SpudFiles

The nitrogen for one of my pneumatics costs .79¢ per shot and the 1.250" ball bearing projectiles cost about $3.75 per shot.

Despite nitrogen being a rather poor propellant gas, you'd still pay less running it as a propane/air hybrid at the same energies. Projectiles generally are quite expensive for more energetic launchers. However, take my last hybrid build: Firing $9.00, 2" diameter ball bearings at ~1700 ft/s and using an oxygen/nitrogen/propane blend (not atmospheric air), it cost roughly $1.80 per shot for propellant. Now to achieve the same performance with helium, we'd need at least 2 cubic metres, even at 2000psi. Assuming that it's being bought in the quantities readily available to individuals, that much helium costs about $50 around here. That, and you now have to contain 2000psi helium instead of 240psi air, for the same ballistic performance.

If one happens to have access to liquid helium it's a much closer match, but to stay fair we'd also have to assume access to liquid oxygen and bulk propane tanks, at which point hybrids still win by a long shot on affordability.

Ah, but for a pneumatic you can use free fuel, the air, if you're not too lazy to use a multi-stage pump. Currently, I'm at about $0.03 per shot for ammo, ignoring build costs.

DYI wrote:Only at the *extremely* low performance end of the spectrum can pneumatics compete cost-wise with hybrids.

Using air limits one to subsonic muzzle speeds, thus placing it firmly at the low performance end of the spectrum, which I mentioned in the above quote.

DYI wrote:Despite nitrogen being a rather poor propellant gas, you'd still pay less running it as a propane/air hybrid at the same energies.

I'm sure you are correct but, personally I don't think economy or velocity should be the primary design criteria for a cannon. The cannon I referenced fires 1.250" steel balls with nitrogen at near mach 1 velocities and 60 rpm. Can a combustion device fuel fast enough to deliver that kind repeatable and reliable performance?

Yes. Look at any high performance racing engine.

High volume compressor, fast valving, fuel injection, ignition. Rates much faster than 60 rps is possible (3600 RPM).

One round per second is slow.

This is why combustion is considered for my design on a future bublegum machine gun. It will take feed from a pressure air source and regulated propane source and will inject both into the chamber.

Technician1002 wrote:Yes. Look at any high performance racing engine.

When you adapt an internal combustion engine (or comparable design) to shoot 1.250" steel balls close to mach 1, let me know.

If I had the interest and the spare cash, I could make something with twice that rate of fire and twice the muzzle energy in a week. Problem for you is I don't have either of those things.

You guys are seriously underestimating the potential for rate of fire in a hybrid.
The only "problem" is ensuring the fuel mixes fast enough (and even that is easy to handle - add fuel first, then add the oxidiser very rapidly. It will mix). Venting is pretty irrelevant - just treat the combusted mixture as more buffer gas and just add the new air/fuel on top of it. Simple.

I may not be a fan of hybrids, but I do at least know what they're capable of. Their current incarnations are nowhere near the potential limits of the principle.

Ragnarok wrote:I may not be a fan of hybrids, but I do at least know what they're capable of. Their current incarnations are nowhere near the potential limits of the principle.

Very true.

If you are interested in modifying an internal gas engine, a simple modification to the cam to open the exhaust valve shortly after TDC will do the job. Instead of a high HP engine with the energy to provide high crank torque, the exhaust could be used to provide high energy into the exhaust/barrel.

With enough delay in valve opening after TDC, the engine could still run as an engine and use the recoil (power stroke) to make the cycle self repeating. (engine still runs as an engine) or you could simply modify an air compressor with an injector in the outlet to provide a fuel air mix on each stroke into a chamber.

The ways to provide a proper mix at high repetition rates is not a problem.

I've no doubt that it can be done... but the fact that it hasn't yet been attempted says to me that there are many more difficulties associated with the actual construction than it seems.

I've considered attempting a high-ish RPM hybrid. The biggest challenge is finding something with enough air-flow at a precise pressure. Something like the GB semi comes to mind, although that suffers from "shoot down" ; The buffer chamber gradually depletes.

I'll do that next summer, like I planned to do it last summer, and the summer before, and...

Technician1002 wrote:One round per second is slow.
This is why combustion is considered for my design on a future bublegum machine gun.

"Considered"? There's a vast difference between "considering" something and actually making it. Comparing the performance of the theoretical bubblegum gun that only exists in your thoughts to a real, tangible cannon cannon that launches 135 gm projectiles every second is like comparing grapes to screwdrivers. Build it to launch 135gm bubblegum every second and we'll see how fast it really is.....after you build it.

"Considered"? There's a vast difference between "considering" something and actually making it

hmm I like your gun but I've got to agree with them. The reason why its ROF is limited is because of the pressure used

a 1L chamber is a 1L chamber, but a 1L chamber at 60 bar = 60L of air @ atmospheric pressure

POLAND_SPUD wrote:"Considered"? hmm I like your gun but I've got to agree with them. The reason why its ROF is limited is because of the pressure used

Poland,
You are correct....the high pressure requirement has always been the determining factor in rof.

My original question was, "Can a combustion device fuel fast enough to deliver that kind repeatable and reliable performance?"

To state it differently.... Can a combustion cannon clear the chamber, etc., and refuel to provide a shot in one second which is sufficient in size to launch a 1.250" x 134gm projectile and near Mach speed?

yeah, maybe but you're being over-sensitive... he didn't say here is my x gun that is way better than yours

Can a combustion device fuel fast enough to deliver that kind repeatable and reliable performance?

the answer is yes.
Since there is less gas that has to pass through the valve, tubing and stuff like that you can fuel a hybid faster than you fill your gun with nitrogen

Sure you've got to eject spent gasses, but I guess that others explained it quite well

repeatable and reliable performance

I know why you mentioned that. Cool, your gun is more consistent than an average pneuamatic and a hybird. Damn, it's probably more consistent than a typical firearm (firearms are technically combustion guns too )
(btw Check JSR's cartridge hybrid prototypes)

You're right. It Probably won't be able deliver that kind repeatable and reliable performance as your gun. Kudoz for building it.

But should he care about it ? It can probably be fuelled fast enough. And it's his imaginary gun not yours so I guess he's the guy who decides whether repeatable and reliable performance is important for him

Your gun uses BBs as ammo - so it's not even that consistency in terms of MV translates to accuracy. Sure still a pretty cool gun but what you're doing

is like comparing grapes to screwdrivers

velocity3x wrote:To state it differently.... Can a combustion cannon clear the chamber, etc., and refuel to provide a shot in one second which is sufficient in size to launch a 1.250" x 134gm projectile and near Mach speed?

Using cartridges as POLAND indicated would give the possibility of launching such projectiles beyond the speed of sound at more than 60 rpm. but again;

velocity3x wrote:To state it differently.... Can a combustion cannon clear the chamber

Doesn't need to. Like I said, venting is not a prerequisite for a hybrid. You're adding fresh air anyway, so it doesn't matter if there's a bit more buffer gas in there.

You can vent if you want to, but it's extra work.
That said, it wouldn't hurt to fire a blast of cold air through the chamber to help keep the system cool - which is why one of my fastest ROF designs does vent.

and refuel to provide a shot in one second which is sufficient in size to launch a 1.250" x 134gm projectile and near Mach speed?

Yes. And I dare say it'd be faster in both senses.