Scenario: Space Technology Corp gets a contract to build the next-generation rocket for the Mars mission. They go around hiring experts in the relevant fields to do cutting edge research. You, having a PhD in Thermo and Fluids and some great research, get hired without an interview.
5 years later: The bulk of the research has been completed and the new (brilliant and revolutionary, thanks to your contribution) rocket technology is almost ready. The managers at Space Technology Corp see their bottom line growing and want to pull a profit, obviously. So they decide to cut out the unnecessary parts of the program. They see you in the research division with your fat, PhD salary and realize that they don't need you anymore. All the cutting edge research is done, what exactly are you contributing? All future models of the rocket will be derivatives of the first, and they've already got your research. They don't need you anymore. So they give you a big thank you, maybe a plaque, and let you go. You try to get a job in another field (supersonic transport, maybe? I dunno) but no, your specialty is rockets; that's what you do. If no one needs to build a rocket now? Well then you're screwed. In industry anyway. I imagine you'd end up in academia or doing something you didn't originally study for.
I'm still a student (also ME, year 3) so this could be bullshit, or it could be true. I dunno. Best thing to do is talk to people with experience, especially people in the relevant industry with PhDs.
"Welding was faster, cheaper and, in theory,
produced a more reliable product. But sailors do
not float on theory, and the welded tankers had a
most annoying habit of splitting in two."