Hyper-Agile Submarines?

Question

Building off of another question I asked about the realism of short-range combat in the future, I was thinking about making combat scenarios with small, hyper-maneuverable submarines. These submarines would need to move about like airplanes (using water dynamics over “wings” to keep motion linear at speed) and would be able to move in a linear direction while able to turn quickly by using flaps or a similar mechanic to rotate in any direction. Is essentially turning submarines into futuristic fighter planes a feasible concept? Would “flight” mechanics work differently than a traditional airplane? I would imagine buoyancy and pressure building with depth would form problems for linear travel in six degrees of motion.

Answer 1

At high speeds (starting from 77 km/h), water acts as an abrasive. Any sub moving quickly gets polished:

When the fastest submarine in the world came to the surface, they could not recognize it - all the paint was completely stripped off, and the welds were smoothed out.

(source: http://avtoexpert--dv-ru.translate.goog/stati/samaya-bystraya-podlodka-v-mire.html?_x_tr_sl=auto&_x_tr_tl=en ). I presume that above 77 km/h, the sub hull and all protruding parts wear down quickly.

Any sub also gets noisy (100 db inside) above 65 km/h; nobody knows yet what to do with it.

As for turning upside down, it requires either

• exremely high speed, as in a barrel roll, so that the sub always thinks it's NOT upside down, or

• a true ability for every piece of equipment to work upside down for a sufficiently long period of time.

High speed has been tried (limitations were found, see above), but not extremely high speed (except for small devices such as torpedoes).

Making all the equipment being able to work upside down is, erm, expensive (again, except for simple devices such as torpedoes). Consider the kitchen, the toilets, water turning into steam in a turbine or a nuclear reactor, steam turning into water in a steam condenser, oil in a combustion engine, fuel pumps having to pump from half-empty fuel tanks, etc. Bear in mind that a sub is already a device more complicated than an orbital space station.

Answer 2

Hyper-Agile Submarines are Neither Practical nor Useful

• Reference: I'm a former Submarine Officer

From the comments, OP seems to want high speed subs that barrel roll, loop-de-loop, etc. That's going to be both Very Technically Challenging and Not Actually Useful.

Design Challenges

The fundamental problem is power. Drag goes up with the cube (!) of velocity. So your drag force increases highly non-linearly as speed goes up.

Power is already a tough question for submarines; basically nuclear is your only choice if you want power dense, air independent propulsion. Nuclear reactors are heavy and large - a modern nuke sub is 350+ feet long, and 30+ feet in diameter, and around half of that space is dedicated to propulsion.

So you'd have to hand wave a truly spectacular power source to get anything like the performance OP desires.

Tactical Challenges

Assuming you did find that power source, you run into the issue of stealth. The faster you go the more noise you make. That noise makes it easier to find you, and harder for you to find other subs (!) because your self noise blinds your own sensors.

A sub trundling along at 5 knots will kill a sub blazing past it at 50 knots, because the slow boat will be invisible on sonar, while the fast boat is highly detectable.

Combat Challenges

Assuming that the fast boat actually hears the incoming torpedo, the ability to "dodge" is not actually that useful. The explosion from a torpedo creates an over-pressure wave that slams into the hull, damaging it - physical torpedo-to-hull contact is not required.

I don't see hyper-agile submarines as a thing.

Answer 3

Supercavitating sub.

https://navalpost.com/a-gamechanger-weapon-supercavitating-torpedo/

https://www.19fortyfive.com/2021/09/the-supercavitating-torpedo-the-one-weapon-the-u-s-navy-cant-match/

Supercavitating torpedos on the other hand travel extremely fast—hundreds of kilometers an hour. Torpedos of all types are slowed down by drag caused by friction with water. One way to overcome this drag is with bubbles.

Supercavitating torpedos travel through the water in a large bubble that encapsulates most or all of the torpedo. Compressed gas stored inside the torpedo is ejected out of the nose, and the torpedo “glides” through the water inside the bubble, remaining relatively dry.

Control surfaces, generally fins, pierce the bubble and maintain contact with the water, allowing the torpedo to steer.

Instead of just a torpedo, your whole sub is supercavitating. It is driven by a fusion engine which provides more than ample power. The gas used to envelop the submarine is steam. Steam is also the propulsive force and is blasted out the rear of the submarine like a rocket engine. Like the supercavitating torpedo, the sub flies through the water at great speed, freed of the draggy draggedness by its bubble suit.

It is much like flying a jet. In the dark, underwater.

Answer 4

TL;DR: Definitely physically and technologically viable, but tactically dubious.

A fighter jet submarine would probably be outrun and outdived by any proper ships that are bigger than it, and it would probably be outmanoeuvred and shot down by any torpedoes that are smaller than it. There's no technical barrier to building a hyper-agile combat submarine— Fluid dynamics follow the same basic laws underwater and above water— But the different environment underwater means that it likely just won't make tactical sense.

