|The Skylab 2 Apollo CSM and Saturn IB launcher stand ready atop the "milk stool" on Pad 39B at NASA's Kennedy Space Center, May 1973. Image credit: NASA|
In the second phase, three astronauts would live and work on board Apollo spacecraft for ever-longer periods. They would use a pressurized Mission Module (MM) launched attached to their spacecraft as a small space station. The third phase would see Apollo spacecraft transport crews to and from an Earth-orbiting space station. Cargo bound for the station would ride in the MM. An Apollo circumlunar mission — a flight around the Moon without capture into lunar orbit — was an option, but was considered unlikely before 1970.
Initially, NASA intended to land NAA's Apollo on the Moon atop a descent stage with landing legs. In July 1962, however, after more than a year of sometimes acrimonious debate, the space agency selected Lunar Orbit Rendezvous (LOR) as its lunar landing mission mode. NAA's Command and Service Module (CSM) spacecraft became the LOR mission's Moon-orbiting mother ship, and to Grumman's bug-like Lunar Module (LM) went the honor of landing on the Moon.
|Before the Lunar Module. Image credit: NASA|
The CM's nose carried a probe docking unit, and at the aft end of the SM was mounted the Service Propulsion System (SPS) main engine. The SPS remained sized for CSM launches from the lunar surface, which meant that it was more powerful than necessary for CSM insertion into and escape from lunar orbit.
|Technical details of the Apollo Command and Service Module (CSM) spacecraft configured for lunar missions. Image credit: NASA|
The SM, which was discarded before atmosphere reentry, included propellant tanks, fuel cells for making electricity and water, fuel cell reactants (liquid oxygen and liquid hydrogen), four attitude-control thruster quads, radiators for discarding excess heat generated by on board systems, a high-gain radio antenna, and room for a Scientific Instrument Module (SIM) Bay. An umbilical beneath a streamlined housing linked CM and SM.
Almost all piloted Apollo Earth-orbital missions were launched atop two-stage Saturn IB rockets. The sole exception was Apollo 9 (3-13 May 1969), which used NASA's fourth Saturn V. All Apollo lunar missions left Earth on three-stage Saturn V rockets.
In early 1970, NASA brought together the parts of AAP that survived — several space station-related Earth-orbital missions — to form the Skylab Program, which was expected to include at least one and possibly two temporary Skylab Orbital Workshops. The first, Skylab A, was meant to receive at least three Apollo CSMs, each bearing a three-man crew, over a period of about nine months.
By late 1970, with just two Apollo Moon landings (Apollo 11 and Apollo 12) and the Apollo 13 accident under its belt, NASA cancelled three lunar landing missions. Apollo 20, the planned final Apollo lunar mission, was cancelled in early 1970 to free up its Saturn V rocket to launch Skylab A. Apollo 15, the planned fourth and last walking mission, was cancelled in September 1970, as was Apollo 19. NASA Administrator Thomas Paine dropped the missions at least partly in an attempt to to gain President Richard Nixon's support for a large permanent space station. The space agency renumbered the surviving missions so that Apollo lunar exploration would end with Apollo 17.
On 27 August 1971, Philip Culbertson, director of the Advanced Manned Missions Program Office at NASA Headquarters in Washington, DC, dispatched a letter to Rene Berglund, Manager of the Space Station Project Office at NASA's Manned Spacecraft Center (MSC) in Houston, Texas. In it, he outlined five Earth-orbital CSM missions for the 1970s that were "still under active consideration" at NASA Headquarters.
Culbertson explained that his letter was meant to "emphasize the importance" of statements he had made in a telephone conversation with Berglund on 19 August. Based on his letter, Culbertson had phoned Berglund in an effort to impress on him the seriousness of NASA's budget situation.
|Space Base: a large permanent Space Station, c. 1980. The nuclear-powered station would have had a crew of from 50 to 100 persons. Image credit: NASA|
By mid-1971, however, it was increasingly obvious that a permanent space station was of interest neither to Nixon's White House nor the Congress. In fact, a reusable space station logistics resupply and crew rotation vehicle — a Space Shuttle — was by then emerging as the preferred post-Apollo program. The space station — if it were built at all — would have to wait until the Shuttle could launch its modules and bring them together in Earth orbit.
Culbertson referred to an unspecified new contract MSC had awarded CSM contractor North American. He told Berglund that, in "the early stages of your contract. . .you should concentrate on defining the CSM modifications required to support each of the [five] missions and possibly more important defining the effort at North American which would hold open as many as possible of the options until the end of the [Fiscal Year] 1973 budget cycle." Fiscal Year 1973 would conclude on 1 October 1973.
Culbertson's five missions were all to some degree station-related. The first and simplest was an "independent CSM mission for earth observations." Earth observation by astronauts was often mentioned as a space station justification. The mission's CSM would probably include a SIM Bay fitted out with remote-sensing instruments and cameras. At the end of the mission, an astronaut would spacewalk to the SIM Bay to retrieve film for return to Earth in the CM.
|A SIM Bay was part of the final three Apollo lunar CSMs. The image above shows the Apollo 15 CSM Endeavour in lunar orbit with its rectangular SIM Bay (upper center) open to space. Image credit: NASA|
Salyut 1, the world's first space station, had reached Earth orbit on 19 April 1971. The 15.8-meter-long station remained aloft as Culbertson wrote his letter, but had not been manned since the Soyuz 11 crew of Georgi Dobrovolski, Viktor Patseyev, and Vladislav Volkov had undocked on 29 June 1971, after nearly 24 days in space (at the time, a new world record for human space endurance). The three cosmonauts had suffocated during reentry when a malfunctioning valve caused their capsule to lose pressure, so the Soviet Union halted all piloted missions while the Soyuz spacecraft underwent a significant redesign.
