From: Richard Aiken > Although . . . one has to wonder why they used helium in the tests rather than hydrogen, if the later is the fuel they're aiming at actually using > for a craft. > > Safety concerns, perhaps? Hydrogen being a tad explosive and all? Eh, it's mostly the liquid oxygen that makes rocket ships blow up so nicely, not the hydrogen. If you're not in an enclosed area, you probably won't get an actual explosion. I suspect the problem is that hydrogen would interact with and erode the metal in the heat exchanger. Other problems I can see: * 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. * Launching to orbit requires a straight line acceleration run of 0.5 * (planetary diameter) * (planetary surface gravity) / (acceleration); on Earth, about 3200 km for 1G. If we launch at 3.2G, we still need a 1,000 km focal array. Required (diffraction-limited) focal array size is equal to (desired range) * (wavelength) / (target size). If the target heat exchanger is 1m across and we're using 1mm microwaves, that requires a 1 km focal array.