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This is a thing that already exists:

Via GfyCat.

A company called SeaBreacher makes them. Although, as their intended application is for recreation and entertainment, the real-world examples seem to have positive buoyancy so they'll automatically return to the surface unless you're actively pushing them down— But the behaviour should be largely similar with neutral buoyancy that is capable of diving much deeper.

That said, I'd want to think hard about the kinematics and energetics of what you're proposing.

Modern combat aircraft are shaped by a variety of evolutionary races:

• Speed of missiles versus speed of aircraft. An example of aircraft winning would be the SR-71, which could simply accelerate when shot at, and outrun any missiles fired at it. Examples of aircraft losing would be the XB-70 and B-1A, which could fly very nearly as fast as the SR-71 but ended up getting shelved because they were expected be shot down with increasingly advanced missile technology.
• Altitude reach of different types of aircraft versus altitude reach of interceptors, missiles, and radar. Related to above, aircraft like the U-2 have tried to stay out of reach of enemy air forces by simply flying higher than interceptors and missiles could reach. When that failed, many bombers and fighters instead switched to flying low enough to use the terrain to hide from radar, prompting a drastic rethink in the design space.
• Manoeuvrability of aircraft versus manoeuvrability of missiles/countermeasures. When aircraft are more manoeuvrable than missiles, you tend to get a lot of close-up dogfights with their internal cannons and infrared/heat-seeking short-range missiles. When missiles are more manoeuvrable than aircraft, a lot more emphasis is placed on having powerful radars and high speed at the cost of agility, as then whoever shoots first can be expected to win. In the early jet age, a lot of US fighter jets were built on the expectation that air-to-air missiles would make dogfighting a thing of the past. Their poor performance against slower but more manoeuvrable opposing MiGs meant that the following decades of US fighter jets (think F-15, F-16) ended up putting a lot more emphasis on manoeuvrability again.
• Buoyancy versus wings. For some time, lighter-than-air airships, not fighter and bomber planes, were the most prominent form of both military and civilian aviation. Their huge disadvantage is that much of their mass and volume always has to be spent just on keeping themselves in the air, while advances in engine technology and aerodynamics allow every generation of airplanes to carry a greater fractional mass of usable payload.
• Sensor range versus stealth.
• Ability to generate kinetic energy versus ability to retain it (thrust versus drag, I guess).

Underwater, a lot of these things are completely different. There's both a lot more more inertia and a lot more viscosity, so the Reynolds number is gonna be different. The fluid's incompressible. You'll get more inertial/viscous/skin drag, but the speed of sound is also much, much higher, so the Mach number is going to be lower and you'll get less wave drag. Radar and optical light won't go far, while sonar doesn't travel at the speed of light and can be confused by your own engines, so you're comparatively blind. Active power isn't needed: Jets have to keep moving, because otherwise they'll fall out of the sky, but you a submarine can silently float in one place basically forever. Buoyancy isn't a cost like it is in airships; buoyancy is basically a given, so the main downside of having a massive ship instead of smaller craft is also negated.

I don't want think too much about this, but altogether, I'm not sure there will be any place for fighter jet-style submarines. Shorter vision, high drag, free buoyancy— All these environmental factors seem to work against the conditions that have led to the evolution of modern combat aircraft above the surface. A big, heavy real-world-style submarine would likely be able to travel further, hide deeper, run faster, stick around longer, and carry more payload than smaller, hyper-agile craft, while a small, automated torpedo or drone would likely be able to outmanoeuvre it without having to carry a pilot or fuel for the return trip. Any role it could fill would be better filled by either something much larger or something much smaller.

You'll notice that despite hyper-agile submersible watercraft already existing in our world, no militaries are trying to use them in any role resembling that of fighter aircraft.

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If you still want fighter jet-style submarines, I recommend making them deploy from an aircraft carrier-style mothership sub that's big enough to take advantage of the conditions I mentioned above, and coming up with some sort of excuse for either why automated torpedoes aren't viable (E.G. no microprocessors, easily hacked), or why automated torpedoes are actually fighter jets and have the same stakes as fighter jets (E.G. remotely driven, pilot executed on loss).

Answer 5

Fish shaped submarines

Yeah submarines have a hopeless design. Under water, better use a robotic construct, shaped like a fish. It can minimize energy consumption by moving in resonance, like fish do.

There are also rigid constructs in design, shaped like a fish,

"A whale shark/manta ray-shaped mothership would be built from super-strong alloys and acrylics, with surfaces which can morph in shape. With hybrid algae-electric cruising power and propulsion technologies including tunnel drives which work similarly to a Dyson bladeless fan, the submarine could travel at unprecedented speeds of up to 150 knots."

https://www.maritime-executive.com/article/royal-navy-considers-fish-shaped-submarines