Command module RCS options | Command module RCS | Forum

Post edited 3:53 am – December 13, 2009 by Luke Maurits

What are our options for command module RCS?

It feels to me like "plain old" thrusters on the exterior of the craft are our best bet.  Compared to more exotic options like momentum wheels or ion engines they are simpler and almost certainly cheaper.

Will we be able to simply release pressurised gasses or will we need to actually burn fuel?  If we can get away with pressurised gas, what factors determine which gas is most suitable?  Obviously the more dense the gas the more momentum we can impart in a given quantity of time, but presumably there are other pragmatic factors to consider like compressibility, freezing point, etc.

If we know the velocity and density of the escaping gas then we can figure out the force that is applied and then estimate rotation rates and from that (once we have a more detailed flight plan) estimate the required amount of gas for the entire trip.  Does anybody know how we would go about estimating the escape velocity?  Does this depend on anything other than the pressure difference between space and our gas storage and the geometry of the thrusters?

Are there compelling reasons to consider an arrangement of thrusters other than the obvious Apollo-esque one of thrusters grouped into blocks of 4, positioned symmetrically around the vehicle and firing in pairs (so that we can achieve rotation without translation)?

Today I learned that the Mercury space capsule used Hydrogen Peroxide for its RCS system.  From Wikipedia:

H₂O₂ can be used either as a monopropellant (not mixed with fuel) or as the oxidizer component of a bipropellant rocket. Use as a monopropellant takes advantage of the decomposition of 70–98+% concentration hydrogen peroxide into steam and oxygen. The propellant is pumped into a reaction chamber where a catalyst, usually a silver or platinum screen, triggers decomposition, producing steam at over 600 °C which is expelled through a nozzle, generating thrust. H₂O₂ monopropellant produces a maximum specific impulse (I_sp) of 161 s (1.6 kN·s/kg), which makes it a low-performance monopropellant. Peroxide generates much less thrust than hydrazine, but is not toxic. The Bell Rocket Belt used hydrogen peroxide monopropellant.

While the performance of this solution is not fantastic, I suspect that it is greater than could be achieved with pressurised cold gas, and it is certainly a simpler solution than combining a bipropellant with an oxidiser and providing an ignition source.  The need for silver/platinum may be a little costly, but it's probably negligible compared to overall project cost.  This is definitely a solution to consider.

I hadn't heard of catalase before your comment, so all I know about it is what I just skim-read from Wikipedia.

With regards to temperature, I note:

Human catalase works at an optimum temperature of 37°C, which is approximately the temperature of the human body. In contrast, catalase isolated from the hyperthermophile archaea Pyrobaculum calidifontis has a temperature optimum of 90°C

Assuming that the 90 degree version represents the upper end of the scale of temperatures catalse can work at, the 600 degree temperature apparently involved is probably well more than can be withstood.  But I know so little about chemistry that I can't rule out the possibility of synthesising a form that can resist higher temperatures.  I'm not sure what the advantage over metal catalysts might be, other than possibly cost.  I know platinum isn't cheap.