Originally posted by Shaira:
Any thoughts? Handwavery required, or is there a proper reason Out There?
IMTU, I have a scheme which tries to minimize the handwaving.
I figure grav drives have two different modes of operation available: direct thrust and contragrav. Contragrav is the simpler and more fuel-efficient mode, and is the one used by air/rafts and other low-performance flying machines (with their ten-week operational durations). Contragrav requires the presence of an external gravity field to "counter", and has a maximum lift/thrust of 2Gs total (as floor plates). Note that the higher one lifts in altitude, the less-efficient contragrav becomes; it's effectively useless in deep space.
IMTU, unstreamlined ships (other than dispersed structures) may use contragrav to perform as air/rafts within planetary atmospheres. Perhaps more importantly, vessels with m-drive ratings of 1G can use contragrav mode to make air/raft-style takeoffs and landings at worlds with greater than 1G surface gravity (up to the 2G limit of contragrav mode).
Direct thrust mode operates as a proper "reactionless drive" and is used to propel a ship at its rated drive performance regardless of local gravity fields, but at a higher power cost. I also hypothesize that grav tech is somehow associated with jump tech -- that one breakthrough led to the other. For ships displacing greater than 100 dtons and mounting computers, it is possible to create a "bubble" or "pocket" with the m-drive that is stable enough to provide acceleration compensation to everything within it -- the gravitics accelerate everything with the field encompassing the ship smoothly. Smaller vessels (small craft), even if they mount computers, aren't big enough to form a stable field, and so rely upon "acceleration couches" for safety. Regardless of whether the ship is big enough to form an acceleration compensation effect,
any spaceship m-drive produces enough of a field to give the radiation and micrometeroid protection noted in Beltstrike.
That's the simplest, yet most comprehensive, explanation I've cooked up... and it has the side benefit of suggesting a reason that j-drives need a minimum 100 dtons of displacement and don't work well in external gravity fields...