[ExI] Something completely different (there is hope afterall)

[Keith Henson]

This is something that has not been covered by news nor (as near as I
can tell) has it been the subject of a press release.  But it is
fairly well known to those in the business.

NASA Ames brought a 1.2 MW, 110 GHz gyrotron for testing beamed energy
propulsion.  They still need the power supply, which is 70 kV at 30 A
but it's a relatively small cost.  Up close it will provide well over
10 MW/m^2.to test hydrogen heaters.

They intend to offer it the same way as the wind tunnels, as a
national engineering test asset.

I think it is an accepted truth that you need single stage to orbit
and that it must be a reusable launch vehicle to get the cost to GEO
down to where power satellites make sense economically.

Given the current state of material science and the best exhaust
velocity you can get from chemical propulsion, neither of these are
feasible.

To put numbers on the problem, it takes 9000 m/s to get into LEO.  For
4500 m/s rocket engines, that's a delta V of twice the exhaust
velocity.  The rocket equation gives a mass ration of 7.4 which means
the vehicle and payload can't be more than 13.5% of the takeoff mass.
For a vehicle to be reusable, the accepted minimum structure is 15%,
leaving less than zero for payload.  (Skylon cheats by burning air
partway up, but it's not enough to get a lot of payload to LEO.)

But if you have 9000 m/s exhaust velocity, which can be done with
hydrogen heated with microwaves or lasers, then the mass ratio is a
little less than 3.  So vehicle and payload can be 36% of takeoff
mass.  If half vehicle and half payload, that's 18% each.  So a 300
ton vehicle with a dry mass of 54 tons could put 54 tons in LEO.

The falling cost of microwave power and laser power makes these
options possible.

Beamed power doesn't make economic sense unless you are talking cargo
volumes in the hundreds of thousands of tons per year.

I should also add that I never thought NASA would do something so
sensible.  Those of you who know Pete Warden might send him a thank
you note..

Keith Henson