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For reference: the nuclear torpedo. It is a torpedo with a nuclear munition for a warhead - exactly what it says on the tin.

Range is 100 kilometers at 75 knots; guidance is sonar, with gyroscope primary backup and broadband direct control secondary backup. Warhead is 50 kilotons; for reference, Operation Crossroads's "Baker" test shot was 23 kilotons. The torpedo is guided by a "dumb" AI (basically a very smart learning algorithm, rather than anything approaching sapience) that will attempt to evade threats, as well as detonate as close to its target as possible.

Presumably, even a miss, if close enough, will still be catastrophic for the target - the overpressure and water waves created will be incredibly damaging.

Is there any way to defend a vessel against such a system other than by destroying its launch platform before it fires? Submarines can dive and stay silent, but a surface warship cannot.

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  • $\begingroup$ you can destroy it before the weapon arms itself. Nuclear Weapon initiation requires precise control--the first thing is that a bunch of explosive have to explode in unison to compress the uranium ball to critical mass. You blew up the nuke before it explode then you just got a small containated area, much better than a crater in the ground $\endgroup$
    – Faito Dayo
    Aug 30, 2021 at 5:24
  • $\begingroup$ How does the guidance system work? (In case jamming is an option). Also, range and speed and size of detonation in your world might be helpful to know. Also also, can you detect when they've been fired, and do they travel at a constant depth, how deep? $\endgroup$
    – Escaped dental patient.
    Aug 30, 2021 at 5:26
  • $\begingroup$ @FaitoDayo What if it arms itself the second it leaves the tube and detonates on contact/in proximity? $\endgroup$
    – KEY_ABRADE
    Aug 30, 2021 at 5:32
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    $\begingroup$ @ARogueAnt. Guidance system is sonar with gyroscope backup and direct guidance secondary backup. Range is 100 kilometers at 75 knots. Size of detonation is 50 kilotons. I will edit OP to reflect this. $\endgroup$
    – KEY_ABRADE
    Aug 30, 2021 at 5:36
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    $\begingroup$ @FaitoDayo No - modern nukes do not blow up if hit, they fizzle. Unless the trigger explosives go off very precisely, the most you'll get is a bunch of radiation. $\endgroup$
    – KEY_ABRADE
    Aug 30, 2021 at 5:37

3 Answers 3

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As the torpedo is nuclear, merely resisting the explosion is a no-starter.

So you need to make the torpedo not explode near enough to your ship to do damage.

Before you can do anything to or about the torpedo, you need to know of its existence.
Fortunately, a torpedo barreling along at 75 knots is very noisy. In addition its guidance is active sonar (passive does not work at that speed), so acoustic detection should hear the torpedo coming from many kilometers away.

Option 1) Destroy it

  • The incoming torpedo is noisy, and its destination is known. Intercept it with your own anti-torpedo-torpedo. Your counter torpedo will be shorter range, and need not be nuclear, so you can afford to send out dozens of them.
  • Alternate: locate the incoming torpedo, drop your own nuclear airborne missile directly over it. As the missile is a magnitude faster, evasion is not an option. Expensive and messy, but very very likely to work on one shot.
  • Close-in defense: deploy a couple (of thousand) tiny drones to swim alongside your ship. These drones are just a fast swimmer, a proximity detector, and a small High Explosive charge. Basically, small smart depth charges that accompany your ship. Have them swim out as far from your ship as they can while still maintaining a 100% overlap in detector and blast-lethal-damage range. When the torpedo approached such a drone, it determines when the torpedo is close enough, and suicides.

Option 2) Divert it
The torpedo is guided by sonar.
Just redirect it to one of your suicidal drones by transmitting a suitable Spoofing signal.
Better yet, hack its "broadband direct control secondary backup" and tell it to return to sender.

Meanwhile, you now have an accurate source for the origin of the torpedo. Send a mr. Bomba to greet the interloper.

Realistically, you would not entrust a nuclear warhead to a longrange 75-knot smart torpedo.
75 knots is way too fast to be stealthy, yet not nearly fast enough to evade interception and retaliation via speed.
Either make your smart torpedo very stealthy, with zero propulsion noise and with a sound&radar-absorbing/deflecting surface, so that it can sneak up on the target,
OR
Make it so ludicrously fast that countermeasures are unable to be mobilized in time.
The russian Shkval, for example, approaches at a leisurely 370km/h. (and that's using 1977 tech)

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  • $\begingroup$ In other words, pretty much the same as evading any other torpedo, just more difficult. $\endgroup$
    – Monty Wild ♦
    Aug 30, 2021 at 11:49
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    $\begingroup$ @MontyWild Nope. With conventional explosives you can (any this is by far the easiest), just string up a net some distance from your hull. Torpedo hits net, detonates there instead of in contact with metal. With a nuclear warhead, that net would need to be 500m++ from the ship, making it very difficult indeed. $\endgroup$
    – PcMan
    Aug 30, 2021 at 11:57
  • $\begingroup$ Exactly my point... conventional maneuvering and decoys can still be used, but the atomic warhead means that the sub just has to be that much further from the torpedo when it detonates. The principles are the same, it's just the execution that's more difficult. The days of having torpedo nets deployed around a ship are long gone. $\endgroup$
    – Monty Wild ♦
    Aug 30, 2021 at 22:48
  • $\begingroup$ Oh yeah? Well my dad has a whole school of anti-torpedo torpedo TORPEDOES! And they are going to swim with the nuclear torpedo and they look just like it and they are going to bust up your little antitorpedo torpedos! And the nuclear torpedo is going to swim right into the mess hall on the sub and blow up in the refrigerator. SO THERE! $\endgroup$
    – Willk
    Aug 31, 2021 at 18:24
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Drones.

