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There are a number of potential options for high-performance engines in the future: nuclear pulse propulsion, NTR, gas-core NTR, nuclear electrothermal, and potentially even fusion rockets.

However, most of these seem to be main drive engines that do not scale down much.

While there appears to be a movement towards cryogenic RCS rather than hypergol, this doesn't provide much improvement.

What options are viable for RCS thrusters with higher Isp than chemical allows?

(To be clear: this is about the higher-thrust-range thrusters typically used in manned spacecraft, spacecraft that dock, and landers. Microthrust electrical thrusters need not apply.)

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  • $\begingroup$ What problem are you trying to solve? RCS thrusters generally aren’t tasked to produce significant delta-v. $\endgroup$ – Russell Borogove Aug 25 '20 at 12:38
  • $\begingroup$ @Russell Borogove I don't know about that - if the main engine produces heavy radiation or is unsafe to use near other spacecraft, or if the main engine doesn't start/stop/throttle very precisely then you definitely will have a need for a fair amount. In any case, 450 seconds is a really low Isp for a vehicle making multiple manuevers and which can't be mostly fill of RCS fuel like it can be of main engine fuel. $\endgroup$ – ikrase Aug 25 '20 at 20:56
  • $\begingroup$ Based on that comment what you are describing is not an RCS which is used for attitude control and small xlational maneuvers. You are asking for an orbital maneuvering system. In any case, it sounds like fantasy. $\endgroup$ – Organic Marble Aug 28 '20 at 1:06
  • $\begingroup$ It sounds like RCS for very heavy ships with capabilities of fast maneuvring, much heavy then ones that we have now. In that case such RSC must have much thrust, not only high ISP. Is arcjet an option? And yes, now it sounds like fantasy, but one day it probably will become a reality. $\endgroup$ – Peter Nazarenko Aug 29 '20 at 12:23
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Some form of electric drive might work if your spaceship had lots of power to spare (for instance if it had a nuclear reactor onboard or as part of the main engines).

Something like VASIMIR or even just a miniaturised coilgun could possibly be made to work using electric and magnetic fields to accelerate plasma or small solid objects (think sand grains) to higher velocities than a thermal rocket can achieve. VASIMIR calims 5N thrust for 40kW, which might be OK for an RCS if the mass of the drive could be made small enough.

Definitely some decades down the technology development track, though, not available now.

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  • $\begingroup$ "plenty of power to spare" tends to be problematic though probably less so for RCS. Basically all electrical thrusters of any kind (except maybe nuclear electrothermal) are hamstrung by the limits of power sources. $\endgroup$ – ikrase Aug 25 '20 at 10:12
  • $\begingroup$ @ikrase I'm thinking specifically for RCS, and assuming there is a power source (nuclear reactor) say, on board for another reason. $\endgroup$ – Steve Linton Aug 25 '20 at 10:29
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    $\begingroup$ @uhoh. a coilgun is a type of linear accelerator. The "coil" is a superconducting permanent magnet and it is propelled by an external magnetic field. Build the coil into a small light bucket, put a small mass (sandgrain) inside. Accelerate it and then decellerate the bucket, leaving the sandgrain flying out. Same idea as an electromagnetic launcher. $\endgroup$ – Steve Linton Aug 25 '20 at 10:32
  • $\begingroup$ okay got it, thanks! $\endgroup$ – uhoh Aug 25 '20 at 12:44
  • $\begingroup$ Mass drivers might make sense as a primary drive, but seem like a bad choice for an RCS thruster that's likely going to need to be used in close proximity to things you don't want to punch holes in. $\endgroup$ – Christopher James Huff Aug 25 '20 at 15:48

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