# Boarding weapons in space

I'm writing a story that's relatively hard Science Fiction. Therefore (Although fancy and high tech) the personal weapons are, at their core, regular firearms.

Now, in boarding scenarios you want to be careful to not pierce the hull (especially if it's a civilian station). I assume that the airlocks and docking areas are fortified and shielded to prevent this, which might allow the defenders some heavy weapons without risking decompression. So my problem is, what do my boarders carry in regards to weapons? Some weapons that would reduce the danger of piercing the hull are:

• Glaser rounds
• Hollow point
• Buckshot

However these would also be negated by the first person to strap on a Kevlar vest. So is there anything the people could carry that would defeat body armor, but has a low risk of piercing the hull? It is preferably something that would also work against aliens (Within reason of course, no energy beings, titans or micro-organisms).

So traditional guns is preferred but not required. I found the glue/foam guns something I hadn't thought of. But it seems the concensus is that the hull would be thick enough to withstand anything that can take out an armored person. So I'll pick an answer.

The purpose of combat is not killing but control. Terrestrial combat uses lethal force because it is the most effective way to take control of a battlefield. But if the enemy surrenders, there's no more need for killing. In fact it's better to be merciful with those who surrender so the rest have less incentive to fight.

Everything in space is counter-intuitive. Shooting a bullet through the hull of a space station is not a big problem. Air only leaks through at a limited rate and the hole can be plugged with a wad of bubblegum. If the hull is hardened against micro-meteorites, the bullet won't even penetrate. The biggest problem with using guns in microgravity is the recoil, which can easily cause any soldier to totally lose control of their orientation and position.

Swords aren't much better. If you swing a sword, the rest of your body will turn the other way. Using a sword would be extremely difficult. A spear might be a tiny bit better because there's not as much rotation involved (though you'd be surprised), but your reach won't be great because you will move further from the target as you trust the spear. Microgravity combat would be largely concerned with securing good anchor points in the environment to enable any kind of effective action without obnoxious physical reactions.

A defensive party that's prepared to be boarded will get themselves strapped in to fixed positions and get ready to fire guns or trust spears. A boarding party will be in free-fall when they enter, which is a huge disadvantage. The defending crew has control of the interior environment. But the boarding force has control of the external environment. The boarding ship can try to knock the defenders unconscious by subjecting them to high accelerations - probably by spinning the ship they are assaulting. This kind of action could kill everyone on board, or force them to detach from their anchors.

The simple fact is, however, that boarding parties in space are probably unnecessary. Having control of the exterior of a ship allows a siege to be completely effective. Cover the target vessel's radiators with reflective insulation, block their sublimators and cook them out. Life in space is so tenuous that it's hard to justify fighting back - or fighting at all. (This is why space colonization is such a great goal for peace.)

Now if you really, really want to have boarding parties in your story, try this tactic: You can't fight what you can't see. Inject a spray of black, low viscosity, highly wetting liquid that will coat the optics of anyone or anything inside. In microgravity, fluid dynamics are dominated by surface tension effects. A blob of liquid touching a surface will tend to completely coat the surface, unless that surface is phobic to that particular liquid. Most if not all materials currently used for optics (glass, plastics, the human eye) are at least somewhat hydrophilic, so a spray of blackened water would easily coat and interfere with these. (Just ask astronaut Chris Hadfield how bad it can be to get something in your eye in space.)

Edit: If your combat environment is subject to spun gravity, it's important to note that projectile weapons will behave differently than when subject to normal gravity because of the coriolis effect.

Let's model the habitat as a cylinder spinning on its axis. Use the right hand rule to define North as the direction you would have to face to see the cylinder as rotating clockwise - or if you point the thumb of your right hand in that direction your fingers bend in the direction of rotation. South is the opposite direction. West is defined as the clockwise direction. East is opposite West.

If you stand on the inner surface of this cylinder and fire a projectile, it's path (as seen by you in the rotating frame of reference) will be significantly different depending on what direction you fire.

If you fire directly North, the bullet will initially go straight, but will start curving to your right as it "falls" to the "ground".

If you fire South, the bullet will fall to your left.

If you fire East, the bullet will fly straight, but will fall slower than you'd expect.

If you fire West, the bullet will fly straight, but fall quicker than you'd expect.

If you fire straight up, the bullet will go straight up, then curve West, then down, and then maybe start heading down and East - it really depends on muzzle velocity and the spin rate of the habitat. The trajectory that you see could end up looking kind of like a "Run for the Cure" ribbon. If you get the muzzle velocity just right, you could get the bullet to come back and hit you - but it would hit you from up and to the West.

