[ExI] Power Satellite payback analysis

[Keith Henson]

I wrote this for the blog OFW, but I thought it might amuse people here.

Someone bitched about me saying that a power satellite would repay the
energy to lift it in

“a bit over two months” with

" Keith, I call rubbish on this. Post the study/paper documenting this
silly claim."

My response:

I can reconstruct this from a few numbers right here.

The mass of a power satellite is around 6.5 kg/kW (my work, but it is
about the center for others),

SpaceX big rocket burns about 4,600 tons of LOX and LNG to take 100
tons to orbit

CH4 takes 2 O2 to burn.

16 and 32 so the LNG is 1/3 of 4600 or 1533 tons of LNG to lift 100
tons. Or 15.3 kg of LNG to lift one kg.

So to lift enough for a kW would take 15.3 times 6.5 or 100 kg.
Methane is 55.5 MJ/kg or 15.4 kWh/kg. 100 kg would be 1540 kWh worth
of LNG. Davide by 24 hours per day and you get 64 days for the lift
energy to be paid back, little over two months.

This does not account for the energy it took to make the power
satellite parts, but that is small compared to the lift energy, even
aluminum is only around 1%.

I also left out the reaction mass to move the power satellite to GEO,
which might push the mass you have to lift up to 10 kg/kW.

But if you burned the LNG for power, the best you can do in a combined
cycle power plant is 60%. This factor makes the payback somewhat
shorter.

So using LNG for rocket fuel seems like a good idea compared with just
using it to make electricity.

If you find an error in my analysis, please let me know.


Keith