After the Apollo-Soyuz mission in 1975, NASA completely shifted all of its resources to the Space Shuttle program. However, NASA still had a large inventory of the Apollo hardware and much of it was given to museums and/or put on display at NASA space centers.
Much of the leftover hardware would have been used to support Skylab B missions.
At one point, the Skylab B was going to be used for the Apollo-Soyuz mission. This was scaled back to just the command module with a special docking port due shifting focus to the Space Shuttle.
The breakdown of the hardware and their current location:
SA-513: only third stage left after the first and second stages were used to launch Skylab. This stage is displayed at Space Center Houston. Originally slated to launch Apollo 18.
SA-514: first, second and third stages with the IU (ie the rocket's computer). The first stage displayed at Space Center Houston, the second and third stages are at Kennedy Space Center. Originally slated for Apollo 19, briefly considered for Apollo 18.
SA-515: first, second and third stages with the IU (ie the rocket's computer). The first stage is displayed at the Infinity Science Center in Mississippi, the second stage is displayed at Space Center Houston, and it's third stage was converted to the second Skylab. Originally slated for Apollo 20, later would have launched Skylab B. The backup Skylab is displayed at the National Air and Space Museum in Washington DC.
Saturn IB (SA-100 series)
SA-109: on standby for a Skylab and Apollo-Soyuz rescue. Late, the cancelled Skylab 5 mission to lift Skylab workshop's orbit to endure until Space Shuttle was ready to fly. (In hindsight, this mission should have been flown considering since it took the shuttle longer than anticipated to fly. But it was not known at the time Earth's atmosphere would expand like it did in the late 1970s, causing drag on the station and reentering the atmosphere in 1979.)
SA-110: was used for the Apollo-Soyuz mission.
SA-111: Unused, possibly slated for a Skylab mission. The first stage is displayed at Alabama Welcome Center on I-65 in Ardmore, Alabama. The second stage is displayed outdoors with Skylab underwater training simulator hardware attached at the U.S. Space and Rocket Center in Huntsville, Alabama.
SA-112: Unused, possibly slated for a Skylab mission. The first stage was scrapped and the second stage converted to Skylab.
SA-113: Unused, possibly slated for a Skylab mission. Only the first stage was completed when the Apollo Applications Program was curtailed in 1969. This stage was later scrapped.
SA-114: Unused, possibly slated for a Skylab mission. Only the first stage was completed when the Apollo Applications Program was curtailed in 1969. This stage was later scrapped.
Command/Service Module (block II CSM-100 series)
CSM-111: originally slated for Apollo 15 when it was a H-type mission, this mission was swapped for a J-type when the lunar missions would be cancelled after Apollo 17. Later, it was used for the Apollo-Soyuz mission.
CSM-115: Incomplete, originally slated for Apollo 19, later Apollo 18. Note: there are some discrepancies in the documents between 115 and 115A. Some list two different CSM and some list 115A as a modification of 115. My research has yet to determined which is the case.
CSM-115A: Incomplete, originally slated for Apollo 20, later Apollo 19. Displayed at Space Center Houston.
CSM-119: on standby for Rescue mission for Skylab and Apollo-Soyuz. Later, the cancelled Skylab 5 mission. On display at Kennedy Space Center.
Lunar Excursion Module (LEM or LM)
LM-9: originally slated for Apollo 15 when it was a H-mission, this mission was swapped for a J mission when the lunar missions would be cancelled after Apollo 17. The J-mission LMs differed from earlier LMs with the inclusion of the lunar Rover. It is displayed at the Kennedy Space Center
LM-13: originally slated for Apollo 19, later Apollo 18. Partially completed, it was later restored by Grumman workers. It is on display at Cradle of Aviation Museum in Long Island, New York.
LM-14: originally slated for Apollo 20, later Apollo 19. It was about 15% complete when it's construction was cancelled, later scrapped.
LM-15: unknown which mission this LM was original for, possibly Apollo 20. It was later intended for modification into Apollo Telescope Mount. It was about 5% completed when it's construction was cancelled and later scrapped.
The ATM was later incorporated into Skylab sans the Ascent Stage.
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