What kinds of weapons produce little or no recoil that could be used aboard a spacestation?

And by "Weapons" I mean ranged weapons specifically. Melee weapons would certainly be useful for close-quarters, but due to the range at which combat occurs in this question, I'm looking for ranged weapons like firearms.

They must function in zero-G, but functioning in a vacuum isn't a necessity due to the fact that most of these fights are going to be taking place inside the space station.

Combat will be taking place in all kinds of distances, from long hallways to open hangars to enclosed residential areas, so any kind of range on the weapons suggested is fine.

• On a station, I'd think not breaching the hull would be a bigger design consideration. – T.E.D. Jun 28 '18 at 5:52
• The amount of recoil a firearm has is greatly overestimated. A 80kg human firing a machine pistol in 0-g would be imparted with velocity negligible compared to leg push against a wall he uses to move around in 0-g. Not even A-10's main gun has a plane-stopping recoil, although urban legend says otherwise. Regular guns would work just fine. – Agent_L Jun 28 '18 at 11:52
• It should be noted here that all projectile weapons are a serious problem for a Space Station, unless they have super-orbital velocity. This is because unless you projectile is stopped (hits the target), hits the planet or leaves orbit, then it will have an orbit that intersects the station's orbit. The only way to be sure of this would be to give the projectile enough velocity to escape the planet's gravity. – RBarryYoung Jun 28 '18 at 14:43
• Is there a reason you don't do like every other sci-fi does, and have magnetic boots? Maybe with an electromagnetic booster for extra grip. – cybernard Jun 28 '18 at 19:32
• Hull breach is a bit like King Solomon and the baby, so whoever wants to save the orbital will want to de-pressurise it before projectiles fly. One good explosive decompression and the whole planet is off limits to satellites for centuries. No winners in this fight. - en.wikipedia.org/wiki/Kessler_syndrome – KalleMP Jun 28 '18 at 20:07

In physics terms, most of your common projectile weapons damage through the application of kinetic energy. Since you're applying force to impart energy onto the projectile, you get the opposite force; that's the principle of recoil. So your options basically boil down to "use a weapon that doesn't rely on kinetic energy", "send the recoil to something other than you", or both.

Rockets and/or gyrojets

Specifically, you want a two-stage rocket launcher. A small charge carries the rocket clear of the launcher, at which point the main motor kicks in. Because the initial impulse only has to carry the rocket a few feet, its recoil is fairly light. "Recoil" from the main motor is actually what propels the rocket; you'll only have to deal with the rocket exhaust. (Wear a mask: it can be pretty hot, and I don't know how exhaust will act in microgravity.)

Explosives in general

A handgun firing tiny explosive rounds, for instance, would have recoil but less than a traditional firearm with the same stopping power. This is because it can be lighter and/or travel more slowly, reducing the force you need to impart on it in the first place.

Thermal weapons in general

Lasers do impart a small amount of momentum from photon pressure, which means they do have a small amount of recoil, but mostly they damage through heat. Flame and (getting a little more fantastical) plasma weapons do the same thing. In all cases, the more damage that things-that-aren't-kinetic-energy are doing, the better your recoil looks.

Recoilless rifles

Well, obviously. Essentially, the recoil leaves in some form of exhaust, similar to a rocket though attached to the gun rather than the projectile, or a counterweight, such as plastic chips or water, that takes the recoil force and is thrown out behind you. (Protip: don't stand behind the guy with the rifle.)

• would a grenade qualify as "explosives in general"? That was my first thought, since most of the recoil happens a long way from the thrower. Hopefully. – craq Jun 28 '18 at 3:17
• @craq Yes, but the way grenades act in zero-gravity is different; they would bounce and roll in very unintuitive ways. If you spent some time practicing and acclimating yourself, it could work. Note that you'd still have to anchor yourself against the recoil of the throw, but that's comparatively mild. – Cadence Jun 28 '18 at 3:37
• You could/should link to further reading material or current-day examples where applicable, e.g. mention the Bazooka as an example for recoilless rifles, etc. – dot_Sp0T Jun 28 '18 at 8:56
• Recoilless rifles are no-no in a closed areas. Pretty much anything kinetic but recoilless is just as deadly to the operator as to the target if fired indoors. (with exception of counterbalance ejection, which is deadly only to the guy behind as you noted) – Agent_L Jun 28 '18 at 11:55
• How about bows/crossbows and .22 weapons? These have little to no recoil. – Hawker65 Jun 28 '18 at 13:57

Lasers

Cadence is right in that lasers do technically have a momentum from photon pressure, but how far have you ever been pushed by light? I have worked with industrial lasers - the kind that cuts through metal - and it's not an issue for those (i.e.: the framework they are assembled on is not built with recoil in mind).

In fact, light, in general, having so little momentum is why a practical solar sail would have to be immorally large to work.

Flamethrowers

These have a little more kick to them as you have to push some gas out, but depending on construction it's more manageable than a firearm. And you can never go wrong with good old fire.

