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Solid fuel revisit

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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Solid fuel revisit

Unread postAuthor: inonickname » Wed May 27, 2009 3:59 am

Well, while searching the nets I eventually found myself watching videos of silly kids trying to make flour bombs. I was thinking of substitute powders, and came across aluminum. Supposedly when in extremely fine particles it is used in flash-bang grenades to create the bright light and a pressure wave of up to 10,000 psi.

Any thoughts of using other solids as propellants? A 1x mixture generating 10,000 psi and extremely high temperatures would no doubt go supersonic, and at a reasonable 20x mixture the power is unthinkable.

I'm unsure of the ignition energy required, and especially with a hybrid I wouldn't use a flame (eg, blowtorch). Because lets face it, blowtorches don't appreciate 10,000 psi+ blowback.

So asides from metering and mixing issues, are other solids such as aluminum plausible propellants? With a lot of work regarding the chamber and barrel a hybrid version could easily pass the power of any firearm, so why not?

Any thoughts? (yes, I know there have been topics similar to this, but I'm not powering my cannon with flour)

Edit: holy shizzle. A 20x air-aluminum mix, based on a 10,000 psi shockwave per mix of aluminum generates higher pressures and temperatures than a theoretical 2000x air-propane mix.

Edit2: After some looking, the number I was given was wrong. What kind of energies would we be expecting to look at from alternate fuels?
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Unread postAuthor: jackssmirkingrevenge » Wed May 27, 2009 4:09 am

Wouldn't the fine particles oxidise immediately when exposed to air, thus rendering them ineffective?
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Unread postAuthor: inonickname » Wed May 27, 2009 4:15 am

jackssmirkingrevenge wrote:Wouldn't the fine particles oxidise immediately when exposed to air, thus rendering them ineffective?


That's the true issue with the reactivity of aluminum, how rapidly it forms an oxide layer. Military flash bangs use Mg or Al and a mixture of a perchlorate oxidizer. Perhaps an easily ignitable light oil that would be ignited by a high current ignition source, allowing the aluminum particles to ignite.

It's starting to look like even more of "it's not worth it" than before..
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Unread postAuthor: jackssmirkingrevenge » Wed May 27, 2009 4:22 am

inonickname wrote:It's starting to look like even more of "it's not worth it" than before..


Not to mention "In breach of forum rules"...
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Unread postAuthor: ramses » Wed May 27, 2009 6:17 am

no, flour and non-oxidizing powders are allowed. but it isn't really worth it.
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Unread postAuthor: inonickname » Wed May 27, 2009 6:18 am

jackssmirkingrevenge wrote:
inonickname wrote:It's starting to look like even more of "it's not worth it" than before..


Not to mention "In breach of forum rules"...


Nope.

That rule specifically states that it's in breach of rules if it contains it's own oxidizer.

teh rules wrote:Discussion of solid propellants, explosive devices or incendiary projectiles fired specifically from a spud cannon is prohibited, and may result in a permanent banning of your account.For the purpose of this forum, these definitions will be used:

1. Solid propellant: Any substance containing all the chemical elements required to enable sustained combustion without the presence of additional oxidizers. This includes but is not limited to gunpowder, thermite, and many commercial and/or homemade pyrotechnic compounds.
1. For this purpose, nitroglycerine, though liquid, would qualify.
2. For this purpose, ordinary flour, though solid, would not qualify.


It's alright, somebody always mentions it. Unless we're mixing in the said oxidizers, which is a bad idea it's not naughty.

It's the kind of thing we discuss but nothing really happens about it.
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Unread postAuthor: jeepkahn » Wed May 27, 2009 10:16 am

Besides, isn't the ETG using aluminum powder, and a lot of the questions about aluminum as a proppelant and ignition requirements would prolly be covered there....
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Unread postAuthor: Ragnarok » Wed May 27, 2009 10:30 am

No, in that case, the aluminium is not the fuel. The energy in ETGs comes from the capacitor bank, not the reaction of aluminium.

On the rules note, Rule 3 is better interpreted as "No use of non-gaseous oxidisers".
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Re: Solid fuel revisit

Unread postAuthor: jimmy101 » Wed May 27, 2009 12:49 pm

inonickname wrote:Edit: holy shizzle. A 20x air-aluminum mix, based on a 10,000 psi shockwave per mix of aluminum generates higher pressures and temperatures than a theoretical 2000x air-propane mix.

Edit2: After some looking, the number I was given was wrong. What kind of energies would we be expecting to look at from alternate fuels?