If at all possible, you don't want to be nuked. Nukes are really bad. As such, you need a wide sensor net.

Drones do that. They can remotely connect and monitor the surrounding area. If someone launches a nuke they can either swim or fly to the nuke and explode on it, or warn larger platforms to deploy countermeasures to confuse or explode the warhead.

Then you have a mess of radioactivity since nukes don't go off if exploded and a very pissed off nation ready to retaliate.

Other submarines and ships

Letting a nuclear sub or ship get within 100 miles of your big expensive ships is a bad idea. As such, you have other submarines sailing near them and if they detect them, they can tell the bigger ships which can ping the enemy submarines with active sensors and tell them to fuck off or get shot. If they then shoot you have more range to shoot them down.

A ship can carry much more powerful sensors than a torpedo, so they have a good chance of detecting your ships early.

Over engineer your ship.

Carriers that got nuked in the past didn't do great. but the data from their failure might help you design a more durable carrier that can survive near misses better.

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If you are looking for some sort of anti-torpedo system similar to the missile defense systems on surface vessels, I am not aware of any that exist. Thats not to say they don't, but submarines are highly classified and if such a system for intercepting incoming torpedos does exist, it hasn't been made public.

A submarine's chief defense is going to be stealth and speed. Being able to out-pace and lose a pursuing vessel is the first and foremost defense a submarine has. This is why in almost any picture of a submarine in dry-dock they will have the propeller of the sub shrouded. The design of modern submarine propellers is top secret because they are designed for maximum speed and minimum noise, making them a matter of secrecy. There are also possible countermeasures to jam incoming torpedoes guidance systems, but my background is with anti tank missiles and I don't really know much about how torpedoes are guided.

The other deterrent a submarine carries is it's own nuclear arsenal. If you fire on one and fail and it gets away, it is very possible a salvo of nuclear ICBM's will be launched at your home country. Even if you succeed in destroying the target, the US military has acoustic and seismic sensors scattered throughout the oceans to detect submarine and nuclear activity. It is so sensitive it can actually identify different classes and models of submarines and weapons used. If you fired a nuclear torpedo at the submarine of a nuclear capable country you would in all likelihood be starting a nuclear war. This is not something any country is going to do outside of being part of a greater strategic first strike operation in a nuclear war.

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  • $\begingroup$ I was thinking more about defending all types ships - surface and submarine - from attack, but I recognize that I did not clarify that in the post. Have a +1 anyway for an answer that reaches outside of the technical and gets into the strategical/political. $\endgroup$
    – KEY_ABRADE
    Aug 30, 2021 at 5:43
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    $\begingroup$ Oh, well, in the case of a fleet they spread out over a huge distance so that a single nuclear munition can't obliterate the entire fleet. Honestly, surface warfare vessels are expected to take the hit for more strategic assets like carriers and nuke subs anyways. My navy buddy was telling me that the survival expectancy in a peer level engagement for his vessel was like, 14 minutes. $\endgroup$
    – TCAT117
    Aug 30, 2021 at 5:54
  • $\begingroup$ Jesus. I guess they'd be dead anyway if nukes were flying. However, what if the torpedo is capable of selecting targets based on their acoustics/draft/movement speed/etc., and it decides to go ignore a frigate and a supply ship and go off right beneath a carrier? $\endgroup$
    – KEY_ABRADE
    Aug 30, 2021 at 5:59
  • $\begingroup$ Then congrats. You've just initiated nuclear Armageddon with the most heavily armed, technologically advanced civilization in human history. But, yeah. I think it's kind of hard to grasp for most folks, but ultimately, in any military doctrine any asset is at some point expendable. If the successful achievement of a mission accomplishes a high enough goal, then the lives are written off. There is no such thing as a non-expendable military asset. Just varying levels of value before that expandability is reached. $\endgroup$
    – TCAT117
    Aug 30, 2021 at 6:27
  • $\begingroup$ @TCAT117 If you are launching a smart selfguiding ultralongrange NUCLEAR torpedo at an opponent's ship, then you have very much already decided that you are in WW3, and are committed to it. That's not the sort of thing a little rogue nation or terrorist is capabe of doing. Remember you just need 11 simultaneous strikes and the "most heavily armed" suddenly has no surface fleet left. The described weapon is exactly the sort of surprise strike weapon that could try to accomplish this. $\endgroup$
    – PcMan
    Aug 30, 2021 at 8:18

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