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An engineering enigma: the useless "wings" behind giant robots.

enter image description here

"Bold design choice."

"What? I like anime."

"Worst-case scenario, we'll end up with the world's most expensive Gundam model."

―Tony Stark and Erik Killmonger

More than an aesthetic choice:

Mecha are a staple memorable aspect of Japanese media. Who doesn't love the sight of implausibly large robots duking it out? In some cases, the creators go as far as to take inspiration from real technology to make their designs feel more grounded in reality. If you know the saying "form follows function" you know where I am going with this. A common design choice in mecha is to endow them with little wings or spikes on their backs (look at the image above). I like details like these, it adds to the silhouette of the robots and makes them look more imposing. While I know it is just an aesthetic choice, I want to uncover their true purpose from an engineering standpoint.

Potential uses?

Looking at the "wings" at face value, they do not participate in any sort of mechanical movement. Typically they move on an x y axis: bending up, down and shifting their angle sideways. Only in some instances do the wings serve as thrusters, usually backing up the main thrusters situated in the legs. The mecha desings tend to have exaggerated busts, possibly as protection for the core, so the wings may serve for balance? They do not seem particularly designed for being antennae either, the head would be a better placement I assume. That leaves us with cooling, which may be accomplished by multiple means. As a rebuttal for that theory we never see any exhaust from the spikes.

The real goal here:

If you have read this far without downvoting it means you are invested in the potential function of mecha "wings". My goal here isn't to ask a random question about giant robots. What I want is a functional application of these limbs to better design my own robots accordingly. Doing "whatever I think is best" isn't my style, I prefer facts and data that adds that sweet taste of realism to fiction.

[Edited] Wait... what were the giant robots used for again? Media portrays them as war machines. I'm using them for my robot designs, especially the larger robots. Their purpose is more mundane like as construction robots or as vehicles to explore hostile environments. The technology level is futuristic. Note that a "mecha" doesn't need to be skyscraper-sized, a human-sized machine can still be considered a mecha.

Answers should present a given function for the "wings" and explain WHY this design choice would be logical from an engineering standpoint.

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    $\begingroup$ @Joachim Marvel's What If. $\endgroup$
    – KEY_ABRADE
    Nov 16, 2021 at 22:20
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    $\begingroup$ Guessing at the reasons would be heavily dependent on the specific location and shape of the "wings" on the mecha, making this question largely unanswerable. In one configuration they might be sensor masts/AESA panels, in another they might be heat dissipaters, in a third they might be aerodynamic control surfaces, in a fourth, they might be counterweights to aid movement, etc. There's just nothing meaningful to be said without specific examples. $\endgroup$ Nov 16, 2021 at 22:20
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    $\begingroup$ thrusters may be a better purpose than you think, especially if thy can move, a thruster out on a mobile lever arm makes for very precise control. $\endgroup$
    – John
    Nov 16, 2021 at 22:53
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    $\begingroup$ This reads much more like a attempt to have a conversation about an aesthetic trope of mecha media than a specific question about building a fictional world. What can you tell us about your mecha? $\endgroup$
    – sphennings
    Nov 16, 2021 at 23:33
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    $\begingroup$ Re "If you have read this far without downvoting it means you are invested in the potential function of mecha "wings"." No, it means I don't have an effing clue what you mean by "wings". All I see is a picture of some boxy equipment with red lights. $\endgroup$
    – jamesqf
    Nov 17, 2021 at 16:40

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A few thoughts:

Your suggestions are actually not as bad as you think. Antennae could be built into such structures, but routine communications are likely handled by the smaller ones mounted on the head. Radiators aren't a bad idea, and they wouldn't vent exhaust because their job would be to get hot, not output smoke or steam.

