Re: Limits and Capabilities.

From: Dani Eder (
Date: Wed Jun 12 2002 - 14:20:36 MDT

> > Space exploration.
> Due to cost limitations, unless the private sector
> gets its act together
> I don't think we can expect to see much in the way
> of space
> exploration. The except would be if the Chinese
> pump a lot of money
> into their space program, in which case we'll have
> another Space Race
> with the US.

The energy cost to put 1 kg in orbit is about 20
kWh, or about $1 at wholesale rates. It takes
about 1,000 kg per person to transport people to
orbit (body mass + chair + share of pressure
vessel, etc). So an efficient transportation
system should run around $1,000 per ticket.
At the efficiency of commercial airlines, for
which total costs run around 4 times fuel cost,
that would be $4,000 a ticket.

The fact that the going rate is about $20M for
a ticket is a testament to the poor efficiency
of current launch systems. For example, did you
know the fuel efficiency of a commercial jet
engine is twenty times that on the Space Shuttle?

One inexpensive way to send people (and other
stuff) into space would use a large pneumatic
tube. Assume a 30 ton projectile carrying a
3 ton cargo needs to be accelerated at 6 g's
(about the limit for humans). If it's diameter
is 2.5 meters, the operating pressure is only
52 psi. That's why I call it a pneumatic tube
rather than a gun. If located on the big island
of Hawaii (which as a shield volcano has long
stretches of constant slope mountainside), you
can get to 2 km/s this way, which is 25% of the
speed to orbit. The projectile carries a rocket
that does the rest of the job of getting to orbit.

The rather small reduction in the velocity that the
rocket supplies has a dramatic effect on the cost.
Without the boost from the tube, the rocket has
to be 83.7% fuel, 15% structure, and 1.3% cargo.
With the boost, the figures change to 74.4% fuel,
18% structure, and 7.6% cargo. So the fuel-to-
cargo ratio drops from 64:1 to 10:1. But more
importantly, the 20% increase in structural
weight results in 100 times the service life for
the vehicle (the fatigue life for aerospace
structures varies by a factor of 10 for each 10%
change in stress in the region close to yield
strength, which is where most of them operate).
So where the Space Shuttle was designed for 100
flights, the pneumatic-launched vehicle would
be good for 10,000 flights (25 years of daily


Having pointed out that space travel can be
relatively inexpensive if done right, I think it
will be a moot point to envision a Mars base
in place by 2030 if the Singularity arrives by
2020. Travel will occur at the speed of light
as your uploaded brain is sent via photons from
place to place. The apparent transit time will
be zero.


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