Usefulness of Kinetic Kill Weapons as Antisubmarine Weapons

[Caiaphas]

I've recently been reading more speculative sci-fi than is probably good for me, and I got to thinking: supposedly, kinetic kill weapons, with decent enough guidance, would be able to easily target and destroy even moving land targets. What would happen if you tried to use these same weapons to kill submerged targets (assuming that targeting isn't an issue and that the round being fired is similar to the Navy's proposed railgun round, which has a muzzle velocity of ~3 km/s and a mass of ~20 kg)?

Knowing that supersonic bullets don't tend to penetrate very far in water, my gut tells me that such a kinetic ortillery round would have similar performance, and would dump most of its energy on impact with the surface. However, it also has a lot more momentum than a bullet, which ought to increase its penetration depth.

Frankly, I have neither the knowledge nor the specific background needed to make a definitive statement on this. Could anyone help, or point me in the direction of resources that could be used to answer this?

[Purple]

My guess would be that it would just shatter when hitting the water and explode uselessly.

[Elheru Aran]

My guess would be that it would just shatter when hitting the water and explode uselessly.

This. IIRC, once you pass a certain speed, most liquids are basically equivalent to solids as far as resistance and what not goes. Something like that.

[Caiaphas]

Hm. With that in mind and given that anything you drop from orbit is going to be moving at stupid high velocities, is there any way to reasonably target and destroy a deep-running submarine?

[Purple]

Hm. With that in mind and given that anything you drop from orbit is going to be moving at stupid high velocities, is there any way to reasonably target and destroy a deep-running submarine?

How are you even going to detect the sub from orbit?

[bilateralrope]

Hm. With that in mind and given that anything you drop from orbit is going to be moving at stupid high velocities, is there any way to reasonably target and destroy a deep-running submarine?

Target any bases it might resupply at. Let the crew starve.

[Zeropoint]

Hm. With that in mind and given that anything you drop from orbit is going to be moving at stupid high velocities, is there any way to reasonably target and destroy a deep-running submarine?

Drop something that will make a soft landing in the water and release a torpedo.

[Caiaphas]

Hm. With that in mind and given that anything you drop from orbit is going to be moving at stupid high velocities, is there any way to reasonably target and destroy a deep-running submarine?

How are you even going to detect the sub from orbit?

...effing hell, I thought I'd forgotten something.

[Borgholio]

Well...the KV would have to be pretty damn massive...but whether it shatters on the surface or not is irrelevant if it's a big enough impactor. The shockwave could be like a decent sized depth charge so the sub would have to be pretty deep to avoid getting hammered.

[Adam Reynolds]

One of the problems with ideas like this is that the default assumption seems to be one of one set of future tech against otherwise largely static technology. If you have the ability to locate submarines with satellites, why does anyone still use them? Submarines actually aren't that hard to kill once you find them, the problem is finding them.

The only thing that it would make sense to build in that environment is surface vessels that are well defended with high power lasers and missiles. Maybe carriers, maybe just futuristic destroyers. It would almost be a situation akin to space combat in which detection is relatively easy.

If satellites don't actually detect submarines, then it becomes a question of why even use this? Satellite guided weapons could make sense in the case of needing to target distant ground targets in a hurry, with targets also identified by satellite, but if you need something closer to detect the submarine, why not just give it the weapon? It would certainly be cheaper.

[Grumman]

If it is possible, I think you'd have to use an active munition and not just an appropriately shaped lump of metal. Something like the Shkval torpedo - an underwater rocket designed to produce a gas bubble extending backwards from the nose to avoid the usual drag from contact with the water.

[Lord Revan]

we should also remember that for most weapon systems there's this dance of attack and defence, basically once you develop a way to attack someone starts to develop a way to counter and you have to develop a counter to the counter which in turns leads to the development of the counter for the counter for the counter and so on.

There's no silver bullet and only a handful of tactical/strategic options have totally disapeared over the years.

[Sea Skimmer]

I've recently been reading more speculative sci-fi than is probably good for me, and I got to thinking: supposedly, kinetic kill weapons, with decent enough guidance, would be able to easily target and destroy even moving land targets. What would happen if you tried to use these same weapons to kill submerged targets (assuming that targeting isn't an issue and that the round being fired is similar to the Navy's proposed railgun round, which has a muzzle velocity of ~3 km/s and a mass of ~20 kg)?

It wouldn't be able to travel straight more then a few hundred times its own caliber before it lost all stability, spun out of control and then sank with gravity. Or something very close to that result. Projectiles are never entirely stable, and as flight conditions change this becomes very critical. When you have something stupendously fast hitting something pretty solid your conditions change rapidly because its going to slow down so damn fast. Even in the air the only reason a 3km/s shell would get anywhere all that far is because you could lob it high into the air to get that drag down. You could solve stability problems with a sphere I suppose, but firing a sphere in the air at 3km/s would result in wild inaccuracy.

