Supersonic sub (split from Speed limit on earth)

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Aircraft can be designed to travel at "supersonic" speeds through the atmosphere.  Would it be possible to design a "supersonic" submarine  to travel though the sea?

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22 minutes ago, Charles 3781 said:

Aircraft can be designed to travel at "supersonic" speeds through the atmosphere.  Would it be possible to design a "supersonic" submarine  to travel though the sea?

The speed of sound depends on the medium and its properties, such as pressure and temperature. The air is ~ 340 m/s at stp. Check what is the speed of sound in water ... much faster than in air..

Edited by Sensei

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2 hours ago, Charles 3781 said:

Would it be possible to design a "supersonic" submarine  to travel though the sea?

That would make for an interesting design exercise.

The speed of sound in water is approx. 1500 m/s.
Streamlining ( finesse ratio )would need to be extreme ( long and pointy ) so the shock wave would disturb a very thin cone of water.
And Whitcomb area rule where any control surfaces are located.

Propulsion would be another problem; the standard axial screw ( propeller ) could not be used as the thrust would need to be faster than supersonic ( no such thing as a supersonic propeller plane ). The only viable solution would be rocket propulsion.

A 'trick' used by Russian torpedoes is to encase the hull n a sheath of gas ( usually Oxygen generated by H2O2 ) to enable faster speeds; that might also be applied. This however, would negate the benefit of the water as a heat sink to alleviate dynamic heating.

Does anyone think anything of the sort would be feasible ?

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The shock wave in water would be an interesting phenomenon.

A problem with streamlining a submarine hull is that it's not the best shape for a pressure hull. Keeping out pressures above atmosphere are a priority for submarines, whereas planes never have to deal with a static difference greater than 1 atm, usually it's even less, and when it's an issue it's about keeping air in rather than being crushed. An autonomous or remotely-piloted vehicle would be required.

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16 minutes ago, swansont said:

The shock wave in water would be an interesting phenomenon.

A problem with streamlining a submarine hull is that it's not the best shape for a pressure hull. Keeping out pressures above atmosphere are a priority for submarines, whereas planes never have to deal with a static difference greater than 1 atm, usually it's even less, and when it's an issue it's about keeping air in rather than being crushed. An autonomous or remotely-piloted vehicle would be required.

An excellent observation  +1.

There is one mitigating factor to this however.

Submarines are not subject to such severe weight constraints as aircraft so the addition of the necessary strengthening bulkheads is far easier.

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3 minutes ago, studiot said:

An excellent observation  +1.

There is one mitigating factor to this however.

Submarines are not subject to such severe weight constraints as aircraft so the addition of the necessary strengthening bulkheads is far easier.

Yes, especially if you don't have people inside. But they still need to end up near neutral buoyancy if you want more than one trip, so there is still a limit. You can fill them with e.g. oil of some sort, or some other fluid that's less dense than water.

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On 10/26/2020 at 10:10 AM, swansont said:

The shock wave in water would be an interesting phenomenon.

A problem with streamlining a submarine hull is that it's not the best shape for a pressure hull. Keeping out pressures above atmosphere are a priority for submarines, whereas planes never have to deal with a static difference greater than 1 atm, usually it's even less, and when it's an issue it's about keeping air in rather than being crushed. An autonomous or remotely-piloted vehicle would be required.

Swansont,  after pondering about the interesting  point you mentioned, "shock waves in water",  I've come to the conclusion that  supersonic submarines can't be compared to supersonic aircraft.  For this reason:

Aircraft fly through the Earth's atmosphere - ie,  through a mixture of gasses.  And  gasses can be readily compressed, and pushed aside. Thus generating a Mach "shock-wave" in the atmosphere.  The atmosphere absorbs and dissipates the shockwave, turning it into a "sonic-boom".  Which may annoy remote ground-based residents.  But doesn't molest the aircraft.

However - water is a different matter.  Water absolutely  cannot be compressed.  It retains its volume no matter how much it's squeezed.  This is indeed, why hydraulic machines work.

Therefore, it seems to me, that if a submarine accelerated underwater at ultra-high speed,  then when it reached under-water Mach -1,  the outcome would be crushing disaster for the sub.

Can you fault my reasoning?

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18 minutes ago, Charles 3781 said:

However - water is a different matter.  Water absolutely  cannot be compressed.  It retains its volume no matter how much it's squeezed.  This is indeed, why hydraulic machines work.

