet? $\endgroup$ – Kreiri Feb 16 '15 at 8:38
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    $\begingroup$ +1 for many reasons, but especially for "A tank is always laying prone compared to a mech and still able to move at full speed." This is a great simple visualization of the advantages of tanks. Back in WWI designers quickly realized the advantage of literally keeping a low profile, even if you're armor-plated! $\endgroup$ – user243 Feb 17 '15 at 1:44
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    $\begingroup$ Robotech (Macross) made the mecha more responsive by the addition of protoculture (magic). So there was more than just authorial fiat, they did some vague handwavium work. Plus, they were designed because the enemy was that size, so they were to give micronians a shot at competing when up against the larger Zentradi. $\endgroup$ – user3082 Feb 17 '15 at 7:49
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    $\begingroup$ @UIDAlexD because the question asked about tanks. A giant humanoid robot really wouldn't be very good at looking around corners either. It would be far more effective and practical to just put a sensor boom on the tank. Also, tanks operating in cities are generally working closely with infantry which can provide the ability to look around corners. If you want something more high tech, an integrated ability to launch multicopter drones for reconnaissance would be far more effective than a humanoid chassis. $\endgroup$ – smithkm Mar 16 '17 at 22:57
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    $\begingroup$ @kat0r: Yes there is...it's called mass. Mounting a tank gun on the arm doesn't just require the mass of the gun barrel. The Rheinmettal 120mm gun weighs 1.2 tonnes by itself, while the weight of its mount and recoil system takes it up to 3.3 tonnes. On top of that, you need an autoloader, and then there's the weight of the ammunition: a sabot round weighs over 22 kilos each, 24 kg for a HEAT round, and they're about a meter long. Where's the magazine going to be? $\endgroup$ – Keith Morrison Nov 29 '17 at 23:30
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The regular options, as smithkm said, are pretty limited.

I think the crux of the problem is this: Mechs are really complicated to build and incredibly specialist vehicles. But they have a form of chassis that is designed (by nature) to be incredibly generalist and adaptable.

Perhaps to reconcile the two, we should try to pull them closer together. I'm going to try and describe how Mechs might work as generalist vehicles.

Starting off with a question... what makes the difference between a soldier and a trech-digger? It's basically whether he's carrying his shovel or his rifle. The reason humans are still used extensively in many (almost all) fields of work, is that they can quickly adapt, use all sorts of tools, and fulfill many different roles.

The only way I see Mechs working, is if they can do the same. The first is to make the designs modular. The army might have tanks, excavators, minelayers, minesweepers, transports, and a dozen other kinds of vehicles, but a Mech army has only one type of vehicle. Every Mech uses the same core chassis, which is designed for flexibility. The arms can be detached and are interchangable and there's a number of plug-in slots for additional equipment (advanced optics, radar, communication, sensors, storage boxes, batteries, whatever a mission needs)

Ideally, most of this material is built not in large factories, but by small craft-robots. Ideally, it's built from components scavenged from the terrain, from other mechs, and by retrofitting existing parts. Of course, the craft-robots can build and repair each other as well. The only larger factories in a Mech army, would be the Mechs themselves (which should be quite capable of larger-scale manufactoring with the right kind of equipment)

This creates a flexible and adaptable force of units. A Mech might not stand up to a Tank in combat, might not dig as fast as an excavator, might not build as much as a factory, and might not carry as much as a transport truck... but if you supply an infantry taskforce with a single Mech it can suddenly do all these things if they come up.

"Everything" is something no regular vehicle would be able to do, but a Mech can take a pretty good shot at it. It makes some sense to equip (off-world?) exploration forces, fast landing troops, and hard-to-reach areas with a few Mechs to make them able to deal with whatever crap comes their way with a reasonable chance of success.

Consider them as flexible tool-wielders rather than machines of mass destruction. Rather than vehicles, which are tools used by people, a Mech is a tool-wielder used by people, but it's what the Mech is equipped with today that determines what it can do.

And of course you will see them in combat. After all, humans are still fighting, even though they are no match for a tank. But in an open battle, they fulfill a support role. And so would Mechs. They would shore up whatever gaps a force would have. Lost your anti-aircraft vehicles? Equip Mechs with surface-to-air missiles. Communication network down? Use Mechs as mobile radar outposts. Need to deliver ammo to an outpost on a steep hill? Attach the climbing hooks and load up a few storage boxes. Whatever the unforseen problems of war are, a Mech and a bit of engineering can at least fix the worst of it.

Mechs would be the among the worst options for a lot of jobs, but they would still be a better solution for any problem than "sorry, we have no specialized vehicle for doing this".

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    $\begingroup$ This. I Like this idea so much. $\endgroup$ – Feaurie Vladskovitz Feb 16 '15 at 8:44
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    $\begingroup$ Unfortunately, Tanks can be, and are already outfitted with bulldozer blades, winches, tow cables, and storage racks, or in combat engineering models, with cranes, excavators, and heavy mortars. The CEV models maintain a degree of commonality with the other AFVs in terms of training and logistics since they are based on the same platform. $\endgroup$ – smithkm Feb 16 '15 at 17:10
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    $\begingroup$ If you can put arms on a mecha, you could put them on the tank as well and have all the advantages and none of the deficiencies. $\endgroup$ – Oldcat Jul 15 '15 at 16:58
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    $\begingroup$ Except for climbing cliffs, traversing rubble filled streets, going through thick woods, and probaly half a dozen other kinds of terrain that require a very specific kind of tracked chassis. $\endgroup$ – Erik Jul 15 '15 at 18:06
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    $\begingroup$ A huge mecha can't do those things any better than a tank can. $\endgroup$ – Oldcat Jul 16 '15 at 0:04
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From a comment on What are the enabling factors for melee combat in modern or future settings? , consider melee or close-range combat in the context of nonlethal combat.

Suppose a very high premium is placed on not killing opponents but instead capturing them alive - religious or other moral reasons, for example. Heavy artillery and long-range indirect fire are ruled out by this, and instead combat moves to close-range incapacitation. "Tank shell vs mech" is no longer a problem as that would be considered a war crime. Meanwhile the tank becomes very vulnerable to infantry prising the hatch open. Also, you can't take infantry prisoner with a tank in the way that you can't with a submarine.

Everyone wears at a minimum NBC suits in order to avoid the obvious vulnerability to gas weapons. Grappling and nets become widespread. People add combat exoskeletons to overcome opponents with physical strength. The exoskeletons get larger and heavier. Conflict is closeup punchups until one mech is disabled and infantry apply can-openers and take the pilot to a POW camp.

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  • $\begingroup$ Wait - you are outlawing bullets as inhumane but allowing poison gas, currently banned by international law? This makes no sense. $\endgroup$ – Oldcat Jul 17 '15 at 19:17
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    $\begingroup$ Giant Mech punches will kill riders just as much as the percussion of an artillery shell. $\endgroup$ – Oldcat Jul 17 '15 at 19:18
  • $\begingroup$ @oldcat rules of engagement aren't necessarily universal. An insurgency, or despot aren't likely to have the same moral qualms as a world power. $\endgroup$ – apaul Mar 29 '17 at 20:17
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I'm going to ignore the science mostly, but here are the "traditional" sci-fi reasons for mecha over tanks based on what I've read:

  1. Terrain. Presumably a bipedal model can go more places than a tank. Whether it's climbing a cliff, fitting through narrow areas in cities, or jumping a pit, your mecha is more capable than a tank at getting around.

  2. Familiarity. A mecha moves like a human does, so if you have the right interface that means you can take advantage of 18-20 years of moving a human body and turn that into experience moving your mecha. Tank drivers start from scratch.

  3. Agility. A mecha can dodge, jump, duck, take cover, hit the ground to avoid a blast, etc. A tank can... tank.

  4. Flexibility. A tank is a weapon. A mecha is a weapon, but it can also double as a construction tool to build barracks, throw up walls, etc.

  5. Ammo. If a tank runs out of ammo it can ram things. A mecha can use a knife, punch, kick, throw rocks, throw enemies, or basically do anything an unarmed human can do.

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    $\begingroup$ What an unarmed human can do on the modern battlefield is mostly just die. $\endgroup$ – Oldcat Jul 16 '15 at 0:03
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    $\begingroup$ Well, there are recon and arty/CAS spotting roles that could be performed unarmed... But that's because they rely on being the opposite of a 25 foot tall 50+ ton turbine/nuclear powered beast. $\endgroup$ – Smithers Jul 17 '15 at 15:20
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    $\begingroup$ Also what makes a mecha more agile? $\endgroup$ – Smithers Jul 18 '15 at 0:58
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I think the question to ask about mechs is how could they be better than tanks, but how they could be different than tanks. For reasons outlined by other posters, the role of heavily armored combatant really favors the tank over giant mech (stability, area of armor facing, ground pressure of a massive vehicle). That leads us toward a non-tank combat role for the mech.

user6511 hit on the possible utility of a small, 9' size mech. I want to expand on this idea some more. Lets say we size our mech somewhere around 10-11' fully upright, less crouching or on all fours. Weighing in at around 3-4 tons. The driver sits in an armored chest, perhaps extending legs into the upper legs of the mech. The legs are a little squat, and the arms are proportionally longer than for a human. This lets it transition between bipedal and a "heads up" quadrupedal movement somewhat like a chimpanzee. Quad movement options give it redundancy in case of limb damage, to help negotiate poor terrain, and to let it lower its profile to make use of cover.

