[ExI] simplifying?

hkhenson 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.

Keith Henson

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