American flamethrowers had a range of fifteen meters during the world wars. In the future, they might have a longer range.

You really, really, really, do not want to use a flamethrower on the same space station as you happen to be on.

You're going to eat up a lot of oxygen. It can spread in any direction, instead of primarily burning upwards. In a forced-air atmosphere, you have plenty of convection to take it everywhere. Smoke spreads everywhere, turning visibility to shit. The temperature will rise really quickly since it's an enclosed space. Shipboard fires already cause the most total losses in naval situations. Space is even worse.

– Daniel B

And

Spitting a flaming liquid around in zero-G isn't going to do you or your buddies any favours either. Sure it might originally go in the right direction but burning humans tend to flail, which would send burning accelerant in all directions.

– Korthalion

So yeah, this might be risky to the point of being suicidal. But if you need to kill everybody in a space station with as little ammo as possible, this might be the way to do it. If you have to die, at least go out with a bang.

Shuriken

Just because of the rule of cool. Also, they won't spread flames nor breach a hull if you miss.

Sonic weapons

These don't shoot stuff, they vibrate the air. No recoil there. There is technology to keep the vibrations focused into a beam.

• You really, really, really, do not want to use a flamethrower on the same space station as you happen to be on. – Daniel B Jun 28 '18 at 5:18
• @Renan Fire + limited oxygen = bad – JAB Jun 28 '18 at 5:53
• It seems a bit of an unfair statement in wake of my suggesting nerve gas. But my main concerns were that fires on a space station are a Really Bad Thing. You're going to eat up a lot of oxygen. It can spread in any direction, instead of primarily burning upwards. In a forced-air atmosphere, you have plenty of convection to take it everywhere. Smoke spreads everywhere, turning visibility to shit. The temperature will rise really quickly, since it's an enclosed space. Shipboard fires already cause the most total losses in naval situations. Space is even worse. – Daniel B Jun 28 '18 at 6:01
• You can use a flamethrower, which includes oxidizer in its fuel already (maybe dual chambers). Thus at least you won't eat nearly as much oxygen. – M.Herzkamp Jun 28 '18 at 9:08
• Spitting a flaming liquid around in zero-G isn't going to do you or your buddies any favours either. Sure it might originally go in the right direction but burning humans tend to flail, which would send burning accelerant in all directions. – Korthalion Jun 28 '18 at 10:48

A taser would work. Short range, but pretty much no chance of overpenetration and the darts are low enough velocity that recoil shouldn't be an issue. Plus sometimes you want to capture someone. Future refinements could increase range/ capacity/ reliability/ lethality.

Flashbangs would also probably be more effective as disorientation in a zero-g space would be much worse.

What about a microwave emitter?

https://en.m.wikipedia.org/wiki/Active_Denial_System

1. Flechette weapons. Flechettes are thin, low-mass darts that travel at considerably higher muzzle velocities than conventional rounds. They have much less recoil than bullets, and as a result, can also be fired at a higher rate without causing your aim to go off too much. Look into the SCMITR: especially in the close quarters of a space station, they will be pretty effective. They also have great armor penetration without much mass, which is nice! They'll probably make holes in your space station, but that sounds like it's not a concern.

2. Gyrojet. These firearms don't fire a bullet, they fire a tiny rocket-like projectile. Both rifle and pistol designs were made in the 1960's. The muzzle velocity is around 100 fps which is not that much more than a nerf dart, and at thirty feet, they're booking it at 1250fps. This will minimize recoil without reducing stopping power. Here are some fun gyrojet ballistics.

3. Sonic Weapons. This is hard to find sources on, because almost all development of this type of weapon is currently hush-hush military shit, but sonic weapons are effective over distances (they've been used for inter-ship combat to some degree, against protesters, and who knows what else, at distances of 1000 feet.) At lower levels, they can induce nausea, disorientation, and rupture eardrums. At higher levels, they can can cause gross tissue damage through cavitation. Look up the LRAD for the commercial version of this. They're very circumspect about the fact that their thing can cause permanent damage.

4. Chemical weapons. You're working in enclosed spaces! That means it's a great day to deploy some nerve gas. This stuff is great because even if your opponents are wearing, say, gas masks and armor, they'll die anyways! It's less effective against space suits, though I'm not sure of the specific data on that. But hey, it's science fiction, so who knows. Maybe combine these with flechettes, so you don't have to count on tissue damage to kill people after you hit them with darts.