  • Steering: If your robot has jet assist, they may act not as rockets, but to deflect the jets in various directions to control flight. While your robots are far from aerodynamic, a few small control surfaces (especially directing exhaust gasses) could really help.
  • Rocket armor: These robots are often portrayed bristling with numerous small rocket launchers. They could either contain sleeves of such rockets within, or deploy in such a way as to cover up such batteries to prevent damage. They might even be able to deploy the equivalent of shields to drop over the most vulnerable spots on the body or to cover up damaged locations where the armor has been degraded.
  • Loading arms: On the same vein, these could be mechanisms used to load ammunition into the mecha. When backed up to an ammunition vehicle, these arms mate with loaders on the vehicle and allow super-fast flow of missiles, shells, bullets, or fuel pods. The high-explosive rounds never have to leave a protected environment. Such dedicated loaders might explain why we never see mecha backed up on the battlefield waiting to reload missiles that they seem to fire off constantly like confetti.
  • The un-sexy backpack: The less exciting use of these arms is to secure fuel, food and ammunition pods onto the mecha so they can operate autonomously away from supply vehicles. Such awkward, ungainly and ugly attachments are used up and dropped, or detached during battle to prevent damage. The arms of the mecha are limited to what they can do. These things can directly pump fuel from pods and pick up items from behind the mecha that the effective but poorly adapted front arms simply can't.
  • Identification: These could be the ultra-modern equivalent to banners carried into battle. All your mecha have the same basic ones, and an enemy would be executed as a spy if he tried to imitate it - it's like putting on a uniform. So what if you don't want them all the same? While jamming can make comm traffic impossible, and IFF equipment can be tricked by electronic counter-measures, the potential pleomorphic nature of these added (and possibly non/low-functional components) parts means that a unit can see that there is Steve with his orange and brown stipes waving in the air like he does. Bob NEVER puts his wings up like that, I think that's really an enemy pretending to be an ally!
  • Folding wings/deployable equipment: What you see is not what you get. These are the mounting points of large deployable wings or parasails that guide the mecha as they fall from drop ships, then fold up or drop off on landing. Or maybe these are where the mecha is mounted inside the drop ships to keep them secured in flight.
  • Tortoise syndrome: Maybe your mecha are less sophisticated than you think. To enhance running and jumping, the limbs have limited functionality. If the thing is knocked over onto it's back, it can't get up on it's own (too much armor and weight). These are actually armatures to hoist the bot up to where it can stand up on it's own.
  • Shock absorbers: On a similar note, the poorly designed head or body is vulnerable to crushing if the mecha is thrown or dropped. These armatures are like huge shock absorbers that point up or back to absorb body impacts, while down and forward are covered by the legs and arms.
  • Optical targeting arms: The modern battlefield has so much EM interference that conventional radar and satellite targeting systems are useless. Even laser scopes have that new (insert gizmo here) that stops them from being useful. Optical sensors mounted out on these extensions allow long-range targeting with depth perception. They are movable to adjust to conditions or shelter behind the body when not in use to prevent damage.
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    $\begingroup$ A nice list full of very plausble ideas, I love it! $\endgroup$
    – Hobbamok
    Nov 17, 2021 at 10:18
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    $\begingroup$ @DWKraus The tortoise syndrome and shock absorber a both really good functions but the un-sexy backpack... it seals the deal. $\endgroup$ Nov 17, 2021 at 14:03
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    $\begingroup$ I would add that having ammunition stored in a separate compartment behind the body means that it is harder to hit, and that if they are hit, it is more likely that the body, or at least the pilot survives. Modern tanks have separate ammo compartments with blow-out panels for this same reason. $\endgroup$ Nov 17, 2021 at 15:57
  • $\begingroup$ dynamic counterweights, so the robot can stand upright, however heavy the weight on its arms $\endgroup$
    – bukwyrm
    Nov 19, 2021 at 22:11
  • $\begingroup$ To add to this list: perhaps a tail of sorts. Many animals, notably cats, use their tails for balance. Go find some slo-mo of a cheetah running and watch the tail. $\endgroup$ Nov 20, 2021 at 5:43
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The fins behind mecha are molten salt heat sinks. Firing directed-energy weapons repeatedly produces a lot of residual heat, unless a superconductor is being used to transfer energy. To dump this heat quickly, you need massive radiator panels.

Instead, the solution is to run a molten salt-based cooling system throughout the mecha, which strips excess heat away from the guns and pumps it into heat sinks so that it can be disposed of over time, rather than all at once. This means that you don't need massive radiator fins.

The fins are behind the mecha to avoid getting shot.

They're long and wing-shaped because, while their core is a heat sink, their surface is a radiator designed to slowly dump heat from that heat sink.

