Since sound cannot pass through a vacuum, ISTM that if you had a navy submarine with two hulls, and a vacuum between them, that internal noise would therefore be unable to transmit to the outside, and you’d have a submarine that would be nice and quiet (and hard for an enemy to detect using passive sonar.)
The practical difficulties would be:
Any submarine still needs to have things connecting from the inside to the outside - periscope, engine shaft, torpedo tubes, etc. As such, the vacuum could only be a partial, not total, one.
In addition to that, you’d need a great many support struts between the first and second hulls, in order to enable the outer hull to withstand the crushing pressure of the surrounding water.
The use of magnets to keep two hulls separate would not be an option because it would create a giant magnetic signature which would be picked up by an enemy using MAD (Magnetic Anomaly Detection.)
But - all the same - might such a hull cut down on internal noise a lot?
There are many problems with this, but what strikes me most immediately is the HUGE amount of buoyancy this would – it may even make it impossible to submerge. The rest of the boat might have to be made out of platinum or tungsten to even have a chance of submerging.
Plus you’d have to support the inside of the outer layer with something. The vacuum would mean the outside forces trying to crush the submarine would be even (relatively) greater. Unless you could do this with some sci-fi magnetic craziness you’d have to have struts between the outer and inner layer, which sound could pass through, nullifying most of the advantages of the vacuum layer. You’d be better off filling the space with some high-density acoustic substance, assuming one exists.
I see. But - it wouldn’t need a lot of vacuum. In theory, a vacuum that’s one inch apart doesn’t permit sound to pass through any more than a vacuum that’s one foot wide. Unless the vacuum is just so buoyant that the sub still could not sink anyway.
Even an inch of space around the surface of the hull of a large submarine would be a huge amount of buoyancy. This is really a solution in search of a problem; modern submarines are already so quiet that under normal running conditions they produce less noise that the ambient environment (often described as “a hole in the water”) and the major sources of acoustic emissions is the coolant loop in the nuclear power plant and the screw(s), both of which have been highly engineered to minimize the noise and vibration they produce, and would not be isolated from the environment by this vacuum hull idea given that you are still going to have to have some kind of standoff structure that would maintain the spacing between inner and outer hull,
Frankly, I like your theory and I feel it might bear some back-of-the-napkin in-a-pub design work, over pints, with lots of animated gestures and a bit of shouting to be heard over the table beside us who are having an animated discussion over the local sportsball teams.
Your design is beyond revolutionary and could not be made to work with current technology. As of this writing, submarines already have two hulls, the “hydrodynamic” outer hull, and the inner hull, built up to withstand the crushing pressures of the deeps.
You’d need a third hull in there somewhere. This will either make the interior smaller (and it’s pretty cramped), or the exterior bigger or, obviously, some combination of the two. One of them adds one effective atmosphere of pressure differential, reducing your maximum safe depth by ten meters or so.
Submarines can already be really, really quiet. Look up “Air-independent propulsion” submarines at some point. Sweden’s Gotland class was revolutionary in this regard. This is the current state-of-the-art with regards to submarine stealth.
I have read about AIP before, such as the Gotland-class. AIUI, it only works for slow crawling speeds, like a few knots. For faster speeds, the Gotland still needs to use the diesel main engine.
@ everyone else: Thanks for the feedback thus far. I knew it was a “solution in search of a problem” as Stranger put it, was just curious what the practical difficulties or benefits were.
Another issue is that, even if you completely eliminate internal noise, it’s still a big thing moving through a medium, and that in itself is going to generate some level of sound.
Not really. Submarines, and particularly modern submarines, are designed only to produce laminar flow, and the motion of the hull through the water produces essentially no turbulence or cavitation. The propeller (“screw” in submariner terms) has traditionally been the source of fluid-interaction acoustics but a lot of effort was put into minimizing cavitation (which is why the blades on modern propeller blades have these exaggerated boomerang-shaped profiles) and why the Soviets spent so much time and effort in industrial espionage to ‘acquire’ the manufacturing information and CFD codes used to design and manufacture those screws for their own use. They can actually run at normal operational speeds without producing any perceivable noise detectable by a passive sonar array at any significant distance, and only cavitate at higher speeds.
