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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.

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  • $\begingroup$ ? Submarines really can travel in any direction they want. And they can rotate around the vertical axis, and can also dip or raise their noses to travel upwards or downwards. And they do have "wings" (which are called the rudder and the diving planes). (Only five degrees of freedom, because they really really do not want to roll. Submariners greatly prefer to have their feet lower than their heads.) The biggest obstacle against "hyperagility" is that submarines are large and moving the water out of the way requires a large amount of power. $\endgroup$
    – AlexP
    Dec 6, 2021 at 22:09
  • $\begingroup$ @AlexP - maybe if the submariner was mounted on gimbals they could roll. The sub would roll around them. $\endgroup$
    – Willk
    Dec 6, 2021 at 22:52
  • $\begingroup$ @AlexP I guess that’s my question. What is stopping a submariner from having their feet above their head? And would buoyancy really not make a difference with changes in depth (to a reasonable degree)? Or would that be easy to compensate for? I’d love the submarines to be able to do loop-de-loops, etc. The subs would preferably be small (car-sized) so hopefully moving water would be less difficult. $\endgroup$
    – Mark Price
    Dec 6, 2021 at 22:53
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    $\begingroup$ Subs are weird - some of the current subs could barely stand on their nose, because the they are so long the pressure differential would kill them. Also many operate in waters where their back would be out of the water if they stood on their nose.. $\endgroup$
    – bukwyrm
    Dec 7, 2021 at 7:01
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    $\begingroup$ @AlexP there are several factors that would discourage barrel rolls and such. The biggest being subs can only operate in a small sliver of the ocean. large combat subs have too much surface area to volume, so they cant survive great depths. Another being the equipment on board wont fair well at different angles, such as lube oil systems, reactor plant systems and fluid tanks. They can do "angles and dangles" which is limited to 35 degrees up or down, but only for a fe seconds. Plus, the sailors do not like flying around the sub much $\endgroup$
    – Sonvar
    Dec 7, 2021 at 14:32

5 Answers 5

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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.

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  • $\begingroup$ Welcome. Please take our tour and refer to the help center for guidance as and when. Nice first post. You had to mention the loos. $\endgroup$ Dec 7, 2021 at 8:29
  • $\begingroup$ @ARogueAnt. Alright, loos changed into toilets. $\endgroup$
    – user91856
    Dec 7, 2021 at 8:39
  • $\begingroup$ I wasn't bothered about the word so much as the mess, but fine. $\endgroup$ Dec 7, 2021 at 8:46
  • $\begingroup$ @ARogueAnt. Aha. So, you think that my answer would be more a mess than the original one-paragraph, unstructured post? $\endgroup$
    – user91856
    Dec 7, 2021 at 13:23
  • $\begingroup$ "Any sub moving quickly gets polished" - except subs that have a hydrophobic coating. $\endgroup$
    – Goodies
    Dec 7, 2021 at 22:43
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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.

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Supercavitating sub.

supercavitating torpedo

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.

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    $\begingroup$ A jet whose skin is boiling, I can't help but notice. This might be useful for a "turbo" button but if it was your normal method of travel, I'd be worried about your thermal load. $\endgroup$
    – Cadence
    Dec 6, 2021 at 23:32
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    $\begingroup$ But the water is on the other side of the steam. Unless your steering vanes are also cooling fins. I don't see a single way that could possibly go wrong. $\endgroup$
    – Cadence
    Dec 6, 2021 at 23:47
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    $\begingroup$ It would also be noisy as hell which, underwater, is the equivalent of having a "Torpedo me!" sign taped to their back. $\endgroup$ Dec 7, 2021 at 6:30
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    $\begingroup$ @GrumpyYoungMan Only if the torpedo can catch you. And if the crew of the vessel with torpedoes have the reflexes to launch in time, which is why navies are so worried about supercavitating torpedoes which are on top of you before you can do anything about it. $\endgroup$
    – Graham
    Dec 7, 2021 at 13:35
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    $\begingroup$ If i could i'd give you a second +1 just for "draggy draggedness" :-) $\endgroup$
    – Burki
    Dec 7, 2021 at 14:37
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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.


This is a thing that already exists:

https://thumbs.gfycat.com/HealthyMilkyLeveret-size_restricted.gif

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.


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).

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  • $\begingroup$ "the behaviour should be largely similar with neutral buoyancy that is capable of diving much deeper" except probably nowhere near as acrobatic $\endgroup$
    – minseong
    Dec 7, 2021 at 17:54
  • $\begingroup$ @theonlygusti And why would that be? Dolphins and other entirely submersed active swimmers can be plenty acrobatic, and wave drag is apparently the biggest source of resistance for surface ships. (IRL subs can often go faster underwater than on the surface because of that.) $\endgroup$
    – Will Chen
    Dec 7, 2021 at 19:15
  • $\begingroup$ Upvoted ! Spectacular toy.. reminding me of dolphins, or shark ! $\endgroup$
    – Goodies
    Dec 7, 2021 at 22:41
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    $\begingroup$ @MarkPrice Depth/pressure shouldn't have much direct effect on agility, as water's incompressible, so viscosity and density shouldn't change too much with pressure. Indirectly: Higher pressure means needing a stronger and heavier hull, which will in turn decrease agility. That said, agility will be pretty poor at any depth compared to aircraft— Water's ~1000X denser than air, so it will be around 1000X harder to move around. $\endgroup$
    – Will Chen
    Dec 7, 2021 at 23:37
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    $\begingroup$ @MarkPrice Maybe? It's still gonna be incompressible, and way denser than air, though. Also, keep in mind that most non-water liquids will eventually freeze into a solid chunk if put under enough pressure. $\endgroup$
    – Will Chen
    Dec 8, 2021 at 1:46
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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,

enter image description here

"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

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  • $\begingroup$ What's "moving in resonance"? $\endgroup$
    – minseong
    Dec 7, 2021 at 14:39
  • $\begingroup$ @theonlygusti before I found that link, I considered having a submarine with an actual fish body and tail. Fish move very effectively on low energy, because they move by wagging their tail in the water, "resonating" themselves forward. Compare human walk, that is also a form of resonance and very effective. $\endgroup$
    – Goodies
    Dec 7, 2021 at 16:37
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    $\begingroup$ I just found this article about resonant motion in fish: royalsocietypublishing.org/doi/10.1098/rsif.2013.1073 Looks like fish have optimal frequency of undulation considering the resonant frequency of their bodies and "the frequency of maximum spatial amplification of unstable wake" in the fluid around them $\endgroup$
    – minseong
    Dec 7, 2021 at 17:51

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