It has a remote weapon station with a medium machine gun and low light/IR optics. This is slaved to a helmet the driver wears, similar to that on an Apache gunship, allowing the driver to look around and aim the machine gun with head movement. The RWS carries smoke launchers and other defensive systems allowed by the tech level of the setting.

It has grasping hands, not dexterous, but strong, useful for building fortifications, moving ordnance, helping stuck vehicles, loading pallets, knocking down barriers, climbing certain types of terrain, walking on all fours, and stabilizing a heavy weapon.

In addition to its head mounted machine gun, it carries a heavy weapon. A .50 caliber HMG, a 40mm automatic grenade launcher, or a 30mm autocannon (probably only a low-velocity, helicopter gunship version). This can be stowed on the back of the mech. A couple of tubes of anti-tank missiles or recoilless rifles (depending on the need and wealth of the mech's suppliers) might be available. All weapons could be detached and used by supporting infantry as needed.

Armor would be centered on the driver's station. It's armored against heavy machine gun fire, with mounting points for autocannon resistant plates in the front chest and top of the shoulders (so if it's on all fours, the part facing the enemy is still armored). The limbs are armored against general purpose machine guns, artillery fragments, and maybe heavy machine gun rounds if armor technology allows it for the weight. They're not cannon-proof, but they're, hard to hit and easily replaceable. An Active Defense System gives it a chance to survive missile and RPG attacks. It doesn't get in fights with tanks without good cover and ambush positions, so it has no protection against main battle tank rounds (but what does?).

What does such a vehicle offer?

It gives infantry forces a source of vehicle-grade sensors, combat engineering support, and mobile heavy weapons that can travel in traditional infantry friendly environments such as urban rubble, dense forests, etc.

It is a combat vehicle that can travel at roughly the height of a walking infantryman, and adapt its shape by crouching or going prone to use cover against heavy weapons, extending its weapon station optics only far enough to peek over obstacles. Combined with its fairly small size, this allows it to use cover in ways that most combat vehicles cannot.

It is a combat vehicle that is easily transported along with the infantry. A couple could be loaded into shipping containers or on the bed of an HEMTT, or in a semi trailer. One could load into a MV22 Osprey with some infantry, or a couple could be carried by a Sea Stallion helicopter. A whole bunch would fit into a C130 transport. It could form the core of a rapid-reaction force that would be much stouter than an all-infantry force could be, but would still be essentially helicopter- or truck-mobile, with all the logistic and planning conveniences entailed by that.

In restricted terrain it would give some armor and heavier weapons to an infantry force to help overcome another infantry force. In such terrain, it could use its flexible geometry to take cover in places vehicles otherwise couldn't, allowing it to make attacks on heavier armored vehicles from an ambush, denying access to these vehicles.

That's how I see it, at least. I don't really know more than anybody else, though.

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  • $\begingroup$ If the mini-mecha is on all fours, then your pilot is no longer vertical, but facing the ground and hanging in his seat. But history shows that things that are not well armored are worse than useless on a modern battlefield - armor makes them slow, small size means less power, thin armor makes them easily detonated with various AT weapons. $\endgroup$ – Oldcat Sep 24 '15 at 22:03
  • $\begingroup$ If you want sensors, put them in a separate pod and stream data to the infantryman's helmet. Then if someone takes out the pod, the guy isn't dead. $\endgroup$ – Oldcat Sep 24 '15 at 22:04
  • $\begingroup$ The sensor pod definitely does not have the driver's head in it. The idea is to use an off-the-shelf remote weapons station. It's basically exactly what you describe. There would be a couple of different positions the driver could change to based on movement. A motorcycle-type crouch in the torso would be a possibility for all-fours movement, but some degree of freedom would be necessary to accommodate different geometries. $\endgroup$ – Michael Sep 25 '15 at 1:51
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Bipedal Mechs are not the Right Choice™

I am noticing a lot of answers focusing on bi-pedal mecha. I think this is a poor choice for many reasons and I will list them and then move to why I think they matter.

  1. (lack of) Redundancy
  2. Limits to locomotion
  3. Complicated joints
  4. Bad Height
  5. Awkward Stability

I agree with @user6511's post about bipedal mecha in some ways, especially for mecha under 9 feet. For small mecha, I think that bipedal may be viable. However, I think that it will fall short of expectations once it is bigger than what is essentially the marine armor from Starcraft.

My proposed solution is a hexapod (or octopod) mecha.
The additional legs would address all of the above points while maybe keeping the weight of the mech down and probably making it easier to produce.

Lack of Redundancy

If one joint is even slightly damaged the mech is going to come down. Human locomotion is extremely sophisticated and a surprisingly delicate balancing act. It could be worse than dead weight if it won't move properly. What is the solution? More legs!

In a hexapod mecha even if one of the legs is blown completely off it will be fine, in fact, it could theoretically still remain mobile with half its legs even if it is slow. That is a lot more than can be said for the biped.

Limits to Locomotion

Because of the square-cube relationship (where things get heavier way faster than they get stronger) its unlikely that a 2 legged mecha will be able to jump or climb very well, they will be dependant on a surface that can be walked on, even if it may be able to be more jagged or steep than a surface a tank could traverse. It would have to have very big feet for balance and to support the vertical weight. What could fix this? More legs!

Hexapods to the rescue. It has more points of contact, therefore, spreading out the weight over more space. Additionally, with all of the legs it has, it's very feasible to say that it could jump. Not as dramatically as a spider, but being able to leap a short distance in an emergency could be life-saving. I theorize that with the extra mobility options it would never have to though. It can easily traverse tall ledges, step across trenches or over roadblocks, and because of its many points of contact, it would definitely be better at handling steep slopes and jagged terrain than tanks or bipeds (although I guess not as well as planes technically).

Complicated Joints

Didn't I already talk about complex joints in point 1? Yes! But not enough!
The very minimum a bipedal mech would need for each leg is 3 joints... wait, isn't that the same amount of joints as in a hexapod mecha leg? Yes, but also no. The knee joint is simple, it just needs one axis of movement, hooray! But if you want your mech to be able to balance itself the ankle needs 2 axes or the mech needs to be moving its arms around a lot to throw its weight into the balance corrections. The arms moving around a lot seems like a bad idea for many reasons even if the weapons are not attached to the arms so I am assuming that the ankle either has 2 axes of movement or its a complex multi-part foot with either 2 joints in addition to the ankle joint or 4 total joints within the foot and no ankle joint. And then the hip joint, even assuming simplicity, will need at least 2 axes of movement. If it doesn't then the robot can't turn its legs unless you want to throw even more complexity into the feet.

So how does the hexapod fix this? Well, each leg needs exactly 3 joints, just like a simple bipod, but each of these joints only needs a single axis of movement for the robot to be able to do everything it needs to do. The joint attaching to the body only needs to be able to move in an arc parallel to the ground, and the two leg joints only need to be able to move perpendicular to the ground as you can see in my incredibly detailed and beautiful diagram here. Hexopod leg joints
(red arcs are arcs of movement)

So while it may have more joints total, the simplicity of them allows a lot more options for what can actually be used to control them and how they are manufactured and serviced.

Bad Height

Bipedal mechs kind of have to be taller because of the nature of how they work. The stubbier their legs are the harder it is for them to balance and move and the less benefit you actually gain from having them in the first place. They are easy targets because of their height, and being an easy target just makes it that much more likely they are going to get shot. Being shot is bad, so being tall is bad.

Hexapods are able to easily stay close to the ground, in fact, they can position themselves so they are only barely taller than the cabin (cockpit) that the legs are attached to and still locomote. They are also able to make themselves taller than their standard resting position if necessary to poke their weapons above a piece of cover or terrain.

Awkward Stability

Now bipedal mechs would have some sort of shock absorber on them for their weapons firing I am sure, so we can assume their own weaponry wouldn't pose balance issues for them, but what about taking heavy fire? Only having two points of contact (and relatively small ones for their mass) is not great when you are taking that much force straight to the armor plating. There will have to be constant balance calculations and adjustments made even for just normal operation out of combat and their stability just gets even more calculation intensive and delicate once they are in rough terrain and required to move quickly while possibly under fire. I am not a physicist but it seems to me that if its armor cant take the hits its dead, and if it can take the hits it might just fall over because of its high center of gravity.

