Fluid Dynamics...

[Tigger]

Don't guns give a bit of a clue? They shoot bullets at far higher velocities than the speed of sound, so the gasses released must be expanding much faster than the speed of sound in air.
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Irrelevant. He's talking about expansion, not explosion.

Yeh, but the explosion provides the expanding gas that drives the bullet.
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But the explosion makes the expanding gas, it's not a compressed gas to start with. Unless you have weird physics in your repertoire. Oh...

[JacksSmirkingRevenge]

Don't guns give a bit of a clue? They shoot bullets at far higher velocities than the speed of sound, so the gasses released must be expanding much faster than the speed of sound in air.
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Irrelevant. He's talking about expansion, not explosion.

Yeh, but the explosion provides the expanding gas that drives the bullet.

^Correct. But those particular gasses are hot. Compressed air, when released from any significant pressure, is freezing cold since the heat energy it had before it was compressed has been dissipated through the wall of it's container prior to being released. - I don't know (can't be bothered to do the maths) how this affects the SOS of the gas.

Sound travels faster in denser substances, so it makes sense that it would travel faster in a compressed gas than in the same gas at 1 atmosphere. <snip>

According to Feck's link -

This equation is valid for liquids, solids and gases. The sound travels faster through media with higher elasticity and/or lower density. If a medium is not compressible at all - incompressible - the speed of sound is infinite (c ≈ ∞).

By way of explanation - I started this thread because I have read claims from air rifle owners and manufacturers of supersonic muzzle velocities which I'm sceptical about. I'm half prepared to believe that some spring piston rifles can achieve such velocities (because the air in front of the piston gets hot) but my feeling is that certain claims made about pre-charged air rifles are exaggerated.

[Feck]

Oh I think you can get a pre- charged to go supersonic remember the Expanding air behind the pellet is a lot more dense than normal air so it's speed of sound is faster ?

[Tigger]

A little drop of diesel behind the slug adds a certain something I'm told.

[Feck]

A little drop of diesel behind the slug adds a certain something I'm told.

Yes soot in the barrel and no fucking consistency !

[Tigger]

A little drop of diesel behind the slug adds a certain something I'm told.

Yes soot in the barrel and no fucking consistency !

It does explode when compressed, but you'll be right there.

[JacksSmirkingRevenge]

Oh I think you can get a pre- charged to go supersonic remember the Expanding air behind the pellet is a lot more dense than normal air so it's speed of sound is faster ?

See your link above.

According to Feck's link -
Quote:
This equation is valid for liquids, solids and gases. The sound travels faster through media with higher elasticity and/or lower density. If a medium is not compressible at all - incompressible - the speed of sound is infinite (c ≈ ∞).

A little drop of diesel behind the slug adds a certain something I'm told.

There are vids on youtube.

[Feck]

Bri a spring piston can so super sonic and essentially the principle is the same it's a compressed gas behind the pellet .

[JacksSmirkingRevenge]

Bri a spring piston can so super sonic and essentially the principle is the same it's a compressed gas behind the pellet .

Yes, as I understand it, the air between the pellet and piston is compressed to around 3000psi in a fraction of a second by the piston - the breech and the skirt of the pellet form a seal which also acts as a detent holding the pellet back until enough pressure has accumulated for the skirt to collapse slightly and release the pellet.

[Animavore]

[Feck]

Bri a spring piston can so super sonic and essentially the principle is the same it's a compressed gas behind the pellet .

Yes, as I understand it, the air between the pellet and piston is compressed to around 3000psi in a fraction of a second by the piston - the breech and the skirt of the pellet form a seal which also acts as a detent holding the pellet back until enough pressure has accumulated for the skirt to collapse slightly and release the pellet.

I do know that small calibres are inefficient in Pre-charged, a .25 can be driven nearly as fast as as a .22 for the same PSI When I overclocked mine it cooked normal pellets in the barrel and they were heavy ones with thick skirts ,Prometheus made some nice heavy ones with solid plastic skirts

[mistermack]

I'm still not convinced of the original principle.
Sound is a wave, where the medium doesn't translate, it just vibrates and passes on the energy.
In a rifle barrel, the air that's pushing the bullet isn't vibrating about a fixed point, it's being bodily moved and accelerated. So long as you can keep accelerating it, it will move faster and faster.
Imagine a dozen springs lying horizontally, not compressed, touching each other end to end. Push at one end, and the disturbance will travel to the other end at a slowish speed, like a sound wave.
But compress all of them fully, and let one end go. The freed end can reach speeds much higher than the "sound" wave, because it's being accelerated over a longer time.

[GreyICE]

Would I be correct in thinking that a gas under pressure cannot expand more quickly than it's own speed of sound regardless of how quickly it is released or what pressure it is at?
Anyone...?

In general? Yes. Is it being put through a nozzle/diffuser combo? Is temperature taken into account? Temperature raises the speed of sound, so the expanding gasses in a bullet chamber (which are much hotter than the atmosphere) have a speed of sound higher than the atmosphere.

On a larger scale, this is how nuclear weapons generate their insane blast winds, which move (near the epicenter) at several times the speed of sound in the atmosphere.

ETA: Haven't read the thread at all, but figured it's going off course anyway

[lpetrich]

The velocity of sound is a = sqrt(dP/d(rho)) for pressure P and density rho, where the entropy is constant.

This is equal to sqrt(gamma*(P/rho)), where gamma is the adiabatic index, the ratio of specific heats (constant pressure) / (constant volume).

If the gas particles have no internal excitation, then gamma = 5/3, but if they have some internal excitation like rotation or vibration, then gamma gets pushed closer to 1.

Using the Ideal Gas Law, we find a = sqrt(gamma*(k*T)/m) where k is Boltzmann's constant, T is the temperature, and m is the average mass per molecule. It's convenient to scale both k and m by Avogadro's number, turning k into R, the ideal gas constant, and m into the average mass in grams per atomic mass unit.

So to first approximation, sound velocity in a gas is only a function of its temperature.

[GrahamH]

The velocity of sound is a = sqrt(dP/d(rho)) for pressure P and density rho, where the entropy is constant.

This is equal to sqrt(gamma*(P/rho)), where gamma is the adiabatic index, the ratio of specific heats (constant pressure) / (constant volume).

Pressure and volume are not constant in this case.