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Chamber and welding questions

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Chamber and welding questions

Unread postAuthor: rp181 » Wed Aug 06, 2008 8:58 pm

For a ETG i am making, i plan on making the chamber out of 1 1/4in steel bar stock, borerd out to 3/4in. In side that, i was going to weld a 3/4in steel rod about a inch in, with a threaded hole in that for the barrel. My questions where:
1) how much constant and peak pressure can it hold? walls are 1/4in and back is 1/2in.
2)Is it possiable to weld 2 different alloys of steel? the 3/4in bar stock is Machinable 1117 Low-Carbon Steel, and the 1 1/4in is Easy-to-Machine 1215 Carbon Steel.
3) i might also use it as a hybrid chamber, how many x's? (how do i calculate peak pressure of certain x with propane and air?)
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Unread postAuthor: DYI » Wed Aug 06, 2008 9:24 pm

I think the question you have to ask yourself is this: Do I really need to go to all that trouble to make a chamber?

If you have a cap bank like Larda's, the answer is probably yes. If you have a bank that the average hobbyist can even come close to affording, the answer is no. SCH 80 steel should be more than sufficient unless you're working in the tens of kilojoules range.

To answer your question: the chamber, even if it was made of a very low grade steel, would burst at over 25000psi at room temperature under slowly increasing pressure.
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Unread postAuthor: rp181 » Wed Aug 06, 2008 9:49 pm

the bank is only about 2KJ ( i almost got a sponsorship for a large bank, i might get it next year), but it actually comes out to be cheaper machining it (and stronger). I also want to try electrothermal-chemical, but thats against forum rules =).
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Unread postAuthor: DYI » Wed Aug 06, 2008 10:07 pm

ETC is what I'm starting on right now, hopefully with a 4kJ or so bank. Finding a fuel for it isn't that easy these days though. I wonder if you could talk about ETCs in the NSGRD forum?

Anyhow, your chamber will be too weak if you're using it as an ETC, unless you have some pathetic fuel like black powder or something...
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Unread postAuthor: rp181 » Wed Aug 06, 2008 10:36 pm

black powder was exactly what i was thinking =) What else could you use? only thing i know that is resonable is rcandy.
For a decent ETC, what pressures should i be expecting and how think walls?

Im doing this with a 3 stage coil gun (~600J ea stage (each SCR can take ~2KJ), same caps make up ETC bank.
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Unread postAuthor: SpudFarm » Thu Aug 07, 2008 12:22 am

you really can't ask about the rated pressure of a weld, a welding seam can be so much
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Unread postAuthor: microman171 » Thu Aug 07, 2008 12:31 am

I think larda used aluminium powder in his ETC. The aluminium turns from a solid to a plasma when the cap bank he has.
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Unread postAuthor: SpudFarm » Thu Aug 07, 2008 12:36 am

that is a ETG not a ETC
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Unread postAuthor: rp181 » Thu Aug 07, 2008 12:18 pm

ETC is basically a ETG, but with a solid fuel.
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Unread postAuthor: Necrosis » Thu Aug 07, 2008 12:25 pm

What if you used... liquid fuel? like alcohol or ether.. mixed with the aluminium.. Or I might be missing something here... I bet I am.
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Unread postAuthor: rp181 » Thu Aug 07, 2008 1:01 pm

yes, that would work too. I was researching fuels and came across this:
cyclotrimethylenetrinitramine (RDX). Here is that patent for it:
"The present invention provides a propellant for electrothermal-chemical guns comprising a dispersion of one or more energetic solids in an energetic liquid phase. The energetic solid is preferably a nitramine such as cyclotrimethylenetrinitramine (RDX) and the energetic liquid component is preferably a homogeneous liquid that is either aqueous or non-aqueous. Aqueous liquid phases suitable for use in the present invention include concentrated solutions containing at least one nitrate salt. Non-aqueous liquid phases suitable for use include those that contain nitrate ester, nitramine, nitro or azido compounds or mixtures thereof.
These propellants provide a high level of energy density because of the use of energetic ingredients and the high loading density allowed with a liquid propellant. The presence of a dispersed solid phase within the continuous liquid phase also permits control of the burning behavior of the propellant by variation of the interfacial area between phases.
"

Of course, this are very dangerous, and the only information i could find on it was at rougesci.

If i were o do this, i would probably start with black powder or potassim nitrate and sugar mix (have that now, was using it to make a rocket for a language arts project).


This is quickly going off to a tangent and going against the rules, so stick with the original questions.
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Unread postAuthor: DYI » Thu Aug 07, 2008 8:37 pm

yes, that would work too. I was researching fuels and came across this:
cyclotrimethylenetrinitramine (RDX). Here is that patent for it:


I have to disagree; it isn't dangerous until you try to set it off :P

I was thinking more like nitrocellulose/NG mix or flash powder. KNO<sub>3</sub>/Sugar won't burn very quickly, so it'll be useless in this case.

If you're sticking to BP, you'll be fine with the chamber you're planning right now. I know from experience.
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Unread postAuthor: rp181 » Thu Aug 07, 2008 8:54 pm

if i were to use flash powder, how thick walls should i use?
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Unread postAuthor: SpudFarm » Thu Aug 07, 2008 8:57 pm

why do you ask that question? just go for the heavyest chamber you can get your hands on..

if it is worth doing it IS worth over doing!
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Unread postAuthor: DYI » Thu Aug 07, 2008 9:44 pm

If you use the best affordable alloys, about 120KSI, with 0.75" ID, you'd want to bore it out of 3" or larger bar stock for a reasonable safety factor. Flash powder burns *REALLY* fast, and it will reach very high pressure very quickly. Apparently about 150 kpsi combustion pressure for Al/KClO<sub>4</sub>. I hope that gives you a good idea of just how dangerous this stuff is. I'd say build it smaller than 3/4". You'll have more power than you know what to do with even at 1/2" bore.
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