A hexapod does not suffer from stability concerns. Even with several legs missing it is simple to move. Additionally, because of its low center of gravity and proximity to the ground it almost cant "fall over" even if it does get hit hard enough that its many legs cant stabilize it.

Hexapods are Boss

The hexapod mech has many advantages to biped mechs. It is able to maneuver through extremely complex terrain, climb very steep surfaces, stay low to the ground, can fit through almost any space that is at least as wide as the cockpit (leg configuration for fitting through narrow spaces), theoretically be straight up faster, and because of their simple joints should be easier to manufacture and maintain.

They would be advantageous over tanks for their mobility in weird terrain (because of their ability to increase their height they would be better at fording rivers. Tanks usually have Deep Wading but it takes setup time, endangers the crew, and any leak in the tank is bad news. Why not just walk above it?), ability to simply step over/onto/into roadblocks and trenches, and with the right attachments have any of the more advantageous aspects of a biped that other answers have suggested.

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Advantages in a large leg-driven vehicle would be the same advantages that are leading people to pursue legged robots: maneuverability in rough terrain. Dense forests, rocky hillsides, and urban cityscapes can all have barriers that prevent conventional vehicles from driving through them. Steep hills, as well, can be impassible for a wheeled vehicle. A legged vehicle or robot, however, can step around or jump over obstacles, and regain its balance if it loses its footing.

Human-shaped vehicles that are also human sized have the additional advantage of being able to maneuver through environments that were built to be inhabited by humans. Stairs, for example, are useless to vehicles, but a person in a powered suit of armor would be able to use them without a problem.

For vehicles smaller than cars, being human-shaped also provides the best protection for the human encased within. Compared to something like a motorcycle or an ATV, something like powered armor will offer greatly increased resistance to hostile firepower for the pilot inside.

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  • $\begingroup$ Don't make the mistake of conflating "legged" with "human-shaped". Most legged robots are not bipedal, but quadra- or hexa- (or even higher) -pedal. This is the reason bears fight on two legs, but run on 4. $\endgroup$ – Rob Kinyon Jul 16 '15 at 1:39
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    $\begingroup$ Also keep in mind, those legged robots are not multi-story, dozen-ton machine. Nor does much seem to have been put into protecting those 50 sensors, go kart motor, fuel tank, and computer bits from even small arms fire. $\endgroup$ – Smithers Jul 17 '15 at 15:25
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In both Neon Genesis Evangelion and Full Metal Panic the use of mecha is explained by the fact that the mecha they can build are much more advanced than what they can build in other form factors.

Basically, if people of the setting have some technology usable for mecha that is far more advanced than their normal level of technology, mecha will be superior to other forms of military technology.

Magic, there might be some mystical reason that requires human shaped mecha. For example the mecha might actually be transformed humans. Or the control system might rely on the mystical resonance between the body of the mecha and the body of the pilot.

External source, the technology is derived from an external source, possibly aliens, that has its own agenda and provides technology in isolated snippets. Aliens would do this on purpose to make reverse engineering more difficult. Other possibility would be technology retained from the lost dark age of technology in incomplete form.

Isolated breakthrough. People might have invented a very efficient and cool technology that they still can't apply in other ways. This happens sometimes, but usually the period is brief. Possible cases for the mecha are the control system and pseudo muscles used. First generation neural link might only work properly if the controlled body is similar enough. Somebody might invent pseudo-muscles that have ridiculously high performance and/or efficiency. I doubt this would last long enough for mecha to get very common or advanced.

More realistically, mecha are really between infantry fighting vehicles/light tanks and helicopters. They are not really designed to be tough and exchange direct fire with MBTs, although they would probably carry anti-tank missiles, they are meant to give higher mobility than most ground vehicles with less noise and expense than helicopters.

This points to some requirements that would help with mecha adoption. The obvious one is need for rapid response units. Mecha would be lighter to airlift than an MBT, could hold their own against light vehicles, would add heavier than man-portable weapons to infantry groups and require less logistics than aircraft. The legs would probably be designed to act as shock absorbers for jumps, so the mecha should be able to do air drops.

Similarly even in urban environments mecha would only have significant advantage if they can jump repeatedly significant heights or distances. Due to obvious reasons of geometry this would put an upper limit on the mass of the mecha. And require at least the legs to be fairly efficient and powerful. A very long legged vehicle, functionally similar to a helicopter that can hover noiselessly and without using fuel, but only at a low height, might also work.

Lack of noise might be important. Modern tracked vehicles can be fairly silent, but a legged vehicle could really sneak on someone. And if the terrain has some obstructions a mecha would have a better chance of navigating them silently and without collateral damage than a conventional vehicle. This might make mecha practical for suprise attacks and peacetime operations in urban areas.

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    $\begingroup$ How would the mecha platform be more sneaky than a similar massed wheeled/tracked platform? $\endgroup$ – Smithers Jul 17 '15 at 15:28
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    $\begingroup$ He could put on a hat and look like an innocent bystander, I presume. $\endgroup$ – Oldcat Jul 17 '15 at 19:19
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The primary tactical use for using combat mecha would be heavy urban combat in areas where it is difficult to fit a tank or other armoured vehicle. Mecha have the advantage that they would be more manoeuvrable over irregular terrain and in tight spaces, and would provide their pilot a significant force multiplier in these situations.

However, Mecha, especially of the humanoid variety, would have disadvantages in open-terrain combat in excess of that of wheeled or tracked vehicles, in that they have a greater surface-area to volume ratio as a consequence of their design, requiring more armour to be carried to protect a given volume of vehicle, thus making mecha less useful for carrying weaponry. Another disadvantage of humanoid mecha is that for a given volume, they would stand taller than an equivalent-volume conventional vehicle, providing a more exposed silhouette to the enemy.

Finally, mecha would have the additional disadvantage of a higher production cost and more complexity in maintenance due to the necessity for controlling limbs.

As to why mecha may have been fielded in the first place, one scenario I posited for an RPG game world was that mecha originated on a continent where cavalry animals became plague-bearers, and where (for various reasons) effective chemically-propelled projectile weapons had not been developed. As knights were unhorsed, they were forced to turn to infantry combat, which led to (literally) mechanised infantry which would use giant versions of infantry melee and muscle-powered missile weaponry.

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  • $\begingroup$ In your scenario, why would there be any drive for giant sized mecha? If you want knights, why not invent a mechanical horse for him to ride without making it 50 feet tall? $\endgroup$ – Oldcat Jul 20 '15 at 21:46
  • $\begingroup$ @Oldcat, because a mech with a human inside it can maintain its own balance without excessively complicated gear, but a mechanical horse must do that by itself and move given much simpler commands than those given to mechs. While neither is simple, the mechs are simpler than mechanical horses. $\endgroup$ – Monty Wild Jul 20 '15 at 23:14
  • $\begingroup$ Not so. The human has no way to forward his balance to the mech. The mechanical horse can always have three legs act as a tripod for stability. $\endgroup$ – Oldcat Jul 20 '15 at 23:18
  • $\begingroup$ @Oldcat, actually, a human would rely on their personal sense of balance, and since they are strapped into the mech, the mech's balance and the pilot's would become one. The pilot could then correct imbalance with natural movements. As for a horse that must keep three legs on the ground: how does it gallop, then? $\endgroup$ – Monty Wild Jul 20 '15 at 23:47
  • $\begingroup$ So can you dance on stilts without practice? Your balance doesn't translate into a huge device at all. $\endgroup$ – Oldcat Jul 20 '15 at 23:57
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It depends on what you mean by "mecha". If you essentially mean a powered suit, then making infantrymen into superheros is a good reason. Getting beyond that you start having to reach for reasons, and the practical answer there is life support (the mech carries the pilot's environment with it).

Incidentally, Heinlein explored both aspects of this to some degree in Starship Troopers (read it if you haven't).

I'm not sure how far in the future you are trying to tackle, or on this world or in/among others, but for terrestrial combat large mechs are just not very practical, so you need another reason -- but that reason could simply be organizational interia. For example, in the tricky and various terrain that exists on and among other planets and planetoids there pretty much isn't any such thing as a non-mech equipped infantryman, and it appears to be simply impossible to delegate all ground-level decision making to AIs (a fact we've continuously been confounded by in the real world). The great flexibility of delegation afforded a military force that can send actual humans to a site of battle (even if primary combat is robotic) will very likely make a mech-style infantry force a reasonable compromise in the real space age of the future. If this were true, it is also likely military industries will be tooled to build large numbers of mechs, but perhaps not many tracked or wheeled platforms simply because the utility of such vehicles is drastically limited in most space settings.

If the above assumptions were to hold, then a driving force behind the use of mechs in a terrestrial setting could simply be that mechs are the primary mode of available infantry. It would be an organizational compromise between general utility and cost effective fielding of units.

