Random idea - possible ridiculous power / velocity.

[Tep]

I had a random idea the other day while fiddling around with the shroud / silencer on my gun.

Basically what I am thinking is:

The air to charge the qev gets drawn from the inside of the perfectly sealed barrel shroud while the end of the [ported] barrel is blocked with a piece of tape (don't worry pictures below).

If you had a pump that was capable of drawing a close to a proper vacuum you could set up a situation where the QEV acted as the rear burst disk like on one of those burst disk vacuum cannons except it also added bucket loads of pressure behind the projectile.

In my imagination this would allow super fast acceleration of the ammunition as there would be minimal resistance to acceleration and because the shroud would act as a vacuum reservoir there would be minimal deceleration towards the end of the barrel and very minimal noise...

I've attached a diagram detailing what I'm talking about

idea

This would have the advantage of being:

Easier to operate than a normal two burst disk vacuum set up - no need for disassembly or anything between shots.

More acceleration than the normal two burst disk set up - as pressure is added (above atmosphere) behind projectile as well as vacuum in front.

Quieter overall as air that pushes round is drawn into the low pressure environment of the shroud.

Could be used as just a standard qev driven air gun without the tape over the barrel burst disk.



BUT....

This all seems too simple and I'm sure that there must be some great gaping gap in my logic somewhere otherwise I feel like this would have been done already.

I need you guys to have a look at the diagram and point out the what I have missed before I attempt this one but if it looks viable I will have a bash at putting together a test rig (will probably take me 100 years to make it work knowing my style.


Factors to consider:
QEV barrel seating face vs uncovered face area - I imagine due to the low pressure in the barrel before firing more surface area will need to be exposed on the piston face. i.e. a 3/4 inch qev running .177 sealing face... probably overkill but I don't know.

Add more ideas here...

Thanks guys!

Tep

[jrrdw]

I was watching a show on tv about a month ago and some collage students built just about what you described in the 1st part of your post. They were shooting ping pong balls through the paddles at about 6"s away. It was awesome!

[Tep]

Damn, I had hoped that the qev idea might have been original. But ping pong balls through a bat = cool - imagine golf ball instead!

[jrrdw]

Found it....[youtube][/youtube]

[Tep]

I guess the difference is that they are using two burst disks (requires pulling the whole thing apart between shots, and high volumes of air. The thing I outlined above uses only one (on the barrel end) and the QEV takes the place of the other one.

But I'm not sure that it would actually work because the force on the QEV piston may be a factor that prevents opening?

I think I'll have a bash at an 8mm version with a galvanised steel shroud; to start with I'll just set it up like a normal QEV + slide valve based launcher, but I'll draw a vacuum into the barrel / shroud before firing with my fridge compressor and see if I get better results than without / does it fire at all...

[jrrdw]

Testing in stages is a good idea.

[Tep]

I'm currently trying this and not noticing an enormous increase in power unfortunately... But it could have something to do with the 8mm barrel and light weight ammunition I have. I'm continuing to experiment. Hopefully I'll work something out...


Update!

There is a definite measurable increase in power to be had here but it's a bit of a fiddly wreck to work out...

I've been using a stack of 5mm corrugated cardboard as a measuring stick and with reduced pressure in the barrel (I've decided not to call it a vacuum anymore - I'm beginning to doubt my fridge compressors ability to pull a perfect vacuum) I can get an 8mm ballbearing through 11 sheets with 200psi behind it. Without the low pressure treatment I only get through 8 sheets at 200psi. Unfortunately I can get even better performance from the rig by just pumping it up above 300psi (blew through the back of 15 sheets of cardboard) so it's not the be all and end all.

This result was repeatable three times so I believe it to be reliable.

I think this would have more effect at higher calibers and with lighter projectiles and lower QEV pressures. i.e. like pingpong ball sized with only 110psi shop air behind

HOWEVER this does defiantly prove in my mind that a QEV can be used in place of the rear burst disk in one of these vacuum driven pingpong ball cannons.

This could significantly reduce the complexity of firing between shots.

Either way I will continue to experiment for a while and maybe try a longer barrel.

I don't have any way to chrony this as my backyard range is max 8 meters so my audio chrony app wont work and I don't really want to shoot this thing in the street,


anyway...

[jrrdw]

Well, a bigger barrel will give you more units to measure, that being said you still will only get the same vacuum pressure that your set up will give you. How ever a longer barrel will give you higher speed up to a point.

My concern with the longer barrel would be running out of vacuum before the ammo leaves the barrel.

[Tep]

My concern with the longer barrel would be running out of vacuum before the ammo leaves the barrel.

Thats why I evacuated the air from both the barrel and the shroud, the shroud in theory acts as a reservoir for low pressure keeping the tip of the barrel constantly at close to a vacuum until the projectile exits the barrel.

Good point about the larger diameter barrel.

I dicked around with this for most of the day eventually making something that was almost exactly as shown in the digram I posted and have come to a few conclusions.

Either the fridge compressor doesn't draw down a full vacuum and it's important for this to have a full vacuum - I have no way of measuring this but I expect it is at least close to a full vacuum so I doubt this is the case.

Or

It just doesn't really make that much difference what you are firing into (1 atmosphere or 0.1atm or a vacuum) when using a heavy projectile with a <350mm barrel because it didn't matter how much I reduced pressure in the barrel and shroud / silencer, I could always replicate the results (as measured on my cardboard layers chrony) by just adding an extra 50 - 100psi to the QEV.