Therefore, it seems to me, that if a submarine accelerated underwater at ultra-high speed,  then when it reached under-water Mach -1,  the outcome would be crushing disaster for the sub.

Hence my comment about the shock wave.

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There are two types of fluid flows in aero/hydrodynamics, compressible and incompressible.
They can both be modelled by little springs between each fluid particle; when you push against one particle, it pushes against the next via the little interconnecting springs, and the fluid, in effect, compresses.
If you push too fast, the little springs reach maximum compression, and the particles begin to 'pile up' into a bow wave.
For air, this happens at Mach 1, and the air effectively becomes incompressible.

Water is nearly incompressible from the get-go ( also, liquids and solids have transverse 'sound' waves as well as longitudinal ). That's why you see a bow wave on a boat doing 10 mph. That wave is the 'shock' of the particles bunching up because they can't move out of the way fast enough.
Another ( huge ) problem would be cavitation. This is usually seen in pumps/propellers where the fluid can't flow fast enough into the area behind the pump/propeller blades, creating a low pressure area ( or even vacuum ) which tends to destroy equipment.
For a plane the shock is the separation where supersonic air is drastically decelerated to subsonic ( inside the shock cone ), and it carries a lot of momentum/energy which is dissipated in the 'sonic boom'.
If the 'sub' was at a shallow depth, with the surface close by, the spray pattern of the water shock into the air would be extremely interesting. to say the least.

So the shock wave of travelling through water would be no different than through an incompressible fluid such as supersonic air, except for the cavitation problem, which I didn't consider on Sunday ( and which may make this idea a non-starter ).

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18 minutes ago, MigL said:

There are two types of fluid flows in aero/hydrodynamics, compressible and incompressible.
They can both be modelled by little springs between each fluid particle; when you push against one particle, it pushes against the next via the little interconnecting springs, and the fluid, in effect, compresses.
If you push too fast, the little springs reach maximum compression, and the particles begin to 'pile up' into a bow wave.
For air, this happens at Mach 1, and the air effectively becomes incompressible.

Water is nearly incompressible from the get-go ( also, liquids and solids have transverse 'sound' waves as well as longitudinal ). That's why you see a bow wave on a boat doing 10 mph. That wave is the 'shock' of the particles bunching up because they can't move out of the way fast enough.
Another ( huge ) problem would be cavitation. This is usually seen in pumps/propellers where the fluid can't flow fast enough into the area behind the pump/propeller blades, creating a low pressure area ( or even vacuum ) which tends to destroy equipment.
For a plane the shock is the separation where supersonic air is drastically decelerated to subsonic ( inside the shock cone ), and it carries a lot of momentum/energy which is dissipated in the 'sonic boom'.
If the 'sub' was at a shallow depth, with the surface close by, the spray pattern of the water shock into the air would be extremely interesting. to say the least.

So the shock wave of travelling through water would be no different than through an incompressible fluid such as supersonic air, except for the cavitation problem, which I didn't consider on Sunday ( and which may make this idea a non-starter ).

Be noisy if nothing else.  Maybe if the water was kept  away from the hull itself though you could avoid the worst of it.

In theory if we forego keeping the interior at atmospheric pressure could streamline the hull. If crewed, atmospheric composition and depth changing would have to be taken into consideration though.

Edited by Endy0816

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On 10/26/2020 at 6:10 AM, swansont said:

Keeping out pressures above atmosphere are a priority for submarines

Why should it be for our racing submarine?  Like a fish, there's no particular requirement to keeps a certain pressure inside. By all means, pressurize it if it helps.  Even humans can take that so long as they don't mind the time it takes to depressurize.

On 10/26/2020 at 6:32 AM, swansont said:

But they still need to end up near neutral buoyancy if you want more than one trip, so there is still a limit.

Similarly, given the power our sub is going to need, the buoyancy of the thing (be it positive or negative) seems a drop in the bucket compared to the sorts of forces we plan for it. Likewise, an airplane going fast need not worry about the fact that it has greater density than the air it displaces.

Propulsion on the other hand is a serious issue, as it was for the early supersonic aircraft. I think fish tails would work better than a propeller, and in the worst case, we can always fall back to our rocket. Nobody said we had to sustain the speed for a long time.

The shock wave is also a serious issue as you point out.  The thing will be shaped like a needle, splitting the water 'gently' to the side rather than compressing it in a shock wave.  Minimum sonic boom. This reduces the problem to one of the increased friction resulting from the greater surface area presented.