A strong parallel to that is the ridiculous vulnerability of MRAPs and armored HMMVs in real war (as opposed to a set of LAPD-style occupation focused mission statements). Unfortunately, though, the broader decision to double-down on Iraq-focused investments in the mid-aughts made this form of HMMV pretty much the only broadly available option for much of the US military, and quite a few aspects of force-on-force combat thinking had to change to accommodate this availability-driven fact. For example, an armored HMMV is so heavy that its original selling points (rough terrain negotiation; soft/hard ground compensation; negotiation of extreme slopes due to a very low relative center of gravity; general agility; utility in airborne (parachute), air assault (helicopter), and air-mobile (injecting forces from planes landed in an area held by the previous two) in soft or rugged terrain; etc.) were completely obliterated, and drastically change what maneuver options are available to a cavalry scout unit or infantrymen tasked with the traditional "11H" mission (I don't have a link for that, but basically 11H was motorized infantry; cav scouts serve a different role but in roughly the same configuration).

In terms of solving the practicality/political reality dilemma you would face in building a world where mechs are commonplace, this is the only reason I can see that is plausible from every angle. Some of the reasons for general terrestrial implausibility are covered in user6511's answer (read it carefully, and also remember that the targeting style of most large weapons is basically "point and click" so the insanely bad visibility of an upright machine is a bad thing).

A big part of the decision making is how plausible is "plausible enough" for you? The idea we are going to build a mech that weighs dozens of tons and can ninja flip, John Wayne slap, or otherwise go Chuck Norris in combat is a bit dense. Consider how much torque and oblique pressure would be applied per square inch on a single foot were a 50-ton mech to kick while standing on, say, anything other than a carrier deck or solid granite. Anything with soil would be a no-go for an even slightly agile mech.

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  • $\begingroup$ Surely if you have to worry about life support, all the pro-tank arguments apply too? E.g. a sealed environment chassis. $\endgroup$ – Sobrique Feb 18 '15 at 16:19
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    $\begingroup$ @Sobrique Indeed they do, but a tracked vehicle is distinctly less favorable when we consider the variety, extreme roughness, low gravity, and fragmentary nature of most of the terrain found among the smaller planetoid bodies likely to host colonies. How would you use a tank in an asteroid belt? A small planet of porous composition? A space station? Inter-ship? In free space? Tracks are simply not worth as heavy an investment in industrial tooling with this in mind. Mechs that can also use a multi-wheel configuration as footers -- this might be the ultimate solution (not sure). But tanks, no. $\endgroup$ – zxq9 Feb 18 '15 at 23:21
  • $\begingroup$ I'll go in for the combination of rough terrain and low gravity maybe. Hey you mention "the insanely bad visibility of an upright machine", but I can't tell from the sentence if you mean that the upright machine provides bad visibility to its pilot, or if it provides a highly visible target to the foe which is a bad thing. $\endgroup$ – Smithers Jul 18 '15 at 0:02
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    $\begingroup$ @Smithers A poor choice of words on my part, indeed. I meant that an upright target provides far too much visibility to the enemy. As time goes on the ability to detect a target tends to equate to the ability to hit it at any range, and I believe we can realistically expect this trend to continue in the future (if we do develop FTL travel, for example, I imagine missile engines will be replaced with a device to warp next to or within a target before detonation). So high visibility == low survivability, even today, and I imagine this situation will only intensify as time goes on. $\endgroup$ – zxq9 Jul 18 '15 at 5:45
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Instead of gigantic mech you can use powered exoskeleton which allows a soldier to carry more supplies (and armor), but require no control interface.

These would be used in terrain too rugged for a tank. And you cab supply deployed units by caravans of pack mules like Big Dog who know "follow the leader".

In less rugged terrain, tank will still rule, because they can pack more power into same cross-section.

One of the advantages of tanks not mentioned so far is defense against armor piercing ammo. One of the defenses is to have armor in shallow angle (deflecting the damage), which is much simpler in tank than in mostly horizontal mech.

Also, some infantry fighting vehicles are amphibious with very little preparation - try that in a mech.

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  • $\begingroup$ Mostly horizontal, or mostly vertical? $\endgroup$ – Smithers Jul 18 '15 at 0:16
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I think the question is not whether Mechas would be better than Tanks or not.Is infantry better than tanks? It depends on the situation. So I think if we can build Mechas armies would use them for warfare too.

Scientists have researched artificial muscles for a long time and I think if they become successful Mechas could be build. Many of you have given square cube relationship as a point against Mechas.According to this new artificial muscles they are at least twice as efficient as human muscles. And I think this would at least negate that point.

If we could build mechas I think they would be excellent against infantry units. Mechas don't have to be very large. Even if they are 9ft tall I think a group of them would fare well against infantry.

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  • $\begingroup$ Not if the infantry is dug in and has good lines of fire they won't. Square-Cube means that even major improvements only give incremental improvements - the cube root of 2 is just a bit over 1.25, so that makes only a 25 percent improvement possible. $\endgroup$ – Oldcat Sep 24 '15 at 22:07
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I think some kind of mech is more likely in the near future than we might think. Two major advances will radically alter ground warfare in the near future. For the last century, offense or destructive weapons have been more powerful than defense weapons. That is about to change.

Body armor is already beginning to reverse the trend to lighter rifle rounds.

Since WWII the trend was smaller bullets, at higher velocities if possible, but always with the idea that a smaller bullet against human flesh was just as good as big round, especially if you put several close together.

But in the last 20 years, body armor has begun to reliably stop the most common round sizes, usually the 7.xmm rounds of common assault rifles. The US has brought back the venerable M14 (7.62×51mm) and even .50cal (12mm) Beowolf to augment the M4 (5.56×45mm)

An exo-skeletopn that let a soldier carry a hundred pounds of extra body armor would be immune to most light arms fire. This would set off an arms race between heavier but slower firing rifles trying to punch through ever increasing mechanized body armor.

Computer assisted aiming will also require body armor. Todays weapons are largely about "spray and pray" compared to automated targeting, because a human cannot spot a target, or part of a target, aim and fire before the target moves. Modern crewed, medium and "assault rifles" rely on the ability to put enough rounds into the air in a small unit of time that some will hit a target. Humans are just not fast enough to take aimed shots at other humans who can move just as fast to avoid exposing themselves to the shot.

But a computerized system move faster than humans. They see in "bullet time" in microseconds and human scale motion appears either nonexistant or takes the equivalent of hours.

There is a bullet stopping system used in VIP protection that detects incoming bullets by radar and fires a Kevlar air bag to popup and catch the bullet. That's how fast these systems work.

A human aimed gun would fire thousands of rounds across a field to try to hit some of the 50 opponents peaking out a trench 300m away. A computerized system would be able to carefully draw a bead on each individual fire one round into eye of each target. It would be the equivalent of firing rate of rotary mini-gun that targets like a sniper with all the time in the world.

With that kind of system your going to hit, so you have to absorb it with armor.

The historical analogy here would overhead airburst in WWI, leading to the return of the armored helment. The burst laid down such a pattern that almost everyone underneath got clipped. The only solution was to armor the head. This time will have to continue armoring everything, especially the face.

DEW (Directed Energy Weapons): These are getting scary good to the point they can incept artillery shells and either destroy them or knock them off course.

Very soon, military units will move under a line of sight dome of an intercept grid through which no projectile above a certain size,not moving at hypersonic speed, will survive.

Now, if you have a powerful DEW system, then crouching out of sight may not be as useful having elevated perspective to see incoming threats so the DEW can eliminate them. Besides, given modern battlefield sensing systems, hiding is becoming increasingly problematic. Instead, you'd want a tower, or tall vehicle to put your sensors and DEW projectors up high to give them most perspective and range.

Naturally, DEWs will become anti-personel weapons despite treaties. A thing that can hit a missile can hit a mobile human hundreds of times before they even begin to fall. Worse, they can hit individual parts of targets precisely as in eye,eye, trigger finger, weld the rifle receiver shut, then set of a grenade on the belt.

So, again, armor is the only solution.

Now, the nature of armor itself might change. It has to face two threats, kinetic weapons and DEWs. Kinetic weapons that move so fast they can penetrate the DEW grid are likely so high energy they can't really stopped, so they may not try. Such projectiles might have to be slender and solid so the best strategy might be to just let them punch though.

DEW weapons don't penetrate, stop them on the surface, absorb or deflect the plasma shock, and your good. Anti-DEW armor is not heavy or dense but focuses no deflecting energy and reflecting plasma from vaporized materials when the DEW hits. Such armors to date are more thick and fluffy than dense and heavy. Sometime a mist of water or certain liquid plastic prove very effective.

So, a tall, tank-analog DEW shielded mech might not be that heavy at all.

