Assume soldiers of the future need to fight person to person underwater, at depths from 30 to unachievable today, like 1-2 kilometer. Today, underwater warfare between scuba divers, involves conventional, gunpowder weaponry, modified to be effective in water (like Russian APS). But what about future? Will future underwater soldiers use same weapons as today, or will they move on to some energy weapons, coilguns or even more exotic means of killing each other?

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    Directed Sonic weapons. A sonar can kill a diver, the challenge is to make it a beam rather than a ball – Bald Bear Sep 26 at 21:08
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    Yeah large focussed sonar beams. Dolphins use this to stun fish. Also.. if we have high density energy storage in the future.. then small.. very fast torpedo like self propelled harpoons.. drone swarms.. very low frequency energy weapons. – Richard Sep 26 at 23:44
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    I would imagine most of the underwater weapons would be taken from your conventional weapons. I doubt underwater warfare would ever get extreme enough to warrant its own branch of infantry weapons. Water covers too huge an area and we humans are pretty inefficient when swimming under water. That and the crushing pressure and low temperature probably can't be solved without submersibles which are now being more automated focused. – Shadowzee Sep 27 at 4:12

Lasers

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As crazy as it may seem there are laser pointer products available with ranges up to 100 meters. Power need not be high to do things like blind enemies, early detonation of incoming explosives. Would have to do more research to determine how much energy you could put into the beam before blooming (the water breaking down to steam and/or plasma) begins to set a practical limit. There's an article here actually researching the subject. Also, here's a link to a place where some other people have given more thought to the subject.

Gravity, Sonic, Anything Concussive

Because water is so dense and is such a good acoustic conductor any kind of focused sound or gravity/force beam would be pretty effective. Likewise, explosives are still very good because water does a good job of radiating the concussive pressure.

Smart / Swarm Weapons

Like a school of fishes, a school of drones or other self-propelled weapons platforms would be very useful.

I think if you had soldiers underwater in the future then you would have underwater drones. And if you are constructing underwater drones you could build them out of mostly clear plastics with similar optical density as the water they will be deployed in so that they couldn't be seen, giving you invisible underwater drones.

To think outside the box: undersea animals, weaponized

If you want to stretch the thinking on underwater weapons a bit, consider the work that in the modern world the US Navy has done with dolphins in terms of finding things underwater. (Granted, it is somewhat controversial among animal rights groups).

Then apply the working dog/attack dog/war dog template from various times in human history.

give "x" amount of progress, and tech, that permits far better human-to-sea animal communication, incentive, and specialized breeding (as has been done with a variety of mammals). The high tech here is those tools that have enabled a far better means by which humans and the various sea creatures communicate, and cooperate as humans have done with a variety of land animals.

There is your underwater weapons system: either the animal itself (sharks, etc) or some animals who have underwater payloads attached to them for delivery (like limpet mines being attached to a ship thanks to exceptional training of the sea animal). This is as plausible as a variety of fancy tech in various sci fi books and movies.

Or go with unmanned/remotely operated armed vehicles, underwater

The less exotic sub sea weapons will borrow from the last 30 years of air warfare: armed drones. Those can be built for underwater applications as well as for air applications. (A wide variety of superb underwater search vehicles exist now. Changing the payload to underwater weapons is hardly a reach even in the present day levels of tech).

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A weapon that fires very streamlined, needle-like projectiles rapidly would be effective if the projectiles could be fired with enough velocity, and is viable within the near future. The projectiles might inject a poisonous substance for increased lethality.

However, if you are fighting at depths of 1-2 kilometers, fighting might be similar to what one finds in Pacific Rim: armored soldiers using close-range weapons. Combat would be based around trying to destroy whatever is protecting the soldiers from being crushed by the pressure.

Apart from directed sonic weapons, maybe also consider relatively small bullets like flechettes, spikes or even bubbles that are shot from a railgun - think pistol shrimps. Or how about electric fields like those in electric eels though this may be more of a defensive field generated by the suits (suits are insulated from their own fields).

TL-DR: There's probably no point improving what we have today other than making it smaller or faster PS, sorry for the long answer... i got carried away

So, we can't say for certain exactly weapons we'll have in the future, so if we take most computer games set in the future our options really consist of:

  • Gunpowder Propelled
  • Compressed Air/some other gas
  • Remote Controlled
  • Laser Weapons
  • Plasma Based (at least most games see this as an option)

Then of course, we have to make a few assumptions, the main ones are what defenses would be put in place to counteract the weapons. Historically weapons that weren't effectively countered didn't improve as fast as those that were. why make a Ground to Air Missile with a range of 50 miles, when Air to Ground Missiles only have a range of 10 miles. wait AGMs have been improved to a range of 60 miles, so GAM need to be improved to have a greater range again and again etc etc

So... I'll start with in reverse order of practicality as i see it:

Plasma Based

These appear in almost all futuristic games and even a few films. Plasma based weapons need to fire a ball of super heated plasma at an enemy. the problem is that the weight of that plasma would be very low, so it would slow down through water exceedingly quickly and the water would drain all the heat from the shot, very quickly boiling the water right next to the person firing. there would be a small range increase from the fact that water in front of the plasma would boil and offer less resistance though. Even s,o definitely the least practical.

Then you have to figure out how to store the plasma ammunition. Store it hot, how? or Heat on site, possible but requires a lot of power so you need to transport massive batteries or a generator.

Laser Weapons

A powerful beam of light that whether single or multiple beams, the water resistance wouldn't slow it down so that's better right...

Well, probably, but not by much. Lasers so there damage to regular material by heating up what it hits, and that would include the water between firer and Target, same problem as the plasma boiling the water right next to the firer.. but also that water diffuses the light significantly, other has pointed out this limit already so i won't go into detail on this.

