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OK, I'm putting together a bunch of concepts for a interstellar military, one that sits between the Colonial Marines(Aliens) and the UNSC(Halo) in terms of technology and equipment. Lasers exist, but are used less by the interstellar forces due to the added complexity, weight, and reduced effectiveness on the planet they are fighting on which has a denser atmosphere with a large amount of particulates. Also because of this most fighting is a relatively short range. Most kinetic weapons are 'normal' by todays standards, and missiles and micro-guidance packages are considerably cheaper than today. Given that they have to lug everything across interstellar space a lot of the equipment and vehicles will be intended to fill multiple roles.

How effective, then, would a tank armed with a Vulcan(or similar) rotary cannon be?

I envision it as similar to the Merkava crossed with the Colonial Marine's APC. It would have decent armour, although maybe not as much as a MBT. Good mobility and speed, perhaps through wheels rather than tracks. And it would be able to carry a few soldiers(4 to 6) as well as its crew.

The idea behind using the Vulcan is that it can scale its firepower. Against a tank it fires on full auto, against lightly armoured targets it can fire burst mode. Also, if the turret is correctly designed it can also engage aerial targets. this should give it greater flexibility than a conventional MBT, although at the cost of making it less effective at being a tank.

The problems I see are the vulcan not being able to damage a MBT without the high angle attack it gets when used in a A-10 attack aircraft, and the turret being to heavy for effective anti-air work. The latter can be fixed with 'its the future! engineering!' but the former is more of a problem. Would it be more effective to have a dual or quad mounting of small bore HV cannons instead, perhaps light gas guns?

In summery;
1) Would a armoured vehicle with a rotary cannon, and the ability to engage aircraft, be a more flexible option for a interstellar military than a conventional tank?
2) Would multiple HV cannon mount be better than a rotary design?

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    $\begingroup$ Why not just use scouting drones to designate targets for rods from heaven? Sounds alot easier than landing craft down, just destroy everything from above before the enemy has a chance to react. $\endgroup$ – FiringSquadWitness Sep 14 '15 at 5:15
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    $\begingroup$ Remember Newton's third law of motion, now your chain gun have to worry about both the recoil and the centrifugal force. This thing is the forefather of rapid firing weapons it shouldn't have a place in interstellar combat anyway I must admit it will look badass on any armored vehicles. $\endgroup$ – user6760 Sep 14 '15 at 6:44
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    $\begingroup$ Like user6760 said, if you want something reasonably believable, you must not forget Newton's third law of motion, or for that matter the tyranny of the rocket equation. $\endgroup$ – a CVn Sep 14 '15 at 7:28
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    $\begingroup$ Why is the whole thing not just done by drones? $\endgroup$ – Tim B Sep 14 '15 at 11:46
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    $\begingroup$ I thought Vulcans were a peaceful people. $\endgroup$ – PyRulez Sep 14 '15 at 18:49

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Just a few general-purpose suggestions to go along with this quote:

“There has never been a military in the entire history of the human race that has gone to war equipped with more than the least that it needs to fight its enemy. War is expensive. It costs money and it costs lives and no civilization has an infinite amount of either. So when you fight, you conserve. You use and equip only as much as you have to, never more.”

― John Scalzi, Old Man's War

1. Minimize moving parts: The terrain described in the question (dense atmosphere, high ratio of particulates) sounds like it would be pretty rough on a rotating barrel assembly. To top it off, if you moved to a planet with slightly too-high gravity, the weapon would jam because there'd be too much downward drag on the ammo feed system. There's a couple options rooted in current technology which could work around this.

a. The Metal Storm system stores all the rounds (which include propellant) directly in the barrel, stacked on top of one-another and fired in sequence. If one round misfires, the next round in the chamber pushes it out. This would allow very efficient ammo storage, and drastically reduce the number of moving parts on the weapon.

b. A Gauss/rail/coil cannon completely eliminates the need for propellant for each round, and also eliminates any moving parts exposed to each planet's atmosphere. This would allow the most efficient ammo storage possible (you'd only store the "warheads"), and would enable the military force to fabricate their own ammunition from any ferrous material on the planet. Combined with a compact nuclear reactor, the crew would have virtually unlimited ammunition.

2. Maximize modularity: For an interstellar military, you'd want a fighting vehicle capable of fighting in a low-gravity jungle one physioday, and a high-gravity tundra the next--it's far cheaper to ship the vehicles/crews directly from one engagement to another, rather than shipping them all the way from a major manufacturing hub or storage facility each time. Being able to replace their wheels with treads for sand travel, or remove armor plating in a higher-gravity environment, would be essential.