The third Earth-orbital CSM mission on Culbertson's list combined the first two missions. The CSM crew would turn SIM Bay instruments toward Earth before or after a visit to a Salyut.
Culbertson's fourth CSM mission would see CSM-119 dock first with a Salyut for a brief time, then undock and rendezvous with the dormant Skylab A Orbital Workshop. After docking with and reviving Skylab A, CSM-119's crew would live and work on board for an unspecified period.
|Image credit: NASA|
The fifth and final Earth-orbital CSM mission was really two (or possibly three) CSM missions. A pair of "90 day" CSMs would dock with the Skylab B station while a rescue CSM modified to carry five astronauts stood by. NASA had funded partial assembly of Skylab B so that it would have a backup in the pipeline in case Skylab A failed. Reflecting uncertainty about the availability of Saturn rockets and CSMs, Culbertson gave no date for the Skylab B launch.
Of the five missions Culbertson declared to be on the table in August 1971, none flew. In January 1972, Nixon called on Congress to fund Space Shuttle development, and Congress agreed. Shuttle costs and continued NASA budget cuts pushed even the least complex and cheapest of Culbertson's five missions off the table.
For a short time, his second mission looked to be within reach. Formal joint U.S./U.S.S.R. planning for an Apollo docking with a Salyut was under way when Culbertson wrote his letter. In early April 1972, however, shortly before finalizing its agreement with NASA to conduct a joint Apollo-Salyut mission, the Soviet Union declared the concept to be impractical and offered instead a docking with a Soyuz.
NASA was disappointed to lose an opportunity for an early post-Skylab space station visit; the Nixon White House, on the other hand, saw the mission as a poster child for its policy of detente with the Soviet Union, so any sort of piloted docking mission would do. At the superpower summit in Moscow on 24 May 1972, Nixon and Soviet Premier Alexei Kosygin signed the agreement creating the Apollo-Soyuz Test Project (ASTP).
Skylab A, re-designated Skylab 1 (but more commonly called simply Skylab), reached orbit on 14 May 1973 on a two-stage Saturn V. It suffered damage during ascent, but NASA and its contractors pulled it back from the brink.
In August 1973, with Skylab functioning well in Earth-orbit, NASA began to mothball its backup. Several plans were floated for putting Skylab B to use in Earth orbit. In December 1976, however, NASA turned the second Skylab over to the newly opened Smithsonian National Air and Space Museum on the National Mall in Washington, DC.
Apollo CSM-111 was the ASTP prime spacecraft, while CSM-119 was refitted to serve as its backup. In the event, the backup was not needed. CSM-111, officially designated "Apollo" (but sometimes informally called Apollo 18), docked with Soyuz 19 on 17 July 1975. CSM-111 did not include a SIM Bay. The last CSM to reach space undocked on 19 July and, after a period during which its crew performed experiments in the CM, splashed down in the Pacific Ocean near Hawaii on 24 July 1975, six years to the day after Apollo 11, the first Moon landing mission, returned to Earth.
|Artist concept of the Apollo-Soyuz docking in Earth orbit, 17 July 1975. Image credit: NASA|
A Summary of NASA Manned Spacecraft Center Advanced Earth Orbital Missions Space Station Activity from 1962 to 1969, Maxime Faget and Edward Olling, NASA Manned Spacecraft Center, February 1969
Letter, Philip E. Culbertson to Rene A. Berglund, 27 August 1971
Skylab News Reference, NASA Office of Public Affairs, March 1973, pp. IV-6 - IV-8
Living and Working in Space: A History of Skylab, NASA SP-4298, W. David Compton and Charles Benson, NASA, 1983
Thirty Years Together: A Chronology of U.S.-Soviet Space Cooperation, NASA CR 185707, David S. F. Portree, February 1993, pp. 9-26 (http://ntrs.nasa.gov/search.jsp?R=19930010786 - accessed 10 May 2017)
Mir Hardware Heritage, NASA RP 1357, David S. F. Portree, March 1995, pp. 33-35, 65-72 (http://history.nasa.gov/SP-4225/documentation/mhh/mhh.htm - accessed 10 May 2017)
"Skylab B: Unflown Missions, Lost Opportunities," Thomas Frieling, Quest, Volume 5, Number 4, 1996
Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter
"Assuming That Everything Goes Perfectly Well In the Apollo Program. . ." (1967)
"A True Gateway": Robert Gilruth's June 1968 Space Station Presentation
Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1970)
A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)
Skylab-Salyut Space Laboratory (1972)
What If a Crew Became Stranded On Board the Skylab Space Station? (1972)
Reviving & Reusing Skylab in the Shuttle Era: NASA Marshall's November 1977 Pitch to NASA Headquarters