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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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Unread postAuthor: jimmy101 » Fri Jul 04, 2008 10:42 am

psycix wrote:
jimmy101 wrote:Combustion guns have other characteristics that make reaching the speed of sound a challenge.

Like?
The only one I can imagine is the lack of high pressures, which can be overcome by hybrids.
The short time the pressure lasts doesn't seem much of an issue to me, unless you want to use a really long barrel.

I believe that the rate of heat loss becomes a significant challenge in a supersonic combustion gun. Gases moving through the barrel at high velocity will be turbulent and will be loosing heat to the barrel very quickly. The faster the projectile is moving tha faster heat is being lost.

A hybrid has a shot at supersonic. A burst disk 1X gun might have a shot at it as well. Maybe a pure oxygen gun also. A standard propane+air gun probably can't do it.

I know there have been claims of supersonic shots from basic combustion guns using propane+air and of "typical" sizes firing "typical" combust gun ammo (1.5" to 2" diameter) but I don't believe'm.

Besides, "combustion spudgun" really means a 1x propane+air (or butane, or Axe, or ...) gun without a burst disk. A hybrid, or burst disk, or pure O2 gun, though technically a combustion gun, really shouldn't be called a combustion spudgun. It should be called whatever it is (hybrid, burst disk etc.). An M16 is a combustion gun. Stick a hunk of spud in the barrel and it is a "combustion spudgun". Obviously, that is not the correct designation for the particular launcher. When someone says "combustion spudgun" you shouldn't come back with "well ya but a hybrid/burst disk/O2/M16 will do ..."
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Unread postAuthor: psycix » Fri Jul 04, 2008 12:59 pm

jimmy101 wrote:I know there have been claims of supersonic shots from basic combustion guns using propane+air and of "typical" sizes firing "typical" combust gun ammo (1.5" to 2" diameter) but I don't believe'm.
Me neither. I bet that guy who said to break the SB in his backyard (what was his name)with that combustion gun probably couldn't get along with his chrony.

You could prove that to model his gun in GGDT using a burst disk launcher with the same size of his, and set the air temp to a certain point (propane+air ideal burn temperature). I bet this will not get near the SB, and then realize that you used the maximum burn pressure, instant burn speed, and no heat loss are involved.
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Unread postAuthor: sandman » Fri Jul 04, 2008 2:40 pm

soren(i forget his forum name, but it starts with a D) broke the SB with his combustion
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Unread postAuthor: DYI » Fri Jul 04, 2008 6:10 pm

Soren (Dongfang) used low level oxygen enrichment to break the SOS with his combustion gun. It doesn't quite fall under the same category as a propane/air gun.

And D_Hall, care to divulge any more information about the LGG you worked with?
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Unread postAuthor: D_Hall » Sat Jul 05, 2008 12:55 am

DYI wrote:And D_Hall, care to divulge any more information about the LGG you worked with?

Not too much to say; it wasn't my design or anything. It belonged to a guy who owned a machine shop in Dayton. We contracted with him to do some impact testing (don't bother asking). We were there for a couple weeks. The first day we were observers, by the end of the two weeks we were doing most of the grunt work to operate the gun (with our help, he could do 4 shots a day as opposed to the 1-2 without our help).

But the relevant part was that his powder charge was completely contained within a 50 BMG casing. Very small by LGG standards, but also very cheap to operate.
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Unread postAuthor: psycix » Sat Jul 05, 2008 8:29 am

4 shots a day!?
Reloading such a thing has to be a whole operation then.
Why don't you design a semi-auto one? :D
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Unread postAuthor: SEAKING9006 » Sat Jul 05, 2008 2:50 pm

Well, that would probably require astronomically expensive heat and shock resistant materials, because of the pressures and mechanical stress and shock the whole thing undergoes when firing. I've never seen one in person, but I can already tell from what I know that those things have a cool down time that is next to impossible to reduce, thanks to the necessary materials either not-existing, or are highly classified and we don't know about them. That being said, the combustion/pressure vessels on those things are huge, about the size of our standard air tanks on a large inline. I don't know HOW you could possibly semi auto something that huge. Although, there was a project about two years ago on spudtech to develop a semi auto combustion, and a few of the designs had individual cartridges that were chamber and everything else, minus the barrel, but the chamber size was still only just over half the size of a standard spray-and-pray. There are a crapload of challenges to overcome before we get a semi-auto LGG, and I think we should first get a working semi auto combustion or hybrid before we even try to build a semi LGG.
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Unread postAuthor: D_Hall » Sun Jul 06, 2008 12:12 pm

psycix wrote:4 shots a day!?
Reloading such a thing has to be a whole operation then.


