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  • maxwelljets commented on iamdenteddisk's instructable Homemade orbital rocket?3 years ago
    Homemade orbital rocket?

    This is actually a common misconception. Rocket engines do not need atmospheric oxygen at all. Small estes motors use black powder, which uses potassium nitrate as the oxidizer. The exhaust gasses are already fully oxidized when they leave the engine. Similarly, larger engines use a propellant mixture called APCP (Ammonium Perchlorate Composite Propellant) which uses ammonium perchlorate as the oxidizer. This also produces fully oxidized exhaust using only the oxidizer contained in the propellant.However, you are correct about these engines not working correctly at high altitudes. The nozzle shape must be optimized for one specific external pressure, or they lose lots of thrust. Using an engine designed for 1 atmosphere of ambient pressure way up near space will reduce your thrust (and ...

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    This is actually a common misconception. Rocket engines do not need atmospheric oxygen at all. Small estes motors use black powder, which uses potassium nitrate as the oxidizer. The exhaust gasses are already fully oxidized when they leave the engine. Similarly, larger engines use a propellant mixture called APCP (Ammonium Perchlorate Composite Propellant) which uses ammonium perchlorate as the oxidizer. This also produces fully oxidized exhaust using only the oxidizer contained in the propellant.However, you are correct about these engines not working correctly at high altitudes. The nozzle shape must be optimized for one specific external pressure, or they lose lots of thrust. Using an engine designed for 1 atmosphere of ambient pressure way up near space will reduce your thrust (and thus efficiency) by roughly half.

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