re orbital repairstation the difficulties of a high isp otv include long transfer times radiation damage from vanallen belts for both the spacecraft and otv arcjets or xenon thrusters require huge amounts of power so you have to have either nuclear power source messy dangerous and source of radiation damage or big solar arrays sensitive to radiation or heavy that make attitude control and docking a big pain if you go solar you have to replace the arrays every trip with current technology nuclear power sources are strongly restricted by international treaty refueling even for very high isp like xenon is still required and turn out to be a pain you either have to develop autonomous rendezvous or long range teleoperation to do docking or and refueling you still can t do much plane change because the deltav required is so high the air force continues to look at doing things this way though i suppose they are biding their time till the technology becomes available and the problems get solved not impossible in principle but hard to do and marginally cheaper than one shot rockets at least today just a few random thoughts on high isp otv s i designed one once steve collins
