Gentlemen, please. Enough with the terrible jokes already. Time for a serious answer.
The Jetstar prototype was originally built for a military application with two 4800 lb thrust Bristol Orpheus engines - that were to be built under license by Curtiss-Wright - and no slipper tanks. The deal to build the Orpheus in the US fell through, and the American military were reluctant to acquire an aircraft with a foreign built engine. Lockheed went back to drawing board and added two pairs of 3000 lb thrust P&W JT12's in place of the single powerplant, added two 560 gallon slipper tanks to counter the increased fuel requirement and as they say in French "Viola!"
Trivia tidbit: The four rear mounted engines configuration Lockheed used was first proposed by Vickers in 1956, for the project that became the VC10.