Now say 200-300 calibers also is already assuming a variable nose, as the ideal shape for atmospheric hypersonic flight is nothing like a good shape for underwater travel. Making your nose change at 3000mps while hitting the water doesn't sound like a great design starting point either.

Another solution to this is supercavitating, it will make a pointy nose for flight in the air rounded off for flight underwater and reduce drag enormously. You can actually already get .50cal, 20mm and 30mm sabot ammo like this for shooting at moored mines and possibly shallow running torpedoes, though the main USN program to mount such a weapon on a helicopter failed because a 30mm Mk44 cannon is just too big for a Seahawk to hold on target firing automatically. Supercavitating could increase penetration depth considerably, but at 3km/s it may be rather hard to implement such a thing. And its basically rocket assist, though not necessarily with a thrust rocket in the tail too. And you need some serious fins, which might as well be guided, at which point you might as well build the Russian supercavitating torpedo.

Its questionable that supercavitating would even work at 3km/s though, and you'd certainly need a fairly large projectile to pack the required equipment into the thing without flying apart.

As far as dumb fire stuff goes this won't get far.

If you fire the gun already underwater you'd be able to make a more idealized projectile but it still won't travel very far, and making the bore able to survive firing flooded with water at that velocity would be no easy feat

A small one might be viable for a gun mounted on a submarine as a last ditch point defense weapon against enemy torpedoes. I can't see any useful offensive purpose.

Knowing that supersonic bullets don't tend to penetrate very far in water, my gut tells me that such a kinetic ortillery round would have similar performance, and would dump most of its energy on impact with the surface.

Basically you could get tens of feet deep and still have enough velocity to do serious damage to anything you hit, but after that its rapidly going to become an ineffective means of attack. Also remember submarine pressure hulls can be to the tune of 2in thick steel stronger in almost all respects then armor steels, though not as tough. And that's buried under a bunch of ballast tanks and the outer casing. So while sure a small shell 3km/s will go through that easy, if said shell was down to 500m/s it probably wouldn't.

Something that DOES make sense is to use a shaped charge as a torpedo warhead. For only a mild reduction in proximity blast effect this really improves the chances of a kill upon direct hit given that even modern diesel subs are now pushing 4,000 tons surfaced displacement.

[Sea Skimmer]

This. IIRC, once you pass a certain speed, most liquids are basically equivalent to solids as far as resistance and what not goes. Something like that.

Sorta yeah. Its a shifting series of ways things can work, but basically if your going over 1000m/s density matters more then anything else, and water behaves in a similar way to the way a solid would as its penetrated. But it still matters that the water was only a super weak surface tension strength.

The main problem with deep penetration in both deep earth impacts and water impacts remains stability though. The damn nose is going to pitch up and then crazy things can happen like artillery shells being ejected back out of the ground. They become unstable, turn sideways as they travel and then ricochet off the sudden pressure spike this causes. In water you end up tumbling.

In the range of 3-12km hydrodynamic compressibility applies more and more, and while water already can compress at fairly low (in these terms) pressures this is a greatly magnified effect, and solid materials begin to compress considerably in the path of the hit and the nose of the projectile. In these situations you won't get tumbling but destruction of the projectile outright before it can even try to tumble. Obviously if this involves a direct impact on the target it will be very effective.

KE impacts below 3km/s actual impact are never going to be all that awesome compared to normal HE bursters. In fact they can be far less effective in some cases. Modern railgun projects are aiming to fire at about 2.5km but the impact velocity will be considerably less, and might not be over 1000m/s depending on the choice of ammuntion.

12km/s meanwhile is the threshold where solid-solid impacts begin to compress so much they vaporize instead of eroding to fine dust. Depending on the materials of course.

The highest anyone had made a shaped charge drive a solid material as of the mid 1960s was 40km/s. Gasses have been driven at 140km/s (same date) but that's kind of the point, the compressive heating already turned in into vapor. These are vastly higher velocities then anyone is projecting with a gun of course but useful benchmarks.

[Terralthra]

If you assume that at that speed, it might as well be a blunt impactor, it's pretty trivial to calculate.

Newton's impact approximation says a fast, blunt-ish impactor will penetrate into a material until it has penetrated depth equal to length times relative density in the penetrated material, regardless of speed (this is a purely momentum-based calculation). The BAE systems currently being developed range from 5" to 155mm in diameter.

Some quick substitution into a cylinder volume and density calculation yields that the surface area being struck is roughly 188 square centimeters. Since water is water, and is the definition of 1 g/cm^3 in density, every centimeter deep of this area masses 188g. The 20kg projectile will penetrate about 1.06 meters before most of its momentum is spent.

If the velocity is low enough that hydrodynamics matter, that analysis is wrong, but it points toward the problem. Water is really massive, compared to air. Pushing projectiles through it at high speeds for any appreciable length of time is really hard.

