A Few Ideas...

[qwerty]

Here are a few ideas thought up by my brother:



The top picture is a simple way to create higher pressures in your chamber.
Air is pumped into the larger side, because of the greater area of the large piston, the smaller diameter chamber is at a much higher pressure.

The Bottom picture is a gun designed to achieve high velocities of small caliber rounds. (He later found out this idea is very similar to a light gas gun.)
A heavy piston (steel/lead filled plastic) Is fired using high pressure air. The piston is shot along the barrel. Before the piston reaches the the hole 9/10 of the way down the barrel, all the pressure built up escapes through the hole. When the piston (With very high kinetic energy) passes the hole, the pressure builds up rapidly and breaks the burst disk/O-ring seal, propelling the projectile out very high velocity.

Obviously the tubing past the hole will need to be able to withstand very high pressures and and a rubber bumper will be needed to remove any energy remaining.

Ask any question if you don't understand.

[saefroch]

I believe DYI has proposed a design like a hybrid-powered LGG, but so far as I'm aware, the gains are minimal, and it seems more likely that with an all-pneumatic setup you'd only experience an energy loss.

As for the top idea... it is interesting, but I sadly cannot see any applications where it would be more effective than a multi-stage pump.

[Technician1002]

On the bottom one the burst disk is not needed and removes some energy.

If you accelerate the piston near SOS, at 1/2 travel the average pressure would be 2atm, but due to the velocity, the pressure at the barrel will be less. The 10% of travel produces the pressure spike.

With compression heating and the proper mass/diameter ratio, the transfer of energy from the slower more massive piston will transfer to the smaller lighter projectile.

A common science museum display uses a falling bowling ball to launch a tennis bell to the ceiling.
[youtube][/youtube]

On the top one, because air is compressible it will take many strokes of the pump to reach high pressure in the chamber. If for example the pump and the chamber was the same volume. At the end of the first stroke, the chamber pressure would rise from 1 atm to 2 atm or only about 14.7 PSI. Hardly high pressure by today's standards. Most often the pump is much smaller in displacement than the chamber. It will work but require many strokes.

This boost in pressure is commonly used in moving liquid into a high pressure. A feedwater pump on a boiler often uses steam to pump water into the boiler.

[qwerty]

If you accelerate the piston near SOS, at 1/2 travel the average pressure would be 2atm, but due to the velocity, the pressure at the barrel will be less. The 10% of travel produces the pressure spike.

i very much doubt the heavy piston will get anywhere near the speed of sound in anything i would make, i think we can ignore any factors due to the low speed of sound in air.
in real light gas guns, a lighter gas is used because of the higher speed of sound, this means the pressure difference can travel faster.

surely a burst disk is nessecary for a piston traveling much under the speed of sound? as far as i can see, without a burst disk, a bb would roll out as soon as the piston started moving and before the pressure could build up.
maximum pressure is achieved when the piston is as close to the barrel as it will get, a burst disk is needed to keep the projectile in place until the point where the projectile/piston has the most potential energy.

[cammyd32]

That (bottom idea) is how a gas-ram air rifle works sorta, (at least I think that is what it is called), its a good idea if you want to conserve gas, cos no excess air is wasted when it shoots, but for the burst disk it is not necessary just take a look at air rifles http://www.arld1.com/ they don't need burst disks, just a tight fit, or a detent will do. which means you will not have add lots of open-able parts that you need to replace a burst disk, making the whole thing simpler.

[Technician1002]

as far as i can see, without a burst disk, a bb would roll out as soon as the piston started moving and before the pressure could build up.

The BB would have a long way to roll at low pressure. The low pressure duration in time is VERY short and generally under 10 ms. It is true that it will start to move, but not fast enough to roll out the barrel.

they don't need burst disks, just a tight fit, or a detent will do.

This can take the minimal motion and eliminate it without a burst disk and hold the projectile until the pressure spike. The last 10% of the piston travel would produce compression heated air (light gas) and the last 5% of the travel can produce temperatures high enough to ignite diesel oil. Diesel engines not using glow plugs and relying on compression heat only for ignition typically have compression ratios of 14:1 to up to 25:1. Compression by the piston of 5% of the volume is 20:1. This is well into diesel engine compression ratios and resulting compression heat.

