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So I have various factions with submarines and surface ships battling it out for control over various underwater resources.

One of the rarer and more lethal weapons is a "stealth torpedo". It presents a serious threat when employed because as its name implies, it is very difficult to detect and, by the time it is, it is often too late for evasive maneuvers. Would It be possible to somehow muffle the sound of the engine for defeating passive sonar as well as bend sound waves around the torpedo hull like a stealth fighter to beat active sonar?

Currently, I have a system similar to the Caterpillar Drive from Red October driving the torpedo. However, as far as I can tell from the research I have done, the few prototypes in existence are too slow, impractical, and not to mention too big to fit in a torpedo.

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    $\begingroup$ Is hiring a squad of kamikaze divers an option? $\endgroup$
    – John Dvorak
    Jun 12, 2017 at 22:38
  • $\begingroup$ Maybe as an absolute last resort. However, If the target sub is below the safe diving depth for divers without a pressure suit, getting said squad outside the sub, not to mention in time to stop the torpedo would be a problem. Also, if the torpedo can't be detected by sonar, the divers would have to acquire a visual in order to intercept which would be next to impossible in anything but well lit, crystal clear water. $\endgroup$
    – Nathanael Williams
    Jun 12, 2017 at 22:46
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    $\begingroup$ Modern diesel-electric submarines such as the German Type 212 are very very stealthy when running on battery. Famously, they have succeeded to "go through all the defence of a U.S. carrier strike group, unseen, and shoot green simulation torpedos at the carrier". When the submarine is so stealthy the stealthiness of the torpedoes themselves is moot; one the contrary, you may want very rapid (even if unstealthy) torpedoes like the Russian VA-111 Shkval. $\endgroup$
    – AlexP
    Jun 12, 2017 at 22:50
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    $\begingroup$ The Shkval and other supercavitation torpedo types feature quite often in the story as they are more common and relatively easier and cheaper to produce than the stealth torpedo. $\endgroup$
    – Nathanael Williams
    Jun 12, 2017 at 23:06
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    $\begingroup$ @AlexP Indeed. US navy routinely "loses" Carriers to diesel subs during exercises. Some of the countries which can claim to have "sunk" one are: Canada, Australia, Great Britain, Netherlands, Chile, Germany, Sweden and China. $\endgroup$
    – M i ech
    Jun 13, 2017 at 3:35

7 Answers 7

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The state of the art for subs is stealth. You can't hit what you can't see. When sneaking around everyone uses passive sonar since active sonar lights everyone up equally. The idea is that you need to minimize cavitation, that's what happens when the spinning prop blade (which has a shorter radius near the center) creates a density differential in the water and forms a bubble. The resulting bubble collapses making noise. Minimize that and you're good. It works better at slow speed, but you can kind of tune your prop for the speed you want. The process is well known, but solving the problem requires lots of number crunching and some good understanding of fluid flows.

Then one day one of the major powers had this break though in out-of-the-box thinking. Stealth is only part of the battle. When things get heated, everybody lights up active sonar, bathes the battlefield in light so to speak. At this point you want to fire the fastest weapons you can. So instead of making a slow quiet torpedo, they made the fastest they could. They put the prop on the front, they maximized cavitation and put a rocket on the back end of it. The little guy swims in a pocket of air and travels like a bat out of hell. Meanwhile the other side worked hard on quiet torpedoes.

In a situation with multiple sources of active sonar and rocket powered torpedoes/torpedo killers, I'm not sure a stealth torpedo is actually going to win any battles.

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  • $\begingroup$ So using fast, super cavitational torpedoes like the Shkval would ultimately prove more useful then? $\endgroup$
    – Nathanael Williams
    Jun 12, 2017 at 23:02
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    $\begingroup$ I disagree. A stealth torpedo has a role on the modern battlefield. Once the enemy knows he's being shot at what you want is speed, but stealth will let you get close before he knows about the danger. You want to shoot at that carrier 100km away? The helos will find you if you try to lie doggo in front of the carrier, your noise will give you away if you try to approach from any other direction. But suppose you get in front, fire several stealth torpedoes and leave. They are trailing antenna wires so they can get updates. When the targets react they drop the stealth propulsion and dash. $\endgroup$
    – Loren Pechtel
    Jun 13, 2017 at 1:08
  • $\begingroup$ High speed torpedo: en.wikipedia.org/wiki/VA-111_Shkval $\endgroup$
    – Antzi
    Jun 13, 2017 at 7:29
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    $\begingroup$ "You can't hit what you can't see." [Citation needed] You certainly can. It may be harder in some scenarios, sure, but that statement is not true! I mean, you can drop an H-bomb on a city and kill thousands of people without seeing any of them. Nor the city. Nor the continent. $\endgroup$
    – xDaizu
    Jun 13, 2017 at 8:26
  • $\begingroup$ @LorenPechtel so you are essentially talking about mines that have the ability to close in on nearby targets? That's what I started thinking of. $\endgroup$
    – Baldrickk
    Jun 13, 2017 at 10:02
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Fishpedo

