I’m a bit late posting this, so apologies if you’ve already read about it, or seen it, elsewhere (and thanks to Jon Goff at Selenian Boondocks, which was how I found it).
This is a lovely sight:
A methane main propulsion system was included in the baseline ESAS design for the Orion Crew Exploration Vehicle. Methane is easier to store for long periods than hydrogen (the main fuel used on the space shuttle, carried in the large orange external tank), and was included for commonality with future Mars missions, because it can be produced on Mars via a simple chemical process. Mars missions are still a long way off, however, and NASA’s exploration plans already face significant budget and schedule challenges. So to save time and money, and to reduce the technical risk, the methane engine was replaced with a storable-propellant system similar to that used by the Apollo CSM and the space shuttles’ orbital maneuvering engines. These have the advantage of being well-understood and mature systems, but have a performance penalty compared to LOX/Methane, and make use of highly toxic hydrazine and nitrogen tetroxide propellants.
But XCOR’s work should go some way toward validating the methane concept. One day a derivative may find itself headed for Mars.