I could see a new future in which tall DEW projectors will be escorted by mech suited infantry wielding slow firing hyper velocity rifles and/or anti-personel lasers. The DEW stops airpower, arching artillery, slower missiles and RPG. The infantry will protect the DEW unit from someone running out a hiding spot and beat it to pieces with a crowbar.

Battles will beginning with low intensity DEW attacks in the thousands targeted at enemy sensor systems which will be constant, followed by precision attacks on vulnerable areas like joints or the unfortunate unarmored. The hypervelocity kinetic weapons will be shot at armored targets hoping their velocity will prevent the DEW grid from intercepting.

(Note that with hypervelocity weapons, hiding behind obstructions or even laying below the line of the ground won't provide much protection, as such weapons can punch through any building and dozens of meters of earth.)

This will not be any place for civilians. The sensor suppression will be blinding. Automatic DEW grids will likely target anything that moves. Even if not, mere reflected energy either from DEWs or shattered material from hypervelocity rounds will threaten all unarmored individuals.

Primary Defenses will BE

  1. Blind enemy senses and sensors with DEW, Jammers, decoys etc
  2. Damage or slow with precision DEW attacks against surface components
  3. Fire hyper velocity dumb projectiles at targets progressively shredding like cannon balls through age of sail wooden ships
  4. Try to intercept of deflect incoming enemy hyper-velocity projectiles.

On factor that will bring large vehicles, mech or not, is that the more powerful energy sources you can lug along the better your DEW defense and offense. It will become a race something like that in battleships from 1899-1936 where the biggest guns and armor won.

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  • $\begingroup$ "Todays weapons are largely about "spray and pray" because a human cannot spot a target, or part of a target, aim and fire before the target moves." Care to back that up? There's a reason infantry small arms are often limited to a burst mode instead of full auto, and are constructed to reduce the cyclic fire rate. Also might review the apparent silliness of comparing a DEW detonating unarmored explosive ordnance which is made to explode vs a DEW eating through armor, at range, with any rapidity... when said target is not moving on a predictable ballistic trajectory. $\endgroup$ – Smithers Jul 18 '15 at 0:15
  • $\begingroup$ The very fact we use automated weapons, especially burst fired at a single exposure of a single target, means we put a lot of rounds in the air to get a hit. That's that taken into account unaimed suppressive fire. DEW cannot penetrate armor though it can cripple and wield surface elements and in some instances cause sprawling. Projectile defense relies less on destroying the projectile, than deflecting it by smacking repeatedly on on side with balls of plasma. This will cause the projectile to veer off or tumble. $\endgroup$ – TechZen Jul 18 '15 at 18:56
  • $\begingroup$ You are NOT describing spray and pray. $\endgroup$ – Smithers Jul 18 '15 at 18:59
  • $\begingroup$ Few active or projected anti-projectile weapon rely on detonating a weapon charge, instead they seek to tumble or deflected the projectile by producing balls of plasma or super heated air next to it or on its surface to knock it around. In one technique, they cause pressure above and vacuum below and just drive the projectile into the ground. In others, they just smack it consistently in one directly. DEW can blind guided weapons and can destroy some lighter control surfaces. Microwave based DEW can fry electronics and sometime detonate charges. $\endgroup$ – TechZen Jul 18 '15 at 19:07
  • $\begingroup$ Well, not by the original or common usage of the term, I'll edit. But in relative context I would argue it is. I suggest you read up on the "SPIW: The most dangerous weapon that never was", (if you can find a copy.) It’s a book about the US Army trying to develop as flechette rifle in the late 50s and 60s precisely to get around the problem of not being able to hit targets with firing a lot of rounds. The M16 solved the problem by putting three light rounds close together, doing as much damage as a single large round. Modern firearms do not do one shot one kill, future automated ones will. $\endgroup$ – TechZen Jul 18 '15 at 19:13
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Roles of the Abrams. Tanks have a significant role in war, if we could build a Mech it would be a boost in fire power and technology plus it could give the US an edge in warfare, (Not saying other countries will do the same) with the abrams and its 120 mm gun, it is a powerfull weapon of pure destruction. Although it suffers in close range combat. you have to rely on the armour to save the crew. the speed of the abrams is quite admirable compared to its weight. The abrams it -self is a heavy tank used mainly by the US military. It is an amazing piece of machinery. its made for pushing the line and killing all of the opposition but at the same time with a surviving factor. Mech's Role in warfare: A mech could be an astounding feat in military tech, we are able to create mech's its just that our military doesn't want to spend time or the money to make a new weapon that may or may not be practical. Requirements Speed: 45-75 km/h weaponry: any and all that will be able to be installed depending on the variation of mech (infantry, Support, Anti-air and long range support) Height: 15 - 25 meters Tall Material; lightweight yet strong. Suggestion: Titanium/cobalt alloy or aluminum/titanium alloy. you need a strong engine for the beast as well. Diesel and Gasoline would make the Mech loud. Go with the turbine-like engine but it is modified to take the weight of the mech. crew: 1 Piolet 5 Pit crew: 2 Mechanics, 1 Electrical and Programing Chief, 2 reloader's There would have to be constant repairs.

Tracks vs. Legs . Tracks: good traction, great with rough terrain. higher speeds achieved on rough terrain (excluding small rocky terrain) High Durability.

Tracks Cons: low traction on ice or other terrains that is flat and slippery. Break when explosives are applied with great force and cannot be repaired quickly on fields. and can be disabled permanently and only made for salvage if damaged enough. Loud when stopping

Legs Pros: Great on hilly terrain and best for urban combat. Open fields will give you your max speed. Better at traversing obstacle terrain by stepping over or around objects such as road blocks. Better manoeuvrability with swaying and left to right movements. Faster acceleration. Can be modified and Repaired with less time depending on the complexity of the leg itself. More responsive. less sound unless running. Optional for modification for various terrains from ICE to SAND (X-shaped feet with spiked pins for ice and flat wide feet with v-shaped trends for sand) Better stability and can stop faster making less noise.

Legs Cons: Constant repairs and evaluation. Can not hold as much weight depending on what type of system it is using ( Pressure, Servo or Gyro). Damage to the leg can be fatal, just like a track, if damaged enough can be disabled, legs have an advantage in this however in the silhouette of the legs beeing long and thin or short and large( medium size or 35% - 45% of the mechs height is in the legs for longer strides. more Parts, A small servo or Gyro could be hit making it off balance.

Weaponry: Abrams: 1x 120 mm cannon HEAT and APDS 2X 50. cal Guns

Mech: any size cannon below 180 mm Missiles. Machine guns. high Calibre rifle (like a 35 or 75 mm High-Velocity cannon for taking out APC's and Smaller Vehicles) Any type of ammo for the cannon.

Mech styles and loadout: Infantry: 2 Missle Pods and 2 high calibre Machine guns (20 mm) Body type: 2 legs, tall and slender for good manoeuvrability Amor: High speed: 55-84.6 km/h

Sniper: high Calibre rifle (57 - 120 mm Cannon) Body type: 2 legs, short and thin for good concealability Armour: medium speed: 45-75 km/h

Support (artillery): 155-180 mm field Artillery Body Type: 4 Legs/ Spider like ( recoil Compensation) Large Bulky but Armour: thin armoured speed: 15-25 km/h

Scout/Recon: small calibre semi-automatic cannon (30-57 mm cannon) for taking out small vehicles and encampments of soldiers. Machinegun's if possible. Jamming capabilities 1 missile pod (4 to 12 missiles) Body type: Short and slender and light-weight Armour: Light Speed: 65-100 km/h

Mech/Tank Destroyer: Large calibre High-velocity cannon (120-155 mm cannon) ATGM's and EMP/jamming capabilities. Body type: Short and bulky Armour: HEAVY (to deal with multiple impacts without penetrating Speed: 20-45 km/h

Anti-air mech: High fire rate (25-35 mm flak) Body type: medium armour for protection from bombs and strafes Speed: Medium of 45- 65 km/h

Tanks have been the dominator of the battle for a long time since WW1.If we had made the time and money to research into mech's we could make a super weapon and get ahead of most other countries. Mechs can have the ability to climb, crouch and other physical Abilities Tanks cannot. They may or may not be practical in a situation but I am sure as hell they could get the job done.

Overall it definitely would be a good idea to spend time into making these machines, we could also use them for construction, medical, firefighting, exploring out deep ocean, ETC . There are endless possibilities for design and purpose. It would cost money but It would be worth getting rid of old tech that is obsolete. The mechs dont have to be huminoiid in shape you can make a sleek or bulky one depending on the job!.

