[ExI] Skylon as first stage.

[Keith Henson]

On Wed, Apr 20, 2011 at 8:12 AM, Eugen Leitl <eugen at leitl.org> wrote:
> On Wed, Apr 20, 2011 at 07:52:39AM -0700, Keith Henson wrote:
>
>> The question is cost, and that's almost all due to the high cost of
>> rockets to get millions of tons of power satellite parts to GEO.
>
> The question is not just cost (by the way, have a look at
> http://www.fastcompany.com/1745113/what-happens-when-solar-power-is-as-cheap-as-coal )
> the problem is EROEI.

I looked at this in some detail from the perspective of a moving cable
(loop) space elevator.  That's the gold standard for lifting stuff to
GEO, being slightly more than 100% efficient.  (Ask if you can't
figure out why.)

Takes (round numbers) 15 kWh per kg.  Reasonable number for power sat
mass is 5 kg/kW.  A good way to look at energy return on energy
invested is how long it takes to get a payback.  For ground solar or
wind it's measured in years.

For a power satellite made with stuff brought up by elevator, 5 kg
will take 75 kWh to lift it.  The 5 kg makes 1 kw, so the payback time
is just over *3 days.*

Chemical rockets are around 2.5% efficient so the payback time is 40
times that long or about 120 days.

For the laser part, it draws around a GW to send 60 t/h to GEO.
(Starting from a sub orbital boost by the Skylon.)  1 M kW/60,000kg is
17 kWh/kg

The Skylon phase burns 66807 kg of hydrogen per launch.  The energy
content for three per hour would be 14029470 kWh (at 70 kW/kg), or 233
kWh/kg.

Together, 250 kWh/kg, (6% efficient) so material for a kW of
production would take 1250 kWh to lift--which gives an energy payback
time of around 52 days.

By renewable energy standards, that's amazingly good.

See any problems with the logic or math?

> When we're looking at kW/kg, we shouldn't forget that the
> kg is at ~Mach 25, and it came from the bottom of the gravity
> well which taxes your EROEI -- unless it came from lunar material.

Later in power sat production it's worth going after lunar material
just so you don't need to be flying so often.  But the way to go after
lunar material is with a moving cable elevator out through L1.  It
takes an investment of around 100,000 tons and pays back the
investment at around a 1000 tons per day (pays back in mass in 100
days).

But you really need big lift capacity from the earth before you go
after lunar materials.

Keith