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maximum projectile weight for a combustion?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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maximum projectile weight for a combustion?

Unread postAuthor: JDP12 » Mon May 07, 2007 8:54 am

Hey all,

Well i want to build some PVC rockets like those on the burntlatke website and weight them, but on spudtech Joel says that you aren't supposed to shoot anything too heavy out of a combustion...So I am wondering what is the maximum weight, or ideal weight for a combustion cannon projectile??

Thanks
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Unread postAuthor: Fnord » Mon May 07, 2007 9:04 am

Well, it really depends on your chamber size, cb ratio, etc. A larger chamber would let you use heavier projectiles than one with a .8:1 ratio.
There really is no maximum weight, short of a projectile that is so large it undergoes a gravitational collapse to become a singularity.
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Unread postAuthor: JDP12 » Mon May 07, 2007 9:29 am

Chamber volume= 152.550

Barrel= 108

about a 1.6:1
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Unread postAuthor: Fnord » Mon May 07, 2007 9:46 am

well, assuming you are using a 1.5 inch barrel, evbec gives the following numbers:

1kg (2.5lbs)--91 fps. kinda crappy. you could probably throw this fast.

500g (1.25)-- 129 fps

250g ( 9 oz)-- 181 fps (getting better)

100g (3.5oz)-- 283 fps
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Unread postAuthor: jackssmirkingrevenge » Mon May 07, 2007 10:07 am

I shot a 3lb steel ball out of my 3" combustion, it barely came out of the barrel but the launcher didn't explode :D
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Unread postAuthor: boilingleadbath » Mon May 07, 2007 2:49 pm

As far as safety goes, assuming a properly built launcher, there's not really a safety max. to the projectile weight.

Obviously, there is an eventual limit to what you can launch - there does exist a situation where the projectile is so heavy that the gasses cool and suck the projectile back in before it exits - but I expect that's about as much a problem as not being able to shoot uphill because your ammo is to heavy to be lifted by the propellant gasses.

As a note, I don't expect those really low EVBEC numbers to be terribly accurate, as the barrel dwell time is really long, and thus the combustion dynamics vs. projectile position situation is very different than that of the latke tests.

Better than darts at a number chart, certainly, but I could see them being off by 30%.
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Unread postAuthor: JDP12 » Mon May 07, 2007 3:50 pm

K so pretty much..I could use a PVC rocket that is weighted like on burntlatke safely in a combustion, while getting good energy and penetration?
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Unread postAuthor: AmYisroelChai » Mon May 07, 2007 3:57 pm

i differ to those who have more experience but again as long as you are not filling them with lead, you should have any safety issues.

Rather you will have to experiement what weights work best with your Chamber:Barrel.

Stay safe and have fun.
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Unread postAuthor: jimmy101 » Tue May 08, 2007 11:02 am

I agree with Boiling, there really isn't a safety issue with a too heavy projectile. A properly glued gun built from pressure rated pipe using propane air at 1 ATM should be able to handle an infinitely large projectile mass, i.e., the round is stuck in the barrel and can't be moved. (Cellular core PVC might have a problem with a very massive projectile.)

A weighted PVC rocket, ala Latke, should work fine. For best results you need a pretty good seal between the rocket and barrel (perhaps requiring a sabot) and there should be a fair amount of static friction between the projectile (or sabot) and barrel.

If your rockets are aerodynamically stable (pretty much the whole point of using a rocket) then they will carry better than a generic spud round, so the rocket will deliver more power to the target at a greater range.


If you look at the mass / velocity estimates _Fnord posted the kinetic energy is the same for all the shells;
<table><tr><td>Mass
(kg)</td><td>vel
(fps)</td><td>vel
(m/s)</td><td>KE
(J)</td></tr><tr><td> 1</td><td> 91</td><td> 27.7</td><td> 1260</td></tr><tr><td> 0.5</td><td> 129</td><td> 39.3</td><td> 1270</td></tr><tr><td> 0.25</td><td> 181</td><td> 55.2</td><td> 1250</td></tr><tr><td> 0.1</td><td> 283</td><td> 86.3</td><td> 1220</td></tr></table>

So actually, the very heavy projectile would be best assuming all the projectiles have the same shape, hence the same amount of air drag. The slower moving but much heavier projectile will carry a lot farther than the lighter, faster moving one. The force of air drag scales roughly as the square of the velocity so there is ~9x more drag force at 280 FPS than at 90 FPS.

But, like Boiling said, the very heavy projectiles with low velocities are probably outside the range of what VBEC can reliably predict.
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