[Sea Skimmer]

I'd suggest you'd use something more rod like, try with something like a 40mm rod, and thus 12.56 square centimeter nose. Its not going to be deep but it should come out better then that. The current BAE project anyway is only 32 MJ, which limits mass fired 3000m/s to about 7.1kg max. But as I do keep point out its kind just a puny thing, and will remain so for a while. Getting any EM weapon working in naval conditions at all is a higher priority then how powerful it really is or effective the actual ammo concepts.

[Terralthra]

The problem doesn't go away with narrower cones, unfortunately. The problem is strictly that of relative density. Steel is roughly 8,000 kg/m^3. Water is 1,000 kg/m^3. That's a density ratio of 8:1, meaning a blunt steel projectile is only going to go its length * 8 deep into water at high velocities. A narrower projectile is pushing less water out of the way, sure, but it's got less momentum because it has less mass. Speed turns out not to matter too much once it's high enough that one can afford not to care about hydrodynamics.

[Sea Skimmer]

No a narrower projectile would be longer and have the same mass but higher sectional density. I don't think you get what you are doing, and in any case Japanese service ammo in WW2 was already a lot more capable then 8 times its own length on underwater trajectories, so what are you calling high velocity?

[Terralthra]

Newton's impact approximation is generally true within an order of magnitude when used on projectiles impacting on surfaces at more than about double the speed of sound in the material being penetrated. The speed of sound in water is about 1480 m/sec, we were talking about 3,000 m/sec projectiles, so it should more or less hold. Did "Japanese service ammo" (for what? a cannon? a submachine gun? a fictional railgun? You'll need to be more specific than a country and a time period) have an impact velocity of around 3 km/sec?

Mind, I can see that the BAE railgun slugs aren't cylindrical, but at 3 km/sec into water, I find it difficult to imagine that mattering that much. When it hits the water going double the speed of sound, the water molecules physically can't "move out of the way" fast enough. There'd probably be all sorts of neat cavitation effects near the shock front of the projectile, but they'd act to slow it even more, not create any sort of air column it could pass through.

[matterbeam]

I think penetrating the water to any useful depth is not going to work.

What you CAN try is to intentionally smash your projectile into the surface at a high enough velocity that the main damage mechanic is the downwards shockwave.

[Elheru Aran]

I think penetrating the water to any useful depth is not going to work.

What you CAN try is to intentionally smash your projectile into the surface at a high enough velocity that the main damage mechanic is the downwards shockwave.

Well, apart from an *almost* necro...

Good thought, but for that to be effective to any great depth, you need either an absurdly huge projectile or an extremely small body of water. In an ocean or even a sea or great lake, you're talking miles and miles of water all around the impact zone that the force is simply going to dissipate into. If a submarine was deep enough, it might get shaken up, pop a few rivets, but it wouldn't necessarily be killed. There's a reason depth-charges don't explode until they get to a certain depth, after all-- otherwise you're just wasting the energy.

[matterbeam]

I think penetrating the water to any useful depth is not going to work.

What you CAN try is to intentionally smash your projectile into the surface at a high enough velocity that the main damage mechanic is the downwards shockwave.

Well, apart from an *almost* necro...

Good thought, but for that to be effective to any great depth, you need either an absurdly huge projectile or an extremely small body of water. In an ocean or even a sea or great lake, you're talking miles and miles of water all around the impact zone that the force is simply going to dissipate into. If a submarine was deep enough, it might get shaken up, pop a few rivets, but it wouldn't necessarily be killed. There's a reason depth-charges don't explode until they get to a certain depth, after all-- otherwise you're just wasting the energy.

There's just so very few threads that aren't 40-page and require that I read them all to get the jokes

As for the levels of kinetic energy required... a 100kg projectile at 6km/s releases 1.8GJ

We can then use the numbers from here: http://nextbigfuture.com/2016/02/underw ... -deep.html

[Elheru Aran]

I think you're missing what I'm saying. Trying to kill something under the water by dropping something on *top* of the water is a different set of equations. Bear in mind that a considerable amount of the energy is going to be expended in a splash (the water will go UP rather than down).

If you want a simple comparison, consider a skinny kid who's holding their breath at the bottom of the pool, when a fat lady belly-flops off the diving board. There's gonna be a big splash, but he won't notice (unless she makes it deep enough to land on top of him, anyway).

I may be completely misunderstanding the physics here, of course. This isn't my forte.

[Simon_Jester]

Well, if you break out utterly ridiculous energy levels, that is to say nuclear weapons, you can kill submarines by launching weapons at the surface of the water. Because eventually, shock waves from the impact/blast will propagate far enough from the point of impact to physically crush the submarine target. However, this is not a particularly efficient way to go about doing business and there are times when it would involve an impractical level of collateral damage.

[Q99]

Well, if you break out utterly ridiculous energy levels, that is to say nuclear weapons, you can kill submarines by launching weapons at the surface of the water. Because eventually, shock waves from the impact/blast will propagate far enough from the point of impact to physically crush the submarine target. However, this is not a particularly efficient way to go about doing business and there are times when it would involve an impractical level of collateral damage.

By the time you're hitting stuff at sub depth, you're getting *really* ridiculous