On Fri, 28 Nov 2003 20:33:25 GMT, (Henry Spencer)
wrote:

In article ,
Robert Clark wrote:
Would this provide a low energy means of creating the plasma required
for ion engines? One means of creating the required plasma is by
irradiating the propellent gas with intense laser or x-ray beams to
strip off the electrons of the atoms of the gas, producing an ionized
plasma. However, these are both high-frequency forms of EM radiation
and therefore require high energy to produce. Microwaves being longer
wavelengths require less energy to produce.

There are already ion thrusters that use microwaves for ionization, and
also some that use lower-frequency radio waves. No actual thruster that
I'm aware of uses lasers or X-rays.

How many use grapes?

Another means that is actually used for the Deep Space 1 probe is to
use electrons emitted by a cathode to irradiate the gas, ionizing it.
How does the energy requirement for the heating element of a cathode
compare to the energy requirement for producing the microwaves?

Both are relatively efficient processes, in themselves. Unfortunately,
that doesn't imply that you get efficient ionization as a result. In
either case, much of the energy gets used unproductively.

Can you get enough energy out of a grape that grapes are usable?
Would you grow grapes in space for long voyages? Can the spent grapes
be used to make wine with? Is there a market for space wine?

(Published numbers on efficiency need to be scrutinized very carefully,
because there is a lot of specsmanship -- often what is quoted is *not*
overall, end-to-end, low-voltage-DC-to-jet-power efficiency, but the
efficiency of some better-looking subset of the process.)

In this, as in many other areas, "figures don't lie, but liars
figure", I suppose.

One reason for interest in Hall-effect thrusters and other plasma
thrusters, as alternatives to ion thrusters, is that they don't need high
ionization percentages and hence can avoid most of the efficiency penalty.

--
Mary Shafer Retired aerospace research engineer