[tt] [Cosmic Engineers] Orbital mechanics problem

[Eugen Leitl]

----- Forwarded message from Keith Henson <hkeithhenson at gmail.com> -----

From: Keith Henson <hkeithhenson at gmail.com>
Date: Wed, 28 Jan 2009 09:34:19 -0700
To: cosmic-engineers at googlegroups.com
Subject: [Cosmic Engineers] Orbital mechanics problem
Reply-To: cosmic-engineers at googlegroups.com


I know this is kind of mundane for this group, but I have a problem
that some of you might have the insight to suggest solutions.

Power sats are one of only two energy sources that scale to the point
they could replace fossil fuels. I just gave a talk at a military
conference on a fuel plant that would use 120 MW of SBSP to make 1000
bbls of fuel a day by reducing CO2 with hydrogen.  (I will try to put
the PP presentation up somewhere.)

The main problem with any power sat proposal is the lift cost.  It has
to get down in the $100/kg range for power sats to make sense.

I have applied work by a number of other people (particularly Jordin
Kare) to this problem.  We call it pop up and push because it uses a
modest rocket (300 tons) to send a 50t laser stage up to 420 km.
While the laser stage is above the atmosphere, a 4GW ablation
propulsion laser pushes it at just over a g into geosynchronous
transfer orbit (10km/sec).

This takes close to 1000 seconds and a 5000 km acceleration path.  The
long path allows the use of a much smaller laser.  The usual metric
for laser launch is a MW/kg or a GW/t.  This method pushes 25 ton
payloads to GEO 4 times an hour using only 4 GW.  That's a 6 to one
improvement and reduces the cost of the laser from $250 billion to $40
billion.

In order for the laser stage to stay line of sight to the laser, the
laser beam has to be reflected (and focused) at GEO.  It takes some
hundreds of 5 t mirrors.  The lasers are best put in a high, cloud
free area where it is possible to draw 8GW from utility lines.  It is
best when the laser light is going the same direction as the desired
acceleration vector, but the light can impinge at as much as 70 deg.

The long acceleration path is close to 1/8th of the circumference of
the earth.  Going over the limb of the earth blocks the laser and ends
the acceleration.  Thus the geometry requires the equatorial launch
site be about 45 deg from the longitude of the laser.  The neat thing
about this geometry is that the launch window is always open.

The problem is that the perigee of the Holman transfer orbit is 90 deg
from the bounce mirrors.  So five hours after orbital injection, when
the payload is at GEO (apogee) and needs to be circularized (1.6
km/sec) it's in the wrong place for the laser beam (behind instead of
in front of the bounce mirrors).

Any ideas?

Keith

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Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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