Waterfall doesn't have multiple passes. You have to have all your requirements before you move on to design. And that has to be complete before you move on to implementation. That's why they use the waterfall metaphor, because waterfalls don't flow back up. And if waterfall seems completely braindead now, keep in mind that the old software guys used to be hardware guys, and it makes much more sense for hardware.

You're right about Spiral though... Spiral is iterative, so its much more compatible.