25 January 1957 – Missile 101, the first flight-ready Thor, arrived at Cape Canaveral in October 1956. It was erected at LC-17B and underwent several practice propellant loading/unloading exercises, a static firing test, and a month-long delay while a defective relay was replaced. Launch finally took place on 25 January 1957. As the vehicle lifted off from Pad LC-17, Cape Canaveral Air Force Station, Florida, it reached an apogee of 6 inches (150 mm) whereupon contamination destroyed a LOX supply valve causing the engine to lose thrust. The Thor slid backward through the launch ring and exploded on contact with the thrust deflector. The vehicle was destroyed by low-order detonation. Serious pad damage occurred. (1).
A film of prelaunch preparations showed crews dragging a LOX filler hose through a sandy area, and it was that debris had entered the LOX, causing the failure of a valve.
The USAF developed the SM-75 quickly, in just over three years beginning in 1956. Interservice competition to control the emerging strategic missile mission meant that the U.S. Army developed its Jupiter missile, which was ultimately assigned to the Air Force, at the same time. Thor’s rapid design and deployment resulted from having much in common with the Atlas ICBM, which was then still in the planning stages. Thor’s engine, guidance, and warhead came from the Atlas program, and only its airframe was new. (2)
The Thor IRBM laid an important foundation for the U.S. space program as it formed the basis of the long-lived Delta rocket family. (3)
Sources: (1) Wikipedia/Portal Aviation; (2) National Museum of the USAF; (3) https://www.spaceline.org/rocketsum/thor.html