Yes, the gun has to be disassembled between shots.

And 4 shots a day is *phenomenal* with a LGG.

Suffice to say that there's one gun I've seen that gets 1 shot off every 2 weeks.
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Unread postAuthor: DYI » Sun Jul 06, 2008 2:46 pm

To reload an LGG, you'd have to open it up, reload the projectile, replace the burst disk, reload the powder charge and igniter, then reset the piston, put it all back together, purge the air out of the chamber, and whatever other steps are in there.

Then retreat to a safe distance, press the red button, and repeat the whole process.
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Unread postAuthor: SEAKING9006 » Sun Jul 06, 2008 4:31 pm

The only way to get a semi-auto LGG so far is to build a revolver. And even that's a stretch. And then you have to reload SIX of them, rather than one.
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Unread postAuthor: D_Hall » Mon Jul 07, 2008 7:55 pm

DYI wrote:To reload an LGG, you'd have to open it up, reload the projectile, replace the burst disk, reload the powder charge and igniter, then reset the piston, put it all back together, purge the air out of the chamber, and whatever other steps are in there.

Then retreat to a safe distance, press the red button, and repeat the whole process.


Uhhh... No.

First you have to disassemble the breech end and machine out the piston. You don't reset the piston. You *replace* the piston. Why? Because when a plastic slug hits the end of the pump chamber at (say) Mach 2, it extrudes itself into the barrel. You have to get it out somehow..... So you disassemble the gun. Usually you end up putting the breech on a lathe and *machining* the piston's remains out (using brass tools so you don't cut the barrel).

The rest is easy. But that whole "getting the piston out" thing is a BEOTCH!
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Last edited by D_Hall on Mon Jul 07, 2008 8:55 pm, edited 2 times in total.
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Unread postAuthor: D_Hall » Mon Jul 07, 2008 8:00 pm

SEAKING9006 wrote:Well, that would probably require astronomically expensive heat and shock resistant materials,

Nothing more exotic than carbon steel is required.

I've never seen one in person, but I can already tell from what I know that those things have a cool down time that is next to impossible to reduce,

I've seen about half a dozen of them and personally used two. And there is no real cool down time any more than there's a cool down time on spud guns. They're physically massive and as a result the heat flux from that one little shot is No Big Deal.

thanks to the necessary materials either not-existing, or are highly classified and we don't know about them.

Who knew that carbon steel was classified? Wow. I learn something new every day.

Stick to spud guns, 'cause you know nothing of LGGs.
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Unread postAuthor: DYI » Mon Jul 07, 2008 9:21 pm

First you have to disassemble the breech end and machine out the piston. You don't reset the piston. You *replace* the piston. Why? Because when a plastic slug hits the end of the pump chamber at (say) Mach 2, it extrudes itself into the barrel. You have to get it out somehow..... So you disassemble the gun. Usually you end up putting the breech on a lathe and *machining* the piston's remains out (using brass tools so you don't cut the barrel).

The rest is easy. But that whole "getting the piston out" thing is a BEOTCH!


Ah... I'd always wondered about what happened there. Because absorbing that much shock didn't seem too feasible to me. Now I know where those astronomical reload times come from... :roll:

If the piston is plastic, couldn't the bulk of its remains be removed from the chamber and barrel by heat, and the leftovers blown out by a stream of heated liquid? There must be some reason that wouldn't work, or they'd be doing it already...
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Unread postAuthor: D_Hall » Mon Jul 07, 2008 10:55 pm

DYI wrote:If the piston is plastic, couldn't the bulk of its remains be removed from the chamber and barrel by heat, and the leftovers blown out by a stream of heated liquid? There must be some reason that wouldn't work, or they'd be doing it already...

Granted, this is hardly soemthing I've spent much time thinking about (my use of LGGs is as a customer who was willing to get his hands dirty and help; not as an owner) but...

Two thoughts...

1) Construction is MASSIVE. Like, a gun with a 6" diameter pump chamber may have an OD of 30". That's a LOT of steel that's just going to be sucking away any heat you input.

2) Depending on how hot you're talking about, and if you hit it hard enough, you may interfere with any surface hardening and such that you've spent lots of money to install into those gun surfaces.

Are these real problems? I have no idea. They're just two possibilities that pop into my mind.
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Unread postAuthor: SpudMonster » Tue Jul 08, 2008 3:11 am

D_Hall wrote:Like, a gun with a 6" diameter pump chamber may have an OD of 30".


Foot thick walls...
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