nick hull wrote:

In article ,
Michael Walsh wrote:

nick hull wrote:

In article nk.net,
"Terrence Daniels" wrote:

URL:

http://spaceflight.nasa.gov/station/...etterlatest.ht
ml

"The combination of a nuclear reactor plus a plasma engine could provide
a
delta V capability of 30 to 100 thousand MPH. This is plenty for
exploring
the solar system."

Is he on-target with this idea? Granted he's talking about The Future,
but I
thought ion engines were a low-power, slow & steady sort of propulsion
system. What about delivering high instantaneous thrust, like when a ship
"burns" for getting into & out of orbit?



Ion engines are low thrust and do not use transfer elipses to change
orbits. They use powered trajectories and spital orbits. Works very
well on long term thrust.

He didn't say an ion engine.

He said a plasma engine.

While current plasma engines are low thrust, although higher thrust
than ion engines, there is no theoretical reason a high thrust plasma
engine could not be build, given a sufficient technological advancement.

I see nothing wrong with what Ed Lu said, although the engineering
could certainly take a while.

Plasma is just ionized gas. If it gets its velocity from electric
acceleration, it's functionally an ion engine. Plasma can be generated
in larger quantities than simpler ions, so it's a tradeoff between a lot
of matter ejected at lower velocities or a lesser amount at higher
speed. True, plasma engines give higher thrust than ion engines but
nothing remotely similiar to chemical thrust - and the closer you get to
big thrust the closer you get to chemical efficiency.

Well, when in doubt go back and read the original source, Ed Lu's
letter. It seemed clear to me that his references to both ion and
plasma engines were about propulsion capabilities after orbit was
achieved.

Delta V is a total measurement that can be achieved either by high
thrust for a short period of time (typical of chemical rockets) or
low thrust for long periods of time. Ion engines have very low
thrusts and high specific impulse. If you calculate anything like
thrust to weight ratio it is truly bad. For a lot of missions within the
solar system your minimum total vehicle weight would be achieved
by using a higher thrust, but lower Isp engine if the total weight of
the propulsion system could be reduced.

I don't know the details of the trade-offs but perhaps the plasma
p;ropulsion system would be optimum for certain types of flight
paths.

For really high thrusts I believe you might need a thermal nuclear
engine and that still falls into the higher Isp but worse thrust/weight
ratio.

In any event, I didn't read anything that looked wrong in
Ed Lu's letter from space.

Mike Waslh