Most of this could be performed in the "mother ship" that takes them from engagement to engagement, but a tiny 3D printer on-board the AFV itself would allow them to create small replacement parts without having to head all the way back to their FOB (or whatever their equivalent is).

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    $\begingroup$ Sadly I can't upvote at the moment, but you get my approval for the '3d printer to make replacement parts' $\endgroup$ – Joe Bloggs Sep 14 '15 at 10:13
  • $\begingroup$ Metal Storm has a high rate of fire, but is it efficient ammo storage? Instead of the bullet and propellant in a thin-walled cartridge case, it has to be stored in a heavier replacement barrel. The railgun sounds better in this regard. $\endgroup$ – o.m. Sep 14 '15 at 10:45
  • $\begingroup$ @o.m. the Metal Storm ammo can just be stored in tubes/stacks that load into the end of the barrel (1:41 in this video: youtube.com/watch?v=zx_9_RgMPCE). It wouldn't be too much of a stretch to imagine a loading mechanism that would eliminate the need to leave the vehicle and actually go to the back of the barrel. $\endgroup$ – Liesmith Sep 14 '15 at 10:55
  • $\begingroup$ The problem with Metal Storm is the ridiculous number of barrels required. You either suffer hugely increased cost or greatly diminished accuracy. Also, I'd guess that the time cost of 3D printing far outweighs the reduction in mass, especially if you have a large number of vehicles with a relatively low part count. $\endgroup$ – 2012rcampion Sep 15 '15 at 1:04
  • $\begingroup$ @2012rcampion You don't need a large number of barrels to use Metal Storm; you can fire the rounds as slowly or quickly as you want. Even if you're only loading three shells into the barrel at once (assuming something like a 120mm cannon), and fire them two or three times as fast as an M1 Abrams could fire. The time to reload that single barrel by hand would be no more than loading the main cannon on an Abrams. For the 3D printer, I'm going to assume that 3D printing technology will improve somewhat by the time we have interstellar travel. $\endgroup$ – Liesmith Sep 15 '15 at 1:28
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Consider this:

  • AFVs can be overarmed, with weapons which can easily penetrate their own armor. Think of a tank-destroyer armored car like the AMX 10 RC or a light tank like the M41.
  • AFVs can be balanced, with weapons roughly in scale with their frontal armor. That's the M1 Abrams, but also the Stryker ICV.
  • Then there are a few underarmed vehicles, with armor in excess of their own weapons. Many are IFVs like the Namer or Marder (if you count just the cannon). Others are specialist vehicles like the M88 recovery vehicle.

Vehicles designed to fight other vehicles tend to be balanced or overarmed, not underarmed. If the armor is strong enough to resist one hit without penetration, it will probably resist a dozen hits, too. Using multiple hits to "chew through" the armor only works if the hit was almost strong enough to punch through. Here is a steel target after a couple of hits. (I was just looking for a nice pic, I'm not endorsing the product or the review on Amazon.) If this was the hull of a vehicle, do you think more shots would have helped?

And all other things being equal, a higher rate of fire increases the weight of a weapon.

  • A 7.62x51mm rifle like the L1A1 is roughly 4.5 kg.
  • A 7.62x51mm machine gun like the M60 is roughly 10 kg.
  • A 7.62x51mm minigun like the M134 is at least 40 kg.

So if your goal is to get a balanced or overarmed AFV, it is unlikely that you can afford a high rate of fire as well. You get one gun, with a relatively low rate of fire. Otherwise the designers would be tempted to reduce the rate of fire just a little bit and to increase the punch, to make sure a hit becomes a kill.

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    $\begingroup$ You may also be able to "chew through" armor in the particular case of reactive armor, which opens up another line of tactics for the OP to consider. $\endgroup$ – Cort Ammon - Reinstate Monica Sep 14 '15 at 18:12
  • $\begingroup$ Excellent points and when combined with Fhnuzoag points on logistics it points to low rate of fire but highly effective weapons being the norm for this sort of situation. Leave the logistics supply train guzzling weapons to the locals. It also suggests how the locals might gain advantage during engagements in such a conflict. Drown the invaders with high rate of fire, cheap to replace items that they can't afford to lose (backpack recon, miniguns, even soldiers as wave attacks if necessary). $\endgroup$ – Jim2B Nov 29 '15 at 19:52
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A rotary cannon is probably the worst weapon system for this sort of tank.

The primary bottleneck for a interstellar military is logistics. The supply chain to ship supplies from one star to another, and then down to the ground and then out to the field is significant. The point of a rotary cannon however is to fire that ammunition off really quickly. Consider that the A10 carries 1.5 tons of ammunition and can fire that in 15 seconds. What does your tank do when it runs out of ammo seconds into an engagement? What happens when your fleet runs out of ammo?