• I'd like to point out that a lot of chemical weapons are quite caustic and will do things like burn through suit seals and damage the shipboard systems, most of the nastier forms of nerve-gas are absorbed through soft tissues and outright bypass gasmasks, hence the existence of NBC suits. They're also a pretty definite violation of the Geneva Convention if you care about that sort of thing. – Ruadhan Jun 28 '18 at 11:35
• That is why I mentioned that even if they wear gas masks, they’re dead. I do suspect that between the nylon layers and the urethane + nylon layers, a conventional space suit would be decent protection. – Daniel B Jun 28 '18 at 13:58
• @Ruadhan2300 I have to nit-pick but I believe it's the Geneva Protocol you would violate, the Geneva Conventions concern the rights and protections of soldiers and civilians. – AmiralPatate Jun 29 '18 at 8:55
• @AmiralPatate You are technically correct, the best kind of correct. – Ruadhan Jun 29 '18 at 9:31

Anyone remember those buzz-saw drone things from Half Life 2? Get a dozen of so 4-inch-wide versions of those, and a targeting laser pistol.

Zero recoil on you, and the psychological impact of a buzzing swarm of death descending on your opponent is pretty intimidating. Since you don't need to worry about keeping them in the air (no gravity!) you can improve the maneuvering systems - with a powerful "straight line" thruster to give rapid acceleration when going in for the kill.

• Damn, I love this idea. – Daniel B Jun 29 '18 at 17:49

Or...

Angular momentum is a problem IN SPAAACE

You really, really, really, do not want to use a flamethrower on the same space station as you happen to be on --Daniel B

Flamethrowers came up, and why this is a bad idea IN SPAAAAAACE got longer than a comment. And it's for more than the obvious reasons...

...fires on a space station are a Really Bad Thing. You're going to eat up a lot of oxygen. It can spread in any direction, instead of primarily burning upwards. In a forced-air atmosphere, you have plenty of convection to take it everywhere. Smoke spreads everywhere, turning visibility to shit. The temperature will rise really quickly, since it's an enclosed space. Shipboard fires already cause the most total losses in naval situations. Space is even worse.

The problem with a flamethrower, or any recoil IN SPAAAACE isn't that you'll be flung backwards, it's that you'll spin. Firing something with sustained recoil like a flamethrowers or automatic weapon in free-fall looks something like this...

Here's why.

Calculating recoil

A military flamethrower, as opposed to just a pyrotechnic, shoots a napalm/gasoline gel propelled by high pressure nitrogen. Using a WWII-era US M2 Flamethrower as our baseline, it carries 10kg of fuel and fires it at half a gallon (1.9 L) a second. This will have a significant recoil. We can work it out. Momentum is $mass \times velocity$ so using the ratio of the mass of fuel being thrown vs the mass of the person holding the flamethrower, we can work out how fast they'll be pushed backwards. It's a simple ratio.

$$m_{firer} \times v_{firer} = m_{gel} \times v_{gel}$$ $$v_{firer} = \frac{m_{gel}}{m_{firer}} \times v_{gel}$$

Napalm is gasoline a thickener, originally palmitic acid. Gasoline has a density of 0.725 kg/L. Palmitic acid 0.825 kg/L. I'll assume the fuel is about 0.8 kg/L. A half second burst is about 1 L, so you're throwing about 0.4 kg per burst. Your average adult is about 60 kg giving a ratio of $\frac{1}{150}$.

It has a range of about 20 meters. In this video of Ian McCollum firing an M2, I estimate it took about 1 or 2 seconds to reach the camera giving a velocity of about 20 meters per second.

At our $\frac{1}{150}$ ratio, the shooter will be pushed back at about 0.133 m/s which is not very fast at all. Similar with bullets, while they're traveling very fast, they are very light compared to the shooter so they don't impart much velocity. Firing a gun IN SPAAACE won't send you flying. But it will start you spinning.

Angular momentum IN SPAAACE

The problem is the angular momentum. On Earth when you shoot a gun you are braced against the ground. In free fall when you shoot a gun you are not, so you spin.

Your typical firearm tries to keep all its recoil in a straight line to avoid "barrel climb" or "muzzle rise". Some do this more successfully than others. From personal experience I can tell you that an M1 Thompson climbs up and to the right quite dramatically. Most pistols have some tendency to climb simply because the heavy moving parts and barrel are above the axis of your hand giving it an angular momentum and a tendency to rotate up.

This is part of why shooting a gun that has a stock is more accurate, the recoil goes straight into your shoulder which you can brace against.

If you fire a gun IN SPAAACE, even with a stock, there is nothing for you to brace against. Instead of the gun rotating, you will rotate. The force of recoil going into your arm or shoulder will cause you to pivot around your center of mass. Your first shot might be on target, but now you're slowly spinning backwards and, if you shoot right handed, to the right.

A flamethrower is so much worse.

With a gun, you can at least get your first shot off before you start spinning, and you can predict how you'll spin. With a flamethrower the gel and pressurized gas are traveling out of their tanks, through pipes, and out in a sustained half second stream. You can see when Ian fires the flamethrower his aim jumps around for a moment until it settles down. Ian is firmly planted to the ground by gravity, and he is a very experienced shooter, so he can control it. IN SPAAACE unless you're firmly strapped to the deck there is nothing to brace against. Once you fire that thing you'll be spinning uncontrollably and shooting flame everywhere.

