yep auxiliary power Unit
my mate james' father worked for rolls royce
seemingly the earliest vulcans used 1 of its 4 jet engines to drive the power for all systems on the aircratft
if you lost that engine you were up the creek without both paddle and canoe
so you had this one
this was much easier to spin up whilst in the air
and it was used as a big generator to give you some time to try to sort out how you were going to crash.
later planes (that didn't use just 1 jet engine as a generator of power) fitted with bigger jet engines used these as extra ooomph for take off, air/exhaust from somewhere else was ducted in so they span up much faster and could get you down again without loss of craft and life if the 4 big ones died during that point of no return on take off
or as emergency power if all 4 main engines failed in flight.
he may be wrong but was realy quite animated by the conversration so am inclinded to belive he knows what he's on about.
my interpretaion of the glut of info. he supplied my well contain errors
Dave
Dave