[ExI] Re :rocket fuel was Re: SpaceX launch

Eugen Leitl eugen at leitl.org
Mon Dec 13 12:29:07 UTC 2010

On Sat, Dec 11, 2010 at 10:31:21AM -0700, Keith Henson wrote:

> > In terms of economy you're likely to employ a hybrid design.
> > A maglev launch stage (e.g. up Mount Chimborazo) instead of a first
> > chemical stage, second stage air-breathing scramjet/rocket hybrid
> > or simple chemical rocket, or laser driven ablation (not sure this
> > will ever work, though current prototypes are sure nifty).
> If you have never looked at the power requirement for maglev, you should.

Most plans assume a ~100 t vehicle, release slightly below Mach 1 at
about 6 km, so it would be around 3 MWh, or 10 GJ. This is equal to about 
a ton of diesel, or about a couple of kilotons worth of charged modern 
supercaps, which can be easily charged from thin-film PV panels adjacent 
to the track.
> > Also, there's nothing fundamentally wrong with chemical rockets,
> > as long as your transport rate is limited, e.g. for a lunar bootstrap
> > using in-situ resources. Suited monkeys are quite pointless, so you'll
> > proceed to teleoperated robots shortly, and whether you teleoperate
> > them from a pressurized can nearby or from rotating ground centers (with
> > the advantage of 24/7 operation) only adds about 2 seconds of latency.
> Dr. Peter Schubert is the only one I know of who tried to put numbers
> on a lunar bootstrap.  He got $2 T and 20 years.

The bootstrap costs are a factor of technology, particularly telecommunication,
teleoperation, automation, and miniaturization. It would be difficult
to put an exact cost, particularly as there will be improvisation
and new technologies developed over the duration.
> Ten times the exhaust velocity takes 100 times as much energy.  We are
> already at $60 B for the lasers, 100 times as much is $6 T.  It would
> take 1.2 TW to power the lasers.  That's more than the installed
> electric power for the US.

The first stage of a Saturn V was average 190 GW, I presume the 
requirements for a 100 ton craft are an order of magnitude lower.
Current solid state lasers achieve 100 kW output, at about 20% 
efficiency, but a 1 MW laser is being assembled for 2013.
Assuming you're leaving maglev at about Mach 1 laser boosting
might help you to get to scramjet ignition speed, so you can go to
high elevations and high Mach numbers at least partly air-breathing
(liquid hydrogen or liquid methane), switching to LOX as oxidizer
when going to LEO.

But you could just launch a conventional rocket stage from maglev.
Especially, if you can go up to Mach 2-3 with maglev alone -- that
might become a bit difficult, though.
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
ICBM: 48.07100, 11.36820 http://www.ativel.com http://postbiota.org
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