Actually, I think that the engine is probably the single most impressive achievement.
You could always get far more efficiency from solid rockets than from liquid fuels. The big problem was that once you lit the fuse, it burnt until the fuel was expended (The SRBs on the Shuttle for example).
THIS thing has a solid fueled rocket...that you can turn off!
I'm already envisiging things like a scale ME 262 powered by one... now wheres my old Tech Drawing set...