The main parachute deployed successfully, however the backup chute came out later as well, but deployed with some delay. As a result, Gagarin approached the Earth surface under two parachutes.
Yes, I understand. I meant that as in contrast with the apollo craft which had 3 parachutes but could still land safely if one of the them failed.
Here, with the backup parachute 'undeployed' until needed you'd presumably have a window where the craft was too low to deploy another chute but high enough for the occupants to suffer injuries or worse.
Anyway, I'm not trying to second guess the engineers here, they obviously seem to know what they're doing it's just that I found the contrast between the two strategies remarkable.
I meant that as in contrast with the apollo craft which had 3 parachutes
Yeah that's an interesting question. I don't have a definitive answer but we can make some semi-educated guesses by googling about.
I think the failure mode designers worried about was not the parachute just up and ripping off - the forces involved and component strengths and safeties required could be computed. They were concerned about parachutes failing to deploy at all or failing to deploy effectively, which is harder to model and predict.
Both Soviet and early US landing capsules (Mercury, Gemini) had single parachute systems. The Soviet safety design choice from the beginning was a fully redundant main parachute system. I don't know what the setup was for Mercury, Gemini capsules had ejection seats in case the main parachute failed.
Apollo Command Modules, designed to go to the Moon, were both heavier than the other capsules of the time and had stricter weight constraints. It seems like NASA determined two parachutes failing was extremely unlikely. So three parachutes, one of which can fail seems like the right choice given the constraints of weight and safety.
The actual recorded failures appear to confirm the difficulties were about deployment, more than anything else.
In 1967, Soyuz 1's main parachute failed to deploy, the backup parachute deployed but got tangled in the drogue and was ineffective. Cosmonaut Vladimir Komarov was killed on impact.
In 1971, Apollo 15 splashed down safely with two functional and one failed main parachute.
http://www.russianspaceweb.com/vostok1.html
The main parachute deployed successfully, however the backup chute came out later as well, but deployed with some delay. As a result, Gagarin approached the Earth surface under two parachutes.