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Apollo, Gemini, (Not Mercury), Dragon, CST-100, the set of American capsules all seem to use three parachutes for landing the capsules. This seems like it makes sense. They are usually designed for landing under two allowing some redundancy.

However Soyuz has only a single large parachute.

What are the design considerations that led to that choice?

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According to spaceflight101:

[The Soyuz capsule] is outfitted with the spacecraft's fully redundant parachute system [...]

I would assume that the designers needed to keep a certain precision for the landing. If they deployed the main parachute and the backup simultaneously, the decent speed may be lowered to the point that they could no longer satisfy their landing area requirement, because a slower decent always means that there is a larger potential for the spacecraft to be moved by winds.

According to Popov's "Surprises in orbit" the backup parachute is smaller, with only 574m² (instead of 1000m²) area, leading to an impact speed of a whooping 8-11 m/s, which is far more than is comfortable. (Thank you, osgx)

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  • $\begingroup$ How is it redundant? They have two chutes, a second to use in case the first fails? $\endgroup$ – geoffc Mar 12 '15 at 16:54
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    $\begingroup$ This paper (repositories.tdl.org/ttu-ir/bitstream/handle/2346/59748/…) talks of a primary and backup parachute failing in Soyuz-1, so yes, I gather this is the case. $\endgroup$ – Rikki-Tikki-Tavi Mar 12 '15 at 18:09
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    $\begingroup$ There are two small pilot chutes to deploy main parachute with area around 1 thousand m<sup>2</sup>. If it fails to deploy, there is smaller backup parachute of 574 m<sup>2</sup>, but it is still safe to land. Source in russian from 1990 book "Попов «Сюрпризы» на орбите": astronaut.ru/bookcase/books/popov/text/03.htm $\endgroup$ – osgx Mar 15 '15 at 1:14

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