Imagine an amoeba, injecting its skin into new territory, and then moving all its interior guts into that new expanded volume, closing it in behind. Just do that a lot faster. They have creatures that word this way.  No propulsion since the skin is effectively stationary relative to the water and need not even be particularly slippery.  All the motion (movement of its center of gravity) takes place internally.

Edited by Halc

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Navigation of the supersonic sub will also present difficulties to the captain.  Travelling sub-surface at a dash-speed of 850+ mph,  he/she will be essentially operating blind.

Sonar will be of little help - aquatic shock-wave effects will delay and distort  echo-returns from enemy vessels.  Thus rendering precise torpedo launches hopeless.

Probably, the best military tactic for the supersonic sub,  will be to come up to periscope-depth near an enemy ship, and get a  directional bearing through the scope,.  Then submerge, accelerate up to supersonic speed, and execute a ramming attack - Jules Verne "Nautilus" style - to slice the bottom off the enemy vessel by sheer kinetic energy.

This will not damage the super sub, as its hull will, of course, be built of titanium/depleted-uranium alloy.

What could possibly go wrong?

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I'm thinking that once the sub got to supersonic under water ( 1500 m/s ), the shock cone would actually become a vapor cone, with liquid water on the outside of the cone, and an immediate change to vapor inside the cone, due to vastly decreased pressure.
The cavitation wouldn't be behind the sub, but behind the shock cone and all around the sub.
That would really make it interesting.

You often see jets with moisture condensation inside their shock cone due to decreased pressure …

( the shock starts at subsonic speeds at 1/4 chord because local airflow, at the wing,  is supersonic )

Looking forward to other members' input, and corrections.

Edited by MigL

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What would the temperature of the surrounding water be? Would it turn to steam due to friction?

If a diver was passed by this supersonic sub, and wasn't killed by the shock wave, would he be boiled alive?

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7 hours ago, Halc said:

Why should it be for our racing submarine?  Like a fish, there's no particular requirement to keeps a certain pressure inside. By all means, pressurize it if it helps.  Even humans can take that so long as they don't mind the time it takes to depressurize.

I was thinking if structural failure. Crush depth won’t be all that deep, and that will ruin your whole day.

7 hours ago, Halc said:

Similarly, given the power our sub is going to need, the buoyancy of the thing (be it positive or negative) seems a drop in the bucket compared to the sorts of forces we plan for it. Likewise, an airplane going fast need not worry about the fact that it has greater density than the air it displaces.

Going down is good for landing a plane. Not so much for a sub, unless it’s single-use.

6 hours ago, Charles 3781 said:

Navigation of the supersonic sub will also present difficulties to the captain.  Travelling sub-surface at a dash-speed of 850+ mph,  he/she will be essentially operating blind.

Sonar will be of little help - aquatic shock-wave effects will delay and distort  echo-returns from enemy vessels.  Thus rendering precise torpedo launches hopeless.

Probably, the best military tactic for the supersonic sub,  will be to come up to periscope-depth near an enemy ship, and get a  directional bearing through the scope,.  Then submerge, accelerate up to supersonic speed, and execute a ramming attack - Jules Verne "Nautilus" style - to slice the bottom off the enemy vessel by sheer kinetic energy.

This will not damage the super sub, as its hull will, of course, be built of titanium/depleted-uranium alloy.

What could possibly go wrong?

This assumes the purpose of the sub is military, as a launch platform or weapon.

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Interesting discussion.

Either a supersonic submarine is possible, or it isn't.

If it isn't possible, what does happen if I put an arbitrarily powerful rocket on the back end of a pointy lump of oil - filled metal with an overall density equal to sea water?

Setting aside the practically, imagine that I can crank up the power as far as I like.

As far as I can tell, if I put enough power into it, the thing will go supersonic- probably not for long before it's eroded away to nothing.

A slightly more practical factor.

The cavitation at the back of the craft may be somewhat offset by the exhaust gases from the rocket.

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9 hours ago, MigL said:

( the shock starts at subsonic speeds at 1/4 chord because local airflow, at the wing,  is supersonic )

Looking forward to other members' input, and corrections.

Nice picture   +1 sorry I can't offer any corrections, I basically agree with everything you have said.

But I can offer some more thoughts.

1) Shock waves are impossible in a truly incompressible fluid since the speed of sound in such a fluid is infinite.

Water, of course is not truly incompressible.

