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Most smaller satellites and other larger but simpler space craft with a less demanding mass fraction tend to use cold gas thruster, importantly with a common or as few as possible pressure vessels, however such thrusters have a tragically low performance, and as such, most lager space craft will use either monopropellant or even bipropellant thrusters. This of course comes at the cost of complexity and a larger mass fraction however. my question is, could a compromise be made between the two mechanisms, where several reaction thrusters could be pressurised from the same combustion chamber, not too dissimilar to rocket engines with multiple nozzles. What would be the practical limitations of such a system- how could the cooling of the pressurisation lines be managed?

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  • $\begingroup$ I don't know enough about propulsion to provide a complete answer. All I can say is that many craft use the pressurant (some inert gas like Helium) as the gas for the cold gas attitude thrusters. When large attitude corrections are needed, the main engines may be fired instead: they may be canted such that the proper selection of these engines allows for attitude control. $\endgroup$ – ChrisR Mar 4 at 5:53
  • $\begingroup$ I’m sure engine gimbal is used to manipulate orientation whilst making a burn, but designated attitude control thrusters are always used for finer and more specific control. It would not be advantageous to forego any attitude thrusters $\endgroup$ – Reuben Farley-Hall Mar 4 at 7:16
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    $\begingroup$ Citation needed on "rocket engines with multiple nozzles". There are multiple engines that share a common turbine, and a few that share a common pre-combustion chamber such as a catalyst bed to decompose high test hydrogen peroxide. But even in the latter case, the combustion of the decomposed hydrogen peroxide with the fuel occurs in separate combustion chambers that each have their own nozzle. $\endgroup$ – David Hammen Mar 4 at 12:03
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You want to use a hot gas thruster with one combustion chamber and several selectable nozzles?

But how should the active nozzle selected? You need something like a valve with one input and as many outputs as the nozzles. But this valve would be damaged by the hot corrosive gas. Problematic for a satellite operating over several years.

Valves for the cold propellants exist for many decades and are known to be reliable. No satellite operator would trust a hot gas valve without successful history.

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  • $\begingroup$ a low enough exhaust temperature could be achieved without too low an isp, after all, it only has to outperform cold gas thrusters $\endgroup$ – Reuben Farley-Hall Mar 4 at 9:44
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    $\begingroup$ If what reaches the nozzles isn't hot, what you have is a cold gas thruster. You're just generating the cold gas on the fly with some chemical reaction instead of storing it in a tank. That might have some advantages in propellant density, but I'm having a hard time thinking of any candidate reactions that produce gaseous end products without being highly exothermic. And you're really just saving a little bit of monoprop-decomposing catalyst per thruster. $\endgroup$ – Christopher James Huff Mar 5 at 1:58

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