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design of a ETC chamber

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rp181

Fri Apr 10 2009, 03:04AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

For next year, we are supposed to complete a personal project, for IB certification. I thought that a ETC would be a interesting project, so I am in planning stages. I am currently trying to decide what kind of forces I am dealing with. My question is on how to determine the approximate pressure that a chemical explosive creates when used. Here is the information I have so far:
The current chamber design is 1.875" C1045 steel, ~6" long.
These dimensions give a yield strength (keet in mind the yield is the point where it changes from elastic deformation to plastic deformation) of 77,000PSI. Once I use barlow's formula:
(2*77000*1.5)/1.875
I get a result of 123,000PSI in the tube to cause permanent damage. This assumes a 3/8" diameter reaction chamber.

As for propellants, I plan to start with a basic flash powder mixture (Mg and perchlorate), but I plan (if design allows) to move to acetone peroxide.

I am not quite sure what the rules area about chemical explosives, So let me know.

Thanks in advance.

DYI

Fri Apr 10 2009, 03:36AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Having done some basic calculations, I can tell you that even the most energetic high explosives cannot build chamber pressures of over 500,000psi, just by basic stoichiometry and gas law application. You've probably already done that though.

In my ETC tests, TCAP created a maximum chamber pressure of around 2kBar (30kpsi). With high-energy electric ignition and no hole in the back of the chamber, you can expect to see much more than that. Apart from that, the shock load on the chamber is not to be discounted. If this is going to be a full-fledged test device, I would suggest trying to come up with a chamber that will resist erosion, and the formation of micro-fractures that will eventually turn your chamber to powder. 1045 is not a particularly impressive grade of steel, and 1.875" OD is cutting it too close. I would use 2.5" if I were you, and an inner sleeve which can be easily replaced if necessary, which I'm almost certain will be the case eventually. High-nickel alloys for the interior sleeves and tunsten or nickel for the electrodes would be another thing to consider. Corrosion has plagued me greatly in my recent research, and I don't even have chemicals to worry about.

On the topic of propellants, TCAP isn't the greatest one to use. There are a few main points against it, considering that you will have enough pulsed power to initiate better propellants. 1: Instability. I think we all know the reasoning behind this one.
2: Difficult to work with (i.e. press, shape, cast, etc...). This stems from reason one.
3. Low temperature: TCAP detonations are not hugely exothermic, which means your propellant gases will have lower particle speeds. This could be a problem in long-barreled designs.
4. Heavy gases. There is some dispute over what the precise detonation products are, but, having smelled them for myself, I can tell you that at least some of them are most definitely not small, pleasant molecules like nitrogen, CO2, and water.
5. The whole "k3wl b0l\/lB3r" factor that TCAP carries with it. I know from bitter experience that this is much more than just a personal pride thing.

Personally, I would suggest ETN as a final propellant choice if you're going for the "massive carnage" end of the scale as you seem to be. It is stable, readily synthesized (with some practice, patience, and a really good respirator), easily initiated by an electric arc, and has decent gas production. PETN would be slightly superior in gas production and likely easier on the steel, but the pentaerythritol synthesis takes a bit more chemistry skills to accomplish, and more difficult to acquire reactants are needed. If you can avoid AP altogether, you will be several steps ahead.

Flash powder will be fine for some preliminary testing, and is easily contained by anything superior to regular steel pipe (and if you screw up like I did my first batch, even regular steel pipe would be sufficient

rp181

Fri Apr 10 2009, 03:39AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

That is true, but right now I am going of off cost. The material to build this is only 20$ with shipping.

EDIT: I forgot to mention that the input will be 5.6kJ.

Wolfram

Fri Apr 10 2009, 08:56AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I second the point against TCAP. Everyone who has played with it can testify to it's great unpredictability. It might usually take a moderate hammer blow to set it off, but once in a while it can go off from just looking at it wrong. This might sound like an exaggeration, but I've been on several pyro forums where people have lost fingers to it.

Anders M.

rp181

Fri Apr 10 2009, 11:50PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I am actually not after power. My main interest with these types of things is efeciancy and looking at other factors, such as pressure,temperature,carbonazation, and other factors, suck as input pulse.
I am aware of the danger from TCAP. The mail reason i picked that was the ease of making it.

I know for a fact that my school has the chemicals and materials to produce better,safer (if you can really say that...) explosives( I had unpacked and took inventory of everything), but my teacher came from teaching criminals (as far as murder), so I highly doubt that happening, I will ask him anyway.

As stated earlier, I am not after power, so I will stick with the chamber dimensions posted above, but I will use a .5" bore, with a replacable inner tube, as well as just flash powder.

How do you guys suggest "packing" and igniting the fuel? Flash powder may be conductive enough to just stick some electrodes in, or I may use a exploding wire. I was reading some articles on the carbonazation of fuel, so I will try putting a inductor in series to lower current.

Sorry for the horriable spelling, I am on a library computer, and it keeps changing things...

Chris Russell

Sat Apr 11 2009, 05:38PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Sorry, but discussion of explosives manufacture is most definitely not permitted. Try creating this topic again, if you like, but leave the manufacture of explosives out of it.

rp181 wrote ...

I am aware of the danger from TCAP. The mail reason i picked that was the ease of making it.

Trading your personal safety (and possibly the safety of others) for convenience is extremely foolish. If you were truly aware of the danger, you would realize that manufacturing it in any significant amount is far too dangerous, unless you have access to proper equipment, like a blast shield.

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