Were all three Mobile Launch Platforms ever in use simultaneously?

Question

With an optimistic view on the future, NASA built a Vertical Assembly Building with four bays and three Mobile Launch Platforms. They were later repurposed for the Space Shuttle and other launch vehicles.

Were the there MLPs ever all three in use simultaneously:

• During use for Saturn launches?
• Post-Apollo?

"In use" can mean for example two in the VAB for vehicle assembly and one on the launch pad, but all three should have (part of) a launch vehicle attached to it.

Answer 1

In August 1990, there were three shuttle stacks at least partially assembled on their MLPs at the same time: Columbia for STS-35, Atlantis for STS-38 and Discovery for STS-41. The picture in this forum thread on NasaSpaceflight shows Columbia and Atlantis, as Discovery was in the VAB at the time.

Answer 2

During the Apollo program, the Saturn vehicle was stacked on the mobile launcher platform. Thus, a platform was in use starting from the stacking of the S-1C stage. This table provides a detailed timeline for each Saturn V launch vehicle and the platform used:

Mission	Stacking start	ML	ML in use*	Launch date
Apollo 4	27 Oct 1966	1	1	9 Nov 1967
Apollo 8	30 Dec 1967	1	1	21 Dec 1968
Apollo 9	1 Oct 1968	2	1, 2	3 Mar 1969
Apollo 10	30 Nov 1968	3	1, 2, 3	18 May 1969
Apollo 11	21 Feb 1969	1	2, 3, 1	16 Jul 1969
Apollo 12	7 May 1969	2	3, 1, 2	14 Nov 1969
Apollo 13	18 June 1969	3	1, 2, 3	11 Apr 1970
Apollo 14	14 Jan 1970	2	3, 2	31 Jan 1971
Apollo 15	8 Jul 1970	3	2, 3	26 Jul 1971
Apollo 16	21 Sep 1971	3	3	16 Apr 1972
Apollo 17	15 May 1972	3	3	7 Dec 1972
Skylab 1	2 Aug 1972	2	3, 2	14 May 1973
Skylab 2	31 Aug 1972	1	3, 2, 1	25 May 1973
Skylab 3	4 Apr 1973	1	2, 1	28 Jul 1973
Skylab 4	31 Jul 1973	1	1	16 Nov 1973
ASTP	13 Jan 1975	1	1	15 Jul 1975

*) At time of stacking start.

Apollo 5 and 7 were Saturn 1B launches that did not use a MLP.

There were two periods were all three platforms were in use with at least part of a launch vehicle on it:

1. From 30 November 1968 (start stacking of Apollo 10) until 16 July 1969 (launch of Apollo 11) all three platforms were in use in a round-robin fashion.**
2. From 31 Aug 1972 (start stacking of Skylab 2) until 7 Dec 1972 (launch of Apollo 17) all three platforms were in use: one for Apollo 17, one for the Saturn V for Skylab and one with a Saturn 1B for the first Skylab crew.

**) The platforms were not continuously in use, for example:

• On 30 Nov. 1968, ML3 was used for stacking of Apollo 10; all three MLs in use.
• On 21 Dec. 1968, Apollo 8 is launched, making ML1 available.
• On 21 Feb. 1969, stacking of Apollo 11 starts on Ml1.

ML1 was thus briefly unused (in the context of the question) for three months from 21 Dec. 1968 until 21 Feb. 1969. Similarly, ML2 was unused between 3 Mar. 1969 (Apollo 9 launch) and 7 May 1969 (Apollo 12 stacking), etc.

Somewhere after 16 July 1969 (Apollo 11 launch) but before the start of stacking of Skylab 2, ML1 was being modified for the Saturn 1B (addition of the milk stool) and out of rotation, but I don't consider that being "in use" in the context of the question.