If you fire straight down, the bullet will curve East and probably hit the ground.

Fire up and a bit East to hit a target on the exact opposite side of the cylinder from you.

If you try to drop an anvil on a roadrunner from high "up", it could take a very long time to get to the big red target you've painted on the ground. The time it takes depends on the diameter of the habitat and how high up you are. If you were fairly high "up" - close to the center line of the cylinder, but not quite at it - the anvil might go all the way East, come back around a few times, apparently picking up speed in the Easterly direction until it finally creams your target going practically sideways.

You can combine these for trick shots, but there's also the complication of air resistance.

The coriolis effect becomes less obvious as the radius of the cylinder gets larger, assuming that the angular velocity of the cylinder is chosen to provide one gee acceleration to a person on the surface. A gargantuan habitat may only rotate several times a day, but a small habitat would have to spin up to several times a minute to yield one gee at the surface. So smaller habitats will have far more pronounced coriolis effects than large habitats. (Earth rotates once per day, so we don't really notice the coriolis effect in our daily lives, but it's there.)

Another neat feature of spun habitats is that you can tell which way you're facing by turning your head. If you face North, then turn South, you'll have the unpleasant sensation that you are spinning counterclockwise - at twice the rate you are actually spinning counterclockwise - and you might tip over if you try to walk too soon after turning around. (You can simulate this by looking up and spinning around until dizzy, then stop and try to walk around without losing your lunch.) Spinning habitats are not much fun for this reason and that's partly why the ISS has no artificial gravity section. It would take a seriously large artificial gravity ring to provide an environment that doesn't feel like a carnival ride.

• I'm totally aware with the distances of space combat boarding ship to ship is highly unlikely. I'm mostly focussing on my abundance of space stations and elevators. Great point how (sudden) micro gravity would make most melee weaponry relatively useless. – Mormacil Apr 21 '17 at 19:46
• If you laid siege to my space station I might deorbit it in wrath; doubly so if my escape pod is ready to go. My plans aren't like ISS plans; the escape pod is completely inside the station and is deployed by crashing through the thin hull if I can't open it. – Joshua Apr 21 '17 at 23:35
• Spin gravity. If artificial gravity is used, they can safely fire weapons. – b.Lorenz Apr 22 '17 at 6:02
• I updated my answer to include details of projectile motion in a spun gravity environment. Reference: youtube.com/watch?v=IOcrHOc23N4 – master_gibber Apr 23 '17 at 10:33

Concerned about firearms piercing the hull? Don't want to accidentally cause an explosion?

The most simple way may be to use older weapons- go for a sword, knives, hatchets, and bullet-proof shields. Here are some reasons why this works:

1. The 21 foot rule is very likely to apply to boarding parties on space-faring vessels. Having large floorplans is expensive in space, and firearms really gain their advantage from being able to hit targets at a distance.
2. A bullet-proof shield will allow the boarders to safely advance into a fortified position. Essentially, the boarders get the advantage of a tank in a human-sized package.
3. Judicial Shields. Hooks, spikes, and other bits on a shield can increase the effectiveness of a shield in combat. Also, they're super scary.
4. Additionally, you don't need to worry about "is there enough oxygen use my weapon? Is there too much oxygen?" A flame of an size in an oxygen-rich environment is bad business for everyone involved.

Here are, however, some downsides to this "medieval" approach:

1. Your troops are only effective if they can close the gap between them and their enemy.
2. Bullet-proof shields can be overcome by stronger and better bullets and guns. This "ups the ante" on boarding parties, and increases the risk of hull breaches.
3. Shields can get in the way if you have a wrestling-friendly enemy, or need to go through especially tight passages.

Upon further thought, I think the boarding parties should be mixed units: you have some shield-bearers, some gun-slingers, and maybe something else. Seems to me like boarding is very ugly business.

• Judicial shields, I like it. I already prepared to use regular balistic shields. Boarding is indeed messy but that's okay. The impersonal clean kill is already show in space battles. – Mormacil Apr 21 '17 at 15:16
• Don't you risk to perforate an oxygen pipe with those sharp weapons ? – EngelOfChipolata Apr 21 '17 at 15:28
• Less then with a bullet I assume, it's confined to how much energy we can put in a swing opposite to an explosion (of the bullet firing). – Mormacil Apr 21 '17 at 15:33
• Two items. First "Is there enough oxygen to use my weapon?" isn't necessarily relevant, as modern gunpowders contain oxidizers and don't require oxygen (they can even be fired underwater). Second, love the answer. Your thoughts mirror mine almost exactly. My first thought when I saw this question was back to Doc Smith's Lensman series, with Peter Van Buskirk and his Valerians armed with their space-axes. – Doug R. Apr 21 '17 at 18:25
• The 21-foot rule illustrates why someone inside 21 feet may be able to stab a defender before she can draw, aim, and shoot her sidearm. I don't think it applies when her weapon is already drawn and either aimed or at a low ready. In those cases, knife-guy is going to have to be pretty close in order to be able to beat the reaction time of gun-girl. – Wayne Conrad Apr 21 '17 at 20:34

