According to this video human-rated launch vehicles can have six guidance computers while cargo-rated launch vehicles seldom have more than two. If these computers function independently for greater redundancy, why don't supply-rated launch vehicles have more than two such computers?

How large are such guidance computers? Is their size/mass or cost a prohibiting factor from installing more?

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    $\begingroup$ I'm not sure he means six fully redundant computers - there might be three running the launcher and three used later in the manned capsule/spacecraft. For example here computer-museum.ru/english/argon16.htm is one of the Soyuz spacecraft computers, which is triple redundant. The shuttle had four redundant computers plus one reserve (unused) - no idea if there was a separate control system in the boosters too. $\endgroup$ – Andy Jul 3 '15 at 12:49

Don't take random youtube videos as fact. That part of that video was incorrect. The unmanned SpaceX Dragon has three pairs of computers used for flight control (so six flight control computers), and has 54 processors in all. The Falcon 9 launch vehicle has another 30+ processors. See http://aviationweek.com/blog/dragons-radiation-tolerant-design. Orbital's Cygnus also uses more than two computers for flight control.

The Shuttle had four primary flight control computers, plus one more as a completely independent backup flight system.

Visiting vehicles to the Space Station, whether crewed or uncrewed, must protect against any two failures creating a catastrophic hazard on the Space Station. You aren't going to get two fault tolerance with only two components.

Crewed vehicles add the additional requirement of being two fail safe (no one on the vehicle dies even if two failures happen).

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    $\begingroup$ Do you have any information about the number of computers of the Progress vehicle? $\endgroup$ – user3049 Jul 3 '15 at 14:13
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    $\begingroup$ The Shuttle had 4 main flight control computers plus a fifth as the backup. $\endgroup$ – Hobbes Jul 3 '15 at 14:54
  • $\begingroup$ @Hobbes - Thanks, that I wrote three was a memory fault on my part. Answer updated. $\endgroup$ – David Hammen Jul 3 '15 at 15:15

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