I'd argue that designing an aircraft still fails badly with waterfall and it is a major reason that aviation is massively behind where it should be right now. Space-x shows that you can build rockets, a lot better, if you ditch that waterfall process.
Agreed, waterfall fails right off the bat at "gather requirements."
If you ask the Air Force how fast a new proposed plane has to fly, they'll say "as fast as possible." Or maybe they'll say "fast enough to accomplish the mission and win wars." But neither of those would be accepted as requirements.
So in reality the Air Force will answer that question with another question: "well how fast can you make it fly?" Which of course depends on a lot of tradeoffs of performance, munitions, and cost. So in reality lots of design and technology tradeoffs go back and forth during the requirements phase, and at some point someone makes a rather arbitrary requirement like "The plane must be able to fly at Mach 2.5." That gets set in stone and drives the program forever after.
So the very notion of "requirements" is complete fiction. Perhaps the plane could have a top speed of Mach 2.2 and still achieve the mission, or go Mach 2.8 and fail to achieve the mission, depending on other aspects of the design.