Tiny Robots

Instead of relatively high-velocity weapons or directed energy weapons, launch small robotic payloads that once they hit their target can seek out a vulnerability in the armor and exploit it. The tiny robots can carry whatever payload you like to defeat armor: tiny shaped charge, drills and acid, electrical shock, work together to dismantle armor/target. Such machines could even change direction in mid-air (space) and determine friend from foe. This concept appears in the book Seveneves by Neil Stephenson as a method of warfare that won't hurt the hulls of spacecraft.

• The smaller the robot, the more limited energy the EMP you need to kill it. There are a wide variety of man-portable devices that could knock out vast numbers of such things, from electromagnetic to electrical discharge to directed EMP. None of these need be high energy enough to hurt the ship itself either. Tiny=hard to shield your circuitry. – JBiggs Apr 21 '17 at 17:28
• I disagree. For one, a smaller target means that it has less cross section to pick up the EMP energy. The killer of an emp is that there are conductors that are longer than the wavelength of the frequencies in the pulse, so the smaller the electronics the safer they are from the direct effects. The catch there is smaller electronics are more easily damaged by the overvoltage that is induced by secondary effects or longer conductors actually picking up the signal. Also smaller electronics means there is more space to be shielded by a faraday cage or to have redundancies to cope with the damage. – Joe Kissling Apr 21 '17 at 17:40

You don't need to kill you need to incapacitate.

STICKY FOAM!!

This stuff couldn't puncture a hull, could totally "take out" a enemy, you could also use it to "seal" areas so you don't have to sweep and clear all the little nooks, just move forward and hose the area.

It needs to be improved a bit, but with better delivery and faster reactions, this stuff could really make a mess.

• Good idea; but even if you are successful, you still have to deal with the enemies you have encased in foam as you clear it out in order to actually USE the ship you are trying to take. Clearly, dealing with them a few at a time is better than all at once. PROBLEM: given that ships will have lots of compartments capable of being independently sealed by bulkheads, how do you immobilize enough of the enemy crew all at once for it to be worthwhile? If you just immobilize a few, you have also blocked your own progress and active enemies might go for YOUR ship while you are playing clean up. – JBiggs Apr 21 '17 at 18:00
• I'd say it makes perfect sense for a civilian space station to have these kind of things as riot control and make shift defenses. Easy to simply fill the breached hallway with the stuff. Sure cleanup takes time but that's a thing for after a fight. – Mormacil Apr 21 '17 at 18:06
• Picture this scenario: 2 ships locked together, a crew goes out and boards the other ship. They start shutting down ship defenders with foam. In so doing, they ALSO block off their OWN ability to progress forward. The defending crew leaves them there, sends out the rest of their guys (behind the single compartment that was filled with foam) and boards the ATTACKER's ship. How does foam really help here? It is basically defensive. It causes blocks and freezes combatants in place, but you can't gain forward tactical momentum. – JBiggs Apr 21 '17 at 18:20
• I didn't specify it needed to work for the attackers. I was looking at ship safe weapons. And I'm more looking into small ship boarding a space station or merchant vessel. Military ship to ship boarding seems unrealistic given the distances of space combat. So I agree it fails as an offensive measure but I didn't specifically ask for that. – Mormacil Apr 21 '17 at 18:25

In any kind of boarding, defense and offense are both going to be in vacuum suits. Gas weapons are out. Shock weapons are out because these suits will be grounded. Medieval type weapons will be tricky.

You are boarding because you want the ship intact, and maybe the crew too. If you did not you would not need to board them.

I propose an automatic shotgun with silly putty slugs. Even if armored, a hit from a slug packs a wallop. Many slugs = many wallops. Silly putty will flatten and deform rather than penetrating the ships hull. Shotguns have a short range which is fine for a ship. If you are too unsilly for silly putty use a regular slug then. Or a riot control beanbag.

I like tranquilizer darts too: something like etomidate for 10 second knockdown and then a heavy sedative to keep your foes staring and drooling while you ziptie them. A long dart will penetrate a vac suit.

