[Spaceships] Reactionless Drives, Pseudovelocity, and NOT breaking a setting (much)

[Ulzgoroth]

Originally Posted by malloyd

This might be a good example of the sort risks you run into with these things. If you assume you are "safe" from an abuse because planetary gravity wells are not portable, you may end up in trouble when a clever player attempts to build an equivalent potential well generator with a pair of (much more portable) charged spheres.

Fair point. So, a drive that can generate potential energy of any kind, not just gravitational, in excess of its input plus acceptable free-energy margin (because small amounts of free energy may break physics and have setting implications but won't break planets in useful time scales) is an RKV-threat unless you have an aspect that prevents it operating when moving at relativistic speeds.

(Though this is premised on considering photon drives plus free power to constitute a setting problem.)

Originally Posted by malloyd

Probably. Actually I suspect a preferred frame may be necessary for a reactionless drive to make sense at all - the relationship between your momentum and kinetic energy will not transform the same way between different frames as the energy consumed by your drive - so if it's "safe" in one frame, it isn't in others. Of course that's a contradiction - it's the same system - so relativity is going to have to give somewhere. With a reaction drive, the relative momentum and kinetic energy of both your ship and your exhaust change, and not in the same way (their relative velocities are necessarily different relative to any other frames you are considering), allowing for some cancelling out.

The dramatic disagreement about how much net change in energy the drive is creating even at much-less-than-c speeds does seem to suggest that, at least for the simpler models, and a lot of the less simple ones can't even be described without overtly using a preferred frame.

But more specifically, I meant an external preferred reference frame. A reactionless drive in the most elementary conception might be preferring the inertial frame in which it is at rest at any given moment, but I think it's worth drawing a distinction between that and situations where the drive cares about your motion relative to, say, nearby major gravity wells.

Having your drive somehow limited relative to something akin to Kerbal Space Program spheres of influence is a (fairly basic) starting point for trying to make it less geocidal.