You want stealth. Then be stealthy. Blend in. The ocean is full of fish. Look like a fish. Move like a fish. Have the sonar profile of... you get it.
The sub cannot destroy every medium sized fish that comes into the vicinity. Have it swim around lazily in a fishlike matter. Then it gets close and BLAM!

The Mossad shark depicted here has a propellor - no, no, no. The slow fishpedo should propel itself by swishing its tail back and forth. The squidpedo would jet along like a squid. The hagfishpedo would do what hagfish do.
enter image description here http://www.keiththomsonbooks.com/blog/mossad-sharks

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    $\begingroup$ Would the fishpedo be made of metal or a carbon alloy? What would be your position of embedding the explosive payload in actual living tissue (not a real animal, just living tissue)? $\endgroup$
    – xDaizu
    Jun 13, 2017 at 8:08
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    $\begingroup$ @xDaizu If you don't pose ethical bounds, it may be even easier to simply train a shark to carry the payload. $\endgroup$
    – T. Verron
    Jun 13, 2017 at 8:18
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    $\begingroup$ @T.Verron Ugh, trusting biologicals is just asking for mistakes in operations (prey, potential sexual mates, the animal hurts himself, etc...). Also, animal rights activists. Also, they take long to train in mass quantities and have a limited effective "lifespan" and may require re-train. Now, torpedos made out of organic matter, with his pseudo bones for structure and pseudo skin for covering... that's a future I'm willing to invest in! Also, it seems like it could have multiple other applications to amortize itself. $\endgroup$
    – xDaizu
    Jun 13, 2017 at 8:22
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    $\begingroup$ @xDaizu what about implants in their brains to remote control them? We've done it with rats and cockroaches... Agree on trusting animals though - look up the russian dogs they tried to train to carry mines under enemy tanks... $\endgroup$
    – Baldrickk
    Jun 13, 2017 at 9:58
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    $\begingroup$ @T.Verron now now, you know the only weapons sharks will tolerate is frickin' lasers. $\endgroup$
    – Carl Witthoft
    Jun 13, 2017 at 15:34
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Even if you could eliminate the sound of the engine and keep the torpedo from reflecting sound back to an active sonar system, it wouldn't make the torpedo undetectable. There's a technique known as acoustic daylight imaging that uses changes to the ambient noise of the ocean to produce an image. To such a system, your "stealth torpedo" would present a clear shadow moving against the background.

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    $\begingroup$ To such a system, how does sea fauna look like? How would a fishpedo look like? $\endgroup$
    – xDaizu
    Jun 13, 2017 at 8:31
  • $\begingroup$ Also, nothing is perfectly stealthy. Even stealth fighters have to know of to fly right to minimize their detectability for different detection systems. $\endgroup$
    – Paul
    Jun 13, 2017 at 19:43
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The problem with a stealth torpedo is that it is moving and displacing the water around it. If it is moving very fast (like a supercavitating torpedo) then the amount of water being displaced will be very large.

We can see some analogues in aircraft, for example sonic booms for supersonic aircraft, but much of the noise an aircraft in flight makes comes from the propulsion system moving masses of air backwards to provide the forward thrust for flight. But that's not all. Even a glider is actually rather noisy when you are inside.

Now water is 800x denser than air, so the amount of movement and displacement is amplified by a huge amount due to the higher fluid density.

This is not to say "stealth" underwater is impossible. A submarine covered in metamaterials optimized to the wavelength of standard or known enemy sonars will evade active detection, and so long as the captain maintains silent running discipline and moves relatively slowly, the submarine itself will be very difficult to detect. But this is actually quite the opposite of a torpedo making an attack run against a ship or submarine.