This also applies to thrown weapons, people have suggested shuriken and spears. A typical overhand, underhand, or even side throwing motion will make the thrower spin. You can reduce this by flicking with the wrist from your belly, but that is much more difficult to aim, has less power, and requires extensive training to do effectively.

A space gun for your belly

You can reduce this somewhat by firing "from the hip" as close to your center of mass as possible, but if you're off by a little bit you'll still impart a rotation. It's hard to aim, but with enough training you can make it work at the short ranges you'd expect in a space habitat.

A real space gun would probably be computer controlled and strapped to your belly. This ensures the recoil goes straight back through your center of mass and imparts no spin. Gyroscopes built into your suit would resist the tendency to rotate, and computer controlled cold-gas thrusters could actively stop any spin.

Better yet, don't hold the gun at all. Put it on a stabilized drone. Give it cameras and an AI to track human targets, and fire it remotely.

As we'll see below, this would best be fired by pneumatic pressure, not a chemical explosion.

Smoke

Even if you use a recoilless gun like a gyro-jet, smoke is a problem.

Firearms, flamethrowers, explosives... anything that burns produces toxic smoke. Unlike on Earth, a space habitat has a limited atmosphere and a limited ability to filter it. A sustained firefight would build up smoke in the atmosphere causing asphyxiation and damage to equipment. Even if you're in a space suit, it would make it difficult to see.

Use pneumatic guns, or if your setting is advanced enough, lasers or particle-beams.

Remote Controlled Mobile Turret Officer's are issued an armed bot with a paired hand unit that is shaped like a gun. The hand unit is only used for targeting, via appropriate sights or optics and it shines a beam on the target. The bot acquires the beam's target and magnetically or otherwise anchors itself. Upon pulling the "trigger" on the hand unit, the bot fires at the intended target.

Alternately, paired gyroscopes could be used instead of targeting lasers

This answer is made with the assumption that the goal is to occupy the space station after combat has resolved. If we're not taking the station, then there is no need for combat on board the station. It's much easier to sabotage it or destroy it.

I'm also assuming that, since there's no artificial gravity, we aren't using any other handwavium technology like force fields, repulsors, or anything that is unlikely to be used in the next 100 years.

In space, whether inside or outside, your options are grouped into kinetic slugs (firearms), missiles (rocket propelled grenades), chemical sprays (flamethrowers), or directed energy weapons (lasers).

Kinetic Weapons

Railguns, bows and arrows, rifles, handguns, flechettes (dart guns), cannons, shotguns, shuriken, throwing axes, etc., are all examples of kinetic weapons.

The main problem with a kinetic weapon isn't just Newton's second law (every action has an equal and opposite reaction), but the fact that you're usually using your arms in some ways, whether holding a firearm or throwing something.

That's going to set you spinning, because Newton's second law is also works right along side the Law of Conservation of Angular Momentum. If the point that you're launching something from is off-center from your own center of mass, you're going to start rotating.

So, rule number 1 of kinetic weapons in a ship: Unless you've very careful to "fire" from your center of mass (which is a bad idea for other reasons, namely you making yourself a bigger target), you must secure yourself.

That's easy, though. Every manned spaceship since the Mercury capsules have had plenty of handholds and footholds. One of the most common complaints of astronauts aboard the International Space Station is that the tops of their feet get sore, from sticking their feet into these footholds.

The second problem with a kinetic weapon is that you're likely to miss.

Yes, you might poke a hole in the ship's hull and let the vacuum of space inside. Even for a tiny ship like the International Space Station, a small arms bullet hole would take a couple of hours to dangerously depressurize the ship. Plenty of time to close a pressure hatch and don a pressure suit so that you can go make repairs. Just don't make a habit out of missing.

More immediately dangerous, and quite likely to make the ship permanently uninhabitable, is if you rupture the coolant system.

Heat is a HUGE problem in space. Yes, space itself is cold. Space also happens to be one of the best insulators, because you can't conduct or convect heat away; you can only radiate it away.

Every light source emits heat, even low energy LEDs. Every human emits heat. Every pump in the life support system and water reclamation systems emit heat. Everything that creates and uses electricity emits heat. Even the refrigerator creates a net increase in heat.

The best chemical for carrying all of that heat out to the radiators, that keep enclosed spaces cool enough for humans to live and work? Ammonia. Not the heavily diluted stuff in window cleaning solutions, but pure, undiluted, toxically deadly ammonia. That's what the International Space Station uses, as did Mir, Tiangong-1, and SpaceLab. (The US Space Shuttles and USSR Burans used water, but their life support was limited to 2 week missions, maximum.)

Ammonia is fine on Earth in small doses. It evaporates quickly, is lighter than air, and is rarely released in large enough concentrations to cause environmental impacts.

