On 28 July 1973, the Skylab 3 crew of Alan Bean, Jack Lousma, and Owen Garriott lifted off from Launch Pad 39B at Kennedy Space Center, Florida, bound for the Skylab Orbital Workshop in Earth orbit. Despite their mission's numerical designation, they were the second crew to visit Skylab; in a move guaranteed to generate confusion for decades to come, NASA had designated as Skylab 1 the unmanned Workshop launched on 14 May 1973, and had dubbed the first crew visit Skylab 2.

The Skylab 3 Apollo Command and Service Module (CSM) separated from the second stage of its Saturn IB launch vehicle and began maneuvers to catch up with Skylab. During final approach to the Workshop, one of the steering thruster quads on the CSM began to leak nitrogen tetroxide oxidizer. The crew dutifully shut off the quad and used the three quads remaining to complete docking without further incident.

On 2 August, a second quad began to leak, raising fears that tainted nitrogen tetroxide had damaged both quads. If this were true, then the remaining two quads and the CSM's Service Propulsion System (SPS) main engine might also be compromised; though the individual quads and the SPS all had independent propellant systems, all contained oxidizer from the same batch. If the leaks continued, moreover, nitrogen tetroxide might contaminate the inside of the CSM's drum-shaped Service Module, damaging other systems. Within hours, NASA put into motion a variant of a plan Kenneth Kleinknecht, Skylab Program Manager, and Lawrence Williams of the Apollo Spacecraft Program Office had described less than a year previously at the Fifth Annual Space Rescue Symposium in Vienna, Austria.

Image: NASA Image: NASA

In their paper, Kleinknecht and Williams explained that Skylab would provide the first opportunity for space rescue in the U.S. space program. One-seat Mercury and two-seat Gemini spacecraft had been too small and limited in capability to serve as rescue spacecraft. Apollo lunar CSMs were much more capable; even so, they each carried only a little more breathing oxygen, fuel cell reactants, and food than were needed to support a three-man crew for the duration of a lunar mission (about two weeks). If an Apollo CSM had become stranded in lunar orbit - for example, by an SPS failure - then its crew would almost certainly have perished long before NASA could have attempted a rescue.

If astronauts needed to evacuate the Skylab Orbital Workshop, then they could board their CSM, undock from the Workshop, and splash down on Earth in less than a day. If, on the other hand, a crew's CSM became unusable while it was docked at Skylab's front port, then the astronauts could wait on board Skylab for rescue. Life support reserves on board the Workshop would be sufficient to support a crew even if the last of the three planned Skylab missions needed to be extended to enable a rescue. This was because the Workshop would be launched with enough oxygen, food, water, and other supplies to support three men for eight months, yet the three Skylab visits were planned to last a total of only five months at the time Kleinknecht and Williams presented their paper.

NASA, meanwhile, would prepare and launch a rescue CSM with a crew of two. It would dock at the radial (side-facing) docking port on Skylab's Multiple Docking Adapter. Kleinknecht and Williams proposed that the CSM intended for the next Skylab crew should be made the rescue CSM. This would, presumably, have reduced by one the number of long-duration Skylab missions flown. A fourth CSM, which would serve throughout the Skylab program as the backup CSM, would become the rescue vehicle for the Skylab 4 astronauts, the third and final crew to live on board the Workshop.

Seating arrangement for Skylab rescue CSM. Image: NASA Seating arrangement for Skylab rescue CSM. Image: NASA

Kleinknecht and Williams estimated that stripping out the rescue CSM's aft bulkhead lockers to make room for a "rescue kit" would require about a day. The rescue kit would include a pair of special astronaut couches, connectors and hoses for linking two additional space-suited astronauts to the rescue CSM's life support system, and an experiment-return pallet. The rescue CSM's two-man crew would recline in the left and right couches; the three rescued Skylab crewmen would return to Earth in the center couch and in the two special couches, which would be mounted below the others in place of the lockers.

The rescue kit would also include a special Apollo probe-and-drogue docking unit that would enable the crew inside Skylab to manually undock the crippled CSM. This would clear the Workshop's front port for any future CSM dockings. Kleinknecht and Williams were not specific about what would happen to the unmanned CSM after it was discarded.

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Though the time needed to install the rescue kit in the rescue CSM would be minimal, the time needed to refurbish Pad 39B and prepare the rescue CSM and Saturn IB rocket for launch would have been longer and would have varied considerably depending upon when the crew became stranded. After each Skylab Saturn IB launch, ground crews would need about 48 days to refurbish Pad 39B and prepare the next Skylab CSM and Saturn IB. If a rescue were judged to be necessary at the beginning of the 28-day first manned Skylab mission (Skylab 2), then the crew would find their mission extended by 20 days. On the other hand, if a rescue were declared necessary late in the first mission, then preparations for the next Skylab CSM launch would be farther along, but would have begun later. The rescue CSM and Saturn IB would, thus, need 28 days of preparation before they could launch. In either case, the crew would remain on board the Workshop for about twice as long as originally planned.

The second and third Skylab missions (Skylab 3 and Skylab 4) were both planned to last 56 days at the time Kleinknecht and Williams presented their paper. Activation of Skylab rescue capability early in these missions would have permitted a rescue before the return time planned when the crew had left Earth. A failure near the end of the Skylab 3 or Skylab 4 mission would see a rescue CSM launched as few as 10 days after the rescue plan was activated.

Skylab rescue mission crewmen Vance Brand (left) and Don Lind. Image: NASA Skylab rescue mission crewmen Vance Brand (left) and Don Lind. Image: NASA

The 2 August failure of the second Skylab 3 CSM thruster quad unleashed a storm of activity. NASA elected to prepare the backup Skylab CSM, not the Skylab 4 CSM, as its rescue vehicle, and tapped Skylab 3 backup crewmen Vance Brand and Don Lind to pilot it.

Almost as soon as the rescue plan was activated, however, analysis showed that the nitrogen tetroxide in the Skylab 3 CSM's propulsion system was not tainted, and that the two thruster quad failures lacked a detectable common cause. Tests also showed that the Skylab 3 crew could maneuver their CSM with a single functioning thruster quad. Though rescue preparations continued, by 10 August NASA had cleared the Skylab 3 crew for a 59-day mission on board the Workshop. The astronauts returned to Earth in the Skylab 3 CSM as planned on 25 September 1973.

Reference:

Skylab Rescue Capability, Kenneth S. Kleinknecht and Lawrence G. Williams; paper presented at the Fifth Annual Space Rescue Symposium Organized by the Space Rescue Studies Committee of the International Academy of Astronautics, 23rd Congress of the International Astronautical Federation, Vienna, Austria, 9-12 October 1972.