hkhenson at rogers.com
Fri Aug 7 01:02:25 UTC 2009
I have had an awful time with this complex
idea. People complain that you have to be a
rocket scientist to read it. If anyone has ideas
on how to simplify the explanation while still
keeping it accurate, please let me know, either here or by private email.
Recently there has been talk about abandoning
NASA's $300 billion project of going back to the moon.
If the goal is a continued and growing human
presence in space, going back to the moon for
flags and footprints won't do it any more than it did it last time.
There are reasons (besides F&F) for a massive
human presence in space. All of the major
problems, global warming (or at least CO2
buildup), energy (including liquid transport
fuels) water, food and poverty can all be helped
if not completely solved with vast amounts of
inexpensive space based solar power (SBSP).
How vast is vast?
In round numbers humans need 25 TW of new SBSP
over the next 25 years starting as soon as
possible. And we would need another 15 TW for
two decades to put 100 ppm of CO2 back in the
ground as synthetic oil (Current world wide
primary energy consumption is around 15 TW.)
And how inexpensive?
Two cents per kWh will under price electricity
from coal or nuclear by half. One cent per kWh
will displace oil with cleaner, carbon neutral
synthetics for about a dollar a gallon. To meet
these goals, the power satellites can't cost more
than $800 million to $1.6 B per GW ($800-1600/kW).
Toward the end of 25 years of construction, the
flow of materials to GEO for power satellite
construction (at two TW/year) will exceed 1000
tons per hour (at 5kg/kW). It seems likely that
most of that will be from extraterrestrial sources.
An initial flow of parts from earth of 100 t/hr
and a cost of $100/kg or less is a near term
design target (build up to this rate in less than
ten years). This side of nanotechnology it's
probably impossible to do with chemical rockets.
To appreciate why you need to appreciate the
rocket equation and "mass ratio." For business
people it is like compound interest at a high
interest rate--ruinous if you need multiples of
the exhaust velocity. A mass ratio 3 vehicle
(100 tons of structure and payload, 200 tons of
fuel) will reach its exhaust velocity. To get to
LEO is about 2.5 times chemical rocket exhaust
velocity and takes a mass ratio of 12. For an
empty mass of 100 tones, the liftoff mass is 1200
tons. A hundred tons of rocket might be able to
hold 1100 tons of fuel, but no payload. This is why rockets are staged.
Laser propulsion is one way to get around the low
exhaust velocity problem. We have understood
laser propulsion for a long time. It is not
efficient at low velocity and requires huge lasers for small payloads.
The combination of a mass ratio 3 chemical first
stage (providing 4 km/sec) and a mass ratio 2
ablation laser upper stage (providing 10km/sec
from an 15km/sec exhaust velocity) looks like it
will get the transport cost to GEO into the sub
$100/kg range. For the same laser, this method
provides payloads of 4-6 times that of laser propulsion from the ground.
Dr. Peter Schubert has done parametric analysis
to minimize cost. Below about $450/kg, direct
from earth construction of power sats cost
less. Above that number, supplying materials from the moon is less expensive.
Even if we start by hauling the power satellite
parts up from earth, in a few years it will makes
sense to construct them partly from lunar or
asteroid materialsespecially if a lot of the
mass of a power satellite is Invar (35% nickel)
or lunar dust used for heat transfer.
In the next decade, it looks like reducing the
cost to GEO using a chemical/laser two stage or
some other method will get space industry
started. (Probably not by the US/NASA though.)
Space habitats happen naturally in the context of
large SBSP production. A crew of 1000 at GEO
building and unjamming automation cost almost
nothing in the context of profits of $500 million
a day. With a materials pipeline of 100 t/hr, supplies are insignificant.
Mining camps at asteroids and/or on the moon make
sense with an established market.
I went into more detail recently. Google Henson
oil drum. A first pass proforma statement
indicated a peak investment of a bit under $60 B.
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