Some musings on propulsion: our requirements and commercial options | Propulsion Workgroup | Forum

Just a few musings on our propulsion needs.

Wikipedia gives the Apollo lunar module as having as ascent mass of 4,547 kg and a descent mass of 10,149 kg.  The ascent and descent engines had thrusts of 15.6 kN and 44.40 kN respectively, leading to accelerations of about 3.4 m/s/s and 4.4 m/s/s respectively.  If we want to get a 4.4 m/s/s acceleration on a 500 kg lander we'd need a thrust of 2.2 kN.

Wikipedia gives the mass of the Apollo CSM plus lander at about 40478 kg and the SM engine's thrust at 98 kN, giving an acceleration of about 2.4 m/s.  If we want to get that acceleration on a CLLARE stack at about 8000 kg we'd need a thrust of 19.4 kN.  That means that if we were to cluster engines identical to the lander to provide thrust for the whole stack we'd need about 8 of them, which is a lot.

Alternatives to this would be to simply use, say, 4 and do slower burns, or to make the lander engine larger and just run it throttled down during lunar landing.  I don't know if there would be inefficiencies involved in doing that.

So now that we have some rough idea of our engine needs: it turns out there are commercial options out there.  Up until now it just seems to have been assumed that we would design and build our own engines, but remember that we are supposed to operate on an off-the-shelf basis and only design our own stuff if there's nothing appropriate on the shelf or if we think we can do it cheaper or better.  In the case of rocket engine design and construction, I'm not sure we can confidently claim (at this point) that we can, so we should probably think about buying engines.  Thus…

Founded in 1999, XCOR Aerospace is a small, privately-held California C-Corporation.  Our headquarters and development facilities are located at the Mojave Spaceport and Civilian Aerospace Test Center in Mojave, California.  XCOR engages in research, development, and production of reusable rocket-powered, horizontal launch vehicles for suborbital, and ultimately, orbital travel. 

XCOR is a leader in the development of reliable, low-cost rocket engines. Since inception, the company has developed a series of proprietary engines, and has built, tested and flown the United States' first privately built liquid-fueled rocket-powered aircraft. XCOR’s achievements to date have been funded with investment capital from angel investors, and from revenue generated from government and commercial contracts and consulting services.

XCOR have a LOX/Methane engine that puts out 33 kN, which would be able to push our 8 tonne stack around with an even greater acceleration than the Apollo SM engine.  They also have a LOX/Alcohol engine that puts out 1.7 kN, which could give our lander just a little less acceleration than the Apollo lander (or the same acceleration if we could get our lander a little lighter).

This is probably something we should think about.  Unfortunately (and bafflingly) XCOR do not have specs for their engines online (thinks like size, mass, cost), but if we asked them in an email they may provide them – this would help us to at least put an upper bound on cost and mass for propulsion systems.

I agree that building our own would be seriously awesome.  Unfortunately awesome sometimes has to take a back seat to practicality.

We can put a commercial option in the plans for now to get solid figures  – if we think we can eventually build our own cheaper we can devote some time to it when we have it spare.