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?

  • $\begingroup$ Have you considered that the comment you got was simply, you know, wrong? Furthermore, size is relative. RCS thrusters are relatively small, I wouldn't exactly call the ones on the Apollo CSM "big". I mean, even the wikipedia entry for the R-4D starts with "The R-4D is a small ...". $\endgroup$ – Polygnome Mar 5 at 9:39
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    $\begingroup$ @Polygnome the comment wasn't technically wrong. After all, there are a lot more satellites out there with monopropellant RCS thrusters than there were Apollo or Shuttle missions. Merely unhelpful, in context. $\endgroup$ – Starfish Prime Mar 5 at 11:18
  • $\begingroup$ @StarfishPrime Not being technically wrong doesn't make the comment right in context. $\endgroup$ – Polygnome Mar 5 at 11:29
  • $\begingroup$ @Polygnome are you saying that most RCS thrusters aren't monopropellant? $\endgroup$ – Starfish Prime Mar 5 at 11:32
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    $\begingroup$ @Polygnome but the original comment was not in that context, and so it was not wrong. This isn't, y'know, rocket science. $\endgroup$ – Starfish Prime Mar 5 at 12:48

I'm going to interpret your question as being:

"What factors drive the choice between bipropellant and monopropellant thruster systems for spacecraft maneuvering systems?"

If that's wrong, advise in a comment, and I'll delete this.

This answer is a summary of the paper Performance Evaluation of Spacecraft Propulsion Systems in Relation to Mission Impulse Requirements

A major factor in this trade-space is the System Specific Impulse (Issp) defined as the total impulse delivered by the propulsion system divided by the propulsion system mass.

For chemical propulsion systems -

  • For small Issp (<1000) a cold gas thruster system is the likely choice.
  • For medium values of Issp (1000 -2000) a monopropellant system is the likely choice.
  • For high values of Issp (2000+) a bi-propellant system is the likely choice.

A very useful graph from the paper shows delta-v required versus the mass fraction of the propellant system relative to the spacecraft.

enter image description here

The paper states in reference to this graph (I'm summarizing and paraphrasing)

  • for delta-v < 150 m/s, pick a cold gas system
  • for delta-v 150 m/s < x < 650 m/s, pick a monopropellant system
  • for delta-v > 650 m/s, consider bipropellant, resistojet, and electric propulsion

If you have an interest in the topic, I strongly suggest you read the paper; it's quite accessible and gives real-world spacecraft examples for different Issps and explains how it's calculated for the different system types.

Also, apart from factors discussed in the paper, real-world factors come into play for real systems: What engines are currently made and what do they cost?; handling difficulties for the various propellants, etc. etc.

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    $\begingroup$ Warm gas thrusters provide a possible intermediate between cold gas and hot gas thrusters, and since the patent expired as of today, perhaps there might be more interest in that technology. $\endgroup$ – David Hammen Mar 6 at 14:38
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    $\begingroup$ @RussellBorogove apparently it is a "Power Augmented Catalytic Thruster", and is some sort of hydrazine-fuelled resistojet. Details seem weirdly thin on the ground. $\endgroup$ – Starfish Prime Mar 6 at 21:19
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    $\begingroup$ @StarfishPrime Ooooh, moderate Isp, poor TWR, -and- high power requirements! Who wouldn't want some of that? $\endgroup$ – Russell Borogove Mar 6 at 21:25
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    $\begingroup$ @RussellBorogove aha, found it... page 57 of ESA bulletin 66, may 1991 (labelled page 55 in the document). It appears to be a hydrazine monopropellant thruster with an electrical afterburner. I suspect you may have identified why we don't hear much about it these days. $\endgroup$ – Starfish Prime Mar 6 at 21:30
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    $\begingroup$ I think has found a home in two circumstances: a) a while ago on communications satellites before bi-propellant systems became more widespread and b) on more recent designs, probably because of an overall cost/performance trade-off, where it was seen as more advantageous to have a monopropellant system for other functions such as attitude control. $\endgroup$ – Puffin Mar 6 at 21:38

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