@futurebird
In an ideal world, adding a 0.75 reliable parallel part increases the overall reliability since the probability of both failing simultaneously is (1-0.8)*(1-0.75), or 0.05. But...this only works if the failure modes are independent.
Real world example: the idea behind NASA funding two different crew vehicles for the ISS was supposed to work as I just described above. But since they share the one available docking port on the ISS, Starliner's failure tied up the docking port preventing Dragon Crew from using it. So NASA painted themselves into a corner where their failure probability was 1-(0.8*0.75), or 0.4, not 0.05. (assuming dragon's success rate is 0.8 and starlinger's 0.75).
https://techhub.social/@artemesia/113602547865193462
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