I understand the disadvantage of monopropellant is mainly dependent on the life of the catalyst. There is a possibility of catalyst failure due to it being subjected to catalytic poison and catalytic attrition. What are other disadvantages of using monopropellant RCS thrusters? Is Hypergolic RCS the best choice for a re-useable spacecraft?

  • $\begingroup$ With a reuseable spacecraft, replacing the RCS catalyst beds postflight is probably pretty far down on your list of worries. $\endgroup$
    – Vikki
    Feb 29 '20 at 19:11

Monopropellants are less efficient than most serious bipropellant rockets, including hypergolics (though hypergolics are themselves less efficient than some other propellant combos). In impulse per propellant mass (specific impulse) and often also impulse per propellant volume (impulse density), monopropellants aren't great. For a reusable craft, this may not matter dramatically (there's generally no need for a great deal of delta-V from the craft itself) but it would certainly matter for systems like a space tug, or systems which require extensive on-orbit impulse either for a lot of delta-V or because the spacecraft has high mass (the Space Shuttle Orbital Maneuvering System and RCS both used MMH/N2O4 hypergolic bipropellants).

Monopropellants also tend to be toxic, especially hydrazine (which is the monopropellant most commonly used in serious rocketry). This complicates fueling before launch, on-pad emergency procedures, and post-landing processing; a hydrazine leak must be checked for and, if present, cleared before any unprotected humans can safely approach or exit the vehicle. There are procedures for dealing with all of this, as most spacecraft (including human-rated ones, both reusable and not) use hydrazine, but it would be desirable to avoid these costs and risks. However, the next-best option (in terms of system mass and simplicity, propellant storage, and other factors that determine suitability in an RCS) is hypergolics (such as the shuttle used), which typically also use some variant of hydrazine as fuel and have the same toxicity hazards.

  • 2
    $\begingroup$ "and have the same toxicity hazards." Worse. The fuel is the same, then you have the oxidizer which adds a completely new set of hazards that are equally nasty (but different). $\endgroup$
    – SF.
    Jan 22 '19 at 11:16

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