Before I continue with my ACES reporting, I wanted to bring up an idea that I had recently. A few days ago, both on Hobbyspace and on one of the mailing lists I was on, someone posted a link to this article about microwave thermal rockets. Now for some serious techno-nerdlichkeit.
Their basic idea is that you use a ground based microwave array that focuses its energy on a heat exchanger on the launch vehicle. The vehicle flows LH2 through the heat exchanger where it gets nice and toasty, and then that now GH2 is flowed through a nozzle, yielding a really high Isp (750-1000s). The nice thing about this idea is that there are currently existing microwave systems capable of putting out this kind of power, and only the heat exchanger has any really complicated parts in it. Also the heat exchanger can double as a reentry shield. The paper documents a 1000kg GLOW SSTO RLV based on the idea.
Now, I’m not so sure if this idea makes tons of sense for the Earth-to-Orbit launch market, due to the really high G’s needed toward burnout, and the really low payload, but it got me thinking about other things.
Basically here’s my crazy idea. Use a ground based microwave system about the size mentioned in the paper (around 300MW, using 300 1MW gyrotrons in a phased array). Have a small (20klb or so) tug in LEO. The tug has a modified RL-10 that can either burn a normal LOX/LH2 mix, or can route the hydrogen through the heat exchanger and directly inject it as a hot gas. Say designed for something like 800s Isp. You then have about 45% of the mass in propellants (mostly LH2 with a few tiny LOX tanks), some 10-15% in structures, and then about 40% in payload (about 8000lb). The whole thing could probably be launched on a Falcon IX and refueled by Falcon Vs or Dneprs. The earth departure burn would be done with the microwave thermal system, and the lunar insertion and earth return burns are done with the LOX/LH2 mode (until a similar system could be setup on the lunar surface). The system would return to LEO using aerobraking. The microwave system would probably be located in the tropics on a high mountain (like say in Ecuador), and the earth-departure burn would be made as soon as the vehicle appeared over the horizon relative to the microwave station.
The interesting thing is that such a system could probably allow for 2-4 lunar flights per month, and could support as many as 12 two-person lunar landings per year per transfer vehicle.
Anyhow, just a crazy idea. We now return you to your normally scheduled program.