[ExI] Fwd: Really off topic, microwave propulsion

[Keith Henson]

This went off to the arockets list.

I really should write up a report for the International Space
Development conference last weekend.

There was a transhumanist panel that might be a bit of fun for most of you.


^^^^^^^^^^^^^^

Sorry to annoy some of you, but this is one of the few places on the
net where I can find people who understand the issues and can help
identify problems.

The May 2014 issue of the IEEE Spectrum has this:

http://spectrum.ieee.org/green-tech/solar/how-japan-plans-to-build-an-orbital-solar-farm

The need for high tonnage to GEO has become obvious.  I am groping for
a way to make that jump real.

I mentioned

http://theenergycollective.com/keith-henson/362181/dollar-gallon-gasoline

some time ago on this mailing list.  It's a proposal to use lasers to
heat hydrogen hot enough to get a 7.5 km/s exhaust velocity (2700K).
It improves the 5% Skylon payload to LEO to 20-25%.

Steve Nixon and I designed a 15,000 ton laser propulsion station (most
of the mass is the heat radiators).  While doing so, we came up with a
way to move it from LEO to GEO.  It uses electric thrusters and the
same microwave link up from earth that powers the lasers.
Hard-to-focus microwaves need a huge rectenna, in this case a full km
in diameter.

There are other problems.  The geography of good runway locations is
one.  Given the available locations, the geometry to the ground power
source doesn't work well. The problem is that the ground location
needs to be 2600 km to the east of the runway and right on the
equator.  The only places that are not far out in the ocean are in
Southeast Asia.

We have been thinking about using microwaves from the ground to power
electric thrusters for the LEO to GEO leg.  This is as an alternative
to using lasers.  (Electric thrust just isn't an option for ground to
LEO.)

In the long run, lasers are much less expensive.  This is because the
higher exhaust velocity raises the payload fraction for the hard
ground to LEO leg by a large factor.

Still, as a temporary method, it may be more practical at first than a
laser system powered from the ground.  It would raise some tens of
thousands of tons of payload from LEO to GEO.  It would put a sunlight
powered propulsion laser in the best location relative to a good
runway location.  That way we don't try to use the microwave power
link to power the LEO to GEO transit *and* the laser propulsion
station.

It means lifting both 15,000 tons of lasers and 15,000 tons of power
satellite to power the laser.  Counting ~20% reaction mass and ~2,000
tons of reusable electric tugs, it will take ~45,000 tons of payloads
to LEO.  That's around 3000 Skylon flights.  Or we could build 5-10
power satellites before building the propulsion laser.  At 30,000 tons
per 5 GW power satellite, it would take 2000 flights each.  That would
be or 10,000 to 20,000 Skylon flights plus flights for the laser and
heat sink.

Reaction Engines expects Skylon flights to go under $200/kg at that
flight rate.  For <$200/kg and a cost multiplier of 1.5 from LEO to
GEO, power cost would come in at 3 cents per kWh.  That doesn't meet
my criteria of 2 cents per kWh, but there are a lot of places in the
world where tens of GW at 3 cents per kWh would have a market.

The main reason to post here is to identify problems.

I know of one, the microwave power beam would sweep over the
communication satellites in GEO at ~10 kW/m^2.  I don't know how much
of a problem this will cause.

My biggest concern is what I have overlooked.

Any thoughts?

Keith

http://en.wikipedia.org/wiki/L5_Society