These are some thumbnail concepts I did for a project a while back. I thought that seeing the process of moving through different revisions of the work might be interesting and give some insight into the process.
I sorta wonder what the practical reasoning behind some of those were, but then I remember that this is sci-fi, there doesn't have to be a reason besides 'it looks badass'. As it goes, model first, explain later.
Ideally, it's really a back and forth kind of thing. You start with a basic idea of what the vehicle should be, 'about this big, mean looking, sleek or funky or whatever' and then make up a design that makes those decisions seem rational. Mechanical design is an element of narrative. It's a way of explaining what something is and does without the need for dry exposition. Once you have a real rough idea of *what* the vehicle is you can get into the million little things that make it look believable. The real magic is in giving the appearance of plausibility to the fantastic. It's a snow job.