[jackssmirkingrevenge]

I think the second idea has potential, it's certainly been suggested before. Essentially you get a very high pressure "valveless" launcher, albeit with a low "chamber volume". Here's how I see it being practical, as a combustion which avoids the need for a fast large bore valve:



I recently made an excel calculator to determine the pressures involved in this scenario (based on simple Newtonian equations and ignoring thermodynamics) and you get some pretty impressive numbers, I would certainly include some form of burst disk or detent in order to maximise the pressure potential.

[qwerty]

That (bottom idea) is how a gas-ram air rifle works sorta, (at least I think that is what it is called), its a good idea if you want to conserve gas, cos no excess air is wasted when it shoots, but for the burst disk it is not necessary just take a look at air rifles http://www.arld1.com/ they don't need burst disks, just a tight fit, or a detent will do. which means you will not have add lots of open-able parts that you need to replace a burst disk, making the whole thing simpler. Very Happy

Gas ram air rifles use compressed gas instead of a spring so the piston can be cocked back. When the trigger is pulled the piston gets slammed forward compressing the air infront of it.

Most air rifles use 2 O-rings to hold the pellet inplace at the breach, I did say "Burst disk/O-ring seal" So you don't have to use a burst disk, just something like a small O-ring. A detent would be a bad idea because it would deform the pellet and give the BB unecasery spin.

I think the second idea has potential, it's certainly been suggested before. Essentially you get a very high pressure "valveless" launcher, albeit with a low "chamber volume". Here's how I see it being practical, as a combustion which avoids the need for a fast large bore valve:

That picture interesting but i cannot see what advantage it would have over a normal hybrid. Also you would need to find a way to refuel and introduce air to the chamber.

My brother will reply to Tech's comment

[jackssmirkingrevenge]

That picture interesting but i cannot see what advantage it would have over a normal hybrid.

The advantage is that the fact that you're compressing air rapidly with the momentum of a fast heavy piston, it will reach pressures far in excess of what a "normal" (feeding fuel at atmospheric pressure) hybrid is capable of achieving.

Another potential use of the idea could be in the manner of the Sims-Dudley dynamite gun:



It is a pneumatic cannon with a conventional chamber, valve and barrel - however instead of being fed by a compressor, it used a magic powder charge to push a piston down a tube, compressing air past a one-way valve on the other end.

To give to an idea of what might be possible, imagine you have a piston in a 3" diameter and 15" long tube, connected via a one-way valve to a 1" diameter chamber 2" long. Given a big enough push, the piston could take your chamber to 1000 psi. As you could however have the air push the projectile directly, it seems like an extra complication to attach a separate chamber and valve, considering it's adding dead space and increasing stress on the system by having to retain compressed air.

Back to the combustion design, here's a concept drawing of what it would look like as a hybrid, with a detent added to the piston to allow a pressurised fuel air mix, and a burst disk to ensure high pressure release.

The things will be under so much stress though that if you actually had the resources to build it, you might as well make a Larda-style "conventional" hybrid.

[Fnord]

If we're going to go this route why not try one step further;

Obviously this has some additional issues to work out, but at least the returns are higher.

[jackssmirkingrevenge]

If we're going to go this route why not try one step further

A hybrid within a hybrid!



HYBRIDCEPTION!

I like it

[qwerty]

Hmmm, ok, who wants to make it? I doubt i will have the resources.

[Labtecpower]

I have the resources to build it, but not enough time. Another factor is that I have 0,0% experience with hybrids

I could also machine the parts, and send them to JSR or someone else within europe with hybrid experience.

This would be machined out of standard steel? (St37 / Fe360)

[jackssmirkingrevenge]

I could also machine the parts, and send them to JSR or someone else within europe with hybrid experience.

Interesting offer, I'm sure proper plans could be drawn up...

[Labtecpower]

CNC lathe could probably be used

I don't know when I would be able to machine it, i'll have to finish some school projects first, if I want to go to the next year.