Re: A venerable workhorse
"Yet with today's trend for large twin jets, one day there will be no large 4 jet airframes left flying"
The 747-8 is still being manufactured. While large twinjets represent the majority of commercial aircraft, there is no indication that four engine aircraft will disappear.
"NASA uses an old 747 as a flying telescope, SOFIA. This is a remarkable piece of kit, extremely useful for a lot of astronomers across the world. The higher it flies, the better it works. A 747 can fly surprisingly high, thanks in part to having 4 engines (lots of surplus power)."
That isn't entirely correct. SOFIA uses the 747SP jet, of which only 45 were made. The 747SP is a special performance version of the 747-100 with a shortened body, so it is much lighter but with the same power as the 747-100. Any normal commercial jet, twin or four jet, is not going to have the service ceiling of the 747SP. But if NASA needs to replace the SOFA jet, they can just chop the fuselage of any large twinjet, and get the same effect. Power to weight ratios aren't rocket science.