Imagine I have a combustion chamber with a nozzle. There is a high pressure oxygen bottle and a high pressure gaseous fuel bottle connected to the chamber. Would the fuel efficiency be lower if I performed combustion in a series of many very short burns than if I emptied the tanks in one go? Since at the end of any burn there's a period where pressure decreases, I imagine that the total impulse of the engine will be lower for the intermittent combustion since it will go through many of these periods instead of just one, leading to increased fuel consumption.
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2$\begingroup$ In many systems, including electrical, intermittent power delivery yealds better performance. You really should test your theory, taking care of thrusting after the air decompression period. But at the same time we ask: why NASA doesn't do that ? $\endgroup$– Lucas BSCommented Dec 4, 2019 at 16:24
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1$\begingroup$ If your choice is between intermittent pulses and running the engine at reduced fuel/oxidizer feed, the pulses will be better because the lower pressure will be only at ignition / flameout, not during the stable combustion period. Tere's a reason why RCS works with pulses, and Falcon 9 runs on 3 out of its 9 engines during the hover-slam. Pulsing reduces efficiency less than deep throttling. $\endgroup$– SF.Commented May 3, 2020 at 9:34
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2 Answers
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Yes, the efficiency would be worse.
The engine performs optimally when operating at the nominal chamber pressure. The time between engine ignition and it reaching the nominal chamber pressure varies with the engine type - in particular, in monopropellant RCS thrusters it's very short, so operating them in many short bursts is perfectly okay. Similarly, electric solid propellant engines can act in pulses as short as 50 milliseconds.
But since you write about a standard chemical bipropellant engine, this is not the case, and its typical initial combustion instability causes a bunch of fuel and oxidizer is used before the engine reaches stable combustion state.
Never mind in majority of liquid bipropellant engines, relighting the engine isn't a very straightforward matter - some can't be re-lit at all, some use a consumable hypergol which they have a limited amount of, some are much harder to re-light when hot after prior burn... but that's all individual and if you keep unlimited re-lighting as a required feature of the engine from the beginning of the design, it will be possible.
Regardless, the specific impulse of the engine is directly proportional to the speed of the exhaust gasses. And that, in turn, is proportional to the chamber pressure. That means if the engine doesn't operate at maximum chamber pressure, it doesn't operate at maximum specific impulse - and both throttled down, and ramping up from zero to max (and down after extinguishing) it operates at pressure lower than that maximum.
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The built-up to momentum in the mass of gas flowing out of the nuzzle and building the design exit Mach and stability of the combustion is better left alone from the disruption of on-off cycles.
Avoiding flame blow out in the combustion chamber and continuity in vector thrust navigation lend better to long bursts.