However the diffusion would greatly reduce the effective range of the weapon, still a lot more than the plasma weapon though. but you still have the issue of powering it, either carry a generator down there with you, or some batteries, noting that cold batteries are much less effective than normal surface temperature batteries.

Compressed Air/Other Compressed Gas

Works exactly the same as gunpowder weapons but uses a compressed gas as propellant. now these already exist, Air Rifles use this pretty effectively, and some spear guns use compressed CO2 as well. However, worldwide this is almost always used instead of Gunpowder for three reaons.

  1. it does less damage than gunpowder weapons. Target shooting doesn't need an expensive backdrop behind the target,
  2. It is quieter, this is usually not the main reason, but its an added bonus, you can shoot rabbits or rats quietly without being disruptive to neighbours etc.
  3. It is less/un-regulated. meaning you don't need to go to a registered store just to grab some CO2 cartidges or it can even be filled at home

However it doesn't change the fact that gunpowder weapons are still deliver much more kinetic energy therefore more damage (usually there are of course exceptions)

Gunpowder Weapons

AS you've already stated, things like the Russian APS already exist. could they be improved on? probably, but is there a need? not yet, but in your story yes there is. so improve the design a bit, increase the range, reduce the weight increase the damage. most people don't even know about the APS let alone know what it can effectively do.

Remote Controlled Weapons

Much more likely the way things are going to go in the future, however consider for a second what needs to be done to make this work effectively, and compare it to current technology:

  • Build a (most likely propeller) engine that can work at depth... Check
  • Provide a fuel tank/battery that can power that engine... Check
  • install a guidance system with basic control surfaces to allow maneuvering.. Check
  • Have a warhead capable of defeating a target... Check

Well done, you've just built a torpedo

All that would change is the size, shrinking it down to make it more man portable, and maybe increase the speed a little

However

I'm willing to bet that the more you think about the weapon and its practical use the closer you get to the thought process below...

So in all cases, you need to carry Ammo, as well as propellant for the weapon. then most likely you'll need Breathable air and Power. Then you'll want some armour, some of it to protect you from enemy lasers and the water that is boiling straight out of your futuristic weapons and scolding your poor skin. the rest is to protect from the extreme pressure. Then you figure that your carrying a lot of weight why not propel yourself around with an engine of some sort, nothing too fancy, though but it will need its own power, or a bigger battery to share it with your future weapons. Then of course, you need your detection systems, whether it be active/passive Sonar or something else is up to you. Then maybe you decide that heating and lighting as well as the detection system would benefit from bringing a generator with you. Then you realize that your actually carrying a huge amount of weight so you need to propel yourself around with a bigger engine engine of some sort. Now you need a big ol fuel tank! even more weight, jeez its getting big now. Now your carry so much you expensive and delicate equipment, having someone with you to fix it if there was a problem, its a long way to the surface after all. and to be fair its now big enough to not be man portable anymore so you need someone to move you around and someone to fire the weapons. But hang on... what happens when i want to sleep??? well, i could put a bed in there for me, the pilot and the mechanic that'll work... Eating? yeah ok so a food store. and maybe a chef, that way we can stay down there for a really long time if needs be. We may as well carry a water maker and an oxygen maker then, save space on storing it, more room for Donuts!!! But what if someone shoots at us? maybe we should fit some countermeasures to confuse their remote control weapons and diffuse their lasers!!! You know that means we need a bigger engine again right? and a bigger fuel tank! Ok so while we sleep, we'll need some other people to control this strange thing and some more beds right? Why not save the big fuel tanks and just fit it why a reactor? tonnes of power... oh and some very well trained people to look after it

Well done my good sir, you've invented a submarine!!!

Things can't move very fast, or anyway it takes tremendous energy to move them real fast. And when you move very fast you leave a big arrow pointing at you.

Munitions can't travel very fast very far. So munitions tend to be short range. Maybe the most effective munitions create a blast, so shear forces in the water tear things apart even from a near miss, but if you're too close the blast affects you too. (Unless it's a little tiny one.)

And oceans are giant. So presumably to have a fight even make sense, your enemy has a specific target you want to take. They have defenses concentrated around it, and you send an attacking force to take it from them. You know in general where their defenses are -- they're defending it.

You send your attacking drones near there. It's most cost-effective in terms of energy to send them on the ocean surface, but they can be spotted. It's fastest to send them by air but they can be spotted. Somehow you get them reasonably close and they try to sneak closer. The enemy detects them and strengthens its defenses against them, weakening the defenses in all other directions. When they are concentrated as much as you can get, you set off a small nuke to create a shock wave that destroys your whole fleet of decoys and their nearby defensive forces too.

They can't just not concentrate their forces because your fleet might not be decoys. But if they are far enough away from the target they want to preserve, they can nuke you.

If you don't care about capturing the target intact but just want to deny it to them, you can probably nuke it.

Not to be a downer or anything but lacking For each of foot in depth of salt water (fsw) you are adding 1 standard atmosphere (14 psi) of pressure. Go 1 kilometer under water and you are at 45,920 lbs / square inch of water weight pushing on every inch of surface area. Humpback whales can survive at incredible depths while holding their breath so maybe if our lungs eventually evolved to be more like theirs we could somehow cope with the compression / decompression but basically at those depths there is very little light and very little life. No matter what you stick a human in anything with oxygen is going to be in great peril just by being at depths below a few hundred meters. Everything below that depth will have to be with robots.

You're going to need some kind of false-image sonar overlay just to percieve anything, which would give away your position -- or turn on lights which might also give away your position but probably less so than sonar because the light doesn't carry as far. Green light can still propagate (it is the last color to get filtered out, longer wavelength), so green lasers can still be used for positioning, targeting, liDar, maybe even energy weapons but the range will be very limited as water diffuses the light.

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