Combustion Theory

Post questions and info about combustion (flammable vapor) powered launchers here. This includes discussion about fuels, ratios, ignition systems and anything else relevant to launchers powered by igniting things like hairspray or propane.

Postby Relax.SA » Thu Dec 14, 2006 4:53 pm


I would like to know why the gas in the chamber actually explodes, and why it does not simply burn slowly. Does it have anything to do with the fact that the chamber is, in effect, sealed up?

Also, what is the maximum pressure likely to be inside the chamber, using a fairly inefficient fuel? I can do all the calculations, but I need to know by how much the fuel (gas) expands. Can anyone help?
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Postby Freefall » Thu Dec 14, 2006 5:08 pm

It does simply burn slowly (also called deflagration), with slow being a relative term. If you could watch the combustion in slow motion, you'd see the flame moving through the gas.

If the chamber were allowed to sit undisturbed until most of the turbulence had settled, a single spark would cause the gas to burn outward from the spark, with a smooth spherical flame front.

Typically, there is some turbulence, which causes the flame front to burn more unevenly and much more rapidly.

In some cases, the precompression caused by the burned gas can cause the unburned gas to self-ignite, or to ignite from radiant heat. When this happens, the flame front suddenly jumps in velocity, up to the speed of sound. This is called deflagration-to-detonation transition, often abbreviated as DDT. This results in local chamber pressures which can greatly exceed the normal closed-chamber steady-state combustion pressure.

That's probably more of an answer than you wanted, but I tried to keep it simple. Reality exhibits many factors which I can't go into detail about without writing a small textbook. But the short answer to your question is that the gas doesn't explode, it burns. It just does so quickly.

In reply to OP's edit:
For propane, the hot combustion products are about 8 times the volume of the reactants, resulting in a theoretical closed-chamber pressure of slightly more than 100 psig, assuming no detonation. (Detonation in a spudgun is unlikely but not impossible).
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Postby spuzi14 » Thu Dec 14, 2006 8:52 pm

Wasn't there just a major discussion on this? I don't remember what the conclusion was but I remember talking about this. It possibly was about creating flame front detonation in a hybrid (that would be insane power!).
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