They're outside the mecha because it gives more internal volume inside the mecha for components, and more surface area outside it for the radiator.

They're also outside the mecha in case they get hit; high-temperature salts dripping through the insides of a mecha would massively compound any damage sustained.

They're also outside the mecha in case they need to be ejected, since they might overheat if the radiator panels are damaged or covered in some kind of substance that reduces their heat-exchanging capabilities.

They're also outside the mecha in case they need to be repaired, since reaching into the guts of the thing is harder than working on an external component.

They're also outside the mecha so that they can be hot swapped out for fresh packs that haven't been saturated with heat energy yet without having to open up any paneling.

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    $\begingroup$ I was going to go with a different medium, salt is pretty corrosive, but yeah heat dissipation is the logical answer. $\endgroup$
    – Ash
    Nov 17, 2021 at 8:24
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    $\begingroup$ I also object to molten salt. Molten salt heat transfer is without a phase-change and therefore only allows for quite slim gradients. A heat-pump based system would allow the critical bits inside the mecha (superconductors for example) to reach almost arbitrarily low temperatures, while heating up the wings to far higher temperatures, thereby increasing the heat emitted there. en.wikipedia.org/wiki/Heat_pump $\endgroup$
    – Hobbamok
    Nov 17, 2021 at 10:15
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    $\begingroup$ @Hobbamok fine then, they are water ice heatsinks $\endgroup$
    – user253751
    Nov 17, 2021 at 11:26
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    $\begingroup$ @Gillgamesh no, salt is used because it does move a lot of heat quickly because it has an impressive heat capacity per volume, aka you only need to pump very little. But heat pumps as I mean it rely on a state change of the transport medium so that the radiator can be actually hotter than whatever we're trying to cool $\endgroup$
    – Hobbamok
    Nov 17, 2021 at 13:57
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    $\begingroup$ In BattleTech, mechs carry extra coolant so in emergency they can flush the hot stuff and inject fresh cool stuff into the system. One more great reason for them to be away from the body $\endgroup$
    – Andrey
    Nov 17, 2021 at 18:57
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I can't resist pointing out that there is a historical parallel of a powerful military unit having mysterious wings for no obvious purpose. I am speaking of the Polish winged hussars. According to Wikipedia:

The Polish hussars were renowned for their huge "wings", a wooden frame carrying eagle, ostrich, swan or goose feathers. In the 16th century, characteristic painted wings or winged claws began to appear on cavalry shields. The most common theory is that the hussars wore the wings because they made a loud, clattering noise which made it seem like the cavalry was much larger than in reality and frightened the enemy's horses; however, such sounds would be impossible to hear in battle. The wings (or wing) was mostly used to block the back of the rider from swords or protect them from getting thrown off their horse.

I am not sure the horse theory is relevant to the robot example.

The noise theory could correspond to some sort of antenna or scrambling device, which has been suggested by other posters.

The "block the back of the rider from swords" theory makes sense to me. If the robots' heads are as delicate as ours, then it makes sense that you'd want to protect them as much as possible (perhaps not from swords, but from falling debris etc. (although why you'd put all the most fragile stuff in the head, I don't know))

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    $\begingroup$ Which horse theory? The 'protect from falling backwards off one's horse' theory seems very relevant to robots, youtube.com/watch?v=g0TaYhjpOfo $\endgroup$ Nov 18, 2021 at 21:51
  • $\begingroup$ @PeteKirkham You just made my day :D $\endgroup$
    – slebetman
    Nov 19, 2021 at 7:44
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    $\begingroup$ I think initially, such wings or other battle adornment were initially to instill fear in opponents that never seen this before. Inspiring visages of monstrous winged creature or flying horses. after time, the foe learns that these are just flashy soldiers. however, the use of this flair become tradition and trademark of the unit and is used for 100's of years after their effectiveness has vanished. becomes a cultural image, but still create images that instill fear in ones foe $\endgroup$
    – Sonvar
    Nov 19, 2021 at 22:01
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Patent avoidance.

The companies that patented mech arm designs didn't consider having nearly useless counterweights and didn't draw them in their preferred embodiments, resulting in a loophole that an overly cosy judge in some district favoured by patent litigators allowed.

