[ExI] Cooling propulsion lasers in space

[Keith Henson]

To get the cost down to where power satellites make economic sense
(half the price of electricity from coal) means getting the exhaust
velocity higher than is possible with chemical propulsion. i.e.,
lasers, big lasers.

In order to make efficient use of the lasers either the lasers or a
redirection mirror needs to be in GEO. It is a hard engineering
problem for the laser beam to go up through the murky atmosphere,
bounce off a mirror at GEO and come down to focus on a rocket.  Less
challenging for the laser is to locate it in GEO.

But then you have to power the laser.  The vehicle requirement is
about 3 GW (considering losses) so the laser input is ~6 GW and you
have to get rid of 3 GW of waste heat at around 12 deg C.

Powering the laser is discussed here:

http://nextbigfuture.com/2013/09/propulsion-lasers-for-large-scale.html

I now have a six page draft design for a 3 GW condensing steam
radiator.  It's 40 tubes, each 25 meters in diameter and 2700 meters
long.

If anyone would like to look at it, let me know.

Keith