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Just reading about manned space-craft. Space travel on record is listed briefly on Wikipedia. The section on successful manned missions is of particular interest.

Successful manned programs have, for the most part, carried upto/exactly 3 crew. For instance

  • Tier One - 3
  • Shenzhou 10, 9, 7 - 3
  • TKS - 3
  • Soyuz-T - 3
  • Soyuz-TM - 3
  • Soyuz-TMA - 3
  • Soyuz-TMA-M - 3
  • First Gen Soyuz - Upto 3
  • Apollo/Moon - 3

The following programs being the exception to that rule of 3.

  • Vostok
  • Mercury
  • Gemini
  • Voskhod
  • Space Shuttle
  • Early Shenzhou craft

The impression one receives is that preliminary craft carried none, one, or two astronauts. More mature missions/technologies carried 3. The space-shuttle is notable in carrying a still larger crew/mission-specialists; as is the proposed ACTS.

What is/was the rationale behind a crew of 3 astronauts? How did the systems evolve to reach this figure of a crew of 3?

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    $\begingroup$ The statistics are skewed somewhat because you're counting all the Soyuz variants separately. I'm also puzzled by the inclusion of TKS and Tier One, neither of which were manned space ships as far as I know. $\endgroup$ – Hobbes Oct 26 '13 at 19:35
  • $\begingroup$ Voskhod-1 is also 3. That constitutes half of Voskhods. $\endgroup$ – horsh Oct 27 '13 at 7:01
  • $\begingroup$ But then there are also the early versions of Shenzhou which did not carry 3; yet IMHO there have been more 3-carriers than not (+: $\endgroup$ – Everyone Oct 27 '13 at 12:40
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From a mathematical perspective rather than historical, I would expect that it stems from the natural constraints of space travel. By far, the biggest constraint is weight. Every pound is expensive, and human flesh and its sustenance is really heavy stuff. So astronauts are expected to keep trim, but also, you want to minimize the size of the crew. This is why unmanned missions are such a bargain.

But why three? Why not one or two more often? I suspect it boils down to psychology mostly. A single crewman in a tin can in the middle of space - well, I don't want to do it. I suspect it carries a high risk of madness of one kind or another, and with something as expensive as human space flight, we don't want to put all our eggs in one basket-case. And two - well, there's a word for when two people are placed in a highly stressful situation together. It's called a fight. I'm exaggerating, but I think it is true that when two people are completely alone under stress for a long time, the potential for conflict is excessive. When you throw that third person in, you have an objective witness, a mediator, a tie-breaker... I think a lot of things go a lot better with three or more. And four doesn't make enough difference to justify the weight.

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  • $\begingroup$ The rationale feels right but I may be prejudiced because that's what I came up with too. The crew count would probably have to have been sold to the bean-counters though ... I wonder whether there any documentation/theory/theories are available for reference $\endgroup$ – Everyone Oct 26 '13 at 6:02
  • $\begingroup$ While I don't disagree with this answer, I think it could really use some references to back it up. $\endgroup$ – Jack Jul 24 at 15:56
  • $\begingroup$ Looks like you're right. Even in the science fiction film (1956) about space flight there were three crew members. youtube.com/watch?v=mXt30Ing3Kk $\endgroup$ – A. Rumlin Jul 24 at 16:21
  • $\begingroup$ Crew selection processes minimize the chances of fights arising. This is not a design factor. $\endgroup$ – Innovine Jul 26 at 9:00
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Mercury was essentially an X-vehicle. The payload of an X-vehicle is a test pilot with a radio, and not much more.

Gemini was a two-man bird. One man flew the Gemini spacecraft, the other guy controlled the Agena docking target. There was other work that had to be done as well.

Apollo was a three-man bird in part because of the rendezvous requirement, which dictated that you needed pilots in both vehicles, and because NASA wanted two guys in the Lunar Module.

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  • $\begingroup$ And presumably for Apollo, two guys in the Lunar Module for crew redundancy, and perhaps also that donning/removing a space suit was not something a crewman could do without some assistance. $\endgroup$ – Anthony X Oct 26 '13 at 17:41
  • $\begingroup$ @John Agreed on the Gemini, and Apollo. There are others though; E.g. Almost all generations of the Soyuz, Shenzhou since their design stabilized ... $\endgroup$ – Everyone Oct 26 '13 at 18:40
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    $\begingroup$ Soyuz for the moon was going to be 2 man, one in lunar orbit, one in a lunar lander. $\endgroup$ – geoffc Oct 27 '13 at 0:38
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    $\begingroup$ Soyuz was originally a two-man bird, and I've seen at least one claim that the name was chosen because it was originally going to be doing rendezvous trials. Someone asked about a three-man version, and a very creative design engineer figured out how to shoehorn a third seat in, IF the third guy was SMALL. I am told that the design engineer was literally the only guy on the program small enough to ride that seat, so he flew. Funny how that worked out... $\endgroup$ – John R. Strohm Jun 5 '15 at 15:52

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