Given the power requirements of ion engines, I would think it would have to be one hell of a large array, or a fission plant. Has there been a power breakthrough with this engine?
Rocket engines have two overall factors
Isp - Specific Impulse, and total thrust.
Engines with a high
Isp, that is they squirt out the propellant really really fast and are incredibly efficient, still may have a low overall thrust. An example of this being an ion engine. The advantage being that they're very efficient, and can operate for very long periods of time, building up acceleration like compound interest in a bank account.
Other chemical rockets, like a solid or liquid fueled booster, the Space Shuttle, etc. may have an incredibly high thrust, but a poor
Isp.
The analogy could be like comparing a garden hose with a spray nozzle vs. a river. The garden hose has a high
Isp, but not lots of thrust. The river has lots of thrust, but a low
Isp. Or if you prefer an electrical analogy, think of the difference between voltage and amperage.
VASIMR has some attributes of both. It accelerates the gas/plasma to the best
Isp ever achieved, but also has a somewhat higher thrust than ion engines. Kind of like a fireman's hose. An extremely high
Isp, but a fair amount of thrust too. And the V in VASIMR stands for "Variable", so it can temporarily generate more thrust forsaking
Isp, and can also go into "cruise mode" where the
Isp is maximized but thrust is less.
VASIMR still takes a ton of electricity to operate, and IMO a fission reactor would be the best choice, especially for Mars, where solar becomes marginal in terms of efficiency, but it's at least got a lot of bang for the buck, or Watt.