Engineers have come to like them, as hitherto the idea of useless space was absolute design heresy, resulting in many late nights and redesigns whenever marketing or corporate HQ asked for a last minute feature. Forgot to allow for a sufficiently larger auxiliary fission battery? In the wings! Corporate HQ wants to make sure that noone flies faster than Mach 7? The speed module goes in the wings! In companies that value their mech pilots, the right hand wing is often reserved for pilots to carry personal cargo. Naturally, such systems are not resistant to neutron bombardment and so there's no way they are going in the torso.

The only downside is having to read reviews of one's mech in Mech Magazine where they gloss over having a 16 GW fusion annhilator beam as well as total mass clocking in under 15T, but it losing 2 stars out of 5 because of an "uninspiring set of wings" that "doesn't match the sensibilities of the 2370 A.D. consumer".

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    $\begingroup$ Yes patent circumvention! I love it. $\endgroup$ Nov 17, 2021 at 18:36
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    $\begingroup$ Or creating an otherwise useless design, just to match it with some patented interface .. This way, the customer cannot buy the airdrop units "M3CH" of the competition, because the "WINGS"-adapter frame to hook them up to the drop ship carrier are patented. $\endgroup$ Nov 17, 2021 at 18:41
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    $\begingroup$ This has historical parallels. The Wright Brothers patented their aircraft control design and prevented others from using it. They controlled their plane by warping the wings. To circumvent the patent other airplane builders used hinges to control the ailerons. The circumvention turned out to be easier to manufacture (though less efficient in theory) and easier to adapt to different materials. $\endgroup$
    – slebetman
    Nov 19, 2021 at 7:48
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  1. Reinforced phased array receivers/emitters for threat detection designed to give 360 degree coverage for the detection of incoming high velocity threats. In order to provide maximum coverage they have to be mounted as high as possible on the chassis while not getting in the way so the top of the back is the best compromise location.

And while one 'wing' would do the job engineers soon discovered that the system worked best with two emitter/receivers working in tandem as this improved accuracy significantly enough to be worth the extra cost/weight. (Two separate systems spaced even slightly apart lets you use trigonometry to get a better fix on a target.) Plus it adds redundancy.

  1. Threat interception pods. Spaced along the leading and trailing edge of each wing behind hinged panels are a number of compact threat interception modules each containing a small number (2,3,4)? of short range countermeasures tied to the threat detection system and designed to be fired automatically at a threat just before impact.

The defense system is directly linked to the detector which, once turned on actives autonomously from other systems onboard the robot. In effect this makes the the 'wings' almost like a second (dumb) robot mounted on the back of the first. This lets the robot focus on other aspects of the battle while the wings do the work of detecting/stopping incoming threats. But importantly real time longer range imaging of the battlefield is also fed continuously to the robot which can then focus the arrays if it chooses to on particular points of interest for a better 'look' at whats going at that location. However doing so for any length of time is however 'risky'. This is because it reduces the radars/lidars coverage of other parts of the battlefield and also reduces the system's ability to detect incoming threats. So if done it's done for short periods of time only. (Bit like sticking your head over a parapet to get a better look at who is shooting at you! Yes you can do it but you don't want to stand there gawping.)

  1. EM Weapon mount although the two main uses of the system are described above if the opportunity arises the phased array emitters can be focused for short periods of time on sensitive targets and used to jam/blind/damage them by focusing high power EM onto/into them. (Again though this reduces coverage.)
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Other than pure aesthetics, there can be some functional reason these robots have these appendages.

The best purpose of these is to help with in flight control.

  1. These can act as control surfaces. many modern airplanes have many control surfaces to assist with control and maneuvering at high speeds.
  2. They can also contain small thrusters to help in vector control, mostly for control outside of an atmosphere where aerodynamics will not work.
  3. Last on control, these can, at least, provide a means of counter balance. These mechas are not very streamlined and not very functional in flight. to counteract the motion of something as an arm or leg in flight, can cause the mecha to get taken off balance. these can act to counter such imbalances. The weight of an arm could be around 10% of the whole weight of the machine, not counting any weapons mounted to it. That much weight shifting in flight can cause extreme disruptions.

