Microwave-powering rockets

Calling this a microwave rocket (see the journal article) isn't ideal, but there's not an obviously better way of describing it. The idea is that you just bring along reaction mass, and let the energy supply be beamed to you. "Furthermore, if the atmosphere around the spacecraft is used as a propellant (in the ramjet or scramjet mode), the amount of fuel that needs to be carried by the rocket can theoretically be reduced almost to zero."

Um, not quite. You could get a huge boost getting out of the atmosphere, but after the first few miles it gets harder to pick up gas to use as reaction mass. Once you're up that high, you need to provide stuff yourself.

Ideally what you'd like your rocket to have on hand is atomic hydrogen, and some magic energy source to heat it up so you can eject it out the nozzle and propel yourself forward. In our not-so-ideal world you have to make do with oxygen and hydrogen, though I suppose fluorine and hydrogen would work too--though I'd prefer to be very far away from the launch. Nuclear rockets use a reactor to heat the fuel, but the shielding is heavy, control is hard, and the erosion you get running plasma through the reactor is scary.

If you can beam the energy from a ground station, receive it and efficiently transfer that to the reaction mass, you have your "magic energy source."

The popular article seems to have gotten a few things confused. The paper says they got 28-36% power generation efficiency (wall power to waveguide power), which isn't something terribly surprising. This is the ground-based stuff--you've got plenty of room to amp up your power generation.

The second part of the problem is power transmission--and they get about 51% of the power getting beamed out in the direction you want, and about 14% actually making it into the thruster.

Side note--they beamed power into the back of a tube with an aluminum nose, to see how much impulse they got--from 22 to 52 mNewtons/second.

With 34% efficiency into actual thrust, they claim a 5-6% power efficiency. That beats rocket fuel.

But that last step bothers me. If only 1/3 of the beamed power goes into thrust, what becomes of the other 2/3? Some is wasted and goes out the nozzle in non-kinetic energy, but not all. And when the rocket trajectory bends and you're not aiming up its backside...

Still, it's a nice measurement, and I'm guessing it would be way cheaper to put together a power beaming station (to send up small rockets) than the huge rockets we make now. But there are a number of little details to deal with in the meantime--like making sure the power transfer to the reaction mass is high performance (even if not efficient), and shielding the cargo and the rest of the rocket fuel, and redundancy in case of a power station failure.