Ammonia is a quick and painful death if you're in a sealed environment, as it will chemically burn your lungs to mush. With no "up" for the ammonia to float away to, it will stay around until a very thorough decontamination is performed. (If the ammonia coolant in the ISS leaks into the living space, plans are to abandon the station and remotely command a controlled deorbit into the Pacific.)

More dangerous than the ammonia, though, is the threat of fire from sparks given off when these kinetic weapons strike something metal. This is less likely, though.

Because of the null gravity, fire burns slow and methodical, and spreads its fuel around before consuming it. The first place that the fire will go is right into the life support system, right to the air intake filters that are full of all sorts of fuel like dead skin cells, crumbs from your lunch, discarded hair, etc... and the filters themselves are unlikely to be flame resistant. Next stop is the CO2 scrubbers, then the O2 outlet nozzles.

The best case scenario in a fire is that your life support system shuts off. Air stops flowing around, which will slow the spread of flame and keep a pocket of oxygen rich air around you. Depending on how big of a room you're in, your most immediate concern will likely be CO2 poisoning (headaches, nausea, disorientation, fatigue, and finally death through toxic buildup) before you'll have to worry about suffocation from lack of oxygen. This will take several hours or even days, so long as the fire is contained and isn't spreading smoke through the entire ship.

But, starting a fire will likely make the station uninhabitable, and will be an undesirable outcome for the attacking party.

Fortunately, little on space ships are flammable, specifically because of how unreasonably dangerous fires are. Accidentally starting a fire from sparks is unlikely. Especially since Apollo 1.

Quick recap of kinetic weapons: You need to brace yourself when using it, it's alright to puncture the hull a few times, but if you hit the wrong thing, everyone loses.

Missiles

Next up are missiles. The least advanced of these are little more than kinetic slugs that have their propellent attached to the bullet, rather than relying on the propellent being consumed all at once inside the barrel of a gun.

The main advantage is that it doesn't impart much of that "equal and opposite reaction" force on the person who fired it, making it possible for a combattant to fire it without being secured to a bulkhead.

Additionally, it makes it more comfortable to fire larger masses at a target, such as a mass that includes high explosives. A typical use case would be a rocket propelled grenade.

With more mass in your payload, you can also add some limited guidance. Probably not helpful in fighting happening entirely on a ship, which will be at extremely close range, but if there were a drone or other small spaceship taking potshots from space, too far to hit with any real accuracy, you could potentially train a targeting laser on it, letting the missile do the hard work.

As far as explosions on a spaceship, a suitably well built station would survive an internal explosion better than any humans aboard. Spaceships are designed to hold pressure in.

There are some very gruesome facts about explosions and mammals that are best left to the morbidly curious. While many people think that the shrapnel from a grenade is the most dangerous part, the actual most dangerous part is the pressure wave.

Because space stations tend to be very long tubes, the pressure wave doesn't have much chance to spread out and dissipate. Because the diameter of the liveable area of the International Space Station is about 7 feet across, a high explosive grenade will be just as deadly 7 feet away as it is 200 feet away (and that wave will bounce back a few times). You're going to want to close a pressure hatch before using high explosives on the other side. At the very least, it will rupture everyone's eardrums.

And even then, even though the station is likely to fare better than humans in case of an explosion, the pressure wave will seek out the weakest points, and will probably find one. It will probably crack a weld and introduce a slow leak, making it necessary to wear pressure suits within a couple of hours.

Chemical Sprays

First, a pedantic note: These impart momentum under Newton's second law, just like kinetic weapons. It's like a garden hose: there's a noticeable force when you have it on full blast.

Anyways, regardless of whether you can reliably aim a stream of liquid or gasses, chemical sprays are just a Bad Idea(tm). You'll want to live in the station. Spaceships have closed life support loops, and air moves very slowly. With no "up," toxic chemicals linger in the air, neither rising nor settling.

If you think about igniting that chemical for some reason, such as if you're using a flamethrower, it would be much easier to destroy the station outright, since nobody will be taking control of it in the near future.

Energy Weapons

Lasers. More pew-pew.

Lasers are probably not viable. At least not if you want your sci-fi to be faithful to basic laws of physics.

At least lasers follow basic laws of physics, though. Lightning guns and plasma bolts are right out. They're not sci-fi, they're fantasy.

I mentioned above that heat is a problem on spaceships. There is no convection or conduction, only radiating the heat.

The general rule of thumb for a laser is that it takes as much heat to make a beam as that beam will deliver to the target.

In order to ablate away enough flesh to seriously wound a person in a blast that's short enough in duration that the person won't just hide away, you need a 4 megawatt laser. That's not a portable laser simply due to the energy requirements. It's either attached to a generator (larger than any camping generators you can find), or attached to a bank of batteries that take up a significant portion of a large room. Best case scenario, you have a monster sized power cord plugged into the station's power.