And that's ignoring the fact that such cannons take a few seconds to spin up...

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  • $\begingroup$ Excellent points and when combined with o.m. points on rate of fire vs. effectiveness of fire, it points to low rate of fire but highly effective weapons being the norm for interstellar invaders. Leave the logistics supply train guzzling weapons to the locals. It also suggests how the locals might gain advantage during engagements in such a conflict. Drown the invaders with high rate of fire, cheap to replace items that they can't afford to lose (backpack recon, miniguns, even soldiers as wave attacks if necessary). $\endgroup$ – Jim2B Nov 29 '15 at 19:53
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There are several good answers here already, but let me throw in a few more ideas.

What is the purpose of the vehicle that you are building? What kind of interstellar world are you building? In both reference points given, Halo and Aliens, the ground forces are used to explore what the fleet does not want to destroy from orbit. Get the fleet tug boats to drag some small asteroids, calculate trajectory, and drop them onto the main military bases. This way you are using resources already available in the area to cause tremendous damage. Think the Seekers from Advent Rising, "They throw rocks." You can kill a whole planet that way without having to land any troops. Our current military is using smart rocks to knock out specific vehicles and targets with minimal collateral damage.

So then, if gravity-powered destruction can be cheaply done on a variety of scales, what is left for your vehicle to do? With interstellar war, ground vehicles are used for final objectives and surgical tactics. The enemy has entrenched itself in the main powerplants, and crushing that will irradiate the habitable portions of the planet, or such.

  • Lighter Recon and transport. Think Warthog. It is quick and has a mounted rotary cannon, but mostly it is used to get the troops from A to B faster. Sometimes you park behind a rock and use the turret to pin down a bunker entrance, but for the most part it is light armor and quick.

Yes, with backpack drones we have the ability to recon anything above ground. But if you want to recon and then also act, you need a few boots in the right place. As stated in one of the other answers, ground troops are tricky and can get into advantageous places.

  • MBT. This design and metric has been discussed in great detail in several answers. It is a good tool when the enemy has an inferior main battle tank, but it takes a lot of storage on your interstellar craft and has limited situations of peak usefulness. Seriously, how far through a Halo mission did you ever take a Scorpion?

Any threatening ground forces can be squished from orbit. Gun installations in mountains or other heavy bunkers could be a problem, but land some troops on the mountain above them and repel down with explosives, ODST style. Your army/navy/peacekeeping force will be thinking from the top down, because that is how their initial assessment of the planet will be oriented. Landing a few tanks in the field in front of the gun bunker is just asking to get shot by those guns.

Really, interstellar soldiers are going to have a few darpa exoskeletons. Mostly those are used for cargo but you could easily mount some Metal Storm tubes to some of those, and use them for temporary artillery.

The interstellar force will save resources (money, cargo space, weight) where possible and try to have an adaptable force. Having a bay full of armor for a long voyage sounds like a great way to increase inertia and reduce your fleet's effectiveness.

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  • $\begingroup$ "Seriously, how far through a Halo mission did you ever take a Scorpion?" the point of HALO is to demonstrate the value of special/black ops personnel in best case scenarios, by definition it focusses on areas where normal units (supported by scorpions) cannot. $\endgroup$ – NPSF3000 Jan 8 '16 at 12:11
  • $\begingroup$ Part of my point is that the kinds of things that you do not want to flatten from orbit are the kinds of things where tanks do not fit. $\endgroup$ – DeveloperWeeks Jan 13 '16 at 18:52
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You don't want a tank at all, but some sort of gunship platform.

Since you are coming in from space, your forces will be limited compared to whatever the planetary defenses can muster, so you want a vehicle platform which is much faster than whatever they have. This gives you more tactical and operational options, and allows you to cover a much greater area with a small force. As well, a flying platform has a much longer line of sight with its sensors (both passive and active), providing you with more situational awareness. You can see what they are up to and quickly mass your forces or move them depending on the situation.

As for weaponry, a rotary cannon is a specialized weapon for a particular purpose (having a very high rate of fire against fleeting targets). You will need to carry several different weapons systems, and since you have limited numbers and limited carrying space, each weapon should be multi purpose. I might suggest a rail gun as the primary weapon since it can deliver a devastating kinetic energy punch against ground, air and even low orbital targets. Missiles can carry explosive warheads for different types of targets, and many missiles are multi-purpose as well (the Starstreak SAM can be fired against ground targets and has a similar effect as a 40mm cannon shell, for example). Missiles can also carry sensors (anti radiation missiles for use against radar, for example) or non kinetic warheads like EMP or chemical agents and smoke.