Though replacing all of the old stuff would create imbalance for the economy in the US, WE would have to make specialised Factories to make the mechs in the first place. also What do we do with all the abrams tanks you ask? use half or 25% of them to be used in a shooting range to test new weaponry of course! again. I could take a lot of time to get the mech project going but it would be a super great idea

Other Info: Mechs need a good armour value to be able to deflect and take shots, though tanks can get better angles a mech still has the ability to make a fear factor and it has better maneuverability (not saying it will dodge bullets). AP rounds can be blocked with composite armour and that's why I suggested, Titanium/Cobalt or Titanium Aluminum. Though we could also use HEAT Shell screen's (look up on WW2 tanks) These would block RPG's and HEAT rounds from tanks. we could also add ACTIVE protection systems to protects against ATGM and Indirect fire weapons such as mortars. the tank will still have an advantage in angle but not manuverability or vewrsatility of a mech because Of the high modification abilities.

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In the Warhammer 40k the Space Marines have Dreadnoughts to preserve combat experience.

Basically only the most honoured of the Space Marines who are mortal wounded are salvaged from the battlefield (the rest have their gene seed extracted) and put into a coffin attached to a Dreadnought.

These are quite like a tank being quite wide, short, and can carry very heavy weapon loadouts.

Another advantage they have over the Space Marines tanks is close combat abilities so that jump jet equipped infantry can't as easily get the drop on them.

more info on Dreadnoughts

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  • $\begingroup$ Haha! I was actually inspired by 40k to ask this question, truth be told. Although more along the lines of Imperial Knights rather than dreadnoughts! $\endgroup$ – Feaurie Vladskovitz Feb 17 '15 at 1:08
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We think of tanks as superior because most of our battlefields in recent memory favor tanks. Iraq, WW I & II, and part of the Korean War all favored tanks. But the fact is, tanks are useless on a huge amount of the earth's surface, because they can basically only travel over roads and cleared terrain (deserts, plains, etc.). In fact, tanks were quite useless in Vietnam. Since most of the land that people who own tanks want to possess also favors tanks, this matchup makes sense. But Vietnam showed us the limits, and almost all fighting was done by infantry and artillery, as well as bombing. So even though mecha-favorable terrain is easy to contrive on other planets/asteroids/ships/etc., there is still plenty of mecha-favored terrain right here at home.

Consider also why nature has not created wheeled creatures: although there might be plausible limits to biology which discourage or prevent the evolution of such animals, we can at least consider the benefit of legged locomotion. Legs have one extremely obvious benefit over wheels: when a leg is injured, most creatures can still move. When a wheel is damaged, most wheeled vehicles are in trouble. In general, the only wheeled vehicles which can continue moving over meaningful distances after wheel damage are explicitly designed for robustness (e.g., the Stryker[1]). If a tank loses just one link of one tread, it is pretty much out of commission until the tread is repaired. This is a pretty dangerous position to be in on the battlefield, and it is usually safer to abandon such a tank than to sit in it while it is used as target practice.

We already have small [6-legged] robots which can learn to walk after losing a leg[2]. Even a bipedal mecha may be able to limp along or fashion a crutch if one of its legs becomes damaged. In a worst case, it can probably crawl, which may still be better than just sitting there spinning on a single track.

Modern technology is quite often optimal-but-brittle. When it works, it works great. When it breaks, it often becomes quite useless. Biology, on the other hand, tends to be good-but-resilient. It usually trades optimal solutions for robust ones, because surviving is more important than that last 10-20% of performance envelope.

Especially if I were trapped on another planet, I would much rather be in a mecha than a tank. If I got stuck in a bad situation and had to run, it may be easy to get trapped via terrain in a tank, and much easier to escape and evade in a mecha, especially if my enemy knows my capabilities. Then you have damage resilience w.r.t. locomotion, and finally you have melee combat.

Humans are not dangerous creatures because we have giant claws (we have puny, pathetic claws) or sharp teeth (we have tiny, brittle teeth) or massive limbs for pummelling (we have fragile hands and feet). Humans are dangerous because they can turn almost anything into a weapon. And that is why humans are at the top of the food chain rather than the middle. So while a tank can indeed ram things, using its momentum and mass as a blunt force weapon, it has no ranged attack other than its main gun and maybe a coaxial machine gun, which have finite ammo stores. A mecha can fight as long as it has energy, even if it has to use boulders and debris as weapons (but as the Japanese have taught us...always carry a sword!).

Of course, a mecha may be able to climb walls/terrain, whereas tanks generally can't do this. And even just jump jets would allow a mecha to clear obstacles which are totally insurmountable for tanks. The simplest way to stop a tank is called the Czech hedgehog[3]. This is nothing more than a few pieces of iron welded together. It doesn't even move! But it pretty much impedes the motion of even the most advanced tanks. Or, you have the even simpler solution of coils of concertina wire[4]. None of these devices would presumably stop a mecha. In fact, trying to devise fixed defenses vs. mecha would almost certainly consume the efforts of a modern war college. Passive defenses would basically be useless. Even a high wall could be defeated by grappling/climbing capability, not to mention jump jets. Only active defenses would be effective (turrets, missiles, directed-energy, etc.). Mines, trenches, and hills at least force a tank into a particular path (e.g., focused-fire deathtrap), but could be easily evaded by a mecha with jumping capability.

Then consider armor. While a tank can presumably carry more armor than a mecha, a tank's armor is integral and is not easily or quickly replaced when it is damaged or consumed (in the case of active armor). And the armor is fixed, and thus must apply to all sides of a tank which might be vulnerable (front, sides and top). This makes a tank much heavier than optimal. A mecha can carry extra armor as well, but limit the armor to just the expected line of fire. Just like a low-tech sword has "infinite ammo", a shield can be just as effective for a mecha as for a tank, but can be trivially replaced in battle. And since the mecha doesn't need to carry the armor on non-threat sides/flanks, it can afford to carry much less of it than a tank (though at the loss of protection from flanking attacks, of course).

Since tanks are cheaper than mecha, the mecha could be deployed like Spec Ops: soften up/disable the defenses so that the cheaper and more numerous tanks can sweep in and pummel the enemy. But the richest actors would just field all-mecha armies, because the mecha are more flexible, whether human piloted or remote or AI. They consume less energy than aircraft, because they don't need to fly most of the time, and thus, they can carry heavier weapons. If you need to cover large distances quickly, you can have transports just like you do for infantry: both ground- and air-based. You may even be able to deploy mecha via low-orbit rocket, drop-ship style. It's not safe or efficient to deploy infantry this way, but it might make sense for mecha.

Finally, consider that mecha may serve a role much like nuclear weapons do today: the threat they pose changes the way nations behave, whether the weapon is actually used on the battlefield. Nations may simply forgo tank defenses and focus on defenses which are also effective against mecha. This might actually work in favor of tanks. If nobody builds mecha, then this approach is not necessary, and nations just bulk up for conventional warfare. If some nations have mecha, then any nation which fears them will have to defend against them, at considerably higher cost than non-mecha armies. Thus, everyone will want to build enough mecha to pose a threat, even if it is not enough to lead a full-scale invasion on their own.

  1. http://www.wikiwand.com/en/Stryker
  2. http://www.engadget.com/2015/05/28/robot-crawls-with-legs-damaged-aww/
  3. http://www.wikiwand.com/en/Czech_hedgehog
  4. http://www.quora.com/What-are-some-effective-tank-traps-in-a-modern-warfare-environment
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  • $\begingroup$ There's a lot here to take on faith. The links are helpful to an extent, though! $\endgroup$ – Smithers Jul 18 '15 at 0:41
  • $\begingroup$ Tanks already limit armor to the expected firing arcs. If a mecha did the same, all its agility would do is expose the relatively unarmored flanks and rear to enemy fire. $\endgroup$ – Oldcat Sep 24 '15 at 22:00
  • $\begingroup$ If you can make jump-jets for a 60-tonne mech, you can make them for a 60-tonne tank, too. A legged vehicle would have better ground clearance than a tank, but I can't quite picture one climbing a cliff, and with feet smaller than tank treads they'd have problems on soft earth. Are you sure a mech would be more all-terrain than a tank, if they both have jump jets? $\endgroup$ – user243 Sep 24 '15 at 23:40
  • $\begingroup$ @Oldcat Giving up armor would only make sense in a scenario where it could be traded for increased maneuverability. You wouldn't give a mecha a portable shield if you wanted it to stand in a circle of tanks/artillery. You would do it to support a fast attack where the enemy can only fire from one direction due to offensive initiative. A tank cannot move all its armor to the front during a frontal assault, but it still pays the weight cost in speed/manueverability of side and rear armor. $\endgroup$ – Lawnmower Man Sep 26 '15 at 19:47
  • $\begingroup$ @JonofAllTrades Tank suspension is clearly not designed for jumping. Re-engineering tanks to sustain jumps would fundamentally alter their design. Legs are made for jumping. An M1 Abrams weighs 60 tons and has tread pressure of just under 1 kg/cm^2. A mech with 2 square feet 5'8" on a side or circular feet 6' in diameter and the same weight would have the same ground pressure. $\endgroup$ – Lawnmower Man Sep 26 '15 at 20:28
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To answer the question of "Why would we make a robot that looks like a person?" we should first ask "Why do people look the way they do?" That is, what problems did the human form evolve to solve?

