From: Richard Aiken

> Plate the pipe interiors with iridium?

Not sure of the chemical reactions involved. Hydrogen reacts with most metals, though, you might want something like sapphire.

>* Power density is limited by the heat transfer rate of your heat exchanger. This tech isn't very useful if T/W is less than about 5.
>
> LOTS of seperate, small-diameter pipes?

You need to be able to get microwaves inside of the exchanger. You should be able to manage a megawatt per square meter in a fairly straightforward way, and ten megawatts per square meter in a less straightforward manner, but a million-newton (100T) thruster at 600s requires 3GW. This doesn't mean impossible, but there's a reason most schemes seem to involve an expendable heat receiver.

> According to wikipedia, the Jansky VLA has an effective focal diamether of 36 km, so 1 km for a microwave array would seem to be doable.

Receivers and transmitters aren't the same thing. This doesn't mean you couldn't have a super-large transmitter, it's just a significant engineering challenge.

Anyway, I'm by no means saying this technology can't work. It's just that it's not obvious that there's anything really noteworthy here.