Re: Man-Rating Atlas V


lou@xxxxxxxxxxx wrote:

I agree that these are all guesswork (after all, as someone once
observed, you get arguments and persecution over religion and politics,
not arithmetic). The question is the size of the errors. Suppose
Atlas has overestimated the reliability by a factor of 2, so in fact it
kills the crew 1 in 500 times, and does not meet the NASA spec. This
is certainly possible, and I think likely. However, it is even more
likely (in my opinion, of course) that NASA has overestimated the
reliability of the stick by a factor of 5 more more. This would result
in a fine record, killing the crew only 1 in 400 times, but it would
not be as good as the Atlas.

Now of course the size of the overestimates is unknown (otherwise we'd
include them in the estimates). But to me the Atlas changes "feel"
minor - better monitoring, new trajectory, redundant electronics, and
so on.

This argument would be more compelling if the Atlas V was a vehicle
with a well established track record. Not that it's a bad record, it
just isn't enough flights to confidently much more than its probably at
least 90% reliable.

And it really is significantly different from Atlas II, with different
tank structure and different main engines.

Nor are the changes for the CEV version *that* minor. Besides the
changes you mention, NASA believes it needs a higher thrust upper
stage, probably with more RL-10 engines. And the three core
configuration, which Atlas has never built. That was a significant
enough change on Delta IV to cause mission failure the first time that
configuration launched. And a assembly line in the US for the main
engines rather than just uncrating the imports.

Will McLean

.

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