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Apollo lunar spacesuits had four hose connectors for air:

The EV PGA, however, is provided with two sets: two [blue] inlet connectors interconnected by a plenum chamber and two [red] outlet connectors interconnected the same way.

Apollo Experience Report: Development of the Extravehicular Mobility Unit

Why are there two sets of connectors?

Armstrong's spacesuit at the Smithsonian

Related:

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    $\begingroup$ Just guessing here, but having multiple connectors provides redundancy and allows uninterrupted operation if something need to be swapped out $\endgroup$ – Dragongeek Mar 27 at 7:00
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    $\begingroup$ @Dragongeek: It does have something to do with emergencies, but not quite as you describe it. $\endgroup$ – DrSheldon Mar 27 at 7:05
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    $\begingroup$ If you know the answer @DrSheldon why are you asking? $\endgroup$ – GdD Mar 27 at 8:59
  • $\begingroup$ The gas connectors were not used for air but for pure oxygen. $\endgroup$ – Uwe Mar 27 at 11:35
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    $\begingroup$ @gdd SE specifically permits asking and answering your own question. stackoverflow.blog/2011/07/01/… Could be OP intends to answer, or it could simply be a good question that isn't already covered on the space.SE site already. $\endgroup$ – Criggie Mar 27 at 22:21
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From History of the Apollo Space Suit by the International Latex Corporation (ILC). PDF

When setting up the suits in preparation for the extravehicular walk on the lunar surface, the astronauts attached oxygen hoses from the Lunar Module (both inlet and outlet) while at the same time attaching to the inlet and outlet hoses of the portable backpack. Once the pressure in the Lunar Module was reduced to the hard vacuum of the moon and the suits were pressurized at 3.8 lb/in² (26 kPa), the backpack would become the sole source for the suit pressure and the astronauts would disconnect from the Lunar Module and descend the ladder to the surface. The suit inlet and outlet fitting for the Lunar Module interface both sealed closed once the hoses were removed. They would reverse the process upon reentry to the Lunar Module.

enter image description here

The double set of gas connectors with pairs of inlets and outlets was used to connect both the LM life support system as well as the backpack of the extravehicular suit. An uninterrupted transfer between both life support systems was enabled by the pairs of connectors.

The intravehicular suit version worn by the CM pilot had only a single set of gas connectors. It was only connected to the suit circuit of the CM and never to a PLSS backpack.

From the APOLLO SPACE SUIT INTERFACE SPECIFICATION, 18 April 1962 PDF.

enter image description here

It was planned to use one PLSS for two astronauts in emergency. The hoses of the other astronaut with the failed PLSS would be connected to the second pair of the gas connectors at the suit of the first astronaut with the functional PLSS.

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    $\begingroup$ Notably, the command module pilot's suit did not have that extra set of hookups. I attended a talk on the Apollo suits during the 50th anniversary of the landing. The speaker showed a photo of the Apollo 11 astronauts and asked if anyone could tell from the picture which one wasn't landing on the moon. An audience member correctly identified Michael Collins. The speaker asked how he knew. "Because he's the only one who isn't smiling." $\endgroup$ – Ray Mar 27 at 21:41
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    $\begingroup$ @Ray - does Neil have an inlet hose connected to his outlet, and Mike is hiding that he has no hose, and is that a slide-rule in his pocket? $\endgroup$ – amI Mar 28 at 5:56
  • $\begingroup$ @aml No, he's just happy to see you. $\endgroup$ – guenthmonstr Mar 28 at 15:28

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