[ExI] Space based solar power again

Keith Henson hkeithhenson at gmail.com
Thu Feb 4 19:09:49 UTC 2010

(reply to a discussion on another list about power satellites)

How you get the energy down from GEO is a problem with a couple of
known solutions.

What has to be solved is getting the parts to GEO (or the parts to LEO
and the whole thing to GEO if you build it in LEO).

Even at a million tons per year (what's needed for a decent sized SBSP
project) the odds are against the cost being low enough for power
satellites to make sense (i.e., undercut coal and nuclear) if you try
to transport the parts with chemical rockets.

You either have to go to some non reaction method, magnet launcher,
cannon, launch loop or space elevator, or you have to go to an exhaust
velocity higher than what the energy of chemical fuels will give you.
The non reaction methods are extremely difficult engineering problems,
partly because we live a the bottom of a dense atmosphere, partly
because of the extreme energy needed.

The rule of thumb from the rocket equation is that mass ratio 3 will
get the vehicle up to the exhaust velocity and a mass ratio 2 will get
it to a bit under 0.7 of the exhaust velocity.  Beyond mass ratio 3
the payload fraction rapidly goes to zero.

So to get to LEO on a mass ratio 3 means an average exhaust velocity
of around 9.5 km/sec

The Skylon gets about 10.5 km/sec equivalent Ve in air breathing mode.
 Laser heated hydrogen will give up to 9.8 km/sec.

So much for the physics, on to the engineering!  :-)


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