ion engine was RE: [extropy-chat] Saving the Hubble

[Spike]

> Robert J. Bradbury

> 
> > And by the way, this business of using xenon as the
> > reaction mass...Jeff
> 
> Spike may correct me but I believe its because Xenon provides
> the greatest thrust for the amount of electric power you
> put into the engine.  It may also include the fact that
> ionizing a gas, particularly a heavy element, is easier
> (requires less power invested).  I'm sure ionizing solids
> is a bit trickier because you have to probably have to
> vaporize them (perhaps with something like a high power
> electric are or high power lasers???).

Yes, well said, all correct.  There have been experiments
with laser acceleration of ions, achieving even higher
*specific thrust* than is available with current space-based
ion engines.  However the total thrust is very low and the
amount of power used accelerate a given mass of propellant
is very high in these designs.  Still they might have their
uses: when we have a looooong time to get wherever we are
going for instance.


>  That would make
> the engine more complex and probably shorten its lifetime.

There is a big headache associated with erosion
in those laser-accelerated designs.  An example is
in one of the atomic oxygen accelerators at NASA Glenn
in Ohio: it accelerates the ionized oxygen down a 
copper tube.  But copper particles erode from collisions
with atoms travelling at over 30 km/sec.  This inherent 
in the design.  A material superior to copper has not
been found as far as I know.  But my visit to the
facility was nearly four years ago. 

> The choice for asteroids might be oxygen and for comets
> might be neon as both would be fairly abundant.
> 
> Robert

Neon?  In an asteroid?  How would that stuff stay
around?  Did you mean nitrogen?  I do agree with oxygen
as a propellant from comets and asteroids.

As for efficiency, I need to do the calcs for something
I thought of as I read your post: there would
be radiation losses from accelerating the charged 
fluid, bremsstrahlung radiation I think its called.  
Amara probably knows right offhand.  

spike