"...in my mind, if you were to put a 747 into orbit, the internal air pressure and the outside vaccum would surely make it rip open? the space shuttle must be made of some strong stuff..."
Not really. When things were going slow for Boeing in the early 1970's, they proposed the 737 fuselage to NASA as the basis of an orbiting laboratory to be carried to orbit by the space shuttle.
The differential pressure capability of the basic 737 fuselage would have been fine for pure O2, partial pressure environment.
Look at it this way: a jet which flies at 40 000 feet has about 4/5 of the atmosphere below and only 1/5 above. At 12 000 m, ISA is 194 mbar
The passenger planes are normally not pressurized to sea level - it would make them too heavy, and humans do have some ability to deal with lower pressure in mountains and elsewhere. Roughly 3/4 of the sea level pressure is regarded as enough: at 2500 m, ISA is 747 mbar.
This leaves 553 mbar differential to be borne by the fuselage. 553 mbar pressure can be found between 4500 and 5000 m.
People live at that height - sleep, drive cars, bear children, without additional oxygen. People who have just travelled there suffer mountain sickness, though. But oxygen can fix that completely.