With that laser, over the course of a second, you deliver 1000 kcal to your target. You also deliver 1000 kcal directly to the station's radiators. Keeping in mind that the ship is a closed system, and all heat has to eventually be radiated away, the 1000 kcal that you delivered to your target also has to make its way to the radiators. Every shot costs the radiators about 2000 kcal of extra work. If the space station is a battle station, then this is fine; the radiators will be designed to handle a few batteries of gigawatt lasers. If it's a civilian space station? Your next step after taking the station will be to replace the radiators, or you'll all quickly roast.

Null-G Combat Tactics

I'm including this section because tactics inform weapon choices.

Good tactics in any combat situation with firearms is to present as little cross section to the enemy as possible. This means using cover and concealment, using covering fire from squadmates when you need to move, so that the enemy is less likely to poke their heads up and take a shot at you, etc.

In null-G, you have an extra advantage: You almost never have to present your full torso to your enemy. Put some extra shielding on your feet and make that the only thing you present to your enemy. Something like 4" thick bulletproof clear acrylic sheets with a small hole that you can poke the barrel of your rifle through. You'll lose some accuracy, but your cover comes with you. (Or, if you care more about accuracy than protection and mobility, you can adopt more of a "Superman" pose than an "Enemy's Gate is Down" pose by putting the acrylic sheet above/in front of you, so that you can swivel your firearm faster.)

And finally, the big question:

Do you want to intentionally evacuate the air in the station during the attack?

All weapons will still work. Firearms since muskets have been able to work in a vacuum. I know, "But there's no oxygen in the vacuum of space!" ... Well, there's plenty of oxygen inside of every explosive that humans make. (Pedantic note: when it's in an explosive, it's called an oxidizer, and is often not found as the gaseous oxygen that we breath.) All of the weapons that I listed absolutely work in space.

There are a couple of differences; explosives don't create shock waves in a vacuum, but they DO fill up an unpressurized space with smoke very quickly.

There are tactical advantages and disadvantages to intentionally letting the vacuum inside. Anyone caught unaware is having a very bad day. Depending on how quickly you can let the air out, people may have as little as 10 seconds to react. That can quickly reduce the strength of opposing forces that you have to deal with.

A disadvantage, though, is that every wound becomes debilitating.

Additionally, your mobility is severely limited, and your vision can be instantly obstructed. Fighting slows dramatically, since just dashing across a corridor becomes an incredibly risky action.

Active Denial System. That means microwaves to make people feel their skin is burning, since it penetrates in the skin just enough to light your pain receptors.

Comes in variety of sizes. https://en.m.wikipedia.org/wiki/Active_Denial_System

Since you are talking about some sort of melee battle inside a space station, one assumption is the people you are fighting are protected by some sort of body armour (or armoured spacesuit). You also have to wonder about the internal fittings of the space station itself, but I will presume the walls are resistant to most small arms.

One other caveat, I would expect a struggle inside a spaceship or space station will take place in vacuum, since the attackers will either have breached the hull with some sort of heavy anti-ship weapon, or used some sort of explosive or cutting device to breach the hull in an unexpected location (no one is coming in through the airlock).

The end result of these factors is the use of a grenade launcher as the weapon of choice. The grenades do not have to be launched at high velocity, and most hand held grenade launchers use a variation of the "high/low" pressure launch system to reduce recoil to very low levels. (The High/Low pressure system was developed by Germany in the closing days of WWII to make lightweight recoiless anti tank weapons. The PAW 600 is the only example to be fielded).

The use of grenades ensures that the explosive payload and effect are delivered as close to the enemy as possible. Standard fragmentation rounds are probably the best for general purpose work, but "shotgun" shells or flechette rounds can be used for close in work. As well, HEAT rounds can be used to penetrate barricades, or if the situation allows, to fire on the enemy spacecraft docked to yours. Since this is a fairly high tech situation, the grenades will likely be enhanced with sensors to detonate when they are close to a target, either determined like the XM-25 (where a laser rangefinder on the weapon is used to program the grenades as they're fired), or some sort of "smart" grenade with it's own on board sensor.

Finally, depending on the situation, the Marines might need a more powerful weapon. Something descended from the "Mini-Spike" Anti Personnel Guided Missile (APGM) might do. It is a missile, so there is no issue of recoil, but using guidance system and presumably a rocket thruster to guide the missile in vacuum, you can deliver a devastating payload into bunkers or other hard to reach places.

No one mentioned railguns yet? I'm using the first answer from this one as a reference: How to build a tank equipped with a 1 GJ railgun

The formula for kinetic energy of a bullet is Ke=0,5*MV^2 The formula for recoil is P=MV

As you can see the kinetic energy grows exponentially with speed, while the recoil grows linearily with mass and velocity. So if you cut the mass to half its size and double its speed you retain the same recoil with a higher kinetic energy. If you want lower recoil but the same kinetic impact you reduce the mass and add less velocity to compensate. IE for half the mass you need to add a quarter of the speed (did not think that one through, correct me if that last statement is a mistake).

