[ExI] Power satellites

hkhenson hkhenson at rogers.com
Mon Apr 27 01:34:04 UTC 2009

At 03:13 PM 4/26/2009, Mirco wrote:


>The project start with a pilot plant (one small SPS, one rectenna 
>field) then you have experience of the problems, what work, what 
>don't, what is cheap, what is not. Then apply the know-how developed 
>and build another.

Alas, if this were possible, SPS would have been done decades ago.

The problem is partly transporting the parts to GEO or alternatively 
getting materials from the moon or asteroids.

Either way involves lifting a lot of material to GEO or 
beyond.  Taking the parts up requires a shipment rate of around a 
million tons per year.  Going after extra terrestrial resources might 
be done with only 100,000 tons, but it would take decades to 
bootstrap up space industry.

The other hard part is that for optical reasons power sats (at least 
the microwave kind) just don't come in small sizes.

If you assume 5kg/kW (a safe number) then for 5 GW (5 million kW) the 
mass is 25,000,000 kg or 25,000 tons (metric tons of course).  At 5 
tons per launch, 5000 launches.  The rocket guys claim that for this 
many flights they could get the cost down from $20,000/kg to 
~$2000/kg. ultimately to ~$500/kg to GEO.  (Less to LEO, but you 
still have to get power sats to GEO.)

At $1000/kg, and 5kg/kW a power sat per kg will cost $5000/kW just 
for the lift, not counting rectenna or parts.  This is not 
competitive with nuclear.

After working through this backwards, it turns out that to really 
compete, i.e., to take a large share of coal and oil market, we have 
to get launch cost to GEO down to ~$100/kg.  That's next to 
impossible to do with rockets and anything close to current 
technology.  The basic problem is the rotten mass ratio, which is a 
consequence of the low exhaust velocity from chemical fuels.

Laser ablation propulsion is not limited to chemical exhaust 
velocities.  Lasers have been considered for close to 30 years, but 
they require really huge lasers to lift even small payloads, one to 
three MW/kg, 1-3 GW/ton.

It is a feature of the rocket equation that matching exhaust velocity 
to rocket velocity puts the most energy into the payload.  (The 
exhaust gets left at zero velocity and all the energy is in the 
rocket/payload.)   Thus rocket efficiency isn't bad up to a mission 
velocity near the exhaust velocity and costs are quite reasonable.

Combining the high thrust of a chemical rocket first stage with a 
laser second stage using high exhaust velocity allows a relatively 
small laser to push a large payload for a long time, (close to 15 minutes).

There may be other approaches that make a case for getting the lift 
cost down to ~$100/kg, but here is at least one.  More at www.htyp.org/dtc.

>If you want government help, the government could offer a reward for 
>the first power plant able to feed 1 MW to the grid, then to the 
>first able to feed 10 MW, 100 MW and so on. The reward could be a 
>sum of money, a tax break, no tax for a 10 years period, etc.



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