I really wish the 747SP had more orders.
A fantastic plane.
I got to fly on it once from Capetown to Joburg to Frankfurt.
Peppy plane, and only 45 made and a handful left in flying condition.
It had a horrible CASM. RayChuang88 says one reason for the SP getting so few orders was improvements to later 747-200's with upgraded engines and additional body fuel tanks that had enough range to fly JFK-NRT nonstop. The upgraded 747-200's could carry a normal passenger load rather than the DC-10 sized load of the SP. I'm sure the 200's also had a higher residual value once it was time to convert them to freighters too.
The SP was the fastest 747 ever built and the highest flying. NASA has converted one into an infrared astronomical observatory called SOFIA precisely due to its ability to fly at high altitudes up to 50,000 feet. Pratt and Whitney has converted a 747SP into a aircraft engine testbed to take advantage of the flight envelope of the SP. These SP's should be flying for a long time to come due to the low number of cycles on them and their special missions.