Saturday, October 24, 2009

Thermite ignition tests

[Disclaimer: I assume no responsibility for use or misuse of this information. It's available elsewhere on the Internet and I'm simply sharing my engineering interpretation of that information. Use these materials with great caution. Do not mix reactants until just before use. Do not mix or ignite them anywhere near flammable materials. Do not look directly at the reaction as harmful UV radiation is emitted. Be careful!]
If you watch Mythbusters then you'll likely have seen their experiments involving thermite. According to Wikipedia Hans Goldschmidt discovered and patented this thermite reaction in 1893 and used it to both cut and weld metal. These powdered mixtures require very high ignition temperatures (2000˚F) but then burn at 4,500˚F swapping oxygen, producing molten metal, and releasing a bunch 'o' heat and UV radiation. Common reactants include elemental aluminum or magnesium with metal oxides like copper (II) oxide (cupric oxide) or iron (III) oxide (ferric oxide).
Many folks in high-power rocketry are successfully using thermite as a source of instant ignition of even the largest rocket motors using a guideline of roughly 1 g thermite per 1000 N•s of total impulse. I'm intrigued both by the reaction itself and its utility in this hobby so I decided to give it a try. I ordered 1 pound each of aluminum powder, cupric oxide, and ferric oxide online. I'm quite certain I overpaid as well so you should definitely compare prices on the Internet(s). :) I was finally able to use my chemical engineering degree and calculated the appropriate mass ratio of the reactants using stoichiometry and atomic and molecular weights. For the aluminum + ferric oxide reaction it's:

Fe2O3 + 2Al → 2Fe + Al2O3 + heat + UV

2 moles of Al = 26.98 g/mol * 2 moles = 53.96 g
1 mole of Fe2O3 = 159.69 g/mol * 1 mole = 159.69 g

So the mass/weight ratio is almost exactly 1:3 Al:Fe2O3

For the aluminum + cupric oxide reaction it's:

3CuO + 2Al → 3Cu + Al2O3 + Heat + UV

CuO: 79.545 g/mol * 3 mol = 238.635 g
Al: 26.981g/mol * 2 mol = 53.962

Mass/weight ratio: 4.4:1 CuO:Al

I set up my video camera intending to share the clips here but the ignitions were so fast that they only show up on about 4 frames at 30 frames per second. As such I've simply grabbed the key frames and assembled sequences for both tests. This camera method also eliminated the need to directly observe the reaction although the emitted UV might have improved my tan a bit. Here a single electric match ignites 0.5 grams of a molar ratio of ferric oxide + aluminum powder thermite:
Ignition was fairly rapid but continued to sputter for about 30 seconds afterward. In this second experiment I again used a single electric match to ignite 0.5 grams of a molar ratio of cupric oxide + aluminum powder thermite:
This second combination appeared to be far more energetic and burned to completion without sputtering. Most folks have been using this cupric oxide+Al combination but I wanted to try the classic ferric oxide+Al combination as well. It's clear that cupric oxide works way better so I now have my empirical answer. Again... exercise caution!!

Monday, October 19, 2009

CD3 ejection ground test for M750 flight

It worked perfectly with only 12g of CO2!

CF minimum diameter build progress

So I've made some progress on the Viciously Mean Machine and unnamed 38mm spike below. I've applied Cotronics 4525 high-temperature epoxy fillets to all fin/body interfaces and laid up tip-to-tip carbon fiber reinforcement on all fin pairs with Aeropoxy. I've also post-cured both tubes in the oven for 1.5 hrs and, let me tell ya, those fins aren't budging even 1mm!!
This week I plan to build/reinforce the payload sections and couplers and start the finishing process. Both should be lookin' purdy by PB.

Thursday, October 15, 2009

Thunder & Lightning mark II pix

Thanks to Jeremy Rich for these shots of the I357/I211 flight covered below!
A reminder that... yes I did get all parts back intact. I don't buy into miracles yet I'll admit that this vessel should not be sitting in my garage ready to fly again immediately!

Saturday, October 3, 2009

Even more carbon fiber success

I was finally able to lay up two new 38 and 54mm minimum diameter designs with 2 wraps of 6 oz carbon fiber cloth and cure them in 1 hour with a nearly perfect finish. The key seems to be plugging in the curing oven right when you place the part and then unplugging at exactly 1 hr. I then let both tubes cool down in the oven for about 1/2 hr then pulled the Mylar™ film and release fabric off while they were still warm to the touch. Here are the finished tubes and I'm tacking the fins on with JB Weld Quick:
I have one batch of high-temp black epoxy left and I want to use it on both designs so all fins need to be tacked down and ready. Knowing that Derek Zoolander stands 5'6-1/2" tall I've built this comparison to my new designs for scale:
I really shouldn't even be talking about these designs because they're really-really, really-really, ridiculously good looking.