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Re: starship-design: LINAC efficiency

Johnny Thunderbird writes:
 > Steve, your objection (although it is of course relevant) is directed to
 > Bussard's concept of using a fusion drive with ramscoop material. Though I
 > have spoken of using proton-lithium fusion, for a drive it only suffices "in
 > system", because its exhaust particles do not achieve relativistic velocity.
 > I do not intend to use fusion rocketry for the main legs of a starship, but
 > as an onboard power source, so this makes its propulsion efficiency
 > irrelevant. For the legs, one uses a tuned linac, as I specified. This makes
 > some of the objections moot, for with an accelerator drive you are putting
 > out relativistic particles, meaning you can achieve an arbitrary thrust from
 > a small amount of reaction mass, if you have the energy to do so.
 > That someone is dissatisfied with fusion efficiency, is no reason not to
 > have a fusion power reactor aboard the ship. Clearly, it is better to have a
 > working fusion power reactor aboard, than not to have one. People might have
 > theories about other power sources, but wishes are not horses. When they
 > show me something that works, we might be talking engineering.

Think about this more carefully.  You have to get more energy to obtain
that "arbitrary thrust"; that means that you have to gather more
hydrogen with your ramscoop, fuse it, then concentrate that energy into
a smaller amount of reaction mass.  But by gathering that larger
quantity of hydrogen, you've just induced more drag on your ship.  This
extra drag ends up counteracting whatever benefit you hoped to get by
producing a higher-velocity, lower-mass exhaust.