Microwaves for interplanetary travel.

[farmerben]

 

The Starwisp design by Robert L Forward uses microwave propulsion.  A specific wavelength between 1 cm and 1 m is chosen, and a net of conductive fibers reflects photons 180 degrees.  

Beam divergence of antenna relays is roughly a constant angle.  So intensity is 1/r from the source.  Theoretically a giant fresnel lens can converge most of the beam, but it has to remain almost stationary with respect to the source and the ship, which is a wee bit difficult.  

So realistically without the lens if you design the ship too have maximum power at 10 km from the source, you would have about 1% of the power at 10,000 km.

A large array of antennas on the moon could broadcast 1 GW of microwaves at say a 10 cm wavelength.  If sails can absorb 10 kW/m2, then a 10m by 10m sail can catch a MW of energy from the source at fairly long ranges.  Giving it over a GJ of power for every flyby of the source.  That is a considerable amount of power to capture lunar orbit, return to low earth orbit, or escape earth SOI even for manned payloads.

Interstellar missions usually envision reflecting photons for maximum momentum and ISP per unit of propellant.  However we could just as easily capture photons and transfer the heat to any material we like for a thermal rocket with high trust, as with the NERVA engine.  A microwave powered ship could use hydrogen, helium, ammonia, or something else as a cooling fluid for the sail allowing it to absorb high power and store up some energy.  

This type of engine would only be efficient near the microwave source at the lunar base so it should stay there.  It could dock with an Apollo command module give it >3 km/s of dV, then drop the payload and reverse thrust to stay in lunar orbit.  This would require less than 5% of the propellant that an Apollo would normally require for the maneuver.  If the propulsion module is very lightweight, it can detach and return to its original orbit, with minimal propellant.

The microwave source is easy to reverse direction, or reduce power.  Such a ship would have vastly more electrical power per unit mass than anything else, via direct photoelectric effect on a net of conducting fibers.  Which means gasses much more difficult to ionize than xenon and krypton are still viable for ion thrusters.