Quote:
Originally Posted by sir_pudding
Sure, but such systems would actually be poor rockets compared to straight antimatter rockets with gamma exhaust. Which means that you aren't going to use the same engine for lift that you use for deep space burns, or at least your engine has two modes, one with significantly poorer ISP.
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No, a photon drive would not be better for taking off and landing on planets, or maneuvering in atmosphere, or most of such activities. The amount of fuel for a gamma-photon rocket is way higher than for seetee-activated reaction mass. For the latter, water is cheap, plentiful, and easy to handle, and an inifinitesimal amount of seetee will turn a few tons of water into a super-hot plasma that makes a dandy reaction mass. You get plenty of thrust for relatively little fuel and very cheap reaction mass.
Nor do you have to use water, you could use any fluid or gas your drive could handle. Water would just be convenient.
(Modulo our current inability to build such a thing, of course. But we can't build a useful gamma-photon drive, either.)
If you pump out enough thrust with a gamma-photon drive to get from Earth's surface to LEO or escape, then with even a tiny spacecraft you need substantial amounts of antimatter to do it, and yes, your drive is also a WMD on a big scale. The energy-to-thrust ratio for photon drives is rotten.
A photon-drive surface-to-orbit/escape vessel is theoretically possible, in the physics sense, but it uses a genuine
death ray as a means of propulsion. I foresee legal and diplomatic issues.
A photon drive is preferable if you're hoping to use antimatter to achieve relativistic velocities, yes. But at intra-solar and planetary velocities the tradeoff is different.