RE: starship-design: Plasma power

["L. Parker" <lparker@cacaphony.net>]

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> In the context, which is of an MPD arcjet with boron
> enrichment from the
> anode which strikes the arc, we are not speaking strictly of
> a "diffuse tail
> of gas" in the jet, for the purpose of the final or "nozzle"
> magnetic coil
> is to collimate the exhaust ions, restricting their flow to a
> relatively
> narrow column in the wake. The fraction of the exhaust plasma which
is
> neutral gas, will of course tend to escape this organized
> column, in diffuse
> fashion by thermal motion. But thermal motion is very, very
> slow as seen by
> a beam of relativistic protons, so even the hot neutral gases
> won't get far
> from the central jet target zone without getting bashed.

Not true, you are considering only thermal MOTION in an normal thermal
regime. The thermal PRESSURE in a plasma is hundreds of orders of
magnitude greater. Unless you can find a way to project the magnetic
confinement beyond the physical scope of your nozzle, you will have to
deal with thermal absorption in the nozzle itself. Current designs
such as VASIMR rely on extremely high density magnetic fields in the
nozzle to keep the plasma from coming into contact with the nozzle,
but radiated energy is still a problem. At these performance levels,
radiated heat absorption is greater then what we are currently dealing
with in CHEMICAL rocket engines. If the plasma were to be permitted to
contact the nozzle it would melt practically instantly.


 Many of them,
> anyway. Our plasma is cold, it is positively frozen, when the
> beam from the
> accelerator comes along. We need to take account of the
> scaling factors in
> speed, contrasting the thermal motions we normally think of as
pretty
> speedy, with particles accelerated to within a gnat's ass of
> C.  Convert
> from keV to Kelvins, that's your comparison.

What you are describing is actually a modified Daedalus engine,
replacing the crude "throw the nuclear bomb out the back" concept with
"throw the target for the nuclear reaction out the back." As was
already pointed out,  this will only work if the reaction occurs
within close proximity to the pusher plate of the vehicle. 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". There is currently research
ongoing that is similar to your concept in that it is still a "throw
the nuclear something out the back and detonate it" approach, but it
is using antimatter as the activation, not an accelerator. This
concept provides many orders of magnitude increase in performance, but
is still far short of interstellar capability.

Lee Parker

He, who through vast immensity can pierce
See worlds on worlds compose one universe,
Observe how system into system runs,
What other planets circle other suns,
What varied Being peoples every star,
May tell why Heaven has made us as we are.

       -Alexander Pope, "An Essay on Man"


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