Other cool aspects of this as regards narrative: by issuing the space marines these nonlethal weapons it is pretty clear that the priorities of command is taking the ship intact. The marines themselves are more interested having a fast battle with minimal loss of space marine life. Plus they are frustrated at fighting with "clown guns" against defenders more formidably armed. So they hack their weapons to make them more lethal / dangerous - with interesting results. They collect weapons from the defenders, who also have different priorities. Later if there are questions about damage to the ship, the space marines can point to the defenders, many of whom somehow failed to recover from the tranquilizer in time to start breathing.

These from the Taofledermaus video on shooting silly putty.

The Silly Putty packs a wallop. Being a non-newtonian fluid it stiffens under stress so it stays together out of the gun. It occurred to me that if the putty were acid all of those little putty fragments you see in the clay would be eating little holes in the vac suit of the person hit.

It is a safe assumption that any boarding operation would take place in vacuum or at least anyone with any sense would be wearing space suits during such a fight. Otherwise you could just flood the ship with fentanyl gas and walk in. Holes in vac suits are bad. Getting it with a slug of putty would feel like getting hit with a bat, and then your vac suit starts to leak.

• automatic shotguns was what I already had. Yeah I figure just brute forcing that would work. Hoping somebody comes up with something novel. But yeah shotguns totally make sense. – Mormacil Apr 21 '17 at 16:49
• This won't work against any kind of hardened space suit (which you have to assume any civilization spending a lot of time in space would use standard). Suits would be impact-armored just because of the rigors of working in outer space. They would also probably feature panels of harder armor to deal with potential micrometeorites or bouncing off a sharp corner of a ship. ANY suit designed for working hard in space would make a "beanbag round" pointless. – JBiggs Apr 21 '17 at 17:32
• That made me think of an answer. – coteyr Apr 21 '17 at 17:42

How about long-range electric shock weapons? If the attackers are good marksman they could incapacitate the defenders with electric shocks and knock them out or tie them up afterwards? Assuming the aliens have a nervous system similar to that as humans of course...

Regarding the comments: The weapons aren't used too much, because of controversy surrounding their lethality. If lethality (or lack thereof) isn't important why not use them? Also, from the above link: The bullets are actually small high-voltage batteries, so no external power pack is required. As long as you carry enough rounds to secure the ship, the attackers should be fine.

Regarding body armor: yes, that's a weak point. You have to target a conductive surface, preferably skin or pierce clothing to reach skin. Maybe metal armor could actually conduct the electricity? In that case an electric weapon would be ineffective against isolated metallic armor or synthetic body armor. So the question remains: would defenders wear full body armor?

• I'll research it some more but wouldn't the round be totally ineffective against targets wearing body armor? Google hasn't been to helpful with an answer. – Mormacil Apr 21 '17 at 15:49
• In a semi plausible ship boarding situation, ALL combatants would be wearing space suits which are shielded against electrical discharge. They wouldn't be wearing modern style "soft" suits either! They would be wearing pretty well armored hard suits with self-healing soft layers. Electrical weapons would be useless (among many other things). – JBiggs Apr 21 '17 at 17:30
• ...until someone comes up with a Taser that can penetrate the armor. They'll have the upper hand in any battle until someone invents a better armor. It's literally an arms race, and you can pick whichever point in weapon and defense development that you wish. – Shawn V. Wilson Apr 21 '17 at 22:29

Now in boarding scenarios of things like space stations or starships with destroyed drives you want to be careful to not pierce the hull.

Why? Are we talking about situations where entire civilian stations or ships are taken hostage? Are we talking about boarding a vessel with a high value target? There aren't many other situations where you care about the lives of the people on the vessel you're boarding.

I don't ask this as point of criticism, but to clarify a very important issue about this topic. The objective. You need to have an objective before you can talk about methods.

• If you need to extract someone (or some people) from a vessel and you need them alive, depressurizing the ship or station they're on is obviously a terrible idea. Your concern for leaving the internal environment intact makes sense in that case.

If you do need to get in a ship or station without breaking the craft's seal to the outside vacuum, you need two things: an airlock that can establish its own seal without help or cooperation and directional explosives.

If you are trying to get through the target's own airlock, your airlock must have a larger mouth than the hatch of the target's airlock. Your airlock will create a seal with the vessel's hull around the hatch, not the hatch itself. (Imagine one of those gross kisses where someone puts their mouth completely around someone else's mouth.) Then you're going to use shaped charges to do something kind of like this. You're going to blow whatever mechanisms are allowing the target's hatch to stay shut.

