Spudtech Forum

MACH 1 !? Here's...
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Author:  Spudtech [ Sun Jul 27, 2003 11:00 pm ]
Post subject:  If you used helium as the gas...

Originaly posted by andeh,
since it has a lower mass, and therefore a higher terminal velocity, you could easily do it with a 10 foot barrel or even less.

Author:  Spudtech [ Mon Jul 28, 2003 1:00 am ]
Post subject:  I Want a Floating Gun!

Originaly posted by Bartman007,
I'm just kidding. I know that it would require a large ammount of helium to lift the gun. Would the helium if pressurized to say, 100 psi, at least make the gun any lighter while carrying?


Author:  Spudtech [ Mon Jul 28, 2003 2:00 am ]
Post subject:  probably heavier...

Originaly posted by boogieman,
since it would be compressed, kind of like a full air tank vs a heavy one... I think, anyone know if this is right?

Author:  Spudtech [ Mon Jul 28, 2003 11:00 am ]
Post subject:  ok, maybe I can help...

Originaly posted by Freefall,
First off, I'll start off by agreeing that a projectile cannot exceed the speed of sound in the propelling fluid. An air rifle will not go above mach 1.

Next, as for a conventional rifle, i.e. one with an explosive charge propellant, the extreme high pressures are not what allows it to go supersonic. What lets a conventional gun go supersonic is the temperature of the gas. As temperature goes up, the speed of sound in that medium goes up as well. (this is how an afterburner works. I won't get into details yet...)

Back to why you can't go supersonic in a pneumatic gun: IsaacKuo states "The simple fact is that air travelling down the barrel of a gun has no idea how fast it's going relative to the outside world...". Well, the "outside world" begins at the inner surface of the barrel. This relates directly to the "gun within a gun" thought experiment. With a gun within a gun, the big gun can shoot the small gun at near supersonic relative to itself, and the same can be said for the small gun shooting the projectile, since the air in the small gun "sees" the barrel surface as stationary, even though it is moving at high speed. If you take away the small gun leaving just the projectile in the big gun, you once again see the barrel of the big gun as the interface between the propelling fluid and the "outside world".

Why, you may ask, does the air care whether or not the barrel is moving? Well, since the barrel surface is not -perfectly- smooth (and by perfectly, I mean polished to the molecular level, with absolutely parallel walls), as the air reaches mach 1, a shockwave will form on any imperfections. As air flows through these shockwaves, it loses energy in the form of heat. If it is still moving at greater than mach 1, another shock will form on the next imperfection, and so on and so forth, until you're once again left with subsonic flow at the backside of the projectile. This applies just as well to conventional rifles, only the speed of sound (mach 1) is much higher in the hotter gas.

IsaacKuo, while you are correct in stating that the speed of sound is not constant as altitude changes, it is not directly due to the altitude. Pressure has little effect on speed of sound. The reason the speed of sound varies with altitude is the temperature also varies with altitude. As you climb, the air gets colder, and the speed of sound decreases. If you continue to climb into the thermosphere, the speed will again become higher. In low orbit, where the average temperature of an air molecule can be in the thousands of degrees, the speed of sound is very high, but since the molecules are few and far between, the efficiency of transmission is extremely low.

If you want a gas gun to exceed mach 1 using air, you need to heat the air quite a bit. I'm planning on making a hybrid pneumatic/combustion, pressurized with an air/propane mix, which will be ignited downstream of the valve. With this arrangement and a long barrel, I think I should be able to achieve the "supersonic spud" quite easliy.

Author:  Spudtech [ Mon Jul 28, 2003 2:00 pm ]
Post subject:  one of those sites had an interesting point

Originaly posted by corncob909,
this is all way over my head, but i'll add this in- if glass flows, then why isn't the glass found in the egyptian pyramids nothing but a puddle (or atleast in a drastically changed shape!!!!!!!!!) >:-0

Author:  Spudtech [ Mon Jul 28, 2003 5:00 pm ]
Post subject:  oops, ya, your right..

Originaly posted by Bartman007,
I thought about that when I posted, but dismissed it. Stupid non-working sleep deprived brain.

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