Test results and postmortem for the HYBRID Taternator II

[Dumbascii]

So I finally got a Shooting Chrony and managed not to destroy it. Covered the sensors with lexan (good thing too). I did several variations of air/oxygen/MAPP mixes at various pressures. Burst disc is two layers of heavy polyethylene that the breech closes against. Barrel is 8 feet long 1.5" aluminum sch. #40. Chrony is 6 feet from the muzzle to minimize blast damage, though it might be better to get closer where the blast radius is smaller. Overall, there was no pressure-stress related damage to chamber, barrel, manifolds or gaskets. At high pressures, it's really freaking me out how much fuel goes in, and in, and in . . .

• Test results
• 466 FPS (Unpressurized Air / MAPP)
• 637 FPS (Unpressurized Oxygen / MAPP)
• 779 FPS (10 PSI Oxygen / MAPP)
• Above 10 PSI Oxygen / MAPP unmeasurable, as the spud exits the barrel as a fine cloud of vapor. Not spud juice like you are used to seeing, but a cloud that forms a dark sticky glue-like film with no visible spud bits and no graininess.

• Several interesting things happened, leaving much work to do.
• The nylon wire ties holding the ignition wires to the fuel distribution pipe burned furiously and uncontrollably in oxygen. The continuous heat from the burning nylon damaged parts of the red silicone tubing.
• Various blasts warped the aluminum fuel distribution pipe into a slight S shape, pressing it very close to the chamber wall. Not sure yet how to make this stronger - maybe a steel rod alongside.
• The last test vaporized the spud and created a freaky gigantic fireball about 4 feet in diameter. I decided to stop after that. I suspect that I had way too much fuel, so it burned in the air as it blasted out. The vaporized spuds are a problem, since I can't test the speed potential of high pressure mixes. Might have to switch to wooden dowels or something, or maybe use some type of sabot to keep the spud together.
• Everything inside the chamber gets covered in a fine powdery white film. Ash from the MAPP gas?

[frankrede]

Wow,hmm, Not sure what the white powder is but those are some good speeds from combustion.

[King_TaTer]

thats awsome. the fine white powdery film is actually the starch residue from the potato.

[MrCrowley]

Yeah i get that white powder in my normal combustion i just get a mop on a stick and shove it in the chamber and it comes off.

[Bluetooth]

OMG!! It can vaporize potatoes. Are you sure it's not just shooting them so fast and far you don't see them?

[boilingleadbath]

This is a burst-disk gun, no?

My personal suspicion is that it's not the high pressure that mashes potatoes, but instead the collision of the disk with the rear of said tuber.

As such, a "grate" of sorts across the barrel behind the projectile might protect it - or maybe a 90* bend...
You can tell this is speculative.

[King_TaTer]

well if you know what a chrony is you would assume that it is inside and that the chrony is by the muzzle to record the maximum velocity, i guess you could do it out side as well though but not from long distances

[Dumbascii]

thats awsome. the fine white powdery film is actually the starch residue from the potato.

No - it's weirder than that and occurs where spud bits never go. It happens only when oxygen is used. Covers the fuel distributor/spark pipe uniformly from end to end. Rubs off easily. Very fine like acetylene soot, not a gritty powder like spud starch.

[Dumbascii]

OMG!! It can vaporize potatoes. Are you sure it's not just shooting them so fast and far you don't see them?

Oh yes. If there were bits flying, they'd be hitting the nearby bushes. The vapor cloud is a big cone shape extending from the muzzle, projects out about 8 feet. It leaves dark grey slime everywhere with the consistency of Elmer's glue. Like glue, forms sticky strands if you lift your fingers from a surface. Was hard to wipe off the lexan over the sensors.

[Dumbascii]

My personal suspicion is that it's not the high pressure that mashes potatoes, but instead the collision of the disk with the rear of said tuber . . .

The burst disc is in perfect contact with the spud at all times. Sorry I don't have a photo, but here's a quick photoshop.

First I cut the spud by closing the breech. Then I reopen the breech to expose the new slug. I hold the poly sheet between the barrel and the front manifold and close the breech again. The leverage forces a tight sealing fit between the OD of the barrel and the ID of the front manifold, with a full inch of sealing surface where you see the shiny machined end of the barrel. The poly sheet is right up against the slightly protruding end of the spud slug.

[Dumbascii]

well if you know what a chrony is you would assume that it is inside and that the chrony is by the muzzle to record the maximum velocity, i guess you could do it out side as well though but not from long distances

I'm not sure what you mean by inside and outside. This was definitely outdoors. The manual for the chrony warns to keep away from the muzzle at least 6 feet. Of course they aren't thinking about spud guns . . . I think the splattery part of the blast may be reduced closer to the muzzle where it hasn't had a chance to expand yet. Too bad the gun failed before I had a chance to experiment more with positioning things.

It'll be fun to use the chrony on various hand loaded rifle ammo that I make.

[boilingleadbath]

Well, I find it surprising that the mere pressure is mashing the taters... but that's the only probable thing that might be causing this that I can think of at the moment.

That said, try to remember to use the edit function... some people are annoyed when you make multiple posts in a row.

[drac]

BLB, I remember someone on spudtech saying that their Supah Valve obliterates potatoes at 100 PSI, and since this has maybe almost the same flow rate with more PSI behind it, it's very well a possibility.

[Dumbascii]

Well, I find it surprising that the mere pressure is mashing the taters... but that's the only probable thing that might be causing this that I can think of at the moment.

BLB, I remember someone on spudtech saying that their Supah Valve obliterates potatoes at 100 PSI, and since this has maybe almost the same flow rate with more PSI behind it, it's very well a possibility.

I think the MAPP is detonating rather than deflagrating, producing a supersonic shockwave, though I don't see any denting or damage at the thinly turned receiver end of the barrel. I suspect the shockwave is liquefying the spud. The liquid is then instantly boiled at 5300°F for a few microseconds under very high pressure, producing a fine, marketable, 100% organic glue (not the usual starchy spud juice). The theoretical non-shockwave pressure here may be approaching 400 PSI.

For stoichiometric oxygen/MAPP:
Adiabatic flame temp estimated at 5300°F
Combustion pressure increase is (10 bar) X (initial bar)
Initial pressure is 2.38 bar (14.5 + 20 PSIG)
10 X 2.38 = 23.8 bar

The next test will be switching down to propane/air and MAPP/air at increasing pressures. That will reduce the chance of detonation and further slow down the rate of deflagration. The oxygen/MAPP is getting too damned freaky.

PS boilingleadbath: Sorry for answering multiple questions in a row earlier in the thread

[mark.f]

It happens only when oxygen is used.

My guess is the white powder is an oxide of aluminum coating the inside of your chamber. If you're injecting oxygen while the chamber is warm, (from firing), or maybe not even used yet, the aluminum is getting that thin coating I see all the time with aluminum. If so, just let it build up and it'll form a "coating" on the inside of the chamber, to prevent further oxidation.