Show us your pneumatic spud gun! Discuss pneumatic (compressed gas) powered potato guns and related accessories. Valve types, actuation, pipe, materials, fittings, compressors, safety, gas choices, and more.
Actually I am going to fiddle with bullets/sabots this weekend to try and settle this... Too bad chronies are so fallible...
Know anyone with a high speed video camera? Using one with a measure and frame by frame is not that subject to error except parallax.
[youtube][/youtube]
Youtube unfortunately scaled the video so the original 1,000 frames/sec is no longer good, but this video gives you the idea. And yes, the orange is traveling close to a foot a millisecond. We confirmed the chroney results with high speed photography.
Actually I am going to fiddle with bullets/sabots this weekend to try and settle this... Too bad chronies are so fallible...
Know anyone with a high speed video camera? Using one with a measure and frame by frame is not that subject to error except parallax.
[youtube][/youtube]
Youtube unfortunately scaled the video so the original 1,000 frames/sec is no longer good, but this video gives you the idea. And yes, the orange is traveling close to a foot a millisecond. We confirmed the chroney results with high speed photography.
That would be awesome if I could get my hands on one...
Update... I was doing some more shooting and this time with the chrony 30ft beyond the muzzle I recorded a 1085fps, 1088fps, and a 1093fps. in order shooting superballs and unregged co2...
If someone who knows the calculations would be so kind as to figure out muzzle velocity from that...
I'm guessing that it'll be slightly under sos unless the balls are scrubbing close to 200 fps in 30 ft...
If someone knows a weight for 1" superballs it would prolly help the calculator...
Chairgun will be useless. It's limited to very specific parameters.
This is odd, because even that far from the chrony (so presumably genuine results), we're still getting results that exceed CO2's speed of sound. Obviously, some odd factors are at play here.
For your back track...
Calculating from polybutadiene's density, a 1" (and genuinely 1") superball will weigh around 7.6 grams - but I'd prefer someone to actually confirm I'm in the right region with a set of scales.
Disregarding a few of those 30 feet because of the lack of deceleration in the muzzle blast, numbers for a muzzle velocity look to be about 1350 fps.
I'm not popping the champagne cork for you yet though. I'm still doubtful, because none of what I know says this is possible with CO2.
Obviously, the numbers suggest otherwise, but results can be mistaken.
What's the barrel length? I tried looking it up, but with no joy. People seem to omit these things with increasing frequency, and it just makes any calculations to back up claimed performance much harder.
Barrel is 66" from breech to muzzle, the balls sit 5/8" into the breech(initial position)... I think the balls are prolly closer to 12grams if I had to guess, and they actually measure and are sold as 27mm... they have to squish slightly to get them into the barrel...
Alsocheck out the decibehind video... I'm sure someone here can isolate the firing sound and impact noise in relation to impact, to the naked ear the barrel moves before the shot is heard but considering thats a 62gram bearing I'm sure it's well below sos...
Last edited by jeepkahn on Mon Jul 20, 2009 4:24 pm, edited 1 time in total.
Then we'll assume they're about the 9 and a bit gram estimation I get from calculating them as 27 mm. The velocity estimations are still similar.
As far as the barrel calculations, we're looking at about 775 Joules from a 840 cc barrel, or 0.92 J/cc.
That kind of thing is easily feasible below SOS in the propellant gas with enough pressure. Indeed, HEAL is slightly above 1 J/cc.
The question is, is that possible above the (basic) speed of sound of the propellant gas?
There are mechanisms by which you might find it happening, but I find it unlikely that you've hit on one by chance - then again, there is another case of that kind of thing happening in spudgunning.
Much thought is needed. Obviously, I'd prefer to do a velocity confirmation of a more reliable nature, but we can't always have what we want.
Is it possible that the layout of the chamber and the chamber/barrel junction are somehow affecting the localised sos??? or the maybe the twin/opposed chambers are in essence allowing multidirectional expansion of the gases? even if it's just for the microsecond that it takes to propagate into the "piston head void"(ie, when the piston head moves back you have gases expanding from opposite directions at 900fps, when the pressure fronts collide behind the projectile it would pressurewise be the equivelant of an 1800fps collision, which I would imagine would also spike the local temp and raise the gases sos allowing for faster than basic sos flow)....
But, Like you, I'd love a more accurate or at least complimentary test on velocity....
It's tough to know. I can't see which of either theory or results I should disbelieve.
Until the results are disproven or reasonable doubt is cast upon them by some mechanism - the nature of which I cannot see at this time - I think we have to assume they're genuine (although we can't totally guarantee my backtracking, but I have little doubt if the results are in the right ball park, MV was in excess of Mach 1.)
And the science behind why you can't exceed SOS in the propellant is still solid. This is that airflow velocity enters the inefficient transonic region where shockwaves form and limit expansion speeds.
That means we're looking for an explanation on why the SOS in CO2 was boosted so dramatically (which would take a temperature rise to about 700 Kelvin) - or how this manages to evade the normal "speed limiter".
How it would manage to go subsonic straight through to supersonic without passing through the transonic region is currently beyond my "past midnight" thought processes, but that doesn't mean there isn't something I'm not thinking of.
jackssmirkingrevenge wrote:... at ambient temperature, but what if the chamber had heated up?
Like I said (in the most recent post), the gas temperature would need to hit about 700 Kelvin, and would need to remain above that - which means it would need to peak even higher to compensate for cooling.
And I can't see where a temperature rise of that magnitude would have come from.
