How do they manage to not have the reciprocating mass (the piston) throw off a space vehicle's trajectory? IIRC the Space Shuttle used magnetic tape because a hard drive would have acted as a gyroscope.
Keep in mind that in the space shuttle era hard drives were a good deal more massive than today. Presumably a modern hard drive wouldn't present much of a problem.
In any case a rotating mass isn't too big of a problem, it's changes in rotational speed that'll mess with your heading -- not your trajectory though. The piston itself shouldn't be too much of a problem either because any motion it makes one way it shortly after makes the other way balancing out the forces over time.
