You need to get into the work of Ron Cobb - he did all the functional stuff for Alien and Aliens (and I think the underwater lab in The Abyss). His designs are always based on an engineers viewpoint and he'll come up with a whole practical theory behind the creation.

There's a lot of stuff he created for Alien that you just won't notice by watching the film - if you're serious you'll need to track down magazine articles and artbooks get to get at his plans/sketches, but the DVDs have his designs in the archive galleries and there's stuff online all over the place, but look for interviews (one of the Alien box sets came with an extended doco featuring some good tech chat from him) as well as nice pictures.

Zer-G: Cobb added stuff like extra handrails up near the ceiling of the Nostromo's airlock to account for Zero-G environments - I think there were other similar things scattered throughout the ship.

Cheers