Comment by eqvinox

6 days ago

TFA is ridiculous with its stance. Yeah, there's this aspect of the design they can't test in their labs. It might even be an important aspect. But safety isn't a boolean "safe"/"not safe", it's a risk assessment, and I'm quite sure there are 100 (or 10000) other things they didn't test for. As long as they're taking all of this into their risk calculations… it's fine.

And if it doesn't blow up due to heat shield failure, TFA (and its references) will be forgotten.

And if it does blow up due to heat shield failure, TFA (and its references) can suddenly claim prescience, all the while this is one of thousands of factors that went into the risk assessment. If one really wanted to claim prescience, it'd need to be a ranking of a sufficient number of failure modes.

To illustrate the problem: I hereby claim they will have a "toilet failure". Now if they actually have one, I'll claim `m4d ch0pz` in rocket engineering.

(P.S.: it's a joke but toilet failures on spacecraft are actually a serious problem, if it really happens… shit needs to go somewhere…)

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eqvinox

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gus_massa  5 days ago

> And if it doesn't blow up due to heat shield failure, TFA (and its references) will be forgotten.

The HN hivemind will remember, he's a well known user with a well known site.

Let's hope there is no accident, but after the landing there will be reports anyway. We will take a look at the report of the shield and see if it shows a problem in spite it didn't explode, and compare with the prediction in the article. He may even write another article after the fly.

For comparison, I remember the Feynman appendix. One important detail was that Nasa said the the probability of accidents was 1/100000, but he concluded that it was closer to 1/100. Nobody expect to fly a hundred Artemis missions to get a good statistic. Even if the current version explosion rate is super high like 1/10, then you can probably fry a few missions without problems if you cross your fingers hard enough.

  • 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.

    • gus_massa  5 days ago

      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

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