Big picture, these mechas are not truly built for flight. Its a purely aesthetic choice to have a humanoid like body plan. To make them functional in flight, you have to make design considerations to keep them functional with your fashion choices.

Auxiliary functions

  1. These mechas use a lot of power and produce a lot of heat. you cannot have the heat sink close to the main body of the machine, or else the operator will get toasty. Having an external component to allow airflow across them will keep the macha cool.
  2. These can carry modular accessories that can be changed out based on the mission it is being sent on. These "wings" can be surface to fit varying sized devices from a suite of sensors, extra armaments' to extra storage space for a camping trip.
  3. the ability to change the RADAR cross section at will. Due to it being a humanoid shape, its not going to be stealthy. Might as well use this to your advantage. A highly articulated "wings" can be shaped to simulate a Cessna, a small jet fighter or what ever shape you want the RADAR operator to think you are.
  4. Damage mitigation. These can act as extra armor or shielding, or be sacrificial material to take damage rather than the core body. Placed on the back can protect the mechas blind spots from damage.

I have seen many anime, such as Macros, Gundam, Exosquad, etc. All these options have been used throughout them.

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    $\begingroup$ Balance for a bipedal robot would be my first guess too. $\endgroup$ Nov 17, 2021 at 8:48
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They could contain cameras that can look around corners.

In the anime series Appleseed, whose mechs and cyborgs have similar sorts of fins, these sort of back-mounted fins mount cameras in the ends that allow the operator to look around corners when breaching buildings, without exposing vital portions of themselves to enemy fire.

Here's a video of them being used this way, in a clip from the 2004 movie:

https://youtu.be/dJEvkKuMzqE?t=292

Killmonger mentioned that he liked anime, and the first Iron Man movie was released in 2008, so it's possible in-universe that he drew inspiration for them from this movie specifically, even.

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    $\begingroup$ Nice example video! The actual fins/ears seem a little too overbuilt to just be camera-mounts though. I get the distinct impression they serve several purposes. Maybe housing for comms-antennae or radar. Situational Awareness as a whole is a big problem when you otherwise only have a few turret cameras to play with. $\endgroup$
    – Ruadhan
    Nov 19, 2021 at 12:33
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In Gundam, MuvLuv and Full Metal Panic franchise the wings have multiple purposes, some of them are heat sinks used to cool vital components. Others act as stabilizers to help the robots maintain balance when they move, or they contain stabilizer or orientation jets that can be used when in flight/to act as jump jets.

Some of this is take from real life where heavy construction equipment will have protrusions from rear of movable component that act as a counterbalance

In the Gundam Wing universe , several models of Gundam has their wings as shields. For example, in Eternal WAltz the lead Gundam wraps its wings around itself to act as a heat shield from beam weapons and for rapid orbital reentry, while Duo Maxwell's Gundam has wing like structures that wrap around itself to act as radar dampeners.

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Even if wings don't have enough lift to fly, they can still 'push' against the air for steering and stability. Like Velociraptors. enter image description here

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Mountings for drop-tanks

Mecha have very limited internal space. That much should be pretty self-evident. So much so that fuel and power is often a recurring plot-point in shows featuring them.
The Evangelion for example was physically tethered to a nuclear reactor and could only operate for a few minutes on battery power.

So, to facilitate longer operating times, most mecha mount droppable fuel-tanks on fins on their backs. The fins themselves can fold away when not needed, but extend out to release the empty tanks and reload with new ones.

It's useful to drop them rather than simply refill because the switchout is much much faster than pumping fuel into onboard tanks.

This same system can also be used for a variety of other liquid and gas resources, such as Coolant, Hydraulic-fluid and even onboard air supplies in hazardous environments.

The upshot is that most mecha benefit enormously from having the capacity to rapidly restock their consumables in the field.

The actual racks of consumables are housed on the back of the mecha to keep them out of the way and protect them. As well as to share mountings with backpack upgrades, and to link to docking facilities better.

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We all have seen how these amazing robots jump and reach the enemy real fast for close combat one-on-one action.

There is where I find it useful, since those little wings are meant to control the body of the robot at those gigantic speeds!

Also, makes it difficult for other robots to apply some krav-maga style killer grab, punch or kick from behind, so is useful as a mano a mano fight shield too.

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