[ExI] Fwd: [Artemis] The Obama-Biden Transition Team looking into Space Based Solar Power.

hkhenson hkhenson at rogers.com
Mon Dec 8 04:48:00 UTC 2008

At 05:44 PM 12/7/2008, Kevin wrote:

>Thanks for the reply.  One thing that nags me though.
>On 12/7/08, hkhenson <<mailto:hkhenson at rogers.com>hkhenson at rogers.com> wrote:
>At 04:19 PM 12/7/2008, Kevin wrote:
>Even with rockets you can make a case, though I think a weak 
>case.   You can make a much stronger case with either the partial 
>elevator that ends a planetary diameter out or using a rocket to pop 
>up above the atmosphere and push the payload with an ablation propulsion laser.
>I've been doing my best to google laser ablative propulsion and I've 
>found a few links but few sources.  If you have any references I'd 
>be happy to look at them.  But I did find one article that seems to 
>ring pessimistic:
>"Cole sees a 21st century where passenger-carrying space vehicles 
>might be powered upward on laser light. That laser would churn out 
>100 gigawatts of power, he admits.
>'That's 10,000 times bigger than any laser that's been built. But, 
>hey, I'll take whatever works,' Cole said."
>So, what size of a laser are we talking about?

4 GW output, 8 GW input. 800 5 MW output solid state lasers or some 
such combination of numbers and power.  The laser beams go close to 
straight up and bounce off 800 five tonne mirrors in GEO.  The 
rockets with the laser stage go up about 90 degrees around the planet 
near or on the equator.

>Does it really take a 100 gigawatt laser to launch a couple of 
>passengers into space?

Doing it straight up from the ground, the standard figure is a MW/kg. 
So a GW launches a tonne and a hundred GW 100 tonnes.  That's large, 
but it depends on what you want to do with it.  It's not enough for a 
Mars Mission.  See Jordin Kare's papers on this subject.  Lofting the 
laser stage to 260 miles with a low performance rocket allows a much 
smaller laser because the rockets have already done the work of 
fighting up through the atmosphere.  So you can get away with 1 to 1 
1/4 g of acceleration for the 15 minutes of sub orbital flight and a 
laser only 1/6th as large.  Jordin says this is a new approach, Dr 
Koelle says it was considered long ago and rejected because of the 
poor performance of lasers in those days.

>Is this a realistic laser we're talking about, or a megaproject as a 
>prerequisite for another megaproject?

Even at 1/25th of Cole's laser, it's a big project.  Solid state 
lasers are $10/W, so the lasers would cost $40 billion.  By the time 
we get all the redirection mirrors to GEO, it going to cost $100 
billion.  On the other hand, if we want to build power satellites at 
a rate where they would actually solve the energy problem we need to 
lift upwards of a billion kg/year.  $100 billion written off over 10 
years is $10 billion a year.  If we are lifting a billion kg, that's 
$10/kg.  The rocket (smaller than the smallest 747) might add $10 to 
$30 per kg to this cost.

The payload at 25 tonnes could be a passenger module for a dozen 
people.  It would not be that rough a ride.  The rocket peaks at 
(IIRC) 3.5 g, the laser injection to geosynchronous transfer orbit is 
just a bit over a g.


Don't hesitate to ask questions.


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