DozyWannabe, my recent experience is with a major aerospace firm and deSitter describes the software development process pretty well. There are (rare) modules that have to be expertly crafted to fit available constraints, but the bulk is produced as he described. No doubt for brevity he left out a bunch of nits, including that the process rewards happy rote-process-followers, depriving the project of the bright imaginative fellow who might've suggested a design for a thrust-and-attitude 'law'.
My usual apologies for treading where I don't belong....