Wednesday, August 20, 2003


What really happens when you ignite a rocket motor? I'm talking about the magic inside that makes everything go whoosh and get gone real fast. Here's the best explanation of the process I've ever seen, courtesy of Peter Clay. I wish he was my chemistry teacher way back when...

Burning Nitrocellulose BP* is sorta like a party that gets WAY out of hand.

Think of the Saltpetre molecule as an unhappy family. Handsome, dashing, not too faithful Mr. K (Postassium) is stuck with homely, unresponsive Ms. N, who is kind of a loner generally but is very protective of her three lovely daughters, all named O. Actually, it was the daughters that attracted Mr. K in the first place. There are eight such families in this party, all exactly alike. Pretty dull, huh?

Think of Sulfur as eight attractive middle-aged women holding hands in a circle.

Think of Carbon as an eligible young sailor, who is not much interested in the ladies in the circle but has his eyes on the lovely daughters.

Still, nothing happens until some additional couples come into the room behaving in a romantic and suggestive manner. Then:

Each Mr. K gets excited, lets go of Ms. N and grabs an S from the circle. Each Ms. N gets disgusted, lets go of her daughters; thus 24 of them are turned loose.The twelve C-men descend upon the now-free O's like wolves, and each one ends up with an O on each arm. Each Ms. N, alas, ends up alone, but it's OK; she's used to it.

Now all the happy new couples are looking for space and some distance from the others. They push hard against everybody else, and rush for the door. If the door isn't big enough, they may just push out the walls.

8KNo3 + S8 + 12C ==> 8KS +8N + 12C02 + heat.

21 molecules that are solid at room temperature have *suddenly* become 28 molecules, of which 20 are gases at room temperature. In addition, a great deal of heat energy has been released, forcing these products to expand further. Of course the reaction is never pure, and further reactions take place after all this is exposed to the outside air while still hot.

* Nitrocellulose BP is simple black powder held together with Nitrocellulose binder. It's the kind of rocket motors you buy at hobby shops, and it's safe and reliable.