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Re: starship-design: LINAC efficiency

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.

Call the propulsion portion covered, for no one has a motive system giving
greater efficiency, than I believe electrical acceleration of particles to
near lightspeed can be. The question remains, how to provide the electrical
energy to power this drive. Nuclear fusion is the most obvious answer.  I
have advocated using stored energy as well, in motor-generator flywheel
devices of overstressed iron rings, spinning in toroidal superconducting

Quibbling over the theoretical limitations of nuclear fusion is not
productive, for the components making up the fuel for the fusion reactor are
continuously being gleaned from the ambient surroundings. If the fusion
reaction is not efficient enough for your tastes, just make it bigger. There
is lots of hydrogen out there, and within it is a teeny taste of lithium, so
this energy is not metered. You can indeed feed your fusion reactor with the
mass equivalent of your starship, dozens of times over,  so in effect there
is no practical limit to the amount of energy you can generate by fusion,
when you are not required to carry its fuel components. The important
modification, that this fusion energy is diverted to a highly efficient
accelerator to comprise your drive, rather than being expended (thermally)
in a fruitless attempt to speed up the fusion products themselves, for
reaction mass, makes a world of difference in the basis of your

Your point on drag is well taken. It clearly must be considered, in
considering designs using ramscoops. Perhaps a partial answer would lie in
reducing the ram cross section, during the critical acceleration phase just
before turnover. This is when the most nearly relativistic ship velocity is
needed, and you would want your highest acceleration capability at this
time. At this juncture, you might shut down the fusion reactor altogether,
to eliminate its demand for mass throughput, and run on your stored power.
You would keep just enough ram scoop area showing, during this final boost
before turnover, to feed the linac. After turnover, the drag force you
mentioned can only assist deceleration, so the greater the ram cross
sectional area the better.

Johnny Thunderbird

>From Steve VanDevender:

> Johnny Thunderbird writes:
>  > Having a hungry linac may be fun and exciting, but we will need to
>  > out what to feed it. I favor the ram scoop method pioneered by Bussard.
> There's a problem with ramscoops.  Yes, you can sweep up interstellar
> hydrogen and fuse it for thrust.  But once your ship is at speed,
> sweeping up that hydrogen induces drag.  If you're trying to put that
> hydrogen into a fusion reactor, you have to bring it up to the speed of
> the ship to get it in there; eventually, the thrust you get from the
> hydrogen only matches the drag of the ramscoop.
> In the case where you have to bring the hydrogen fully up to the speed
> of the ship before you fuse and expel it, it's expected that the ship
> will top out at about 0.1 c.  The incoming stream of hydrogen moving at
> 0.1 c relative to your ship is exactly balanced by an equal mass of
> outgoing fusion products moving at 0.1 c.  At that point you may as well
> shut down the ramscoop and coast.