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Re: starship-design: LINAC efficiency

In a message dated 2/15/01 2:48:54 PM, stevev@efn.org writes:

>Johnny Thunderbird writes:
> > Steve, your objection (although it is of course relevant) is directed
> > Bussard's concept of using a fusion drive with ramscoop material. Though
> > have spoken of using proton-lithium fusion, for a drive it only suffices
> > system", because its exhaust particles do not achieve relativistic 
> > I do not intend to use fusion rocketry for the main legs of a starship,
> > as an onboard power source, so this makes its propulsion efficiency
> > irrelevant. For the legs, one uses a tuned linac, as I specified. This
> > some of the objections moot, for with an accelerator drive you are putting
> > out relativistic particles, meaning you can achieve an arbitrary thrust
> > 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
> > 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
> > theories about other power sources, but wishes are not horses. When
> > 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.

And again, our analysis here was that you couldn't scoop up as much mass as 
the mass of the scoop over any nearby mission.