We know that the development of intelligence guided our species' evolution. As our ancestors became smarter, the ability to make good use of intelligence became increasingly valuable. To prehistoric hominids, that meant tool use, which required appendages that could effectively manipulate the environment in a wide variety of ways. So the most obvious reason for having mechs is that you need machines that can effectively use a wide variety of tools that are similar to those that humans would use, but on a much larger scale, or with more durability or strength. This need for adaptability is going to underlie most of the reasons presented here.

To give a plausible near-future scenario, let's say for instance that we want to send a construction team to Mars to build a colony for the first civilian inhabitants. An individual mech might be larger than an individual construction vehicle, but it can do the jobs of many different vehicles. Weight is an extremely important factor when it comes to space missions, and a team of mechs would add up to weighing far less than an entire fleet of construction vehicles.

The example of Mars also brings us to another possible reason: you're fighting on a planet that has very different gravity than Earth. With human soldiers, you'd need to train them how to move and fight in many different gravitational environments, which could be very difficult: you could be on an Earth-like planet for one mission, a rocky "super-Earth" for another, and then after that you could be on a dwarf planet like Pluto or Ceres. With mechs or robotic soldiers, instead of having to retrain your soldiers, you can just recalibrate them for the new gravity.

Another reason could be preservation of combat experience, like with Dreadnoughts in Warhammer 40,000. Perhaps military training or willing soldiers have become extremely rare, but an injury in battle has put them beyond saving by conventional medical means, to the extent where they can only be kept alive by extensive prosthesis or life-support. A Dreadnought needs to be as large as it is because it has to contain an entire suite of life-support systems.

Now, regardless of which explanation you choose to use, you are going to need to also explain two things: first, why not just use an artificial intelligence, and second, why is an on-site rather than remote-control pilot necessary?

Answering the first question isn't too difficult. There are sound ethical arguments that have already been made in the real world arguing that general AI should be avoided. For instance, there's the classic objection that creating a human-like AI to perform a specific task of your choosing amounts to slavery. Perhaps the risk of tech-savvy hostiles hacking your AI is too great. Or perhaps you'd go for something more like in Warhammer 40,000: AIs, lacking a "soul" or some other essential and intangible element of humanity, are not able to feel the Emperor's presence and therefore are much more vulnerable to corruption by Chaos. Or maybe they just haven't been invented yet.

As for the necessity of the pilot being on-site, this can also be explained very easily. An enemy could, with only very simple tools, easily hijack or simply jam the remote control signal. In the real world, this has already been done with UAV drones, and the US military invests considerable time and effort into keeping their UAVs safe from being taken over by hackers.

So while we might not be seeing Gears or Gundams or Titans anytime soon, it's by no means the most outlandish thing to ever appear in science fiction.

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More then likely the evolution of legged vehicles in combat will not go directly to mechs. More then likely you will start with a kind of power armor ranging from 8 to 10 feet tall. Their systems would not be fielded until they were at least as maneuverable as a humans. The major advantage of them would be increasing speed, strength and hopefully survivability of the pilot.

The increase in strength is what would make tanks extinct. Right now a man portable AT missile is very heavy and bulky and the average man can carry 1. Now imagine a suit of power armor that is at least as fast as a man but more then likely more so with anywhere between three and six AT missiles on its back you now have advanced infantry that can kill six tanks for every one man and realitively hard to hit with weapons that will cause significant damage in one shot.

As these suits are refined and perfected, wheeled and tracked fighting vehicles will become more and more rare because the power armor is so effective. You will likely see normal vehicles relegated to supply and transport duties. So then what happens when everyone has power armor you start trying to outdo the next guy eventually that will lead to suits being specialized with assault suits growing in size to hold more armor counter measures and weapons until eventually they may be the 3 story tall monsters we see in fiction and the other direction stealth or recon suits getting smaller lighter maybe even capable of flight to the point they are almost a second skin.

So it would take years for tanks to leave the battlefield but when they do which will probably be because the anti tank systems are so deadly in smaller more maneuverable power armors they won't come back because people will see wheeled and tracked vehicles to be old fashioned ideas and will prefer to build and expand on at that point existing power armor mech technology.

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Versus a tank, in conventional tank-warfare situations, a mecha is likely to lose, for all the reasons above.

So – where might it be useful? Let's look where we don't have tanks warfare:

  • much of the work done by allied forces in Afghanistan etc. was done without tanks, but smaller vehicles. Why? I think manoeuvrability in a city which you're trying not to destroy with big tanks. A mecha here is good. Top speed isn't an issue, but fairly quick direction changes, fitting into more awkward spaces, etc. could be more valuable.

  • psychological warfare. Often, tanks are used as a big threatening thing against small people / vehicles. (Think many coups, patrolling an occupied city, ...). A mecha could have an even stronger psychological value, due to the increased height, and the familiar humanoid silhouette.

  • fighting smaller things than tanks. Tanks can't bring their big weapons to bear very close, due to the way they're designed (the turret's elevation). Whilst obviously countermeasures exist, they're relatively vulnerable to humans with satchel bombs, etc. A mecha can be designed to bring all its weapons to bear at any range, any angle. It can kick humans. So against a mix of smaller troops / vehicles, it could be at an advantage compared to a tank.

  • extremely difficult terrain. A multipedal tank in particular should be able to traverse much more difficult terrain than a tracked tank. (e.g. GITS's think-tanks). It could potentially climb buildings and similar. This could allow heavy armour to reach all kinds of places you can't normally get it to (mountainous regions, etc.)

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There's some good answers here, and pretty fundamentally - if you use 'real world' mechs just don't work.

So I'll offer instead an excuse why you might use one, despite tanks being generally better. With reference to Dune - in the setting, a lot of combat is hand to hand, because personal shields exist. Personal shields deflect projectiles moving at any significant velocity, and make the user largely immune to guns and explosions as a result.

So they resort to close combat, because of the relatively lower velocity, but higher force-per-unit-area of swords/knives etc. *

I daresay you could apply a similar fudge in your world. Invent the ubiquitous shield generator, making people move away from ranged combat to melee, and suddenly your mech-with-sword type combat unit becomes more viable.

For bonus points, have the optimal velocity/force to penetrate a shield related to size, which means there's a useful strategic advantage to bigger mechs.

You do still have the pretty fundamental square-cube law problems as other answer-ers outline though. So you'd need something pretty compelling to over-ride that.

* Other weapons do show up - there's artillery when shields aren't used because of sandworms, and lasguns which aren't used normally because they explode when they shoot a shield. But both are only effective in an environment where shielding is artificially inhibited by environmental factors.

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    $\begingroup$ You could still mount your close combat weapon and shield on a tank body and have the low profile advantage in this model. $\endgroup$ – Oldcat Jul 17 '15 at 19:23
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Bipedal mecha are not better than tanks, in any size (see square/cube and also see balance).

Best will be quadra-, hexa-, or higher -pedal robots that can deploy in semi-autonomous swarms. A single 50-ton "super-unit" will never beat 50,000 1kg units, especially when those 50,000 units are coordinating across dozens of meshes.

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  • $\begingroup$ the problem with 'ants' is that their small size would likely limit the power they could exert on a tank, just like your 2 year old punching you doesn't hurt much. $\endgroup$ – Oldcat Jul 17 '15 at 19:21
  • $\begingroup$ Enough 1kg 'ants' that are actively trying to worm their way into every crevice and jam every moving surface or blind every sensor will quickly overwhelm any larger opponent, especially when they're working in concert. $\endgroup$ – Rob Kinyon Jul 17 '15 at 20:20
  • $\begingroup$ still aren't strong enough to break through thick armor readily, and how do you outfit and fill with gas 50000 units with any facility? And countermeasures would be relatively easy. $\endgroup$ – Oldcat Jul 17 '15 at 20:22
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    $\begingroup$ And who provides the intelligence? nothing in this world posits intelligent robots of any sort. Mechas and tanks are run by men. $\endgroup$ – Oldcat Jul 17 '15 at 20:23
  • $\begingroup$ Armor is useless without sensors or movement. Those are always the weakest points of any military force, especially armored platforms. "sem-autonomous" swarms. These sorts of robots have been under development for decades. You give the swarm a problem to solve and adjust the parameters, then let the swarm determine (through trial and error) the best paths to solve the problem. No "intelligence" required. $\endgroup$ – Rob Kinyon Jul 17 '15 at 20:25
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I only read the first view answers so I'm not sure if anyone mentioned this in later replies but the square vs cubed scaling problem mentioned in the first couple of replies assumes the Mech would be solid, or mostly solid, throughout like an animal body. But would they be solid? Most machines aren't.

