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In a rocket engine - specifically the SOLID FUEL ones, the "combustion chamber" burns the propellant, creates hot exhaust gases, which pass through the diverging nozzle and create thrust. In a liquid fuel engine, the fuel cannot catch fire in the tank even if combustion flames reach the tank, since there is no oxygen. However, in case of a solid fuel rocket, both are present simultaneously. What mechanism does the engine have, to prevent fire in the combustion chamber to travel back to the fuel-oxidizer mixture and cause an explosion? How do we control the combustion so that the propellant burn slowly, in a controlled fashion, and not explode like that in a cracker?

Niranjan
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  • Is your question about solid motors or liquid engines? If liquid, see https://space.stackexchange.com/q/17961/6944 and https://space.stackexchange.com/q/38500/6944 – Organic Marble Oct 17 '22 at 04:37
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    The question seems to indicate you think the propellant in a solid rocket is located somewhere other than the combustion chamber. The "fire in the combustion chamber" doesn't "travel back to" the fuel-oxidizer mixture because the fuel-oxidizer mixture is already in the combustion chamber. – Christopher James Huff Oct 17 '22 at 14:28
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    Have no idea what you mean by fire travelling back to fuel-oxidizer mixture in a solid rocket. In this rocket, the fuel and oxidizer are mixed with a rubber type binder of different types and materials into a semiliquid mixture that is poured/cast into a cylinder to which a closure with a nozzle is attached The fuel and oxidizer are uniformly mixed into the mix with the binder. The mixture is most often poured around a mandrel in the casing to generate a particular shape. The shape of the cast o/f mixture determines the thrust/time curve from simple geometry. – tckosvic Oct 17 '22 at 17:03
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    A little further amplification relative to the question. Upon ignition the burning gases are initially started by the ignitor at the headend, escape through the nozzle and set up a flow field in this direction. Thus the net flow is towards the nozzle exit. This prevents "upstream" flame propogation, i.e., the net flow is downstream to the nozzle, and mostly combustion products which reduce flame propagation speeds are available in the upstream direction. – tckosvic Oct 17 '22 at 17:25
  • @Christopher James Huff: YES. I think the propellant mix in case of solid fuel, is located just above the combustion chamber. Thats what a few of the illustrations have shown. That is certainly the case for liquid fuels, but that is not the point. The basic question is how do we prevent burning of most of the propellant stock (if not all) in an instant / very quickly, as if in an explosion. In other words how do we control the rate of combustion. Different patterns of packing is one way, but has limitations. Only a controlled burning would produce the kind of continuous thrust we look for. – Niranjan Oct 18 '22 at 16:29
  • @tckosvic: As I understand, solid or semi solid fuels do not flow the way liquids flow towards the nozzle. Let me clarify my question in different words. How do we prevent burning of most of the propellant stock (if not all) in an instant / very quickly, as if in an explosion. In other words how do we control the rate of combustion. Different patterns of packing is one way, but has limitations. Only a controlled burning would produce the kind of continuous thrust we look for. Hope I have been able to clarify myself. Thanks. – Niranjan Oct 18 '22 at 16:36
  • @Niranjan I have never seen such an illustration. The propellant in a solid rocket is absolutely not "above" the combustion chamber. The "combustion chamber" of a solid rocket is typically within the propellant grain, with the grain burning from inside out. The motor casing is a pressure vessel that keeps the motor from blowing apart under the pressure produced, and at the end of the burn is the only thing you could consider a "combustion chamber". – Christopher James Huff Oct 18 '22 at 18:36
  • And nothing burns instantaneously, even high explosives detonate at a finite speed, though the detonation velocity is greater than the speed of sound. Chemical reactions take time, and propellants that are suitable for use in solid rockets burn relatively slowly...slowly enough that the propellant geometry is designed specifically to increase surface area to get a faster burn. If that's what you're asking about, this does indeed seem to be a duplicate of the other question mentioned. – Christopher James Huff Oct 18 '22 at 18:43

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