How ever John's rocketsubmarine would encounter other effects. In particular you would get flow separation between the compression half cycle and the expansion half cycle.
In the compression half cycle ice would form around the sub. In the expansion half cycle the solid and liquid would not expand to fill the void.
Cavitation is indeed an issue.

It is worth noting what a 'shock' wave is.

It is a discontinuity in the pressure field across a very narrow region.

Such a discontinuity  entails an enormous value for  $\frac{{du}}{{dx}}$  so that even for a low friction substance such as water viscoscity effects would be significant.

The second law of thermodynamics requires that entropy does not decrease across the shock. Since entropy is proportional to $\ln \left( {p{p^{ - \gamma }}} \right)$

ift follows that

$\frac{{{p_2}}}{{{p_1}}} \ge {\left( {\frac{{{p_2}}}{{{p_1}}}} \right)^\gamma }$

Finally an alternative view is that of specific energy  -- The energy per unit volume (or mass) of the fluid.
Consider a flow regime and gradually increase the flow velocity.
The specific energy increases as a result.
At some 'critical' point the specific energy exceeds the ability of that flow regime to sustain it.
So there is an abrupt change to a different regime.
In liquids this is responsible for phenomena such as the hydraulic jump, which does not occur in gas dynamics.

Edited by studiot

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Interesting ...

1 hour ago, studiot said:

The specific energy increases as a result.
At some 'critical' point the specific energy exceeds the ability of that flow regime to sustain it.
So there is an abrupt change to a different regime.

Is this what Reynolds number indicates ?
I always thought t indicated where 'laminar' flow changes to turbulent ( or ratio of inertial to viscous forces in a fluid ).
Is this the specific energy interpretation ?

Back to supersonic sub design …
As John suggests, given enough power, it has to go supersonic; we are just trying to determine ( and mitigate ) resulting effects.
I suppose the easiest way is to shoot hi-power bullets into water; at least initially, they would be travelling at 1500m/s.
Never actually done this, but seen video of ( slower ) bullets travelling through water, and they do leave a vapor trail.
Heat and pressure energy are quickly attenuated by the surrounding medium. Unlike a sonic boom, water is a good heat sink, and expansion takes care of the vapor pressure.

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8 hours ago, swansont said:

I was thinking if structural failure. Crush depth won’t be all that deep, and that will ruin your whole day.

Going down is good for landing a plane. Not so much for a sub, unless it’s single-use.

This assumes the purpose of the sub is military, as a launch platform or weapon.

Rightly so. The purpose of the sub is not these things.  It can use GPS if navigation is a goal at all. We probably want to know our depth at least.

But you're making the same mistake with the first two statements. You're assuming a purpose of the craft is to keep a human alive, or to float when unpowered.  We just want it to go fast on its own power. That's all.  I also assure you that if it experiences structural failure at 1500 m/sec, the survival odds of a hypothetical occupant is not going to be a function of the internal pressure of the life support system.

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1 hour ago, Halc said:

Rightly so. The purpose of the sub is not these things.  It can use GPS if navigation is a goal at all. We probably want to know our depth at least.

GPS isn’t going to work while submerged.

1 hour ago, Halc said:

But you're making the same mistake with the first two statements. You're assuming a purpose of the craft is to keep a human alive,

Yes, especially if you don't have people inside.”  is something I said, so, no, I’m not necessarily assuming that. The OP was not clear on this one way or the other

1 hour ago, Halc said:

or to float when unpowered.

It needs to be able to if you want to keep using it

1 hour ago, Halc said:

We just want it to go fast on its own power. That's all.  I also assure you that if it experiences structural failure at 1500 m/sec, the survival odds of a hypothetical occupant is not going to be a function of the internal pressure of the life support system.

It can experience the failure at 1 m/s if it’s below crush depth, which is not as deep if you don’t have the traditional hull shape, and this has nothing to do with the pressure of a life support system.

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7 hours ago, MigL said:

Is this what Reynolds number indicates ?

I think you will find it's the Froude number you need.

A typical analysis might study the 'force' (called specific thrust) needed to halt the stream at a stagnation point.

Fanno and Rayleigh curves can be drawn from this approach.

Edited by studiot

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7 hours ago, Halc said:

The purpose of the sub is not these things.  It can use GPS if navigation is a goal at all.

Only at depths where it could navigate by the stars in the way that sailing ships did.
Attenuation of 1.5GHz GPS signals by water  will be worse than attenuation of light.

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