The original Navy contract with Douglas for the D-558 high-speed research program specified six turbojet-powered aircraft of various configurations centered on a straight wing design. It was amended in January 1947 to come in two parts- Phase I was for three turbojet-powered straight wing aircraft (which became the D-558-1 Skystreak) and three turbojet/rocket-powered swept-wing aircraft for Phase II, which became the D-558-2 Skyrocket. Although the Skyrocket did set records, the main emphasis of the Phase II program was data collection as opposed to chasing records.
One of the major areas of study in the Phase II program using the Skyrocket would be drag reduction. With a new generation of supersonic aircraft already taking shape on the drawing boards of aircraft companies, the bulky and boxy shapes of the external stores like bombs and fuel tanks of the Second World War generation still in use needed an overhaul lest they negate the high performance of the aircraft to come. Legendary Douglas Aircraft designer Ed Heinemann and his engineers set about developing a new aerodynamic shape for external stores that would be flight tested on the D-558-2 Skyrocket's wing pylons. In addition, Heinemann was tasked to develop a data set on the optimum shapes for external pylons.
The two basic shapes were very similar in profile to the fuselage of the Skyrocket- the first shape was an external tank that was 180 inches in length and could hold 150 gallons of fuel. The second shape was a general-purpose bomb that was 120 inches long and weighed 1,000 lbs. Both shapes were extensively flight tested on Skyrocket number three, validating Heinemann's calculations on drag reduction. The pylon shape was also tested, both with and without the stores on the Skyrocket.
The flight tests were so successful that the layout of the external tank formed the basis of a whole family of external tanks of different sizes used by US military and the 1,000 bomb became the now-common Mark 80 series of bombs that over the years have permutated into different variations from the Snakeye air-retarded bomb to the Paveway series laser-guided bombs and the GPS-guided JDAM munitions.
Interestingly, the shape and size of the external tank tested on the Skyrocket was very similar to that of the Mk-7 nuclear bomb. Sandia National Laboratories, the developer of that nuclear store, followed the Skyrocket external stores tests very closely.
Source: Skystreak, Skyrocket, and Stiletto: Douglas High-Speed X-Planes by Scott Libis. Specialty Press, 2005, p78-80.
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