This could revolutionize interplanetary flight.

Nice. At 60 km/s, we could launch 24/day if they only need 1 hour to get up to speed. The up to 300 km part would make that economically infeasible to do too many. As they say in telecom, it's the last mile that kills you.

This seems to me to be a rather ineffient way to direct your reaction mass. Wouldn't it be better to narrow the beam and point it into a cylinder filled with a volatile solidified gas (more of a nozzle then a flat plate). I don't know too much about microwaves so I guess the question is how tight can you get a high powered one at a few million miles?

Instead of waiting around for the super-ultra-mega-microwave, why don't they consider a lower wattage for the beam, a smaller surface area for the propellant and increasing the burn time.

I don't know enough about microwave disapation to speak intelegently, but I would guess a solar powered generator in orbit could do the job with much less power and could be used for many launches??? Where are all the physics majors when you need one?

2 thoughts. First, why not make it so that the solar heat itself causes the bleed off instead of waiting for the Earth to rotate for the microwave to be in position again. Second, the return trip will still have to go the long route unless we get a big microwave on the Mars end of things.

This seems like a good idea. It is decoupling the spaceship from its propulsion power requirements.

But I have a question similar to Jardinero's. Why not just use a high-power laser to transmit electrical energy to the spacecraft and then use a high-powered ion engine for propulsion? Is it just a matter of not having advanced the laser transceiver technologies enough?

I note that we're getting into the several kilowatt range lasers nowadays.

"unless we get a big microwave on the Mars end of things"
Power-beaming satellites circling around the celestial bodies we wish to visit ? Of course. Its just sad that theres so little R&D going on in microwave power beaming, it would really come in handy in all sorts of space development scenarios.
BTW, it looks like Japanese havent dropped this ball:
http://groups.yahoo.com/group/solarpowersatelliteplace/message/176

Maybe I missed it in the article, but how do they get back to earth?

I have some concerns about this idea.

First, I am worried that it's pushing limits on either the temperature tolerance of the sail or the fraction of the sail that is low molecular weight reaction mass (that is, probably hydrogen).

If the concept is depending just on heating at the sail, it will have a rather limited Isp, even with hydrogen (since the sail can't get too hot without itself being destroyed). Reaching 60 km/s would require a large mass ratio, even with hydrogen dissociated to atomic hydrogen and heated to the temperature limit of carbon.

The idea would make more sense if one could heat the outgoing gas after it left the sail. One way to do that would be to embed a coil in the sail to create a magnetic field, and use electron cyclotro heating in the gas. I don't know if the magnetic field you could get would be high enough for this to work at the wavelengths he's thinking of, though. (Ditto for heating at the electron plasma frequency.) One might be able to do something like transmit two closely spaced frequencies and have them mix at the sail (via rectennas or some other non-linear process) producing a lower beat frequency that could drive plasma heating.

Dan asked: why not laser powered ion engines? The acceleration of that approach would be much lower. This scheme is promised to reach 60 km/s in an hour, which is an average acceleration of 1.7 gees. Ion engines (even ignoring the mass of the PV array) achieve accelerations orders of magnitude lower.

(Rand: your spam filter made me hyphenate 'non-linear'.)

Sorry, Paul. Can't spell n0nlinear without 0nline, you know...

I had always had this idea that one could attach a giant solar sail to one end of a spacecraft. Then, put a ion drive pointing on the other end. Just make the solar sail out of organic solar cell material to capture the particles and convert to electrical energy to power the ion drive. Not only would you receive the benefit of the solar wind applying pressure to the sail but also absorbing and redirecting that collected energy into a supplementary propolsion system. Since the sail itself is the solar energy collector it would produce power and propulsion even when tacking into the sun for the return trip.

Recent advances in organic solar cell technology would allow one to actually make the sail itself out of flexible electrical energy producing materials.

I'm in the camp with the return to earth problems. This may be great for transporting unmanned probes, but for human interplanetary flight, it's hard to get humans to want to fly a month in a vehicle without their own means of ensuring a return flight home.

In the old Mars Reference Mission, the great idea was to not carry the return fuel with the spacecraft. Now the new idea is to carry the fuel but not the engines. We may already be there if we could carry the engine and nuclear fuel.

Paul: I realize that ion engines plus laser photovoltaic receivers will have lower thrust. On the other hand, you could use the propulsion method going both from and to Earth, and you could use the method for both acceleration and deceleration. I would be interested in knowing how much more time it would take the ion engine/laser setup to make a trip to Mars.

On getting to and from planetary surfaces.

This should only be viewed as half of a system. To me this craft would function more like a cycler going to and from various planetary orbits, constantly. There could be an entire fleet of these things constantly in motion. With enough of them you could have something like a train schedule. Even if their wasn't one going direct to your destination, say Saturn, you could catch the going by Mars and wait a few months for the one going from Mars to Saturn orbit.

You would than use a different craft to ascend and descend from different planetary surfaces.

Of course Leland we could always make sure the ship has an engine and fuel for a one-way trip, then the sail takes them out without using fuel. So the return trip will take half a year or whatever they figure, it's still makes things far easier to get the astronauts there in a jiffy.

And shooting a microwave tranmitter might be worth it in the long run.

The real key to this is on missions like the one to Pluto or one to Jupiter. We could send a probe out there in a fraction of the time it takes now.

A microwave radiator array whose beam is electronically steerable is the active component of a phased-array radar. The most poweful such radar currently operating in the U.S. is the COBRA DANE radar in Alaska. It's about 1/4 as powerful as the Benfords' 60 megawatt requirement.

I'm sure, given COBRA DANE's 30-year-old technology, that a 60Mw device, optimised for space sail pushing, could be built using modern components for much less than COBRA DANE cost - particularly if it's built somewhere with a less hostile climate than Alaska.

Building an equivalent device that could be sent Mars-ward and auto-depoyed in Mars orbit, though, would likely be quite a pricey proposition.

I don't think a phased array transmitter is needed for this application, since the beam doesn't have to slew at high rates. A collection of mechanically steered dishes (with their phases matched) would be adequate, I think.