A target vessel's airlock may not be particularly convenient for quick breaching or it may be too easily accessible for the responding forces who want to keep the invaders (you) out. However, you have an airlock that can form a hard seal with bare hull, so you don't care. You can simply attach anywhere on the ship and do something like this or this. Of course, if you're resorting to just digging through the hull by cutting, you probably have no element of surprise. You'll be giving your enemies ample time to figure out how to shake you off their craft before you breach the hull or just lots of time to get all their guns march to the breach point before you get through.

So is there anything my guys could carry that would defeat body armor but not immediately pierce the hull?

That depends on the hull. Is it a thin sheet of aluminium (like on the ISS) or is it the thick hull of ship designed to take small impacts from micrometeoroids traveling much faster than any bullet? In the latter, you don't have to worry about (personal) armour piercing rounds getting too far into the outer hull. However, you don't need 100% avoidance of hull punctures. Small punctures aren't going to cause a pressurized craft to explode (or unravel if it's an inflatable). Assuming you're quick with you're boarding, you could secure the craft and plug the holes before the air runs out.

• Starship astronomical logs, encrypted or not. Space elevator access. Orbital factories. Refueling stations for starships. Are all targets you want to capture as much in one piece as possible. Just venting civilian installations is a one way ticket to a galactic occupation force. Humane treatment of POW and civilians will be a part of my story. How barbaric would a future civilization actually wage war. – Mormacil Apr 22 '17 at 3:27
• @Mormacil Astronomical logs, space elevator access, orbital factories, refueling stations, none of these being taken require survivors. If anything, survivors complicate things. People that haven't been killed before they can respond could delete their logs. Factory workers under occupation can stage sabotage. "Just venting civilian installations is a one way ticket to a galactic occupation force." Now you're referencing information you haven't given us. Why would a breaching party worry about intervention? The only people who you're talking about breaching ships are nonmilitary actors? – CircleSquared Apr 28 '17 at 15:30

Need to board enemy ships and deal with defenders without damaging the hardware or causing explosive decompression? Worried that gas won't work against space suits, tiny robots are vulnerable to enemy EMP or electrical discharge weapons and anything like a shotgun can EASILY be defeated by even normally-armored work hardsuits (hardened spacesuits) much less military grade combat armored suits? No problem! Your worries are over. I give you:

The Vibro Sword

The Vibro sword is a simple concept: a blade roughly similar in shape and design to a Roman short-sword has an outer edge designed to allow for high frequency vibration that turns it into something like a cross between a sword and a chainsaw. A Vibro sword easily cuts through anything organic, plastics, interior ship insulation, fittings, and even control panels, but is unable to damage the heavy duty alloys used in modern ship construction. Even if you slip and accidentally drive one into the floor, it isn't going all the way out into space.

Facing armored opponents? Of COURSE you are! The need to have humans out in space for long periods working on ships or exploring planetary environments with a wide variety of dangers has led to ALL standard spacesuits being armored against most normal penetration. Self-healing layers are supplemented in any normal suit with hard armored "shell" pieces which can be replaced, which will protect a worker against many types of common threats (like a loose bolt bouncing off him in the space dockyard at high relative velocity.) These kinds of suits (and the MORE armored military grade suits) make weapons like shotguns basically useless. Bean bag guns, tasers, and a wide variety of "non lethal" weapons are pointless against defenders in hard suits, but really powerful weapons like man-portable lasers, micro-sized railguns, or modern chemical explosive powered assault rifles pose a threat to the ship you are trying to capture.

A vibro sword easily deals with this problem! How? Simple. A short sword (designed to be used in close quarters for obvious reasons) can be used to thrust into weak points in an opponent's armor. Aim for gaps between the chest plate and the arms, and open them up like a crab! Obviously, using a bladed weapon requires a higher level of skill than most firearms, but then so does space boarding itself! You wouldn't take just any old standard ground infantry unit to board an enemy ship!

To get close to your enemies, you rely on your OWN military grade armor and the knowledge that your opponents have the same issues you do about blowing apart their own ship to kill you. We also make a handy-dandy riot-style shield to help you close the gap in tight formation!

Once you get close and start creating gaps in enemy suits, your options really open up (pardon the pun) nerve gas clouds, or even simple things like low-velocity pistol rounds do nicely to finish off defenders who have had a nice hole made in their suit and are probably lying prone. (This is where you put your less skilled fighters, behind the main "avant garde" line).

Skilled men can cut through a dozen heavily armored defenders in a matter of minutes with vibro swords. This is because armor will almost ALWAYS have weak points, but they will usually not be exploitable with ranged weapons.

Welcome to the age of space warfare.