The other sources—coolant pumps in the reactor, mechanical operation of ancillary machinery, and crew noise—are dealt with by creating reactor coolant systems primarily using natural convection, using ultra-quiet impeller pumps when necessary, and putting using anechoic materials inside and out. Again, the modern attack or ballistic missile submarine actually produces less acoustic radiation than the ambient environment, particularly in littoral regions, and electric subs can actually be so quiet that there almost is no sense of movement or the operation of machinery except for a barely perceivable hum of electric motors and the operation of the HVAC system.
Sort of… they’re regular diesel-electric subs with additional AIP units- they can either run directly on it at about 5 knots, or they can use the AIP unit to charge the batteries (in the Gotland class, it’s a 75 kilowatt AIP unit), which means that they can creep away, recharge the batteries and remain a threat, without making themselves vulnerable by surfacing or snorkeling.
The other issue I could see with this is that any leak into the vacuum space between the hulls would rapidly fill with water unless you had a shit-ton of bulkheads separating difference sections (which would defeat the purpose of trying to isolate the inner and outer hulls). On a normal submarine where this happens, you would just close off watertight bulkheads and hope it all holds together until you can ascend and call for rescue, but in this vacuum hull you’d now have a bunch of extra ballast flooding in that you’d have to offset with additional buoyancy capability notwithstanding the weight of the extra pressure hull to begin with. It’s like wearing waders in deep water; everything is great until water comes over the top of the bib, and then it fills up it feels like you are wading through concrete.
It also may actually be counterproductive from a sound suppression standpoint. People commonly think that sonar and ambient sounds reflect off of steel (or titanium, in the case of some Russian subs) hull, but the density of the hull is less than an order of magnitude higher than the surrounding water, whereas the air inside the hull is almost four orders of magnitude less than the hull; as a consequence, this dramatic difference in density is really what causes acoustic waves to reflect. An evacuated hull will reflect all incoming sound waves (except for that that are damped by hysteresis of the hull) so it will actually reflect sound more efficiently. I supposed you could fill it up with silica aerogel or some other very low density material with good damping properties, but then, you could just cover the inside of the pressure hull with such a material and mechanically isolate the mechanical and habitable spaces and get largely the same effect without the weight, complexity, and risk of two concentric pressure hulls.
Thank you for the explanation. What if we filled it with a heavy gas like sulfur hexafluoride which gives a very low sound speed and will presumably attenuate higher frequency noise ?
Well, I guess that would sequester “the most potent greenhouse gas currently known”; in the end the problem is going to be that you’ll need so much supporting structure between the inner and outer hulls that I don’t think you’ll actually get as much isolation as you’d like, but I like the novelty of the thought experiment. Maybe former professional submariner @robby will offer an opinion.
I vaguely remember reading someplace a few years ago that this was the next major field of acoustic processing technology in sonar: not detecting a point source of non-ambient sound (like plant noise or screw turns) but a point attenuation, a “hole in the water”. If you can sweep for ambient noise, you can catch an unnatural and sharply-delimited reduction in that ambient background and localise it. There’s your Ohio-class hiding.
That is one of those things that is theoretically possible but good luck maintaining a track on such ‘source’. It’s like looking for a jet-black cat at midnight. You might catch it in silhouette against a lighter background, but once it moves it is just another blob of darkness.
ISTM that searching for the lack of something will always be far harder than searching for the presence of something. You get far more false positives when looking for a gap than for a thing.
But no more buoyancy than air, right? Air is already one thousandth the density of water.
I know submarines are cramped, but is the volume of air already such a problem that it’s difficult to add the density required for them to achieve neutral buoyancy?