ELECTRICAL PROPULSION: REQUIRED FOR ANY TRANS-LUNAR MISSION
Electrical propulsion is so far superior to any chemical option that it should be used for any deep space mission of more than about 50 days. The mission must start from orbit. The very low thrusts make it totally unsuitable to achieve orbit. Classical rockets from a 300 mile orbit to Mars may be launched only about once every year and a half to take the lowest energy Hohmann transfer ellipse. That takes 277 days with no major variation. With typical chemical rockets the payload is about 12-14% one way. Electrical propulsion has no launch window restriction. You go and come when you wish. The time from worst to best position varies from about 100 to about 140 days. The velocity at mission half time is so high it is not really in an "orbit". It can exceed solar escape velocity! The payload is fairly constant at 25%. But that can be a ROUND TRIP payload. The system requires 50% of the weight be an electrical power system plus structure. That can also be used while at Mars, a bonus. The weight ratios are roughly 25% payload, 50% power plus structure and 25% "fuel" expended to get thrust. The key is very low but steady acceleration. The acceleration depends mostly upon the power per mass ratio of the energy source, but for now 0.01 g is optimistic, and 0.001 g is quite pessimistic. Current solar panels will do that well. Another advantage is that emergency mission changes are possible, and all it does is make the mission longer. You use the remaining fuel at full available power and you still "get home" if you have enough air and food. It just takes longer.