[ExI] Low energy payback time

[Keith Henson]

The Skylon/laser combination gets the energy payback time for power
satellites down to about 53 days at ~6% energy efficient getting parts
to GEO.  (Rockets are ~2.5% efficient.)

This is from a preliminary look at a system that uses a cargo plane
for the first ten km, microwave heating and burning methane to 25-30
km and mach 5-6, microwaves alone heating methane to around 7 km/s,
then hydrogen heated with a laser from there on out to GEO.  60 t/h to
GEO with expansion to 360 t/h at somewhat lower cost.

It looks like ~11 percent energy efficient, which gives an energy
payback time of under a month.

This is made possible by recent cuts in the cost of microwave and
laser beamed energy.

Keith