Cancelled American Missions...

Mercury cancelled missions (1963)

After the first six successful Mercury manned missions, another three aditional flights (Mercury 10 to 12) were originally planned, all to lift off before the end of 1963. These were cancelled for budget reasons and to implement quicker the first flights of the Gemini program.

Gemini 9 (1966)

Elliot See amd Charles Basset were in line to fly as Prime Crew for Gemini 9. However, on February 28 1966, both were killed when their T-38 Talon crashed into a building at the Lambert Field airport in St. Louis, Missouri before Gemini 9 flew. The backup crew became the new prime crew:

See and Bassett (seated) were killed in a T-38 before their Gemini 9 mission. The backup crew (Stafford and Cernan) made the flight.

Apollo Program (1967/74)

Apollo 1 is the official name given to the never-flown Apollo/Saturn 204 (AS-204) mission. Its command module (CM-012) was destroyed by fire during a test and training exercise on January 27, 1967 at Pad 34 (Launch Complex 34, Cape Canaveral, then known as Cape Kennedy) atop a Saturn IB rocket. The crew onboard were astronauts selected for the first manned Apollo program mission and all three died in the accident: Command Pilot Virgil I. "Gus" Grissom, Senior Pilot Ed White and Pilot Roger B. Chaffee.

Due to budget constraints there were various cancelled Apollo missions during the Apollo program. Along with Apollos 18, 19 and 20, which received some level of planning, there were a variety of later planned flights. Some of these were incorporated into the Apollo Applications Program, of which the only result was the Skylab space station.

Originally, NASA produced fifteen flight-worthy Saturn Vs; inclusive of two unmanned tests, this was enough to provide thirteen manned missions; these would have been the missions that took place plus Apollo 18 through Apollo 20.

The first mission to be canceled was Apollo 20. On January 4, 1970 NASA announced it was canceling the Apollo 20 as its Saturn V rocket was now needed for the Skylab space station and budget restrictions had limited the Saturn V production to the original 15 flight models. Then on September 2, 1970, NASA announced it was canceling what were to be the Apollo 15 and Apollo 19 missions. Apollo 15 was originally meant to be an H mission — like Apollo 12, 13 and 14. These cancellations meant that Apollo 15 became a J mission — three day stay on the moon with the lunar rover and that Apollo 18 would no longer be launched.

Deke Slayton was the Director of Flight Crew Operations and effectively chose the crews for the flights. During the early Apollo missions he had used a rotation system of assigning a crew as backup and then three missions later they would be the prime crew. However, by the later Apollo flights, this system was used less frequently as astronauts left the program, Slayton wanted to give rookies a chance, and astronauts didn't want to take dead-end backup positions.