Variable specific impluse
Hi EP
Of course hi-Isp is just one setting for a VASIMR - low revvs, so to speak, can be as low as 1,500 seconds. That means ~ 50 kW/kg, though more like ~ 75 kW/kg due to lower plasma-energy conversion efficiency. Much lower and ionisation energy wastes most of the power going into the jet.
For low-gee planetary lift-off there are high-thrust nuclear thermal rocket designs around. For the Moon lower-thrust high-Isp NTRs aren't disadvantaged, unlike for Earth liftoff.
Adam
BTW in a 1975 "Analog" article Donald Kingsbury did a brilliant popularisation of the difference between NERVA and DUMBO NTRs - the old NERVA design just didn't crack the heat-exchange problem efficiently. I just received that issue and it's an interesting read.