[extropy-chat] Space elevator numbers III

[Eugen Leitl]

On Thu, Feb 15, 2007 at 09:46:15PM -0800, David Masten wrote:

> > LOX-Hydrogen for the fuel I presume? 
> 
> Not economically efficient, but I'm guessing kerosene use runs counter
> to what you are doing, so sure. At any rate, the trade-offs between
> LOX/LH2 performance and LOX/Kerosene energy density give approximately
> equal payload to propellant ratios.

There is no reason why an (to be developed) air-breathing scramjet 
couldn't go most way to LEO, which would need liquid hydrogen (not 
sure liquid methane would do). Since the packages for lunar bootstrap 
would be under a ton, a large number of small automatic craft would do. 
If we're talking about 300 kg packages to LEO (how many of those 
required for a bootstrap?), current launch methods should be cheap 
enough.

The more local production capacity you install the earlier, the
less total mass you need to soft-land (notice that electronics
survives artillery launches, so "soft" is basically anything not
involving leaving a large glowing crater in the regolith).
 
> On a conservative SWAG, a doubling of strength should give about 20%
> improvement in mass. Modern carbon composites can reach 2 GPa tensile,
> so 5 doublings gives us the strength necessary for a tether (I think, I

I understand the tether design is a pure CNT ribbon 1 m wide, and aready well
in touching distance of the theoretical tensile strength. This does
strike me as only borderline doable, and perhaps not at all doable.

> >  So lifting ten 
> > thousand tons will take 400,000 tons of propellant.  Give me real numbers 
> > and I will recalculate.
> > 
> > Ignoring liquefying the gases,
> 
> Why? Standard industrial processes (liquefying O2 and cracking H2 from
> natural gas) for making the propellants are much more efficient.

Liquid oxygen is dead easy, liquifying hydrogen (96% from methane reforming,
only 4% from water electrolysis) takes 40% of the energy capacity of the 
product (methane is a bit better).
 
> > That's not actually bad, but you are up against a *one* day energy payback 
> > for the space elevator.
> 
> What is the initial capital outlay required, the amortization period of
> the elevator, and the maintenance/operation costs? I have found one
> estimate of ~$10 billion for initial construction.

10 GBucks? Ridiculous. How about one order of magnitude more than that.
Or two. 10 TBucks, chump change.
 
> > Electrical power in to mechanical power out is typically 95 % or better for 
> > large electrical motors.

Which is the reason why a linear motor on Luna (2.37 km/s escape velocity
at Moon surface) doesn't need reaction mass, just photons and recycled
electrons, impulse absorbed directly by the solenoids and thus the Moon.
You might not get a 95% of total efficiency, but it will be high.

Notice that unlike the 6 kT aramide you don't have to launch the motor.
You only launch the actual packets.
 
> Ah. Learn something new everyday.

-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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