Comment Re:Always left out... (Score 1) 201
People often miss that the problem with ion drives and other electrical drives is that the exhaust velocity is too HIGH, not too low.
The higher the exhaust velocity, the more power you need for the same thrust. Making high specific impulse drives is easy - a microwave source can be ~80% efficient and has an exhaust velocity of the speed of light. The problem is that the power requirements are enormous.
Sure, energy from the sun is "free", but the mass of the solar cells to collect that energy is not free. With a speed-of-light drive the thrust to weight ration is exceedingly small for conventional power sources (which includes nuclear and solar) so the acceleration is too small to be useful for most applications.
If you imagine a solar powered spacecraft, you need to be sure that over the lifetime of the mission you would get more total velocity change out of the solar cells and electrical drive than you would from the same weight of chemical rockets. For ion drives and long missions this is true, but for a photon drive (or any other propellant-free drive it is not true for any reasonable length mission).