Solar for a Mag-Beam

The Mag-Beam propulsion concept was developed by Robert Winglee and his fellow researchers as a means of propelling quick interplanetary shuttles without mucking around with rockets. Basically it’s a big plasma rocket turned into a big plasma gun, that fires a fast, hard ion-beam at a magnetically ensheathed shuttle vehicle. This gives the shuttle a big shove and sends it on its way – to be slowed down at the destination by another Mag-Beam, or a bit of adroit aerobraking if it’s slow enough.

Problem is the Mag-Beam wants a lot of juice to do the job – for example, a 20 ton shuttle being accelerated to 20 km/s needs a 300 MW Mag-Beam firing at it for about 4 hours, which is a lot of battery mass (3,000 tons at 400 of battery.) Once the beam has fired the massive battery pack can be recharged via solar power over a few weeks or days before the Mag-Beam is needed again. But is the battery pack needed at all?

Geoffrey Landis designed a Solar Power Satellite that beams 1 GW to the ground @ 33% efficiency and it massed just 1,300 tons which means its in-space power output is 3 GW – ten times the power needed by the Mag-Beam. Thus an in-space SPS power source for a 300 MW Mag-Beam need only weigh 130 tons. That’s a mass that could be launched in one piece by an Energia or Saturn V class launcher – like the new Ares launchers for NASA’s Return to the Moon. To do everything the Mag-Beam is required to do the power has to be delivered to multiple Mag-Beam plasma-gun stations. What made Landis’ SPS so light was that it remained in an orbit perpetually pointed at the Sun, so there was no need for a rotating power transfer collar from the array to the rectenna, and the same reflectors used for power gathering act as rectennae for power transmission in the Landis design. To keep that simple design, and power Mag-Beam stations in multiple locations, Power Relay Satellites – really just microwave wave-guide horns for changing a beam’s direction – might be needed. Or we might just bite the bullet and have a rotating connection between array and station. Both add mass, but it’ll be a LOT less than 3,000 tons of batteries. Plus PRSs can point at rectennae beaming power up from the ground to transfer back down to the ground at another location, allowing a PRS to do multiple roles and make money transferring power to areas of peak demand on the ground.

Aside from propelling shuttles to Mars and elsewhere a Mag-Beam can also boost a sub-orbital vehicle ( a modified Virgin Galactic SpaceShip, for example) to orbital speed. The power required maybe higher, but as a shorter burst. If the sub-orbital vehicle can boost to a horizontal speed of ~ 1 km/s, then another 6.8 km/s is needed for low orbit. At an acceleration of 20 m/s^2 that’s a boost for 340 seconds, just under 6 minutes. In energy terms it’s the equivalent of 120 minutes of a 300 MW beam. Perhaps the higher power can be supplied from the ground?