Assuming you can build your Mech very big (if the metal can withstand the stresses - ask an engineer because I don't know) it's overall density could be low, very low. That's why ships float and planes fly - their density is low because they are mostly empty space.

Following is a simple calculation for a 30 meter sphere made of steel at a density of 7.85 kg/litre:

Volume = 113,097,335.5 litres Weight under 1 gravity = 887,814,083.9 kg pretty damned heavy, right.

But in reality it's not going to be solid metal, though it would make an awesome wrecking ball if it were.

We would want it to be heavily armoured so lets say the shell is 30 cm thick solid steel.

The inner empty sphere then has volume of 109738231.6 litres.

The volume of the shell is 113097335.5 - 109738231.6 = 3359103.9 litres

Under 1 gravity the sphere would weigh 26368966.1 kg

Density = mass divided by total volume

Density = 26368966.1 kg / 113,097,335.5 litres = 0.233 kg / litre

Density = 233 g / litre

Less dense than water, less than the density of a human, it would float on water, just like a ship. A man squashed into a ball would not float as well.

Now it's mostly empty space, so that leaves a lot of room inside to put some very big engines, which of course it going to need and of course they are going to increase the weight but even if they weigh as much as the hull, it's still going to be a lot less dense than a man.

A sphere is the best case scenario but the same logic would apply for any shape as long as it's mostly empty space - give it solid hammer fists maybe but keep it mostly empty everywhere else.

Would the metal fail under it's own weight at that size, thickness and weight? I don't know - ask an engineer. If steel couldn't take the strain are their any alloys that could? Or substructures within the steel that would make it stronger?

If the independence day aliens could solve the problem and build an intricately designed mothership with mass equivalent to quarter of our moon's (so over 10^19 tons - they must have built it using degenerate matter from a collapsed star) that somehow didn't collapse in on itself, hopefully we can solve the problem for a comparatively lightweight materials like steel.

Then again why bother, hurling a giant wrecking ball weighing 887,814,083.9 kg at your enemy is going to do more damage than an army of giant mechs, if it connects. Better not miss though because it won't be easily retrieved.

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Intimidation could be an advantage. A mecha could serve as a symbol of military superiority. Also take into account the psychological impact of seeing a giant metal humanoid towering over a defended area vs a tank. The tank is still intimidating but a giant humanoid shape would tap into a primal fear of someone being much larger and stronger than you. It probably wouldn't be worth the cost to make an army of large mecha but if a few were made and used strategically they could be useful for crushing enemy moral

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The only real reason for mecha is for show. It's big, powerful and shows the enemy how technologically advanced you are. It's just like aircraft carriers. They are more show than anything else in a real war.

Tank are really more for show when a modern soldier can carry weapons capable of destroying them.

The key to future warfare is cost and numbers. Why build a giant robot as big as a building when you could build a million explosive drones for the same money?

See Slaughterbots

The only possible real use for a mecha I can see is perhaps as a mobile drone command centre. It could also back the drones with artillery fire against hardened sites.

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Who says 2 legs mean you need to walk?

Hopper.

frog hopping http://www.montananaturalist.org/frog-jump/

The hopper can go places a wheeled vehicle cannot. It can go places a walker cannot. It can throw itself over impediments blocking the way of wheels and legs. It can throw itself over holes and trenches. It can soar up, shoot from above, then land and hunker down.

Using large surface area feet that unfold like umbrellas, it can move along the tops of trees or across swampy areas.

The movement of a hopper is difficult to predict and so it is harder to hit.

Downside: your mech does not look like a badass giant samurai. But your pilot can still wear a badass samurai costume if you want.

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There is one thing missing in all the answers : all weapons always change in reaction to enemy. Modern values in warfare turn around the idea, that you are going to fight fellow human with similar level of technology. This means battles are waged at long distances and with idea of air superiority.

But imagine enemy, that is able to deny you the air superiority and that would be able to quickly close that distance even after hard shelling and still be able to trample you. How is tank helpful when you have 5 big-jawed enemies 5 meters away and closing? How is jet helpful, when both his wings are gone?

But what if you had a mech, that can use jets to hover 5m above the ground, move quickly around and get in and out of enemy group to be able to strike into enemy's back and while using the enemy itself as a shield?

Exactly how mechs work in Muv-Luv.

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    $\begingroup$ Keith Laumer's Bolos had anti-personell charges mounted around the perimeter for just that problem. Real tanks have machine guns. And if the mech could use jets to hover, so could a tank. Dave Drake's Hammer's Slammers have hovertanks, although they don't go high. $\endgroup$ – Oldcat Jul 15 '15 at 17:02
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    $\begingroup$ It's hard to imagine any technology to fight creatures that could destroy a tank with jaws. $\endgroup$ – Deolater Jul 17 '15 at 15:53
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    $\begingroup$ It's hard to imagine any technology to fight creatures that could destroy a tank with jaws--which would suddenly be useful when applied to a mecha chassis. $\endgroup$ – Smithers Jul 18 '15 at 0:30
  • $\begingroup$ It's also hard to imagine creatures that could destroy a tank with its jaws. $\endgroup$ – Oldcat Sep 24 '15 at 22:09
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First off, mechs with arms which are most commonly referred to as a gundam styled mech, uses opposable arms which match the shape and combat style of people. But by mimicking the shape of a person, that limits the possibilities a single mech can hold. Keep in mind the mechanics that are required for functional arms and legs. By mimicking people, the arms will have a limited carrying weight. The legs which would be limited by the over all weight of the mech and arms, plus whatever is equipped to the arms. This is a lot of hydraulic strain for two legs plus the arms. So gundam styled mechs, while possible, would be not be helpful on a battle field.

Some mechs could have arms that have little to no movability. This means less hydraulic strain on the arms and more strength can be focused on the legs. Not to mention, if body no longer goes by human shape, mechs could have a tank like, turntable torso, which allows for 360 degree visibility, as well as quick utilization of weapons instead of turning the head to look, and then the torso to react.

The torso contains a lot of important electronics and equipment like the reactor, and all the main electronics to deliver power and hydraulic fluid to all the parts of the mech. Then the legs, which supports literally every part of your mech. They need to be VERY strong. The strength of the legs is what will limit what can be equipped to each mech. On the left and right are the arms or weapon pods. Mechs don't have to have arms. The best example of this is the UrbanMech. (Picture below) Urbanmech example

The arms will most likely be where the majority of your weapons will be located, as well as maybe one or two on the torso. With both arms having weapons, one can be sacrificed to keep harm from the torso and cockpit.

Now, with size and speed differences of each mech, there will be separate classes. There are light, medium, heavy, and assault mechs. Light bots would be small, fast, and have small weapons. They are commonly scouts, or deployed in groups to take out heavy and assault mechs. They could usually outrun most larger mechs if need be. Light mech example - locust

Next are medium mechs. These mechs are used to fill the gaps between the small and fast, and the big and strong. They would be used as foot soldiers mostly, but can fill a variety of roles. Medium mech example - Nova

Then there would be heavy mechs. These mechs would be slow, and very, very tough. Their size often hinders them against light mechs. They could carry heavier weapons than most mechs.

Heavy mech example - Hunchback

Then there are assault mechs. They are about as heavily armored and slow as heavy mechs, but can carry more weapons instead of just bigger ones. Once again, someone in a proper light mech can out speed them, but just like heavy mechs, they are best taken down by at least a medium mech.

Assualt mech example - Kodiak

And that's pretty much it. The real world application of mechs such as this could be limitless. They carry, and can survive hits from weapons way larger than what is carried by a tank and survive bombardments of missiles.

Sure two legged robots are usually unstable, but look at not only how big they are (it would be hard to knock them down with really anything) and the base of their feet are quite large. While they would cost more than most other vehicles of war, all other ones would really become obsolete. A simple AMS (Anti-Missile-System) could defend against anything airborne, while enormous guns would enable the mech to protect itself. The engineering is there, but the military hasn't put forward the effort to make these war machines reality. They could be more dangerous than anything we have ever created.

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  • $\begingroup$ The question is asking "Will a walking mecha ever be a practical option, relative to a tank or plane?" This reads like a fluff piece for the categories of mechs in a particular franchise. $\endgroup$ – sphennings Nov 29 '17 at 0:44
  • $\begingroup$ You need to pay attention to what I said about size comparisons versus tanks. They are bigger, have heavier, thicker armor, and as you can probably imagine, the legs make them more maneuverable. I cant do all the thinking for everyone. It provides an answer by giving a picture of where all equipment will be put and the uses of different classes of such machines in comparison to tanks and planes, as such mechs can carry larger than normal weaponry, and withstand more abuse than modern war machines. This was all explained. $\endgroup$ – Micah Da Canon Nov 29 '17 at 18:05

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