Mass flow and De Laval nozzles for your spuddy

[Lentamentalisk]

as for a supersonic paintball, you could just use helium, but that is kind of defeating the point I guess. Helium really is the answer to any problems anyone is having with the speed of sound, and is far easier than trying to figure out a De Laval nozzle, though as Rag said, easy is boring (though it is sometimes awesome )

[jackssmirkingrevenge]

Small transfer ports are the ideal...

... unless you're aiming for sheer power, that was my point

[Gippeto]

Thought I would put up the article I found, and see what you make of it.

The details are sketchy as noted, but it does seem as though they went about things with an eye toward accurate data collection.

http://www.rose-hulman.edu/Catapult2004 ... roup05.doc

[jackssmirkingrevenge]

Interesting, but not seeing any nozzle attached/nozzle removed comparative data though...

[D_Hall]

De Laval nozzles are also used in wind tunnels.

Yeah, we've got one at the office.... We also heat the air in that wind tunnel to about 1500 F before we shoot it through the nozzle.

Seriously.

High speed wind tunnels most certainly make use of massive heaters. There are various flavors, of course. Some use hydrocarbon combustion (our's does). Others use megawatt class arc-heaters. Exactly what you're trying to do has a big effect on what sort of heater you would use, but the point is that heaters are integral parts of high speed wind tunnels.

[Gippeto]

Done deal then, "massive heaters" are not in the cards.

Thanks for saving me some time.

[jimmy101]

Done deal then, "massive heaters" are not in the cards.

Why not? A combustion spudgun is basically a "massive heater". "Massive" is a relative term. A wind tunnel with a ten foot diameter takes massive heaters to heat the huge volume of air pumped through / around them.

A gun only needs to heat the air in the chamber and combustion typically does a pretty good job of that. Wait, a gun also needs to heat the air in the barrel to get the round going greater than ~1100 FPS. But the air in the barrel and in the chamber and the ammo isn't actually supersonic in the gun since the speed of sound goes up with the gas temperature. IIRC, at the temperature of a propane in air combustion (~2200K before any heat loss) the speed of sound in the gun is about 2200 FPS.

I believe the same is true with a real rifle. The ammo is never supersonic when it is in the gun. It is only supersonic after it exits the barrel and gets into cool air where the SOS is much lower than it was in the gun.

EDIT: According to GasEq, adiabatic propane in air combustion gives a SOS of 3250 FPS, not the 2200 FPS I posted.

[Gippeto]

It might be of benefit with a combustion, but I'm working on a pneumatic.

I've limited myself to 400psi air, and no heating other than pumping provides.

My goal is to get a reading 5 feet from the muzzle of 1125fps+ with a .12gram airsoft bb.

I am sitting at 1059fps. (peak reading to date)

I've torn down the launcher, and have made some changes that I hope will improve airflow.

We'll see what happens when it's back together. Should be sometime monday.

[D_Hall]

I've limited myself to 400psi air, and no heating other than pumping provides.

If the goal is to break the sound barrier with THOSE limitations, one would think that a simple two stage design would do it with ease.

Basically, think of a two stage gas gun, but imagine that both stages are filled with air. The first stage, your 400 psi. The second stage, ambient or maybe a couple psi above (to begin with).

Upon firing, the piston would cause adiabatic compression (read: high temperatures) of the second stage as well as potentially MUCH higher pressures.

'Twould require a very robust gun (read: steel construction), but it should be pretty easy to break the sound barrier.

[Lentamentalisk]

can you explain how a two stage gas gun works? I think I understand, but a diagram would help.
is it just air from one chamber, pushing one piston, which effectively compresses a second chamber, shock heating it, and accelerating the projectile?

[D_Hall]

<img src="http://physci.llnl.gov/Organization/HDi ... llisbx.gif">

Just replace "burning gasses" with 400 psi air and "hydrogen" with ambient air, and you oughtta have a supersonic-capable gun powered by nothing more than 400 psi air.

[Lentamentalisk]

Awesome, that looks quite easy, though you are right about it needing a robust construction... With cheap materials, either you use a very light piston, that destroys itself every few shots, or you use a strong piston that destroys the fittings and all. I suppose since the piston doesn't have to seal perfectly, you could just use a little balsa dowel that fits snugly inside, and just cut out hundreds of sections of the dowel...

[D_Hall]

I've no idea for an application like this, but in the "real" versions of the gun, they use robust materials and the piston is utterly destroyed. IE, each piston is good for exactly one shot.

Oh, and if you want increased pressures, the piston HAS to have a certain amount of mass. You need it's momentum to drive the second stage.


edit: It occurs to me that (ironically enough) a spud would probably make a good piston!

[Lentamentalisk]

lol oops, ya, I blanked out on that part...
Good point on the spud! That gives me an idea. What if you used a squishy, heavy material for the piston, that would destroy itself, but not injure the gun? Examples being: potatoes, wax, other vegetables or fruits, condoms filled with mercury etc.

[D_Hall]

What if you used a squishy, heavy material for the piston, that would destroy itself, but not injure the gun?

Well, that's why I mentioned the potato.

And yes, this is the way it's normally done. The piston is sacreficial and the gun is built very robustly. But then, normally these guns operate at say... 120,000 psi and a solid nylon slug is "squishy" compared to the steels used to build the gun.