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Dead volume and the sound barrier

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Dead volume and the sound barrier

Unread postAuthor: Ragnarok » Thu Dec 19, 2013 8:34 pm

Given I've got into some discussions about supersonic pneumatics recently, I figured it was actually worth experimenting with the theory regarding the adiabatic heating in the dead volume.

I'm starting with rather exaggerated example - fitting HEAL with the extended (175cm) barrel, and having this "piston"...
CIMG0340-400.jpg
CIMG0340-400.jpg (56.15 KiB) Viewed 1893 times
... in the breech, and this sabot carrying a .177" pellet...
CIMG0339-400.jpg
CIMG0339-400.jpg (56.58 KiB) Viewed 1893 times
... 75cm further down the barrel. The space between the two provides a very large dead volume, which shouldn't mix with the cooling air from the chamber.

This could be looked upon as a variant of the spring piston airgun (or light gas gun), although without a distinct compression chamber and barrel (as they have the same diameter and the piston can continue chasing the projectile out of the barrel).

Test #1 was to have a shakedown run and try to chrony it via sound recording. While normally inferior to a light-gate chrony, having the microphone behind the target makes it easy to prove whether the projectile beat sound over the 10 metre distance and inherently takes into account the speed of sound not being a fixed velocity.
(And if I can't achieve a supersonic average over that distance... well, then I'd be too close inside the margins of error to be sure anyway).



Unfortunately, the shot was not on target, so the velocity is uncertain. But based on the state of the recovered piston and sabot, I am defaulting to considering the test a failure - they do not appear to have been strong enough to have functioned as intended.
For future tests of this set-up, a slightly heavier and stronger version will be used. Given the gases in the chamber already mass around 18 grams, going a bit heavier on the piston and sabot won't dramatically affect the accelerating mass.

After that, I'll switch to smaller dead volumes and see what happens. Eventually, I'll reach a comparison of HEAL's normal barrel with the sabot in the normal breech and test with/without the "piston" (adjusting sabot mass to keep the overall mass the same) to separate the dead volume*
*Which, on HEAL, is full bore and straight, so it's easy enough to put a "piston" up against the valve sealing face to completely segregate the dead volume.

"With" should mimic a perfect case scenario, where the dead volume gases aren't at all mixed with the cooling chamber gases.
"Without" relies on the assumption that the flow without the piston is turbulent enough to mix the gases to the point of negating any temperature rise, but this should be at least strongly true.

The difference should be reasonably telling about how much a good "laminar" valve could reasonably capitalise on this compression effect.
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Last edited by Ragnarok on Fri Dec 20, 2013 11:08 am, edited 1 time in total.
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Re: Testing the sound barrier

Unread postAuthor: jackssmirkingrevenge » Fri Dec 20, 2013 6:58 am

I'm not getting the configuration, care to draw a diagram? Does the barrel have two different diameters?

This could be looked upon as a variant of the spring piston airgun (or light gas gun), although without a distinct compression chamber and barrel (as they have the same diameter and the piston can continue chasing the projectile out of the barrel).


You say that but the sabot and "piston" seem to have very different diameters.

Also, I don't think this would be valid without a proper chrony - or at least getting another gun and making sure the sound chrony is properly calibrated.
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Re: Testing the sound barrier

Unread postAuthor: Ragnarok » Fri Dec 20, 2013 11:06 am

jackssmirkingrevenge wrote:I'm not getting the configuration, care to draw a diagram? Does the barrel have two different diameters?

No. One completely normal length of 22mm copper pipe - about three hairs over 20 mm diameter the whole way through.

Not to scale:
Sound_Layout.jpg

Any appearance that the sabot and (rather than calling it the "piston", I'll start calling it) the separator are of different diameters is a trick of the camera. They are essentially identical in all but the sabot being fitted with a way of holding the pellet.

Also, I don't think this would be valid without a proper chrony

If a sound recording (with the microphone by the target) hears the target being hit before it hears the muzzle blast, that is pretty much its own calibration, as the intent is to determine the velocity as related to the speed of sound.

There will be a "double report" (a small one from the dead gap, and a larger one from the main chamber), but this should be distinct enough on the waveform to remove that as an error.
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Re: Dead volume and the sound barrier

Unread postAuthor: jackssmirkingrevenge » Fri Dec 20, 2013 11:20 am

Ohhhhhhhhhhhhh... lol I thought the sabot was the small diameter part and you just put it on top to the other piston for some reason

Image

Pistons should be thicker to avoid flexing in the barrel.
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Re: Dead volume and the sound barrier

Unread postAuthor: Ragnarok » Fri Dec 20, 2013 12:12 pm

Indeed, it's the most obnoxiously oversized sabot going.