• What's to keep the "weak points", which rightly so are the places that are not an actual solid plate of armor, from being reinforced with a space aged version of chain mail? A layer(s) of say tungsten carbide or pick your carbon allotrope rings looped together with a shear hardening material within that matrix. Slices won't work and stabs would have difficulty piercing, much less breaking, the loops thanks to the shear hardening material. – Joe Kissling Apr 21 '17 at 17:53
• It is not possible to engineer a perfect suit of armor. There are always trade offs. This gets into a much bigger discussion. In medieval times there was a continual arms race between weapon and armor designers. Chain mail stopped cuts good, so they moved to stabbing weapons that can be pushed right through it (that's your answer, btw) so partial plate came out and they moved to things like horseman's picks to punch right through armor... etc etc etc. It never really ends. Main point: at very close range, even MODERN military doctrine considers knives better than pistols. – JBiggs Apr 21 '17 at 17:57
• I'm not entirely convinced a non-handwavium vibro blade works. But a sword itself is a good idea. Obviously it's impracticle to issue hard suits to everyone on a ship. I imagine everyone wearing a skintight bodysuit and widely distributed face masks for breathing in case of a breach or leak. – Mormacil Apr 21 '17 at 18:10
• @Mormacil, I think the technology to make something along the lines of a "vibro blade" is basically something we could do now if we really thought we needed it and wanted to invest the engineering expenditure. As far as it being "impractical" to have suits for everyone on a ship, I TOTALLY disagree. That's like saying that it's "impractical" to include life boats capable of carrying every passenger on a ship. If we have the kind of technology where you would ever even THINK about an actual space-based boarding action, the cost of a space suit is NOT a big problem. – JBiggs Apr 21 '17 at 18:16
• @JBiggs I'd argue a hardsuit is closer to a diving suit on a ship/submarine or a parachute on a plane. Lifeboats are more like escape pods. Sure our boarders will wear hardsuits, that's a given but our defenders are less likely unless they're prepared and not beyond the first line of defenders. As for the vibro blade, the science doesn't work to my understanding. – Mormacil Apr 21 '17 at 18:23

Grenades Not your usual fragmentation grenades, but ones designed to do damage through concussive force, such as the US Mk3a2 concussion grenade. This would prevent serious damage to ship and would also not rely on piercing whatever armor the defenders have. Other types of grenades exist, and flash-bangs would be very useful for a boarding party as a first attack. Additionally grenades could be filled with some kind of chemical like white phosphorous and burn the enemy. White phosphorous is also highly poisonous to any human that breathes it in.

For a more fantastical grenade, you could have a type that releases some sort of liquid or gel that attaches to your enemy and then hardens or otherwise immobilizes them. Finishing them off with a close combat weapon would be simple afterwards.

• In any plausible ship boarding scenario, ALL combatants will be wearing space suits. Very likely ARMORED space suits. How effective is a grenade based on concussive force being transmitted through a consistent atmospheric medium going to be against pressurized and hardened suits? Probably sort of (knocking them off their feet, sure) but probably not NEARLY what you would really need. – JBiggs Apr 21 '17 at 17:53
• It entirely depends on what armor they are wearing and distance from the explosion. For example, look at armor spalling. Even if the force of a blast does not penetrate your armor, the force can either break pieces of your armor off and into your body, or it can impart kinetic energy, breaking ribs and causing internal bleeding. Plenty of examples of cops walking away from being shot with broken ribs. Real world armor that is designed to survive small explosions, like bomb disposal suits, rely on dispersing the blast through kinetically resistant clothing. That would be bulky to wear in space. – Vasily Kushakov Apr 21 '17 at 18:07
• You're talking "flash-bangs?" – Doug R. Apr 21 '17 at 18:20
• @DougR. No a "concussion grenade" is lethal. A "flash-bang" is a less-lethal M84 grenade. – ChrisW Apr 22 '17 at 10:42

Overall Delivery mechanism could be a breaching charge or grenade launcher (from a brace position) to bypass the zero-g knockback effect.

Generally whatever payload you're firing, the projectile cannot be simply a slug in Zero-G which will be defeated by armor. So explosive/poisonous/electroshock/heat will need to be the death/disable mechanism (stored energy is more dense than kinetic energy requirements).

Category: Conventional Weapons:

Small Phosporous/thermite tipped grenades (amount insufficient for hulls breach, but sufficient for body armor).

Explosive Tipped ammunition that detonates on contact. Powerful enough to generate internal injuries, but not hull damage.

Category: Non-lethal:

Restrict Movement:

Any sort of sticky or expanding material should be able to lock their movements.