The "10,000 PSI shockwave" may well be accurate. You just have to remember that shock wave pressure isn't the same as the static pressure. That very high shock pressure only lasts for a tiny fraction of a second, probably much less than one millisecond, perhaps even less than a microsecond. Shock waves really aren't much use for accelerating a projectile. That is why guns (cannons etc) don't use fuels that detonate, they always use fuels that deflagrate.
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Unread postAuthor: ramses » Wed May 27, 2009 3:28 pm

peak pressure is what the pipe needs to be rated to, average pressure determines force, thus power. (energy for people who don't like the word power) and they are CORRECT
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Unread postAuthor: psycix » Wed May 27, 2009 4:31 pm

Solid fuel is only allowed when you are doing a "dust-explosion".

Wouldn't the fine particles oxidise immediately when exposed to air, thus rendering them ineffective?

Thats why people tend to throw a teaspoon of graphite into the ballmill before grinding the aluminium down to a high mesh.


But it would be very hard to have the right amount of aluminum in your chamber, especially atomized. It will drop down to the floor within seconds, and may go "thermite" instead.
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Unread postAuthor: Brian the brain » Wed May 27, 2009 4:35 pm

I imagine the first hybrid was a bit tricky to get right as well..
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Unread postAuthor: jimmy101 » Thu May 28, 2009 12:45 pm

ramses wrote:peak pressure is what the pipe needs to be rated to, average pressure determines force, thus power. (energy for people who don't like the word power) and they are CORRECT

You get into some semantic difficulties with shock waves. The "peak" pressure is actually the shock wave pressure. But you don't have to have material that can handle that pressure. As the very short durations of the "peak" pressure the momentum of the pipe contributes significantly to what "peak" pressure it can withstand. So, things get more complex with shock waves, detonations etc. A pipe rated to 200 PSIG static pressure will generally handle much higher shock wave "peak" pressures.

For spudgunners, it is usually pretty difficult to get a fuel mixture to actually detonate so the peak pressure is rarely due to a shock wave.

I would also add the the pipe needs to be rated to the worst case peak pressure, not the typical peak pressure. For a generic combustion spudgun the typical operating peak pressure is in the vicinity of 60 PSIG or so. The gun should be designed for the worst case scenario. The worst case scenario is that the round jams in the barrel and the gun is now a closed chamber. Closed chamber peak pressure for propane + air at 1X is ~120 PSIG. So, the gun should be designed to at least 120 PSIG.

Of course, combustion spudguns are rarely designed that way. The pipe and most of the fittings are generally good to quite a bit higher pressures than 120 PSIG. It's the cleanout plug that is the problem. Cleanouts are generally DWV parts and aren't rated for use in pressurized water systems.
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Unread postAuthor: jimmy101 » Thu May 28, 2009 12:55 pm

psycix wrote:But it would be very hard to have the right amount of aluminum in your chamber, especially atomized. It will drop down to the floor within seconds, and may go "thermite" instead.

Can't go "thermite" without a metal oxide present. "Thermite" is typically aluminium metal plus the oxide of another metal. The oxide is the oxidizer and the reaction is a "trans-oxdiation" or oxidation exchange. For example aluminium plus iron rust;
Fe2O3 + Al = Al2O3 + Fe + heat
The reaction is not all that energetic compared to using oxygen as the oxidizer;
4Al + 3O2 = 2Al2O3 + lots of heat
releases a heck of a lot more energy than does a thermite type reaction.

The useful characteristic of thermite isn't really related to how energetic it is. It is related to the fact that none of the reactants or products are gases. Thermite reactions don't tend to blow themselves apart and all the heat is released in one concentrated volume. That's why thermite is used to do things like melt the breach of a gun, or cut metal, applications where a typical explosive wastes to much energy in it's expanding ball of hot gases.
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Unread postAuthor: Technician1002 » Thu May 28, 2009 2:23 pm

jimmy101 wrote:
psycix wrote:But it would be very hard to have the right amount of aluminum in your chamber, especially atomized. It will drop down to the floor within seconds, and may go "thermite" instead.

Can't go "thermite" without a metal oxide present. "Thermite" is typically aluminium metal plus the oxide of another metal. The oxide is the oxidizer and the reaction is a "trans-oxdiation" or oxidation exchange. For example aluminium plus iron rust;
Fe2O3 + Al = Al2O3 + Fe + heat
The reaction is not all that energetic compared to using oxygen as the oxidizer;
4Al + 3O2 = 2Al2O3 + lots of heat
releases a heck of a lot more energy than does a thermite type reaction.

The useful characteristic of thermite isn't really related to how energetic it is. It is related to the fact that none of the reactants or products are gases. Thermite reactions don't tend to blow themselves apart and all the heat is released in one concentrated volume. That's why thermite is used to do things like melt the breach of a gun, or cut metal, applications where a typical explosive wastes to much energy in it's expanding ball of hot gases.


It's also used by the railroad to weld rails.. I got to watch one being done. :D This is not my video, but this is how it is done.
[youtube]http://www.youtube.com/watch?v=nR6K90cR8Lg[/youtube]

Thermite would be useless for spudding unless you are trying to destroy evidence.
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