Garmaine Staff asked 2 years ago

In a recent discussion, I asked about using a certain bipropellant system for RCS thrusters (I was actually thinking about it as fuel for an MMU jetpack or the "scooter" commonly imagined in science fiction, or for small orbital drones). Someone remarked that RCS thrusters are almost always monopropellant, sometimes ion engines, and generally extremely small.

This immediately sounded wrong to me, so I looked it up, and it turns out that the R-4D thruster, which is used for the main RCS on the Apollo CSM and LM, the Space Shuttle, and the Orion capsule, is a bipropellant (hypergol) engine. The same is true of the smaller 100N thrusters used on the Japanese ISS logistics freighter (forget the name) and the Draco thrusters used on the SpaceX dragon. (I couldn't figure out what fuel Soyuz and Progress use.)

What these have in common, is: – They are either man-rated, or "quasi-man-rated" (i.e. they dock to an inhabited space station). – They dock (or, at least, fly within range to be grabbed by a robotic arm) – They are pretty large and heavy, and a significant mass / volume payload fraction is a fundamental driver of their design.

Does this discrepancy indicate a distinction between a "navigational" and "docking" RCS? Is it more about performance? Or modern man-rating?