What technology would be necessary to allow soldiers to ascend rapidly from deep in the ocean and to the surface?

Question

In my world, submarine technology and ocean exploration is highly sophisticated. A large sub can house hundreds of people in addition to smaller subs and they are used to travel, kind of like airplanes. They are also utilized in warfare. Much like paratroopers, there are soldiers trained to rapidly ascend from submarines in the deep sea and reach the surface in a very short period of time without suffering any ill effects. What kind of technology would be needed to make this possible?

Some info:

Technology is about 500 years ahead of our current time (2021).

Some example of the technology used in this setting :artificial wombs that is almost exclusively used to birth the humans in this world. Gyrojet guns are the primary form of firearm, but large railguns are used for artillery.

This is an earth-like planet, but is not Earth. It is not an Earth colony, rather an alternative universe

The maximum they would ascend from is 3000M

The soldiers rapidly ascending are in a sort of half biological half mechanical mech

Ascent to the surface should take about 10-15 minutes

These soldiers ascend to the surface but they never leave the water

Stealth is desired on occasion, but is not always necessary

Answer 1

3000 metres is deep

Pressure underwater is a serious problem for humans. The record for diving without a rigid pressure suit is slightly over 500 metres (using special gas mixtures), while the record with a rigid diving suit is 610 metres. This is nowhere near the 3000 metres required, and the required ascent speed of 3.3-5.0 m/s is also very high.

Humanity has built submersibles to operate at a depth of 3000 metres or more, but they are very specialised vehicles. For comparison, an Ohio class nuclear submarine has a test depth of 240 metres.

There are living creatures that exist at 3000 metres depth, but few of them come to the surface. Sperm whales dive to depths of 2250 metres (though some sources say 3000 metres) over the course of a couple of hours. However, even sperm whales do not ascend quickly enough to meet the required parameters - as per this article, data loggers showed a maximum ascent speed of 3.15 m/s.

Given all of that, there are a few possible approaches:

1. Genetically engineer human bodies to be like sperm whales. This one is marginal. It would be very hard to give a human sufficiently sperm-whale-like characteristics without going the whole way (see option 2). It also leaves the difficulty of making all of the gengineered combat swimmer's equipment able to survive the pressures required and the pressure transitions.
2. Genetically engineer sperm whale brains to be like humans. Frankly, it's probably easier (over the next 500 years) to get a sufficiently smart and dedicated sperm whale that's willing to work for humans than it is to make a human sufficiently like a sperm whale, given that they are staying in the water when the reach the surface. Especially if you frame the mission as the whales having a chance to whale on the surface humans in revenge for all the centuries of whaling. However, there are still the difficulties in hardening the combat payload that Moby and his pals will carry.
3. Construct rigid dive suits that can handle 3000 m depths. With 500 years of development in materials technology, this is probably feasible. The major challenge will be giving the joints sufficient flexibility, but if the limbs aren't required until above 600ish metres depth then use smart materials to either make the limbs fully rigid at greater depth or keep the limbs retracted within the main frame until required.
4. Use larger submersibles to deliver combat swimmers to surface (frame challenge). Frankly, paratroopers are not much of a thing any more, most airborne deployments are conducted by helicopters or other aircraft. Paratroopers always sustain a certain level of casualties and equipment damage even in peacetime drops, so except for special circumstances (eg special forces doing a covert HALO drop) it's safer, faster and more reliable to deliver troops in a vehicle. While there may be special forces combat swimmers using stealth to insert into a target zone (especially using option 2, which is very difficult to distinguish from normal ocean life in an area) it's much easier to deliver combat swimmers with heavy weapons using "conventional" submersibles.
5. Send in the drones (frame challenge). I understand the drama of using "people", but real life people, both now and 500 years in the future, would rather sit at a console a long way away from the nasty strangers with guns and control a submersible drone that goes into harms way. This basically lets you build option 3 but without the technical challenges of keeping a squishy human alive, instead use a bit of space for the communication link and either make the "suit" either smaller or more heavily armed.

Answer 2

As mentioned in the comments, the problem is pressure.

• In a sufficiently science-fictional setting, your troops could be genetically engineered to survive large pressure differences without getting the bends.
• The alternative is to keep both the submarine and the ascend system at sea level pressure. Either there is an atmospheric diving suit for the ascent or something like the escape capsule on some subs.

Answer 3

If they don't breathe pressurized gas, they won't get the bends.

Deep divers breathe pressurized gas. They must, to inflate their lungs against the pressure of the water outside them. Pressurized gas means pressurized gas dissolved in the blood, which then comes out of solution when the pressure decreases. It is like the bubbles coming out of solution when you open a bottle of soda. That is the bends.

But your divers are in submarines. These have rigid hulls to withstand the pressure. Inside the hulls they can breathe gas at atmospheric pressure.

When they leave the hull and enter the water, the exterior pressure will push their bodies in and collapse their lungs. That happens to free divers. This dude did a 3 minute free dive. You can see his chest has been compressed. If your people have 3 minutes to get to the top, that is 1 km/minute or 60 km/hour. Very doable if you have a machine to tow you up!

https://www.redbull.com/int-en/join-a-three-minute-free-dive-in-dean%E2%80%99s-blue-hole

You could make it more scifi by having your free ascenders breathe a lungful of oxygenated perflubron before exiting for the ascent. That is 1980s tech. This will provide an incompressible bulk in the lungs and also a reservoir of oxygen for them to use on the way up. They will have to excuse themselves to cough that out once they get topside.