"Jeff Findley" writes:

I'm not so sure. When I was a kid, I flew a small RC airplane which had a
glow-plug on top and seemed to fire quite a few times a second. Certainly a
non-toxic RCS thruster based on combustion of some fuel with LOX would be a
challenge, but from a physics point of view, it doesn't seem
impossible.

Well, there are lots of things that aren't exactly impossible from a
physics point of view, but still really complicated. Repeating all the
uncertainties of ignition (and to do this with changing temperatures of
the systems) hundreds of times a second is surely not going to give you
more accuracy, I think. And for things like docking to a space station
or other hardware you really, really need that. A pulsed system with
liquid fuels in which all the fuel ignites is quite perfect here.

Another interesting possibility is resistojets (or similar) which at one
time were proposed for use on the space station. A non-toxic propellant
like LN2 or even LH2 is heated by some means (resistive heating element or
other means) and then the resulting hot gas is expelled through a nozzle.
This sort of a thruster could be pulsed many times a second to achieve the
sort of attitude control you're talking about. ISP may not be extremely
high (LH2 would be better than LN2 in this case), but the toxicity would be
zero.

For nothing but fine attitude control this would be nice, yes. If you
want to have the very same system double as an OMS in a pinch (as the
shuttle does) this is a bit limited, though. A hypergolic RCS can be
pulsed for really fine control and additionally used in a continuous
mode for getting some real work done (as long as you have enough fuel)
while being less precise then. As a backup for de-orbiting your craft
this can be very handy to have. And it still can be quite simple. There
are good reasons for using such fuels for RMS. They suck for things like
quick turn-around, but as long as this is not a real hard requirement...


Jochem

--
"A designer knows he has arrived at perfection not when there is no
longer anything to add, but when there is no longer anything to take away."
- Antoine de Saint-Exupery