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I was thinking of designing a very simple and small Paraffin rocket engine as an experiment. One thing that I don't understand about these designs is how you efficiently use your oxidizer without a large amount being shot through the rocket unburned. With a solid fuel like this, how do you efficiently mix it to ensure it burns in the right place where it can add thrust?

EDIT: I liked the answer down below, but found this great public chapter about hybrid rockets that had some good illustrations: https://web.stanford.edu/~cantwell/AA283_Course_Material/AA283_Course_Notes/AA283_Aircraft_and_Rocket_Propulsion_Ch_11_BJ_Cantwell.pdf

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  • $\begingroup$ After reading that paper I'm not at all sure my answer is correct. ¯\_(ツ)_/¯ $\endgroup$ – Russell Borogove Nov 22 '18 at 5:57
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    $\begingroup$ Paraffin is a mixture of different alkanes with the general formula CnH2n+2. There are soft and hard paraffins with different melting points in the range of 50 to 60 °C, some even to 70 °C. May be a special paraffin mixture with high melting point is used to balance the combustion between too fuel rich and too oxygen rich. But the length of the hybrid rocket may have an influence too. $\endgroup$ – Uwe Nov 22 '18 at 16:06
  • $\begingroup$ @Uwe - That is very useful to know. Was hoping NASA would publish a full paper on their design, but weapons laws would probably make them drop many of the details. $\endgroup$ – David Nov 22 '18 at 16:40
  • $\begingroup$ The flow of oxygen into the hybrid rocket may be controlled and reduced if necessary using a valve. But the amount of molten and vaporized paraffin per time may not be controlled that easy. $\endgroup$ – Uwe Nov 23 '18 at 11:23
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Keep in mind that for bipropellant chemical rockets, fuel-rich combustion is generally preferred: it keeps temperatures more manageable than stoichiometric combustion, and tends to produce simpler molecules in the exhaust, which leads to less kinetic energy held in vibrating intra-molecular bonds and more kinetic energy going straight down the nozzle.

Conveniently, keeping the oxidizer flow rate down to maintain fuel-rich combustion also helps minimize wastage; the combustion chamber should maintain a surplus of hot, atomized-or-vaporized fuel, eager to consume what oxidizer is available before it escapes.

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