Disable Senses:

Flashbang, squid ink, Sonic/Heat Beam Pain weapons

Category: No Geneva/Hague Conventions in Space!

Gas Them: Assuming not all personnel are in enclosed spacesuits, then they will be vulnerable to chemical/heat attacks. They would have to otherwise vent the section of the gas-bourne toxic/igniting agents.

You could render your personnel immune or have them sit tight from outside/another room while the opposition expires. Examples Dry Ice to starve oxygen (Excess Carbon Dioxide)/fires will burn oxygen. Ammonia + Bleach reaction produces some noxious fumes for a condensed payload.

This could be an effective killbox setup for defenders (preset defense room with gas defense).

## The lack of defence

Contemporary spaceships designed with thin hull and without any defence measures because there are no real threats from aliens/human enemies. They are similar to yacht (for some reason I failed to insert image here http://12knots.ru/img/aggregator/mmk/564224430000101637_Bavaria_41_ext.jpg):

• designed for several people
• has so thing hull that lady with stiletto heel shoes could damage it

Space station designed to survive in a war could be similar to a warship with many measures against different threats:(another picture https://en.wikipedia.org/wiki/King_George_V-class_battleship_(1939)#/media/File:KGV-Armor_Scheme.jpg):

• top-level weapon to bring heavy damage on a long distance
• medium size weapon to protect from small targets which dangerous on a short distance
• armor against enemy's top guns
• additional armor protecting vital areas
• with bulkheads dividing ship to separate areas
• several thousands people on board
• airplane, anti-aircraft battery, small boats etc etc

Total armor weight for Bismark was 18,700 tonns which is 40% of Titanic tonnage. Only armor, without guns, ammo etc!

Probably your space station is more like a carier (with medium armor, medium guns, 1,000-3,000 people). Carier is war-designed so it has construction intended for survive with severe damage (bulkheads, protection against torpedoes etc).

For both battleship and carier there is no problem to repel boarding attack. Either there is no problem with using firearms except special areas like ammunition magazines.

Space station could be totally non-military like tanker

• no armor
• no weapons
• crew is 25 people
• Tonnage twice than warship
• hull
• made with thicker steel than small yacht
• can withstand a shot from hand weapons
• will be crippled by any anti-ship weapon

In this case defence from invaders is assigned to another ships. In case battleships somewhere else you have two options:

• dozen of people could protect from invaders with light weapon (like pirates)
• most efforts are aimed to prevent boarding
• surrender to any spaceship with anti-ship weapon

There are exclusions, f.e. cargo ship has sunk careless submarine, but it only confirms the rule

So options are:

• you have station prepared to war and boarding and using firearms is not an issue
• you have non-military station ergo you have no abilities and reasons to defend from strong enemy - it's task someone else

## Ways to prevent onboarding

Anyway, situations with boarding are possible with more or less madness scenarios like this.

My first point that for large spaceship the hull should be thick enough so using firearm would not be an issue. Like tanker vs yacht hull, big space station should have more durable hull than ISS to carry himself and to withstand loads while maneuvering.

Second point there are many hazards. Depending on its targets invaders could:

• change orientation and/or orbit of space station to disrupt its usual operation. It's easy to do with several boosters
• it couldn't serve as space elevator
• it couldn't get energy from its solar arrays
• it couldn't communicate with planet
• cancel artificial gravity
• damage the hull so seriously that people could survive only in vacuum suits
• made a small hole to go inside at unexpected place

Measures to protect against these threats are very different and out of scope to your initial question. The main points are

• don't allow to dock at all. If enemy are on the hull, you probably lose
• guys with any weapon is the last line of defence. At this line no matters how much damage the hull could get. Just destroy them all using as powerful weapon as you has.

Let's have some fun with Chemistry!

Create a caustic chemical, maybe some sort of binary agent might be best for storage. Design the chemical to damage the joints or faceplates of armored/hardsuits, the goal is to compromise seals and such.

Breech the hull of the vessel you need to board, but do it carefully. That's going to send the defenders into the suits. As you advance to clear, level by level, hit the defenders suits with the solvent to cause pressure loss. the defenders suffocate. Weld some hull repair in place over your deliberate breech and you have a ship ready to re-pressurize.

Have the caustic agent neutralize itself over time, so that cleanup and repairs of the newly acquired ship is fairly easy.

This technique is going to have a limited shelf life though. It would likely only work for a space version of the Dread Pirate Roberts who leaves no survivors. One ship gets away, and samples of the caustic agent can be studied and then defended against.

Problems that are common with projectile weapons are reduced, as the solvent will be low-mass and move at fairly slow speeds. Surface tension effects actually help the solvent penetrate and lodge to the critical joints.

