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RE: RE: starship-design: Plasma power
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> "Thrust drops off rapidly as the distance from the pusher
> plate increases.
> VASIMR is a much more low tech concept that is capable of providing
> the same performance and is "tunable". "
> Forgive me if my question is very rudamentary (I have a hard
> time reading all the way
> through some of these messages,) but this sounds terribly
> innefficient to me. If you're
> talking about varying the thrust by changing the distance
> from the pusher plate, doesn't
> that mean that you're expelling the same amount of energy, no
> matter how fast you want
> to go?
Sorry, I guess I DID run a few thoughts together there...
In the text quoted form the previous message, even though the same
amount of energy is expelled, the amount of it that does WORK is
different. The farther from the pusher plate the explosion occurs, the
less useful work that can be extracted from it. The energy is still
there, just wasted to open space.
Project Orion envisioned a craft that literally threw small hydrogen
bombs out the back and set them off, resulting in a "push" against a
heavy, shock mounted plate at the back of the craft which also served
to shield the rest of the craft from radiation. When I say small, I
mean around 1 Kiloton, still large enough to level a small city, but
terribly inefficient as much of the energy was wasted to open space.
Nevertheless, this crude concept could still reach a respectable
fraction of the speed of light, and what it lacked in efficiency it
made up for with thrust, it could propel HEAVY payloads.
Today, 40 years later, we have achieved a mastery of the fusion
process impossible to even imagine when Project Orion was first
conceived. We can, using a small supply of anti-protons initiate
fusion on a microscopic scale and liberate within the confines of a
magnetic nozzle a miniature hydrogen bomb. The efficiency of this
process is thousands of times higher than Project Orion and still
maintains a respectable thrust. Of course, this is "bleeding" edge
research, meaning that it is not quite ready for prime time.
VASIMR is an entirely different approach. It does not seek to achieve
fusion, just the thrust and high efficiency that are traditionally
thought of as being achievable only by a fusion engine. It works by
ionizing a fuel and then feeding it into what amounts to a large,
extremely powerful microwave tube where energy is pumped in until a
plasma is formed. The plasma then escapes into a magnetic nozzle which
accelerates the escaping plasma even further before it finally exits
For interplanetary purposes, this engine is ideal. Since it is
controllable in the amount of fuel and energy applied, it can be tuned
for either high thrust or high efficiency, or somewhere in between. A
cargo flight for instance could be programmed for a high thrust
transfer that would reach Mars in a few weeks at very high but brief
acceleration. A crew carrying vessel on the other hand could
accelerate more gradually, and continuously, without using up all of
its fuel. This is a rather simplified presentation, but essentially
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