beowulf in space

Robert G. Brown rgb at phy.duke.edu
Fri Apr 25 12:21:27 EDT 2003


On 25 Apr 2003, Dean Johnson wrote:

> In terms of setting up mining operations on other celestial bodies, cpu stability
> and radiation protection are amongst the least of your worries. What will be
> needed is "diplomacy hardened" and "bureaucracy proofed" processes. Technical
> issues are largely tractable, one way or another, but the institutional and 
> international issues will be like herding cats on crack.

<ignore topic="science fiction" comment="off topic as hell"> 

Well, there are also the umm, "economic" issues as well.  As in no
matter what you do, no matter what you say, the theoretical minimum cost
of lifting something out of the earth's gravity well is on the order of
a 100 megajoules per kilogram (mgR_earth is "escape energy").  Ignoring
things like the 2nd law, call it a first-law cost of a buck purchased as
raw electricity at commercial rates.  

However, using rockets to provide lift, the net 2nd law efficiency is
some appallingly low number, as one has to lift the fuel to lift the
fuel to lift the fuel to lift the payload, and then there are things
like drag forces and the fact that failure has a very high cost so
everything is overengineered, and the fact that you have to build a
REALLY BIG vehicle to deliver a REALLY SMALL payload, which kicks in
several orders of magnitude in cost (like 5?).  As in, we are never
going to "explore space" on chemical rockets more than slowly and
infrequently, period, at $100K/kg.  We can't afford it.  Sorry, that's
just a fact and I don't see it changing, not even if we figure out how
to make electricity from fusion reactors and drop the fuel costs.

Electromagnet mass drivers (a la Heinlein) would get rid of the lifting
of reaction mass/fuel problem (which would make a BIG difference) but
leaves you with lots of OTHER problems (like accelerating something to
order of 10 km/sec against drag forces and without exceeding (say) 3-4
g's or cooking the contents with eddy currents, punching it through the
thicker lower atmosphere against nonlinear turbulence that makes the
stuff that ripped up the space shuttle seem like kid's stuff, and more).

This approach would require a huge capital investment, new technologies
galore (and maybe a bit of new physics), and might not ever work.  We
could spend a significant fraction of a terabuck just finding out.  IF
it worked, though, it could reduce the cost (ignoring the amortization
of the initial investment, which was a mostly-ignored hundreds of
gigabucks for chemical rockets and NASA as well, truth be told) to
perhaps $100/kg, which is at least in the not-completely-insane range
(assuming that one could achieve 1% efficiency, which is open to doubt).

I see nobody designing earth-orbit mass drivers.  I see little serious
investment in the entire concept (although my physics students love the
idea and regularly do exam questions on it:-).  Until somebody does, the
space program will be restricted to rare manned big ticket "exploration
of space" trips and lots of unmanned earth orbit flights with a
predictable economic payoff (weather and comm and military satellites).
This goes for the indefinite future.  Just buying the fuel to fill a
shuttle mission has to cost a literally insane sum compared to the
weight of the orbital payload, and the cost of that energy (viewed as
energy) literally defines the value of money and cannot ever become
"cheap".

So sorry, although I >>love<< space exploration and have read a
signficant fraction of all science fiction, the tragic thing about being
a physicist is one has to really work to suspend that disbelief thing
when one can do the math.

Look on the bright side.  With mass drivers it at least >>is<< feasible
to contemplate exploiting here to the moon, maybe even near solar system
(although the cost issue starts creeping in again when you get outside
the moon, as do lots of other things like time of travel).  Until you
hear of work being done on them (with some degree of success) then
forget mining the moon.  They may well have to wait on other
breakthroughs, as well (like viable hi-T superconductors) -- in addition
to atmospheric drag forces and friction, there are eddy currents to
consider, where resistance in the lifted shell is a "bad thing".

</ignore>

   rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu



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