Big advantage: the ammo is now lighter and smaller allowing either more ammo or faster turn ratio's and movement in zero-G.

• I feel like either your math is off or your conclusion is. A big heavy object moving slowly can produce the same recoil as a small object moving fast, but by reducing velocity and mass, all you get is less recoil and less impact-damage. So not ultimately very helpful. you might as well take a low-calibre pistol and call it a day. – Ruadhan Jun 28 '18 at 11:32
• @Ruadhan2300 I said "add less velocity", not substract velocity. You want to add less velocity to make sure you dont increase the recoil despite having reduced the weight. But try to read the reference, or try to make sense of this one that contains the same formula's: en.m.wikipedia.org/wiki/Recoil – Demigan Jun 28 '18 at 12:57
• So your answer boils down to "use low velocity bullets"? – Ruadhan Jun 28 '18 at 14:15
• @Ruadhan2300 exactly the opposite. Lets say a bullet goes 360km/h. You reduce the bullets mass by half and double its velocity to increase the kinetic energy, this adds velocity. But if you want to keep the kinetic energy the same despite reducing the mass you need to ADD LESS velocity compared to doubling it. IE anywhere between 361 and 719km/h. I'll write up an example of the given formulas when I'm not on a Phone. – Demigan Jun 28 '18 at 17:08
• Ah, I think I understand, I see your reasoning now. Not a bad idea either. The ultimate expression of this is probably a particle-beam weapon, virtually recoilless but packing enormous energy on impact. But either way, a railgun or a particle beam both require huge amounts of energy to fire, vs a more conventional weapon which has far better energy density. But assuming future-tech it'd answer the question I think. I've upvoted your answer accordingly – Ruadhan Jun 29 '18 at 8:39

I would suggest a long spear as a ranged weapon. It could work on a space station and you could poke at walls to move yourself around in zero G.

Also, on a tangent, I would create a cannon that the player holds but as it fires all of its recoil expels through a jet engine on the back of the player so the player stays in place by moving forward and also gains the benefit of damaging backwards at the same time.

Have fun!

• aside from the fact that usually who stands in front of a fighter is enemy and who stand behind is a friend, and firing on friends is nonono, since when poking walls has become a space combat approach? – L.Dutch - Reinstate Monica Jun 28 '18 at 10:24
• if friendly fire is disabled then only those trying to flank would be hurt by the backward repulsion. i figure this is just game theory anyway. the wall poking would be a way to move faster. have fun! :) – sycx1 Jun 28 '18 at 10:32
• you have missed the combat tag. there is no game tag – L.Dutch - Reinstate Monica Jun 28 '18 at 10:35
• and what you're suggesting is a recoil-less rifle, they already exist and would not be a good thing on a space station. also you can't disabled friendly fire in real life – Blade Wraith Jun 28 '18 at 11:05
• If you try to poke somebody with a spear in free-fall, you'll just push yourself backwards. You'd have to be braced against something very much limiting your reach. Top tip: "Don't go near the guy with the spear who's stuck themselves to the wall". And how do you get it around tight corners in a space habitat? – Schwern Jun 28 '18 at 19:02

Recoil is not certainly bad

In a battle with ranged weapons you don't want to cross through an open space with a predictable direction and speed and you really really don't want to take a long time doing it. By firing a weapon with recoil while on a ballistic course you make targeting you slightly harder by changing your velocity.

If the open spaces are large and critical you have to worry about air resistance as much recoil. Having something on hand to give you velocity in whatever direction would be nice.

Recoil is not certainly relevant

A handguns recoil is in the 10J range and a human's jump in the 200J range. A couple shots will be within the variance of direction and power between jumps. Presumably handguns for space would be even less because loud noises in confined spaces are bad, and knocking holes in the walls would be very bad. Which follows that carrying a shield of whatever the walls are made of would probably be impervious to anything sane opposition will hand out to combatants. This is likely to be heavy enough to matter in momentum math reducing the effect of recoil on a ballistic path.

In general staying on the walls gives you mobility, a place to dissipate recoil and fewer angles of attack to be worry about.

• I would imagine that in larger spaces, combatants would be using thruster-packs to move around and not having to expend precious fuel to compensate for their own gunfire would be very useful. but yes. Realistically you don't want to be far from the walls anyway. – Ruadhan Jun 29 '18 at 8:47

Chemical Gas. Will incapacitate living targets. Ineffective against targets that may use private O2 supply through their helmet.

Ultra high spectrum lights. Will incapacitate targets with helmets good for gas but not sufficient for light filtering.

Increase or decrease temperature. For smart enemies having both breathing and lighting defenses on their helmets. :)

Magnets. Will incapacitate top tech attackers that have O2 supply, light filtering AND isothermic suits.

Foam. Lots of. Sticky, hard and increases volume when kinetics apply. Will hinder movement and all actions no matter the tech of the enemy. :) :)

Electromagnetic pulse (EMP) hidden mini turrets. Ray, focused, not spheroid. Will not affect the actual enemy but disable all his equipment, including his awesome pulse rifle.