Using the same platform as the basis for a troop carrier provides the means to take and hold ground, the ultimate reason you are invading rather than nuking the place from orbit (it's the only way to be sure...). Think of the movie Aliens and you have an idea of where this is going.

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It really depends on what you are going to use this vehicle for, how you're going to deploy it, and what do you envision it to do/engage.

A multi-role vehicle, while seemingly smart/more financially efficient, will sacrifice many capabilities that a purpose built vehicle has. Especially combat vehicles. So let's say, your vehicle is designed to transport infantry, engage ground/low level air targets, and provide fire support. Then yes, the vehicle you design will probably work well, especially if the opposition has little/no heavy armor, deploys primarily soft skinned/light armored vehicles. But as soon as it meets an enemy that deploys considerable numbers of heavy armor/MBTs, its going to get shredded. Tank armor are typically designed to withstand hits from much bigger bores than what rotary cannons have.

This is why modern IFVs (Infantry Fighting Vehicles - which is what your vehicle sounds like) are also equipped with ATGMs (Anti-Tank Guided Missiles). Will a force primarily equipped with IFVs (even with ATGMs) be able to counter a armored thrust of tank units? No. At least not in open, maneuver warfare. IF that force manage to suck the tanks into close terrain (say cities), and IF the opposition tanks do not have the training and doctrine to operate closely with its infantry in such terrain, and IF the tank force do not deploy IFVs/screeners of its own, then maybe it'll work.

ETA: Also, depending on how this force is deployed, then your IFVs might need support from orbiting platforms i.e. the ships from which they deployed. Remember: A spaceborne force is limited in mass in order to be viable. You cannot deploy heavy armor easily from space because of the sheer weight. A force that is waiting for them on the ground has the advantage of not being so limited. They can concentrate their heaviest armor/units, defenses, etc. where they are needed the most.

You will then have to design a 'space-air-land' battle doctrine for your troops.

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How effective, then, would a tank armed with a Vulcan(or similar) rotary cannon be?

Effective against what kind of target? Soft or lightly armored targets? Yes. Main battle tanks or heavily armored positions? No. Not unless you got really really lucky. An MBT is more heavily armed and armored and thus will dominate in open combat.

Would a armoured vehicle with a rotary cannon, and the ability to engage aircraft, be a more flexible option for a interstellar military than a conventional tank?

Yes, in absolute terms, it is more flexible but the tradeoffs required to achieve that flexibility may be too great to make it effective. MBTs have a single mission, kill other tanks and APCs and not die in the process. To achieve that mission they have large guns and thick armor. An APC

Would multiple HV cannons be better than a rotary design?

The fewer moving parts in a mechanism, the better it is. In light of extremely long supply lines as one might expect to find in interstellar conflicts, getting spare parts and ammo to the front lines is difficult. 3D printing of parts may alleviate this problems to considerable degree but it's not a perfect solution. No 3D printer can print everything.

Alternatives to the rotary canon

Railguns

Assuming rapid deployments to distant star systems, this civilization has access to very advanced technology such as superconducting materials and compact fusion reactors. Tech like this facilitates rapid fire hypervelocity rounds from railguns/gauss guns that require no propellant and minimal space. Hypervelocity rounds achieve kills purely through kinetic energy; no explosives are needed. With a compact fusion plant onboard, the power requirements for rapid firing shouldn't be too hard to meet. Barrel wear may be an issue.

Drones + Guided Weapons

For the last 60 years, there's been a growing trend towards more capable sensory networks coupled with increased accuracy. As targets gained more degrees of freedom to evade incoming munitions, the greater the need for weapons with terminal guidance. Modern day examples include laser guided bombs, JDAMS, cruise missiles, torpedoes, and air-to-air missiles.

In the far future, this trend toward better terminal guidance and better sensory networks should only continue. Thus, pervasive drone/sensory coverage of any battlefield will be required to drive the one-shot-one-kill guided munitions of the future. This strategy complements the need to make every shot count in the context of infrequent/expensive resupply. If each round fired results in a kill or knock-out then fewer rounds are needed; and resupply requirements are less egregious.

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Your first question is, in my opinion, inherently flawed. A vehicle with limited abilities to engage ground troops, armored land vehicles, and aircraft, is by definition going to be more flexible than a conventional military tank. What it's not going to be as good at is providing superior firepower to friendly infantry forces and survivability. I know you're envisioning a Merkava-style tank/APC hybrid, but it's important to note that Israel has a major technological advantage over its immediate geographic rivals, and can afford to make a tank/APC hybrid because they're not sacrificing so much in either capability to lose superiority in either capability against a military like Egypt's, Jordan's, Syria's, etc.

