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Let's say we have a monopropellant, pressure-fed rocket engine. We don't want the combustion in the chamber to propagate back up the fuel line and into the fuel tank or we will have a big kaboom.

(Blow-back is always theoretically possible with monopropellants, because they burn on their own. It's not like bipropellant kerosene and oxygen; in this case, combustion can't propagate back up either one because a given propellant line has only the fuel or oxidizer, not both.)

So how is this prevented in monopropellant pressure-fed engines? Do we have to install a bunch of super-strong check valves everywhere in the fuel lines? Especially right before injection into the combustion chamber? And if so, wouldn't that imply that the engine can only fire in pulse mode (non-continuous)?

The only thing I can think of is that the pressure is so much greater than the pressure in the combustion chamber, such that the liquid (or vaporized liquid after injection) flows through the chamber so fast (speed of sound maybe, which might prevent some thermodynamic things from flowing backwards). Is this right?

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Sometimes there are solenoid start/shutoff valves after pressurant tanks, and at the injector (although in the simplest cases only one control valve is used before the injector). Check valves are also present - mostly to guard against the case of propellant tank overpressurization and relief valves being triggered.

Typical N2H4 catalyst bed

Source

Please note that the monoprop decomposes in the presence of a catalyst (say, Shell 405 for hydrazine monoprop), thus there's no much hazard of decomposition wave going backwards. It may be a problem for high-test peroxide monoprops and in fact there are known cases of HTP causing deadly accidents.

Regarding the pulse vs continuous firing (the wording of your question isn't clear), there are two major constraints: a) avoidance of overheating, b) total thrust required. If you need to have large thrust delivered, gas-pressurized monoprop is decidedly not your choice - you'd do better by switching to a bi-propellant engine, and, at the higher end of the scale, by adding a turbopump. Longer burns also lead to higher gravity losses. At such a cost, you may consider installing a higher specific impulse ion engine instead.

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  • $\begingroup$ but doesnt hydrazine decompose at around 800 deg C when passing over the catalyst? And isnt 800 degrees big enough to sustain the decomposing reaction? therefore, the "decomposition thermal reaction" (have to say that instead of combustion) could propagate backwards, couldnt it? Also, Hydrogen Peroxide monoprop also uses a catalyst too, so if hydrazine is blow-back safe thanks to a catalyst, why wouldn't HP be so too? P.S. I edited my question with more thoughts. $\endgroup$
    – DrZ214
    Jun 5, 2015 at 23:28

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