Actually, Don - the first two airplanes were twins, the next 202 were quads. Number 2 was converted to four engines, leaving only the Prototype as a Twin. It went on to fly for 25 years as Kelly Johnson's personal airplane.
I have that airplane now at the Museum facility in Everett -KPAE - where I hope to make it airworthy for one last flight to the main Museum at KBFI (doing the same with the 727 Prototype.)
http://www.rbogash.com/jetstar.html
There were actually three different configurations of those slipper tanks used on the Jetstar. Kelly invented all of them and holds the patents on them. He was a big fan of external fuel, and also holds the patents on tip tanks - a wide assortment of them - first used on the P-80 /T-33 and later on the XF-90, F-94, Super G Constellation, C-130, etc.
The slipper tanks were readily removable and you could fly with or without them, although I think in practice, they stayed on. Slipper tanks were also adopted by deHavilland on some versions of their Comet airliner, and the Vickers Viscount also had some airplanes with slipper tanks.
The twin wasn't suffering at all for noise or range - it was the superior configuration - witness essentially all of the biz jets out there these days, as well as most airliners. The original engines were Bristol Orpheus made in the UK. There was a USAF requirement for all American made, so Bristol was working a license agreement with Allison to build the engines in the U.S. The agreement never got made, and Lockheed evaluated two alternate engines and finally settled on the PWA JT12 - which had only 3000 lbs of thrust - and they needed four. Late in the production run, they switched to fan engines with many airplanes retrofitted.
Only four of the Orpheus engines were ever built - one blew on take-off from Burbank, leaving three remaining - we have all three here.
My website section on the Jetstar will tell you all that and lots more!
Bob Bogash
N737G