Now, because this is an interstellar military, weight constraints definitely play a role, but I think you should still consider giving your military a main battle tank. They're awesome, they're very effective at accomplishing the objectives they're designed for, and they provide major morale boosts to friendly infantry and forces opposing troops that are outgunned to either be destroyed or withdraw. Rather than, say, mounting a .50 caliber machine gun on top of this tank like is done today, you could put a small unmanned railgun, laser, etc., that can provide anti-personnel support as well as anti-aircraft firepower. Alternatively, you could put a swarm of small drones on the sides of the tanks that serve as both extra armor, and when deployed can help clear a building or down enemy aircraft.

I think your second question has been better answered by others so I'll leave that be.

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What you have is not a tank, but a mobile anti-air artillery platform. It can provide some help against helicopters and straffing airplanes and some anti-infantry support, but won't win a single battle.

Scenarios

  • Against enemy AFV: Even if the armour is good, your tank is unable to harm any enemy tank. So, the enemy tank can try to move around you at leisure, find your weak spot and BAM!. If you say "I put more armour in the weak spot", it leads to a desigual arms race:

    • the enemy tank upgrades his main gun (one part of the tank) to increase his penetration power

    • you have to upgrade all of your armour to protect against that.

    It is not difficult to see that your armour will increase in weight way faster than your enemy gun.

  • Against enemy infantry: here you are able to use effectively your gun. The enemy can oppose two kind of weapons:

    • small arms fire, for which you do not need so much armour

    • anti tank weapons, which is a return to the previous point (you are safe as long as you present your "strong" point, so you are not free to roam and cause mayhem into the enemy lines. Another reason for not approaching infantry is that soldiers are sneaky and can move into a position where then can try to hit the upper and lower parts of your tank.

  • Against enemy aircraft, but only against those who try to straff your friends. For fighter/bombers flying high and launching "intelligent" missiles and bombs, you are just another target.


In essence, your lack of firepower makes your vehicle almost useless except for fighting infantry, and for that it weights way more than it needs to (instead 2 super heavy anti-infantry vehicles, 4 medium/light anti-infantry vehicles would provide more fire power and better flexibility). I do not think it is a good idea.

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What about high speed vehicle that rely on thermooptic camo to get to the battle. and are used in urban combat? (tall building and subways) would protect from orbital strikes and aircraft I recall ghost in the shell with there small nimble smart tanks as way modern armor might survive such a conflict. They will support soldier with indirect fire support weapons mortals, ECM, heavier weapon than soldiers want to carry and maybe drones.

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  • $\begingroup$ I'm not sure how this answers the question. $\endgroup$ – bowlturner Sep 17 '15 at 2:07
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A vulcan cannon is a terrible choice for a sci-fi tank. This forces the tank to carry massive amounts of extra ammo weight that will get exhausted quickly.

How to build a sci-fi tank that doesn't fail hard:

  • Gravity considerations: It must be designed to work properly in any amount of gravity that doesn't outright cripple the crew. So you're looking at anything from "no gravity" to perhaps 1.3 Earth gravity. Rugged wheels with a highly adaptive suspension are a must. You also have to account for the case where there's too little gravity for wheels to be useful, in which case it's advisable to include an impulser engine so the tank can fly. Another advantage of a flight engine is that the tank can be safely airdropped.

  • Primary (anti-personnel) armament: Definitely an energy weapon (laser or blaster type), because ammo adds excessive weight.

  • Secondary (anti-vehicle) armament: This might seem like an odd choice - take an EMP projector or ion cannon. This allows you to burn off shields and disable enemy vehicles without needing to carry shells or missiles which add weight. As an added bonus, the EMP projector/ion cannon automatically intercepts incoming guided missiles whenever possible.

  • Small missile rack - perhaps around 4-6 missiles. Because sometimes you just have to demolish something when "disabling electronics" and "fill it with laser holes" doesn't cut it.

  • Extremely tough shield generator backed by dual powerplants. Space marines don't have time for vehicle failures, and focusing on energy-based protection decreases required survivable hull weight. Energy shields also self-repair, decreasing the overall attrition rate.

  • Medium-lightweight stealth hull. This increases maximum speed and agility. Rely on the shield generator for primary protection. The stealth hull makes it difficult for missiles to lock on. Sure you could down them with an EMP or ion cannon - then again, why bother?

  • Bonus feature: Heatsinks to nullify infrared signature for a limited amount of time.

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