Re: Interstellar Propulsion idea using an Asteroid and a few comets!

From: Grimble Gromble (grimble.gromble_at_virgin.net)
Date: 08/30/04 

Date: Mon, 30 Aug 2004 13:29:34 GMT

"AA Institute" <abdul.ahad@ntlworld.com> wrote in message
news:adbf5bc1.0408291046.1b3c3daf@posting.google.com...
> Let's say we have a starship of biosphere volume 3km x 3km x 0.2km
> (ceiling), population average of 2,000 people in total carried on this
> hypothetical voyage to Alpha Cen. How much Uranium (kg) would be
> required to power the day to day electricity needs of this comunity of
> people, plus all biosphere heating, lighting, etc needs over 4,300
> years continuously?

Not sure why you're thinking in terms of a rectangular biosphere as gravity
won't be much of a consideration but I have estimated a sphere of radius
750m would provide roughly equivalent volume.

One thing that bugs me about how much they reckon you can save on
electricity using energy efficient bulbs is that, for most of the year in
this country, what you save you then have to spend on heating to replace the
energy the old bulbs gave out in heat. Similarly, it doesn't matter whether
the energy is initially used for lighting or for pumping air and water
about, it will all end up as heat. Consequently, you can calculate the power
requirements simply by considering how much heat is going to be radiated
from the external surface of the asteroid. Without insulation, such a
biosphere at 300K would radiate (and thus require a power source of) 3.5GW.

Assuming the biosphere occupies the centre of an asteroid, made of rock with
a thermal conductivity around 2Wm^-1^K^-1, a little bit of modelling allows
us to calculate the external temperature, and hence the power requirements
given the size of asteroid.

A fairly modest wall just 2m thick reduces the power requirements to 0.7GW.
30m brings it down to 90MW and given an asteroid 2Km in radius, 8MW will
suffice. At this size, the external surface would have a temperature of 40K.

There is probably little to gain by going much larger as the power
requirement doesn't drop much below 6MW which will sustain a 6.4Km asteroid
with an external temperature of 21K. The mass however increases with the
cube of the radius.

Given how much better we can do by replacing the rock with some decent
insulation, and how much mass can be saved by doing so, it would suggest we
don't bother with an asteroid at all! Sorry.
Grim

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