Whenever it comes to building something in a factory for sale to the public you they have to weigh production costs to profit to life expectentcy.
A friend of mine is a engineer, Dell computers, the box that your computer comes in cost more to manufacture than your computer.
BUT your computer is still in brand new condition and works when you take it out of the box even after being dropped, stepped on, bounced around in the back of a semi trailer for 1,000 miles and ran down a series of convor belts and tossed like a football by the package handlers at the warehouse.

 

Spike...if what you said is true, I wonder if it's because the production of computer components is mostly done with robotics. I know at one point my brother-in-law worked at a factory where they handled corrugated cardboard, maybe that process is really labor-intensive...I don't know. Interesting tidbit though, I'm a "systems engineer" and I deal with Dell computers on a daily basis.

Actually a few weeks back I was watching a history or discovery channel show on the Titanic and its' shortcomings. The naval architects were talking about how the Titanic pushed the threshold of engineering for the period. One gentleman who had a theory on the achilles heel of the Titanic said "Engineering isn't about making something strong...any idiot can do that...you just throw a bunch of material at the problem. Engineering is about building it with the least amount of materials for the cheapest cost." (or something to that affect). Which really struck home with me personally...

I just bought a new home and during the home inspection, the inspector looked at the front deck and said "HOLY S!*%, this deck is built to withstand world war III!!!" It's a nice looking deck and the seller did a great job with it, BUT he put waaaay more concrete into the job than he needed to. I'm not faultiing the guy because I would have probably built in the same way because I don't have a great understanding of forces and building materials and all of that.

So really the point that I was trying to make is, it's entirely reasonable that you could replace many steel structural components on the Discovery with aluminum and suffer no loss in strength. The main guarantee here is that the cost is going to be much more than what it would cost to do in steel.