Microwave Thermal Rockets for a Lunar Tug?

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.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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6 Responses to Microwave Thermal Rockets for a Lunar Tug?

  1. Kelly Starks says:

    Of course if you had a system like that, you could SSTO it and save the LOX/LH boost for takeoff and final insertion. And if you launch a SSPS you can boost craft to Luna a lot easier.



  2. kert says:

    Isnt this more efficiently doable with lasers and a beaming station on HEO ?
    No environmental issues and “free” powersource once launched.

  3. Jon Goff says:

    Not really. Beaming power up from the earth doesn’t really have any “environmental issues” that I’m aware of. Second, no laser of that power level has ever been developed. Phased array microwaves systems of the required size, however are a (to my knowledge) well-proven technology. Third, conversion efficiency to laser light is a lot worse than conversion to microwave energy (so you have to deal with a lot more waste heat). Forth, since this would require a SPS-sized power platform to work, it’ll be a while before it happens.

    In the long run, having a ground based (or possibly LEO) based system on one end, coupled with a LUNO, L1, or GEO based system on the other end might make a lot of sense. If done right, you’d get most the benefit of solid-core nuclear thermal, without the issues.

  4. PhysBrain says:

    The system you are describing sounds similar to what I’ve heard about the VASIMR. It also utilizes microwaves to heat up a plasma of pure hydrogen fuel stock. The only difference is the power supply is on board the spacecraft so it does not require the LOX on either end of the trip.

    Another advantage of the VASIMR is the variable specific impulse (I’ve seen values in the range of 1000 – 30000 seconds). The rocket can run with a low thrust and very high specific impluse in a very fuel efficient manner, or the thrust can be increased at the expense of fuel economy and specific impulse. See this page for a chart relating the propellant rate vs. thrust.

    I’m sure the power and weight requirements for a VASIMR propelled spacecraft are prohibitive at the moment, but if the technology can be refined I think this is the engine that will make interplanetary flights a practical reality.

  5. Anonymous says:

    What would be the efficiency of a system like this but using LOX instead of LH?

  6. Phil Fraering says:

    I _DON’T_ have a reference or link, but I remember reading articles in the J. Spacecraft and Rockets on various electric propulsion schemes that worked by using a microwave generator and a resonating cavity of some sort (sometimes with permanent magnets…) It may be possible to build some sort of rocket like you’re describing but without the heat exchanger.

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