Back in the early '70's, as part of a study I did in my second go-around at college, I used methane collected off a municipal sewage processing plant that produced natural gas as a byproduct of its chemical process to fuel a laboratory methane/GO2 rocket motor. The natural gas was about 60 percent methane, with CO2 and traces of other combustables. The rocket was a 1 lbf motor. My study concluded that IF one could set up a biomass in a spacecraft, you could produce enough methane to power thrusters, etc., plus, if you could break down the CO2, and had H2 aboard you could create more methane, water, etc. As I'm an engineer, not a chemist, I left the how-to to others. The project had earth-bound practical applications, as you could heat 10,000 houses with the natural gas by-product, if you put all of the then-population of Denver, Colorado, on the anerobic digester type sewage treatment. They never have done it, but DO tap landfills for methane.<br /><br />When I was a kid, we had another kid who had a tendency to be a bit...uh, er, flatulant! We joked that if we stuck him in the tail end of a rocket ship, and fed him beans and Ex-Lax...well, you get the picture! Might have had a problem with a high rate of nozzle erosion, however!<br /><br />Ad Luna! Ad Ares! Ad Astra!