SpudFiles

Hmm...Looks like it did alright

http://www.sentex.net/~mwandel/cannon/my_cannon.html

I hope you saw the "failure in cannon barrel"
The chamber can fatigue over time, and when ABS fails, it does that. If It was identical, but PVC, it would probably explode.

Wow, that old video surfaced. It was one of the reasons I decided to build the ABS test cannon AKA the Apple Cannon. I had good results except no failure so far as a Pneumatic. ABS has a much safer failure mode.

Link;
http://www.spudfiles.com/forums/abs-cel ... 17968.html

Moonbogg wrote:Hmm...Looks like it did alright

Well, not really. The ABS failed even though the gun probably never reaches all that great of an internal pressure. His 180m/s velocity is pretty impressive for a gun that size. So the peak pressure in the chamber is probably a fair bit higher than a generic combustion gun.

From the cite:

The ramrod isn't really necessary when shooting just a hardwood projectiles - I always make these so there is about a quarter millimeter of margin, so they slide in and out freely.

So the ammo has some blow-by and very little static friction. The peak pressure in the chamber probably isn't anywhere near the theoretical value for propane + oxygen + air, it can't be, combustion is too slow and the ammo moves to easily and there is some blowby.

I suspect that if he shoved a solid projectile into the barrel then bolted it in place then something would fail.

Overall, it is really a fairly crude canon. All that balloon stuff makes fueling more consistent than a spray-n-pray but it is probably still not all that close to a stoichiometric mixture. There is no chamber fan so the mixing is suspect. There is blowby around the ammo so if you wait a while the mixture will drift. etc.

jimmy101 wrote:

Moonbogg wrote:Hmm...Looks like it did alright

Well, not really. The ABS failed even though the gun probably never reaches all that great of an internal pressure. His 180m/s velocity is pretty impressive for a gun that size. So the peak pressure in the chamber is probably a fair bit higher than a generic combustion gun.

Don't make assumptions of flame speed in an oxygen enriched atmosphere. It's a good way to get killed.

Supersonic shockwaves are created.

Technician1002 wrote:

jimmy101 wrote:

Moonbogg wrote:Hmm...Looks like it did alright

Well, not really. The ABS failed even though the gun probably never reaches all that great of an internal pressure. His 180m/s velocity is pretty impressive for a gun that size. So the peak pressure in the chamber is probably a fair bit higher than a generic combustion gun.

Don't make assumptions of flame speed in an oxygen enriched atmosphere. It's a good way to get killed.

Supersonic shockwaves are created.

Also, don't assume that just because the gun survived it is because ABS is such great stuff. The fuel ratio sucks, the mixing sucks, the performance is not that of a properly executed gun.

And, a supersonic shock wave is not necessarily a big deal. It depends on when and if the shock occurs. What's the run-up distance? If it takes combustion of 90% of the fuel to get to DTT then that last 10% of the energy, even if it is in a shock wave, is really no big deal. Heck the inertia of the gun's wall be enough to withstand the shock load.

Bottom line is that regardless of how fast the combustion is, it isn't reaching DDT in this gun. Nor is it ever likely to. Folks (as in folks that know what they are doing and are getting paid to do it) that have tried to get guns to DDT have a heck of a time doing it. It ain't going to happen in a gun with a ballon based meter system, incorrect fuel ratio and 1x charge pressure.

Even in an ICE predetonation isn't all that big a deal. Indeed, most modern cars will predetonate on occasion, and do it for a couple motor revolutions until the electronic controls can correct it. Again, it depends on when it happens. And, it depends on exactly what the peak pressure is.

Besides, just because the shock wave pressure is 1000 PSI doesn't mean it'll be a problem for something rated for ~200 PSI static pressure. Shockwave pressure is not the same as static pressure and materials react to shock wave much less than they do to static pressures.

jimmy101 wrote:

Technician1002 wrote:

jimmy101 wrote: Well, not really. The ABS failed even though the gun probably never reaches all that great of an internal pressure. His 180m/s velocity is pretty impressive for a gun that size. So the peak pressure in the chamber is probably a fair bit higher than a generic combustion gun.

Don't make assumptions of flame speed in an oxygen enriched atmosphere. It's a good way to get killed.

Supersonic shockwaves are created.

Also, don't assume that just because the gun survived it is because ABS is such great stuff. The fuel ratio sucks, the mixing sucks, the performance is not that of a properly executed gun.

And, a supersonic shock wave is not necessarily a big deal. It depends on when and if the shock occurs. What's the run-up distance? If it takes combustion of 90% of the fuel to get to DTT then that last 10% of the energy, even if it is in a shock wave, is really no big deal. Heck the inertia of the gun's wall be enough to withstand the shock load.

Even in an ICE predetonation isn't all that big a deal. Indeed, most modern cars will predetonate on occasion, and do it for a couple motor revolutions until the electronic controls can correct it. Again, it depends on when it happens. And, it depends on exactly what the peak pressure is.
.

Tell that to a mechanic.

http://www.theultralightplace.com/pistons.htm

Technician1002 wrote:Tell that to a mechanic.

http://www.theultralightplace.com/pistons.htm

Ya, so what? Like I said nearly all ICE's knock and ping on occasion. It isn't a big deal. It only becomes a problem when the engine does it consistently and for a long period of time. There is a huge difference in the stress on a structure when you compare "it is fired once every several minutes and it's total number of firing cycles is less than 100" and "it is fired a couple thousand times per second and it's normal number of firing cycles is in the billions". What happens in an ICE that consistently knocks has pretty much zero relevance to what happens in a gun.

Cool points here. Yeah the cannon in the video is totally not chamber sealed so its nothing like a hybrid etc. I thought that damage was caused by recoil on his cannon however. Well either way, i'd never use any plastic for a hybrid or pure o2 gun. I was just showing it because I thought it was interesting. At least it generated discussion, and thats always good when people can learn from reading a post.
As far as the pressure spike thing goes, maybe its a good idea to make sure the cannon can safely endure maximum theoretical pressures, including DDT spikes. That way the only thing DDT will do is give you more velocity

Moonbogg wrote: As far as the pressure spike thing goes, maybe its a good idea to make sure the cannon can safely endure maximum theoretical pressures, including DDT spikes. That way the only thing DDT will do is give you more velocity

I think you would be hard pressed to design something that is static rated to the transient pressure that DDT might cause. Steel pipe, welded joints etc would be required.

BTW, DDT really won't give much more power. Indeed, if the gun was designed properly DDT won't help at all. The energy released is the same regardless of deflagration versus detonation. The only difference is the time frame. Fast burning will boost performance but you have to get fast burning (or detonation) of a significant amount of the fuel for it to make a measurable difference. Even if DDT gave a pressure spike of 10,000 PSI that really won't boost the rounds velocity all that much since the spike only lasts for microseconds.

Someone will probably mention that the shock wave from a conventional high explosive does indeed transfer significant energy, therefore a shock wave from a combustable gas mixture must be similar. That really isn't the case. The energy released from a solid explosive is so much greater than the energy released from a flammable gas in air that the two sets of results really can't be compared. To compare the damage potential for a gaseous combustion reaction and a solid high explosive you need to take into account the vastly different energy densities.

I don't think it was that smart to use a balloon as a metering system, those things gradually leak out air.