[ExI] Skylon as first stage.

[Eugen Leitl]

On Wed, Apr 20, 2011 at 09:46:34AM -0700, Keith Henson wrote:

> 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.)

This is all very well, but we don't have such elevators, and
the number of launches required to build such an elevator, even
if we had theoretical-strength SWNT ribbons (which we don't)
would seem enough to wreck major havoc on this planet (look
at the environmental footprint of the Shuttle, and Russians
are only slightly better).

Meanwhile, on the Moon you only need commercial aramide.
 
> 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.

EPBT for current CIGS or CdTe is under a year (EROEI is 40:1 at the
moment). The trend is is of course towards better, though it will
turn asymptotic at some point.
 
> 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.

Chemical rockets currently means kerosene/LOX or maybe liquid
methane/LOX or even liquid hydrogen/LOX. Synfuels will screw with
your energy, and e.g. you waste half of energy in liquid hydrogen
to liquify it. Including rocket construction, supply chain, the
EPBT for chemical rocket-launched SPS is probably never. EROEI
needs to be better than 5:1 to bother, so I think chemical rocket
SPS is stone cold dead. The only reason you want to do it is
to supply global wireless power for military applications, where
prices are less relevant.
 
> 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

Yes, but Skylons don't yet exist. It is not obvious we can
make scramjets to work, though of course I hope we will.
 
> 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.

I think we'll be at EROEI of 100:1 and payback times of few
months for terrestrial PV within 20 years or less. I think that
would be pretty good. Of course, long-term is self-replicating
machine-phase photovoltaics both on Earth and in space.

> 
> 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

I think we'll first get chemical rockets from in-situ synfuels
from lunar cryotrap water, and then maglev launches. The advantage
of maglev is that it scales, and can produce effectively continous
stream of material, and is self-amplifying since you can beam
down more power from Earth-Moon space SPS as you run out of 
available flux.

> 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.

The whole point of boostrap with ISRU is that you need minimal
amount of material until you go near self-rep closure of unity,
and none after you're above. The ultimate free lunch, long-term.

-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
______________________________________________________________
ICBM: 48.07100, 11.36820 http://www.ativel.com http://postbiota.org
8B29F6BE: 099D 78BA 2FD3 B014 B08A  7779 75B0 2443 8B29 F6BE