Hi Philw -

Chang Diaz's works on entirely new propulsion systems are always interesting. Another of his schemes, the one for a plasma bottle which stores propellant on Earth until launch, is very intersting indeed.

One big problem with plasma and ion engines is the power supply. If you're using solar generated electricity, then you have conversion efficiencies. In this regard, in the case of a comparison with solar thermal engines, nearly everything comes out as way less efficient for plasma and ion engines, since with solar thermal there is no conversion to electricity, but instead the light is converted to heat. This also means that solar thermal engines are far simpler mechanically, which means they both cheaper to build and less subject to failure.

The other big problem with ion and
plasma engines is with plume impingement - you're dealing with charged particles, so naturally they look for balancing mates, and in the near vacuum of space, the spacecraft is the largest thing around -

Personally, I kind of wonder if maybe there isn't some way that both charged particles from the separation could be accelerated, and then recombined, with the energy of the separation turned once again into useful thrust -

Ah well - my thinking here is probably as bad as it is in some other fields. My guess is someday we'll both be surprised by some new engine which the boffins have developed -

EP