Or

The short barrel length just doesn't give the system enough time to act on a projectile to it's full potential and we need a longer barrel. I suspect that this is the case.

So...


Some time this week I'll make a 1000mm barrel version (I'll have to go to the hardware shop) and test again to see if the gains are exponential.

In my imagination something around mach 1.5 or about 500m/s would be cool which would let you hit a target at about 100m without the 2g ball going subsonic and loosing stability. Like I said in my imagination

Either way I'll post up when I have the new rig built and tested.

[jrrdw]

http://www.amazon.com/b?ie=UTF8&node=15707481

Give that page a look over, you should be able to get something suitable at your local auto parts or hardware store.

[D_Hall]

There's nothing new about pulling a vacuum on the barrel of an air gun....even a QEV powered air gun. It works for pretty much any gun wherein the mass of the air in the barrel is a significant compared to the mass of the projectile. That is to say that if Mair/Mproj > 0.1ish, you're going to see significant improvements. The bigger that number, the bigger the improvements, obviously.

Note for supersonic guns its pretty simple to get a very close estimate of what the vacuum-assisted velocity is. Just treat muzzle energy as a constant and do the math with and without additional mass in the barrel.

With all that said... Never heard of a vacuum pulled on a supressor before. That's a decidedly interesting idea!

[Tep]

Right I see!! thats why I don't get huge improvements in power using a 2g projectile in such a short barrel, the mass of air in the barrel would only be about 2.4 - 3mg I think.

I guess greater improvements could be seen with a longer barrel and a lighter projectile like a 0.2g plastic airsoft pellet.

The whole idea with pulling the vacuum into the ported barrel shroud was to make sure that any air that leaked past the projectile while in the barrel wasn't slowing it down towards the end of the barrel towards the burst disk and I had hoped it would also make the shot way quieter but the second part wasn't really noticeable. I'll report back after the larger rig with increased shroud volume is made though.

(It was also super easy to manufacture by just mounting a barrel in a galvanised tube with a "T fitting" on it, one part of the T was the vacuum hose attachment and the other housed the barrel sealing part).

In theory then...

with:

0.8cm barrel internal
100cm length
Dry Air = 1.2g/L

0.4^2x3.14x100= 50.27ml barrel volume

1.2x0.0527=0.063g total weight of air in barrel.

SO.

0.3(Mair)/2(Mproj)=0.15 this being the ratio for the 1m barrel.

0.0632/2=0.0315 this being the ratio for the 300mm ish barrel I initially tested with.

0.0632/0.2=0.316 this being the ratio achievable with a 0.2g airsoft round I imagine this would equal high velocity if you had 600 - 800 psi in the QEV.


Design Ideas are welcome / improvements on ideas stated.

tep

edited for terrible maths...

[D_Hall]

Having a bit more time to elaborate at the moment. OK, suppose....

Projectile Mass (Mp) = 1 kg.
Air Mass in barrel (Ma) = 0.1kg.
Muzzle velocity measured (Va) = 100 m/s.

Muzzle Energy is often stated to be 1/2 * Mp * Va^2. This is, however, inaccurate. While it's the energy you're interested in if you're worried about how far your projectile goes or how hard it hits, it is NOT the energy delivered by the gun. After all, the gun had to push all the air in the barrel out of the barrel too. That air was moving (more or less) at the same speed of the projectile. As such, your ACTUAL muzzle energy is...

0.5 * (Mp + Ma) * Va^2 = Energy

OK, great. So what if we pulled a vacuum on the barrel but kept everything else constant? What would the muzzle velocity in a "vacuum shot" (Vv) be? Well, the energy available to your gun really hasn't changed. As a result, your actual muzzle energy is going to stay pretty constant. So....

0.5 * (Mp + Ma) * Va^2 = Energy = 0.5 * Mp * Vv^2

Simplifying and rearranging we get....

Vv = sqrt(((Mp + Ma)/Mp) * Va^2)

Using our example numbers.... sqrt(1.1*100^2) = 105 m/s.

Not a huge difference, but significant (5% increase). However, as stated previously, the bigger the ratio of Ma/Mp, the large the difference it will make.

Note: I'm assuming no issues with choked flow or the like. Still for a lot of system I play with as part of my day job, this quick and dirty does a remarkable job.

[Tep]

Wow that's a great explanation - thank you!


Running the calculations for my theoretical 0.2g at 100m/sec in a 1000mm x 16mm barrel gives me

0.5 * (Mp + Ma) * Va^2 = Energy
or
0.5 x (0.0002 + 0.00003) x 10000 = 2.5

and with vacuum in barrel

Vv = sqrt(((Mp + Ma)/Mp) * Va^2)

Vv = sqrt(((0.0002 + 0.0003)/0.0002) * 10000) = 158m/sec (minus any diminishing returns encountered)

thus: 0.5x(0.0002+0.0003)x 158^2 = 6.24 249% energy increase


Verses the 2g steel ball where


0.5 x (0.002 + 0.0003) x 10000 = 11.5

Vv = sqrt(((0.002 + 0.0003)/0.002) * 10000) = 107.23m/sec

thus:

0.5 x (0.002 + 0.0003) x 11498 = 13.2 114% energy



I know this makes some broad assumptions but it gives a ball park for better visualisation of the possibilities.


Edit: Bad maths x 10000000000000 times but the principal stands.

[farcticox1]

Like this