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Supersonic BB's using compressed air?

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Supersonic BB's using compressed air?

Unread postAuthor: Necrosis » Mon May 09, 2011 4:10 pm

I was wondering;

How fast could one theoretically fire a bb using just compressed air?

(No helium or whatever just pure unadulterated 200 bar breathing air)

Thanks in advance!

Arien.
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Unread postAuthor: jackssmirkingrevenge » Mon May 09, 2011 4:13 pm

This is as fast as I got with less than 30 bar, so you should hit the speed of sound with a long enough barrel and fast enough valve ;)
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Unread postAuthor: Necrosis » Mon May 09, 2011 5:19 pm

Barely breaking the speed of sound huh, I wonder what the expansion rate of air is..

I was planning on using a solenoid powered hammer valve (daystate cfd imitation yay) at 200 bar, and 330 bar once i get my good scuba tank..

with a barrel length of 1.2 meters, bullpupped so I get something that is still.. manageable.

I put a .12 bb in my 6.35 hatsan at44 on steroids and definately got a sonic boom, I just wonder what the maximum theoretical speed is, as air kind of expands at a set rate (it does right? or is this again pressure dependant... I beleive it was just over 500 meters per second..)

It'll be a long term project but hey, it'd be mighty fun to build and probably to shoot, but if I can get a .40 gram BB out at 500 m/s that'd only stay supersonic for about 30 meters, my intended goal was atleast 50...


Ah well, I'll wait for some more answers here then eh?

Thanks for replying, When I repair that steroid at44 I'll chrony the .12g bb for you guys.
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Unread postAuthor: saefroch » Mon May 09, 2011 6:12 pm

Necrosis wrote:the expansion rate of air
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There is probably some limit you can find using GGDT for your specific setup. Just be warned, D_Hall says it gets rather optimistic when you go transonic. I know I've gotten transonic predictions out of GGDT, and that was without correcting for the higher temperature in the chamber, and using a close-fitting projectile. At 200 bar, you should have no trouble getting supersonic with a reasonable valve, since you'll find valve opening time becomes pretty close to 1ms, and is near instantaneous.
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Unread postAuthor: Gippeto » Mon May 09, 2011 8:34 pm

I believe there's a fellow getting around the 1600fps mark with a .25 condor....seems to have dropped from sight of late though.

Really like my AT44-10....outstanding value IMNSHO. 8)

What's wrong with yours? :?
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Unread postAuthor: Necrosis » Tue May 10, 2011 3:46 am

Oh right now the fill valve kind of went poof.

Previously my valve broke in half as did my sidelever, allthough both are fixed now. :P

It propels barracuda's a lot faster now at any rate.

At any rate making a new fill valve comes first and then, well yeah I'll post some pics for you guys.
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Unread postAuthor: Necrosis » Tue May 10, 2011 10:19 am

Eh yeah, 430 meters per second...

that's in a way too large barrel... eh..

Baracudas at 300 meters per second at any rate..
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Unread postAuthor: warhead052 » Tue May 10, 2011 10:45 am

Necro, I have never tryed this before, or have any experience with it, but try the rubber washer with the small hole trick to build up pressure till the bb pops out at a high speed. I have never tryed it, but it may be worth a shot!
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Unread postAuthor: qwerty » Tue May 10, 2011 2:32 pm

SOS in air is:
340 m/s
1125 FPS

What valve are you going to use?

Baracudas at SOS is going to be very hard. I doubt the Condor could get those kinds of speed. I had 200 of them and fired them out of my QEV rifle. They were going fast but not that fast.

Ah well, should be a fun challange :)
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Unread postAuthor: Crna Legija » Tue May 10, 2011 11:24 pm

warhead052 wrote:Necro, I have never tryed this before, or have any experience with it, but try the rubber washer with the small hole trick to build up pressure till the bb pops out at a high speed. I have never tryed it, but it may be worth a shot!



with 300 bar he could use Teflon as the dent :lol:
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Unread postAuthor: Necrosis » Sun May 15, 2011 7:57 am

qwerty wrote:SOS in air is:
340 m/s
1125 FPS

What valve are you going to use?

Baracudas at SOS is going to be very hard. I doubt the Condor could get those kinds of speed. I had 200 of them and fired them out of my QEV rifle. They were going fast but not that fast.

Ah well, should be a fun challange :)


Hammer valve of course!
:D

And while I may or may not get good speed with BB's, I could allways swap out the smoothbore barrel with a .223 and shoot 40 grainers.. or.. .25 and shoot 60 grainers.. both at over 280 m/s. <_<

Done that before, allthough not in a completely homebuilt one, just my modded BAM B51.

@crna I could probably use a conical chamber and force a soft bb into a .223 barrel. haha.
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Unread postAuthor: DYI » Sun May 15, 2011 8:50 am

There is probably some limit you can find using GGDT for your specific setup.


GGDT is a 0-D simulation. That's a good deal of the reason why it's so terrible when approaching the speed of sound in the propellant gas. GGDT sims are almost totally useless for a situation like this.

@Necrosis: the particle speed distribution in air follows a Maxwell-Boltzmann distribution, and you can look at those for further insight on the matter. With a barrel and perfect-flow valve ground to the best tolerances possible with modern manufacturing processes and an extremely lightweight BB being pushed by room temperature air down an appropriate length section of evacuated barrel...
I wouldn't be overly surprised if you hit 1000m/s.

With your limited pressure and likely non-evacuated barrel, a ballpark estimation is, for me at least, not really possible. You could attack this problem with a standard 2D finite differencing approach or some more advanced method of solving the system, but considering the nature of how this system is limited by the particle speeds in the gas, something more along the lines of a DSMC could be more appropriate, at least for the propellant gas side of the system. You'll also have quite a shock wave building up in front of the projectile.

This would be an interesting problem to work on, but it'd take less time to simply build the thing and find out, especially when the imperfections in the valve and barrel surfaces are as important as they are in the kind of flows you're interested in.
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