The best sort of "stealth" torpedo is actually targeted using passive means like hydrophones or MAD, and either air dropped by helicopter or airplane, or launched at the enemy sub by a rocket and drops into the water quite close to the target. There the enemy has very little time to react to the sudden appearance of an active "fish" making a run, since they did not realize they had been spotted and never "saw" the torpedo coming until it hits the water.

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  • $\begingroup$ A glider is also noisy when you are outside of it. If you get a chance to watch gliders come in from a competition, when they cross the finish line at high speed (for a glider) you can hear them make a sound like they are ripping the air apart. $\endgroup$
    – Wayne Conrad
    Jun 13, 2017 at 3:25
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The stealth torpedo exists today. It called a sea mine. They don't make noise. They can selectively attack vessels. And, covered with sound absorbing material, active sonar has a difficult time spotting them.

The design of submarines is to get close enough, without being detected, to fire a weapon. Once the weapon is fired, everyone knows where you are. The design of torpedos is to have such a great range and velocity that the target has to focus on evading the torpedo giving the submarine, or subma to aficionados, slink away and hides again.

Ship killers fired by submarines sound like locomotives bearing down on you -- at least that is what the sonarman told me. The only way to make them quiet is for them to move very slowly. But, the torpedo ejection mechanism is very loud, so a slow-moving torpedo after a big loud sound of the launch is what subma's call very very bad.

In many ways, submarines are like artillery. You shoot, then you scoot, if you want to stay alive to shoot again.

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The japanese used oxygen propelled torpedoes, which were hard to detect from the air, since the oxygen bubbles were absorbed by the water, in contrast to other gasses. You could maybe try to extend this system and use it to reduce the moving parts and use a jet system as propulsion.

What you also have to consider is the following. A traditional torpedo works via a displaced water and shockwave, not via the impact. That is why they are actually quite big.

So your goal should be to reduce their frontal profile as well, to reduce the water displacement and the possibility of active sonar detection.

If your target is something like a carrier or a strategic sub, you could make the torpedo "smart" replace the payload with something like a long shapedcharge, that attaches to the hull, and rips it open, via lance explosion (think of ebene emael and the nazis), or the cut (cooper plate method used in demoltion), or a real small nuclear device, (if you are in the near future)

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  • $\begingroup$ Do you have a link or citation for that? I think the quantity of gas is far in excess of what can be desolved, and water already has oxygen disolved in it to a large portion of its capacity. $\endgroup$
    – JDługosz
    Jun 13, 2017 at 14:20
  • $\begingroup$ Soluabilitynof oxygen: 40 mg/L nitrogen is 20. So indeed that’s more, but is that enough to destroy the bubbles? $\endgroup$
    – JDługosz
    Jun 13, 2017 at 14:24
  • $\begingroup$ @JDługosz ill do some refrence searching later, but it did give the japanese an advantage, atleast visually. (saw it in some us army training short from the era). I dont know if the bubbles are small enough and the count low enough to be significant for passive sonar, but a gaseous system without moving parts, is certainly less noisy then a whirring motor or gas driven motor. $\endgroup$
    – Git
    Jun 13, 2017 at 15:52
  • $\begingroup$ It is a gas-driven motor. The air pressure turns the shaft, just as with modern pnumatic tools. $\endgroup$
    – JDługosz
    Jun 13, 2017 at 16:13
  • $\begingroup$ as i said, you could use a gas jet propulsion system, and take advantage of the solutabilty of the driving gas $\endgroup$
    – Git
    Jun 22, 2017 at 7:17
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There are two paths you can take to bruteforce your way around detection.

Supercavitation

There are some torpedoes that through sheer thrust cause the water around them to vaporize. They literally fly inside a bubble and reach speeds which regular torpedoes cannot match.

With future tech it would be doable to make them faster than sound underwater. And you can hear them coming if they are coming faster than sound.

Early preparation

Another russian invention, the Poseidon is a torpedo with a payload of four Tsar Bomba's. If you haven't heard of the Tsar Bomba, you haven't browsed this site enough.

The idea of the Poseidon is not to be fired during a battle, but to be activated up to a a couple decades prior to the battle. It makes its way toward coastal cities slower than a sleeper shark, which makes ot very stealthy. Once an enemy power threatens you, you just send the signal, and then your enemy is a few cities shorter than it was a few minutes before. There is no better deterrent to aggression.

Some conspiracionists say that there are already many Poseidons stationed worldwide after its unveiling last year by Putin. Of course no one can either confirm or deny that right now.

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