Determining the "piston" strength and reaching a middle ground with mass is part of the shake-down testing.
The first upgrade will probably be enough, but I'll start throwing balsa at the problem if extra upgrading is needed.
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Re: Dead volume and the sound barrier

Unread postAuthor: ToasT » Fri Dec 20, 2013 9:03 pm

This concept is similar to one I have been wanting to test for some time, its ironic that this discussion seemed to gain traction in the thread initially based on my existing gun.

I think that you are in a position where you can test this more easily than me.

As far as my brief thinking has gone on this debate, could you use the "piston" to compress air and force it through a nozzle/reducer into a smaller diameter barrel. The air will then be pushed out rather than a pure pressure driven flow. A burst disk could be used to further maximize gains.

This air in the smaller diameter tube should also sharply increase in temperature allowing for a faster pressure wave. All in all, higher muzzle speeds.
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Re: Dead volume and the sound barrier

Unread postAuthor: Ragnarok » Sat Dec 21, 2013 8:23 am

I already have something that does that:
Image
:D

Reducing to a smaller diameter certainly works in spring piston airguns. Although the above one is strictly subsonic, there are others which can comfortably go supersonic. (I wouldn't actually want a supersonic air-rifle though. The accuracy is pretty horrible).

This particular testing is strictly about what effect can be had without a reducer though.
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Re: Dead volume and the sound barrier

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Re: Dead volume and the sound barrier

Unread postAuthor: jackssmirkingrevenge » Sat Dec 21, 2013 8:38 am

Ragnarok wrote:I wouldn't actually want a supersonic air-rifle though. The accuracy is pretty horrible


*without the right pellets. If you're going supersonic at 12 ft.lbs then you're probably using sh;tty pellets.

A proper FAC air rifle firing heavy slugs at supersonic velocities can be very accurate.
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Re: Dead volume and the sound barrier

Unread postAuthor: Ragnarok » Sat Dec 21, 2013 9:00 am

Ah, but I didn't say at 12 ft-lbf, did I? :P
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Re: Dead volume and the sound barrier

Unread postAuthor: jackssmirkingrevenge » Sun Dec 22, 2013 5:23 am

Ragnarok wrote:Ah, but I didn't say at 12 ft-lbf, did I? :P


The point still stands, if you're not getting good accuracy from a supersonic airgun, assuming it's consistent then you're probably using sh;tty pellets.
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Re: Dead volume and the sound barrier

Unread postAuthor: Ragnarok » Sun Dec 22, 2013 12:59 pm

Yes and no.

There's no inherent repeatability advantage that can be claimed by going supersonic, but it is well known that decelerating back through the sound barrier has negative effects on projectile stability.

There is potentially a range and time-to-target advantage (so theoretically affecting wind*), but given the relatively small margins by which airguns go supersonic, subsonic trajectories (particularly when benefiting from heavier projectiles and the lower drag coefficients of subsonic velocities) can nearly match this out to beyond what counts as sensible airgun ranges - if not actually flatter at the longest distances.

*However, heavier projectiles will also resist wind better.

Ultimately, "horrible" was overstating it, but many of the claims of supersonic airguns are achieving it through using rubbish projectiles; even of those that aren't, if they do seem to be shooting more accurately at supersonic velocities, it's almost certainly the effect of internal or intermediate ballistics rather than external ballistics.

~~~~~

Anyway, I had another test yesterday.

Given the separator plate was recovered with a 5mm hole through it, I suspect the pellet got loose in the barrel and was hit by the separator.

Further sabot redesign needed, I think...
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Re: Dead volume and the sound barrier

Unread postAuthor: Ragnarok » Tue Dec 24, 2013 2:50 pm

Well, strengthened separator and sabots in hand...

CIMG0348-600.jpg


... aaaaaand they didn't hold up:

CIMG0353-600.jpg

(I painted them to make them easier to find afterwards).

Well, my first guess from studying them is that I'm getting closer. It looks like rather than the sabot tumbling in the barrel, they exploded from the pressure spike and ditched the pellet off at a wild angle.

Still, there's things that don't add up.

The state of the sabot (purple) is considerably worse than that of the separator (yellow) - the damage is more like it's hit something at speed. Indeed, there's a rather large dent in the HDF target holder that's smeared with magenta paint. (Fired conventionally, the sabots normally just bounce off the target intact at these distances).

There's also something going like a bat out of hell in a few frames of video and a sound spike that matches well.
It's plausible, although not confirmed, it might have been fast enough to have gone supersonic. More testing as and when possible.
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