Bobby traps. Countless flavors. Blades and spears launched from the hull piercing enemies apart. Multi use. Do not like blood? Hammers do fine bludgeoning damage, run, boom, sleep. Trap compression chambers, invisible hatch opens, enemy fly to space, hatch closes. The wire, my favorite. Thin and sturdy, placed at edges of a corridor along the way, will convert an entire squad to ham slices.

Gain time. Do not incapacitate, but gain time. Labyrinth. Special corridors that change routing. Just after their mapping technology finishes readings. Very embarrassing. Make enemies believe their mapping equipment is faulty. With a little luck you can have them make harmless circles forever. The "almost done" technique. Smart doors: No enemy squad will blast a door that just opens with the button. Waste of time. Press button, door responds, suddenly stops with an irregular sound. Looks like that, with a little pushing it will freed. Indeed, 1 guy catch the door and pushes - after a little go up door stuck again. 2 guys try. Go little more up. With a little tuning you already lost time to blast, and when you finally prepare charges, door opens fully.

And a super weapon: Like reverse gravity. Changes the gravity field upside down. Change again. Change again. Make sure your personnel all are in the central room with seat belts. Do not worry for gravitational boots. They are designed for no gravity no reverse gravity. They won't hold a fully loaded attack soldier upside down.

With all the above techniques you have one great advantage: You control ENEMY FIRE. Because no matter how safe equipped and trained your squads will be, you have no guarantee what weapons enemies will carry nor how they use them. The heavier enemies are equipped and the lesser trained they are the more damage will do to your station.

• Forgot the nano-bees. Nice top tech 0.001 cm bees. Preferred targets: Eyes, ears. 1 may not stop an attacker, but 10 certainly will. – Stefanos Zilellis Oct 7 '19 at 11:55
• Did i mention the water-looking ultra strong glue? Conveniently fluids at a glass windowed large hall area. So by the chance of sticking down badly, enemy troops will have a great view of reinforcements arrive, 200 top special force marines with combat droids out at the hangar... pointing their extra large super huge demolition cannons just at their sticking locations... I believe they accept surrender before realizing it was not a glass but the very new top tech 'almost real' new glass tv merged to some of the glass windows... – Stefanos Zilellis Oct 7 '19 at 12:02

Bows and arrows.

You really don't want something that can accidentally puncture a hole in the hull/windows of the space station.

Electric assisted draw bows and smart arrows to be precise.

• Bows and arrows, no matter how automated, are not recoilless. Granted, the mass of the arrow is quite a bit less than the mass of the human, but the force of the bow's spring is still against the human, who is pushed backward when the arrow moves forward. – JBH Jun 28 '18 at 2:06
• @JBH I'd like to specifiy that I said "Little or no recoil", and that a Bow and Arrow, while producing recoil, is far below the amount a firearm would produce. The effect on the human firing a bow is negligible, meaning it fits the question's criteria – Sydney Sleeper Jun 28 '18 at 2:31
• @SydneySleeper, This is curious. Your arrow-to-human mass ratio is 25.7e-3 with an average 320fps, so the human is moving about 1 inch-per-second. A 9mm pistol slug-to-human mass ratio is 10.7e-3 with an average 1,245fps, so the human is moving about 1.6 inch-per-second. Are you sure about that specification? or is my math wrong? – JBH Jun 28 '18 at 2:58
• In all fairness it is probably my math that is wrong. I wouldn't have thought a bow and arrow produce nearly as much recoil as a firearm, but it turns out I was wrong – Sydney Sleeper Jun 28 '18 at 4:13
• actualy it makes a lot of sense. It's basicly the rocket problem backwards. The impulse (aka the recoil) is I = mv while the energy of the projectile is e= 1/2 mv². So for a fix amount of energy you can remove 75% of the mass if you throw it twice as fast. but since the impuls is linear to both mass AND speed you get half the impuls(recoil) in the second case. – m.fuss Jun 28 '18 at 7:12

Crossbows are good, and with some modern/sci-fi mechanism, should be able to fire many shots before needing to reload a magazine. If your technology allows, you could let the crossbow carry some sort of minor payload. The power might be able to be adjusted, allowing less damage to fragile structure, or for longer range.

If your society is advanced enough, you could just make it a mini-railgun, with about the same results, but at a much higher energy cost with the benefit of relative simplicity.

• Crossbows have recoil. – Turksarama Jun 28 '18 at 5:25
• Crossbows have comparable effective recoil momentum with handguns. – Daniel B Jun 28 '18 at 6:27
• All you need is some minor bracing and you are good to go. And besides, handguns have alot of recoil for their mass & size. – Joe P Jun 28 '18 at 11:54
• Better see the discussion over on Michael Kutz's answer. – JBH Jun 28 '18 at 14:13