Your space pirates may want to get fancy with timing the breech and how much air gets out, you don't want the liquid freezing, or worse, sublimating in vacuum. That is, unless, you can use some handwavium in the formula that would prevent this.

An alternative to this approach, but keeping with the theme, pinpoint where the ship's environmental systems are. Particularly the ventilation duct work, downstream of the filters. Pop some nasty nerve agents in there via a very small hull breech with a device that maintains a seal so a pressure loss isn't noticed. Wait a bit, then simply open an airlock to vent out the nasty stuff, chuck the corpses after, and sail on with your newly acquired ship.

Instead of having a weapon that is unable to penetrate the hull, just don't let it! Your gun is fitted with a with visible/IR cameras and real-time machine vision processing, and will only fire if it is confident it is aimed at someone or something that is capable of absorbing the kinetic energy. It even adjusts the muzzle velocity based on armor thickness. And when that someone stops being capable of absorbing the energy, it firing. As a bonus, this saves ammunition.

If gun-mounted cameras aren't sufficient, your boarding party can release a swarm of tiny camera/sensor drones to build dense, real-time 3D models. Defenders of course can have higher-quality devices permanently mounted in locations with good vantage points, and more powerful computers processing the data, which works out OK, because the defending crew probably values the ship more than the attackers, and thus will need more confidence to fire a weapon than an attacker would need.

This could lead to some interseting dynamics where different groups, or even individuals, could have different confidence thresholds set for their weapons, based on how much they value the integrity of the ship vs their own objectives. You could have hot-heads setting their thresholds lower because they're sick of the gun "jamming up" when it's "clearly pointed right at the guy".

Since the ships are so valuable, there may even be some sort of open standard (or de-facto tolerance of hacking) to allow data-sharing between opponents' systems; if the norm is for only a small percentage of fighting forcs to be killed before one side surrenders, it may be "safer" for all involved if everyone is shooting more accurately. This seems less likely though.

How to solve the old 'gun on a spaceship' problem? already addresses bunch of non-kinetic weapons, but doesn't mention rockets and torches, both of which can be used to disable and cut through opposing force like engine, hull, and crew.

One could also tow and land an enemy ship to board with gravity, or as mentioned elsewhere, spin up the ship.

# Infrasound

How about an infrasound device? They are quite big, and so you would have to weld it onto the outside and sort of stalk the ship for a little bit while you wait for its effects to kick in. Infrasound is sound at the 1hz-20hz range. It can cause hallucinations at high decibel levels (as it turns out the resonant frequency of eyeballs is 14 hz) They can also cause uneasiness, fear, a feeling like there is a spiritual presence. Of course you would have to wait for everyone to go crazy or kill themselves somehow like in the Dyaltov Pass Incident, which one of the more likely explanations is infrasound generated by the winds there.

This page has some information on the effects of infrasound, and a diagram of a infrasound device. "the frequency that is thought to be most dangerous to humans is between 7 and 8Hz. This is the resonant frequency of flesh and, theoretically, it can rupture internal organs if loud enough ... according to results published by NASA researcher GH Mohr, frequencies between 0Hz and 100Hz, at up to 150-155dB, produced vibrations of the chest wall, changes in respiratory rhythm, gagging sensations, headaches, coughing, visual distortion, and post-exposure fatigue. 5 Subsequent research has determined that the frequency that causes vibration of the eyeballs – and therefore distortion of vision – is around 19Hz." These are a few key things I decided to pull out.

Also, armor and earplugs are completely useless against it. "Even with industrial ear protectors, sound waves are able to enter the head via the nose and mouth which are, in turn, linked to the ears by the structure of the skull." (this quote is from the same article). As sound can't travel through space you would be perfectly fine as long as you shut it off before you board.

## Frame Challenge - Go Nuts and Fire Away

Space ships and stations will need to be built pretty tough to survive. Micrometeorite impacts are a serious problem. Radiation shielding (for vessels outside LEO) would require even bulkier amounts of armor.

And assuming you're constructing your ships / stations in orbit, the cost penalty for being heavy goes WAY down. So everything is going to be well shielded. You probably won't pierce the hull even if you try.

But even so, atmosphere in space is a tenuous thing. Ships are going to include air tight bulkheads that can be closed in the event of a leak. Ships are going to have low pressure air piping that can supply O2 to passengers in an emergency.

So punch holes in the ship. Go nuts. Most defenders will probably already be in space suits by the time you do, but if not... well, they are going to be distracted trying to don suits when they should be fighting back!