Vacuum hulls for quiet navy submarines

I don’t understand the point of complicated double hulls that can be punctured losing a vacuum or gas and leaving a space to flood with water eliminating a huge amount of buoyancy. Why not just use acoustic foam or other material lining the inner hull?

And can’t there be technology like noise blocking headphones to change the sounds emanating from a sub so they seem more like an unidentifiable noise? I understand the problem is that it’s still a noise that can be located, and if it doesn’t sound like a whale or school of fish then it’s not really any help, but maybe that can be managed also.

And how about little noise generating torpedoes that are like chaff, in this case to clog up a sonar readings instead of radar?

But of course, it’s not a maglev vacuum thermos, it’s a bungee thermos. Huge difference.

But in fear of being serious for a moment, I didn’t know you were shock-mounting noisy machinery. I’ll admit I hadn’t thought about it, and it makes perfect sense.

There is a common misperception that active noise cancelling headphones can neutralize all sound, which leads to complaints about how the don’t block conversations or other sharp, low frequency noises. In fact, headphones with active noise cancelling sample the ambient acoustic environment and cancel mid- and high-frequency pseudo-“white” noise (like the humming of an engine or aeroacoustic vibration of an airframe flying through the air) but do little if anything for conversational noise or mechanical sounds.

Launching a torpedo is one of the immediately recognizable sounds a submarine can make because of the high pressure air to push the fish out into the water. Acoustic sonar arrays are highly directional, so sound made in one direction does not really affect sounds coming from another direction; you’d literally have to disrupt the acoustic environment across a broad frequency spectrum in a wide area to “clog up” the sonar picture. One tactic if another submarine fires upon you is to launch a torpedo back at them, hammering them with active pinging and forcing them to cut the control wires so they can turn away, which I’m sure is unnerving but even that won’t really prevent being able to track other submarines.

Stranger

They already have that (although it goes on the outside).

As for active noise cancellation, I guess you could do that, but submariners and submarine designers are already pretty good at not making any noise in the first place.

But… but, nuclear energy is EEVIL! :wink:

I apologize. I didn’t mean to malign you with magnets. I don’t even believe in “magnets”.

“It’s quiet. Too quiet.”

My first job out of college was working on a power supply for the Trident fire control system. There was a lot of concern regarding conducted EMI getting into the boat’s hull, which would theoretically allow detection of the state of the fire control system at some distance. The EMI filter was huge, about 20% of the volume of the entire power supply.

Yes indeed. Plus, you’d need that additional thickness (and likely a bit more) for the pressure hull anyway. The vacuum will effectively pre-stress the outer hull in compression, requiring that it be a little thicker to create extra sectional modulus to prevent buckling at depth.

In other words, if you want a vacuum-isolated-hull sub, the pressure hull will have to be significantly thicker to handle the extra stress, and that thickness is almost certainly enough to offset the extra buoyancy from the vacuum. A vacuum sub is impractical, but not because of the extra buoyancy.

————

On a separate point, even fairly stiff connections between the pressure hull and the inner hull could produce enough of an impedance mismatch to reduce some of the acoustic signature. But my understanding is that the impedance mismatches already in place (by design) mean that any acoustic improvement from a vacuum hull would get lost in the noise—but I yield to the expertise of the board’s submariners on that point.

You can’t shock mount the hatches, the periscope, the torpedo tubes, etc - those things have to penetrate both the outer and inner hulls, and they have to stay aligned.

Most new design submarines use an optronics mast that doesn’t require a pass-through for an optical tube. Hatches, torpedo and missile tubes, and of course the screw shaft(s) and other controls have mechanical passages between the pressure hull and the outer hull.

Stranger

Sure, but does it need to protrude (I suppose intrude) into the main body of the sub when it is lowered?

Read the linked HowStuffWorks article; the periscope well for the AN/BVS-1 photonics mast is entirely contained within the sail outside the pressure hull.

Stranger

Fair enough. The point still stands for all the other stuff.

Personnel hatches penetrate, but they’re not operated when submerged and they’re not hard-connected to noisy people and noisy machinery.

Torpedo and missile hatches are noisy, but so are torpedoes and missiles.

How about instead of vacuum, fill the gap with those noise-cancelling headphones?

Isn’t that another way of saying the idea of a vacuum interhull is unnecessary? In a sub of this hypothetical design, the hatch opening would be a rigid, sound-transmitting link between the inner and outer hulls, if there’s no noise in the inner hull to be transmitted via such a link, then there’s no need for this vacuum idea.

Not the most recent one, but I think you mean this:

I sure hope I didn’t come across on the pro-bungee-vacuum side of this discussion :stuck_out_tongue_winking_eye:

The “vacuum airship” is completely different, though. In a regular airship, you have equal pressure of about 1atm on both sides of a semi-rigid membrane, and changing one side of the membrane to a vacuum is obviously a drastic change in the pressure differential, requiring a fully rigid membrane that would be way too heavy to be supported by the buoyancy of the air displaced.

But water is 1000 times denser than air. So what’s relevant in a sub is:

(a) buoyancy, and the difference between an air space and vacuum space is only 0.1% of the density of the water being displaced, so adding an evacuated space is no different than making the air-filled space of the sub a bit bigger;

(b) a hull that is already designed to withstand a pressure difference of >25 atm, so increasing the requirement by 1 atm is not a huge change.