Inspired by the current thread on breaking the sound barrier in air.
Would it possible for a manned* craft to break the sound barrier underwater? Or do incompressibility effects put paid to that?
I’m aware there are torpedoes that travel at very high speed but if I understand it correctly they do so by forming a layer of bubbles in front of them, which while they’re certainly doing it underwater doesn’t seem quite the same as an object moving directly through the liquid medium.
The VA-111 Shkval torpedo and its descendants are super cavitating torpedoes originally developed by the Soviet Union. They are capable of speeds in excess of 200 knots (370 km/h or 103 m/s) a long way short of the speed of sound.
Are you allowing supercavitation type technology or purely a craft traveling directly through water? If the latter, then sure, in theory you could propel a manned craft through water at the speed of sound in water (good luck turning it though). If the later then…I’m not sure. I’m not sure of the physics of water at that sort of speed and temperature or what the pressures would be. My WAG is, it would be something like supercavitation anyway, as at those sorts of speeds the water would flash to steam anyway, meaning your craft would be essentially moving through a much less dense media than water anyway, but maybe some of the board physics types can weigh in.
I know the US was developing a few years ago a supercavating sub that would be able to move at higher speeds through water, but no idea where it’s at (probably been scrapped at this point)…I don’t believe they were going for supersonic speeds with the thing, but you could I suppose if you wanted to spend the money and just wanted to do it to do it. I doubt it would be practical except for some sort of burst speed to get away from an attack or something along those lines.
The pistol shrimp closes it’s claw at a speed of about 97 km/hr while the speed of sound is 1.4 km/s in salt water. While tiny the cavitation bubble reaches ~4800 C as it collapses, any craft traveling a significant percentage of that speed would have serious issues with cavitation in the reverse direction of travel.
Supercavitation wouldn’t help, but normal cavitation would be an undesirable phenomenon.
A crude calculation shows that the ram pressure on any flat forward-facing surface at the speed of sound in water would be about 163,000 psi. You can reduce this a lot with streamlining, but it’ll still take a lot of thrust.
Water is about 833 times as dense as sea-level air, and the speed of sound in water is about 4.2 times as great as air. Ram pressure scales with density and with the square of velocity; if you’ve got a vehicle that can achieve sonic speed in sea-level air with a given amount of thrust, you’re going to need about 14,900 times as much thrust in order to achieve sonic speed in water. This assumes only pressure/form drag, and disregards viscous skin drag, which is much more substantial in water than in air.
Being the tinkerer I am, I have actually tried a water rocket in my youth. I was into model rocketry in the 1960s and made an attempt at a supersonic model rocket.
For the water rocket I used only the upper stage from my (air) supersonic attempt. It was a very thin, needle nosed rocket with the highest thrust engine Estes made. We called those engines sledgehammer engines. They had very high thrust for short times. Launched by itself in air it would accelerate so fast you could barely track it. I have no idea how high it went in a few seconds.
I weighted it to neutral buoyancy and waterproofed the ignitor. The guide rod was embedded in a small concrete block and laid horizontally in the pond. I touched it to the battery and a furious cloud of smoke and steam arose.
I was wondering about a craft traveling directly through water rather than super-cavitation, my question was inspired by reading about going through the sound barrier in air and the air molecules piling up in front the plane.
Thanks, that pretty much answers it, pretty much impossible with today’s technology.
That’s pretty dramatic, why did it slow so rapidly? Or does Machine Elf’s post explain it?