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

From: Grimble Gromble (grimble.gromble_at_virgin.net)
Date: 08/29/04 

Date: Sun, 29 Aug 2004 10:04:33 GMT

"AA Institute" <abdul.ahad@ntlworld.com> wrote in message
news:adbf5bc1.0408281155.5061fbbf@posting.google.com...
> The asteroid's solid, rocky body would provide insulation against the
> surrounding cold space temperature of just 3 degrees Kelvin (-270 C)
> from creeping into the crew habitat carved out of its interior, or
> more accurately, from the heat escaping outwards.
Going purely on memory (which runs permanently on empty, so don't believe a
word of it), absolute temperatures are measured in Kelvin (no 'degrees').
Temperature differences might still include degrees (less sure about that).

> Does anyone know those Physics things like specific heat capacity...
> thermal insulation stuff when it comes to 3 or 4 miles of rock
> surrounding a heat source?
Basalt has a typical thermal conductivity of about 2 W/mK at room
temperature, granite slightly higher. I'm not sure about rocks, but ceramics
such as aluminium oxide conduct heat better (significantly so) at lower
temperatures.

Snow typically comes in around 0.2 W/mK though that includes air.

Silica aerogel (again including air) at a density of 0.1 g/cm^3 does a
lovely job at around 0.003 W/mK and is pretty well constant over a very
large temperature range (down to 70 K in my book). Regrettably, I don't have
any figures for the amount of stress an aerogel can withstand - probably not
a lot - otherwise you might want to consider turning the bulk of your
asteroid into an aerogel (consider the impact protection).

Assuming you want to keep your asteroid at a constant temperature, you can
ignore anything to do with specific heat capacity as this is only relevant
to changing the temperature of a body (e.g. heating up the inside to a
comfortable temperature before you set out). if you're not generating power
to maintain a constant temperature, then it will become a factor in the
cooling rate of the interior. Happily, a lot of materials with low thermal
conductivity also have a high heat capacity.
Grim

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