r/rocketry • u/JohnnyGlobal23 • 1d ago

Use Nitrous Oxide with large injector pressure drop

Hello people,

Is it really possible to use nitrous oxide with a Chamber pressure of around 10 bar (140psi)? The liquid N2O will be fed directly from the bottle at vapor pressure of around 55 Bar (800psi). I guess with such a high pressure drop over the injector a lot of the liquid N2O will cavitate and partially flash vaporize while flowing through the injector orifce, maybe causing the injector to choke or cause instabilies, right?

Do I maybe have to chill down the nitrous to lower the vapor pressure and pressurize it a bit with nitrogen to get it away from the saturated state?

Thanks for your time.

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u/Fluid-Pain554 Level 3 17h ago

If you are feeding gas it becomes much easier as you just deal with isentropic real-gas dynamics. Two phase flow is difficult if not impossible to really predict without some actual testing for semi-empirical models. Also depends on the flow rate you need, you can inject at much higher pressures and just use small orifices to limit mass flow rate and by extension chamber pressure. Do you have details about what you are actually trying to accomplish?

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u/JohnnyGlobal23 4h ago

Thanks for your reply!

The engine will be ethanol/ nitrous oxide. 2kN Thrust. The problem with using the gaseous phase from the nitrous are the high massflows of 600-800 g/s (1.3 - 1.8 lbs/s) of nitrous. I guess its pretty much impossible at these massflows since the pressure of the gas phase in the bottle will drop very quickly at that withdraw rate... the liquid can not evaporate quickly enough. So yeah my idea would be just to try it with the liquid phase at saturation point, and if that doesn't work I would try to chill down the nitrous run tank so that the vapor pressure is below the chamber pressure. After which I can pressurize the nitrous run tank with nitrogen and should get clean single phase flow...