David Findlay wrote:
Why are aluminium modules still being used? Surely even kevlar or similiar
would be lighter.

Because in very rough terms 2/3 of the mass of a station module
is something other than its hull, and all the pain and suffering
involved in making composite structures 14 feet in diameter and
40 feet long was not seen as worthwhile to save the 10% of gross
mass that you'd get by doing so.

The term 'false economy' jumps to mind. Saving a pound that costs
two thousand dollars to launch at an engineering and fabrication
cost of close to a million dollars (don't laugh, that has happened
before) is a really poor overall tradeoff.

Optimize *mission cost* for a given capability, not launched mass.
Mass is cheap. It's several times more expensive than it should
be given reasonable launch providers, but it's still cheaper than
engineer time. NASA spacecraft programs typically cost 3-5 times
what their launch cost is. Making them 2x as large but only half
as expensive otherwise to develop (larger margins, etc) would
go from not changing the cost much (for a project which costs
3x launch cost) to saving 20% or more (for a project which costs
5x launch cost). And the real tradeoffs with mass are usually
a much better payoff than 2x as large for half the cost,
it's often 2x as large for 1/10 the cost if developed right.


-george william herbert