I understand what x-tricks is saying, but I have to agree with neurovore. From experience, one of two things happened when I tried to do the writing first. (YMMV) I either relied too heavily on 'real world' physics and couldn't accomplish what I set out to do or wrote my plot and put myself into a corner. ie. I will explain how they can do this later, but oh, wait, if they can do it, why can't the bad guys. and oh, no, I just contradicted my half thought through rules and the whole thing is falling apart.
I'm not saying that something like that can't be salvaged, but only through lots and lots of rewrite, ususually to the extent that it is nothing like the 'first draft' and I would have been better off doing the research first.
That said, I don't think you have to spend months and months working out every minute detail about the world/universe you are creating. I (now) use the plotting method described in Scene and Structure (Elements of Writing) by Jack M. Bickman. I don't have to have the research to plot out the story, but it helps to identify essential areas that need to be researched before I get to them. Eventually you will want to define the whole world/universe, but you don't have to have it complete before you start writing the story.
my 2 cents
-paul
