Comment by eqvinox
5 days ago
I'm not super happy with the pattern of thinking in these numbers; arguing 1/100000 vs 1/100 for "accidents" is again a boolean thing. They probably had a very specific definition, which does make this viable, but we don't have that definition here. So the numbers are meaningless to us. And not having that definition, "accidents" is a sliding scale… e.g. I'm pretty sure astronauts injured themselves banging various bodyparts against various parts of the spaceship. That's technically an accident.
And in this concrete example — the heat shield isn't boolean either. I don't know how steep the gradient between pass and fail is, but it certainly exists, and it's possible they come back successfully but with it singed significantly outside expected parameters. (Even "less than expected" would indicate a problem here IMHO.) That does mean it's not necessarily a question of having enough boolean datapoints.
A high enough threshold for "accident" is a death. In the space shuttle the "accident" rate was 2/135 that is somewhat close to 1/100, but it shows that 1/100000 is too optimistic.
You can pick a more strict safety criteria, but the result would be (something)/135 >= 2/135 > 1/100 >> 1/1000000
Yes… for the space shuttle. But where is what on this for Artemis II? Especially if it has more heat shield failure modes that are not immediately deathly, the whole consideration becomes non-obvious. Is it possible the astronauts will arrive back "well toasted" but alive and OK other than heatstroke? (Probably not, but that's the kind of question involved here...)
(Also, technically [you may begin your eyeroll] even death isn't a simple boolean condition. I guess brain death should do it.)
Anyway all I'm saying is that all of these considerations need more qualifiers to be really useful.