Space Shuttle Missions - Flight Numbering

Flight Numbering

The U.S. space shuttle program was officially referred to as the Space Transportation System (STS). Specific shuttle missions were therefore designated with the prefix "STS". Initially, the launches were given sequential numbers indicating order of launch, such as STS-9. This scheme continued for 25 launches and eight cancellations up to STS-33.

Subsequent to the Apollo 13 mishap, due to NASA Administrator James M. Beggs' fear of the number 13 and consequent unwillingness to number a forthcoming flight as STS-13, beginning in 1984, each mission was also assigned a code, such as STS-41-B, with the first digit indicating the federal fiscal year offset into the program (so 41-B was scheduled for FY 1984, 51-L originally for FY 1985 and the third flight in FY 1995 would have been named 151-C), the second digit indicating the launch site (1 was Kennedy Space Center and 2 was Shuttle Launch Complex (SLC) 6 at Vandenberg Air Force Base, although Vandenberg was never used), and the letter indicating scheduling sequence . As with the sequential numbers, these codes were assigned when the launches were initially scheduled and were not changed as missions were delayed or rescheduled.

Although the codes were adopted from STS-41-B through STS-51-L, the sequential numbers were used internally at NASA on all processing paperwork. Flights were assigned with sequential numbers from STS-9 through STS-33. With the resumption of flights in 1988, NASA restarted with STS-26R, the "reflight" suffix to disambiguate from prior missions. This continued through STS-33R.

After the Challenger disaster, a sequential numbering system only was used, with the number according to counting from the beginning, although, unlike the initial system, the assignment of numbers was based on the initial schedule and may not reflect launch order. The letter indicated that the intermediate numbering system applied, and e.g. flight STS-51 (a mission carried out by Discovery in 1993) was many years after STS-51-A (Discovery's second flight in 1984).