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On 2022 Sep 26, Artemis I was moved from its launchpad back to its "garage" to shelter from a hurricane.

It was reported that

NASA managers have said the system is rated for two more rollbacks to the VAB, so after this hurricane decision it appears the stack will need to launch after its return to the launch pad.

  1. What constrains how many rollbacks Artemis can endure? (Wouldn't whatever it suffers in terms of vibration, temperature, and forces be peanuts compared to a launch?)

  2. After its last permissible journey on the crawlerway, what happens next? Must it launch, no matter what? Or might it be taken apart, reinspected, and recertified for another n rollbacks -- if so, how much work is that?

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    $\begingroup$ 3. They decide it really is good for more than that. $\endgroup$ Sep 27, 2022 at 16:35

2 Answers 2

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You wrote [bold emphasis mine]:

It was reported that

NASA managers have said the system is rated for two more rollbacks to the VAB, so after this hurricane decision it appears the stack will need to launch after its return to the launch pad.

The key word here is "rated". What that means is that NASA's engineers and safety officers, Boeing's engineers and safety officers, Northrop Grumman's engineers and safety officers, ULA's engineers and safety officers, Aerojet Rocketdyne's engineers and safety officers, Lockheed Martin's engineers and safety officers, Airbus's engineers and safety officers, as well as the engineers and safety officers from all the other contractors, involved space agencies, the Eastern Range, etc., when they made their safety checks many years ago, thought it would be enough to run their risk calculations for 5 rollouts.

So, for now, all that this means is that after 5 rollouts, they need to run their calculations and assessments again, need to inspect the vehicle(s) to check what the actual status of the wear and tear is compared to the worst case assumptions they based their earlier assessment on, and write a new report.

It is highly likely that the result of this report will be that they extend the rating to a couple of more rollouts, simply because it is very unlikely that every single one of the previous rollouts met the worst-case criteria that they used in their calculations. In fact, it is highly likely that even in worst-case conditions, the stack can support more than 5 rollouts, however, they didn't bother to run the numbers because they didn't think they need it.

You can imagine that they calculated 5 rollouts through a hailstorm during an earthquake with the crawler breaking down halfway or something like that. (Note that I completely made those conditions up, but conceptually, that is how risk assessments work.) Since none of those things happened during the rollouts and rollbacks, it stands to reason that the stresses on the stack were much lower than assumed, and thus additional rollouts can be performed.

Here are just a couple of other examples where the actual performance of a vehicle, engine, or spacecraft far exceeded the rated performance:

  • The Falcon 9 booster was designed for "at least ten" reflights with minimal refurbishment. The fleet leader now has flown 14 times.
  • The Sojourner Mars rover was rated for a week to a month, but ended up operating for almost three months.
  • The Spirit and Opportunity Mars rovers were rated for 90 days, but ended up operating for 6 and 14 years.
  • The Curiosity Mars rover is rated for two years, but has been operating for over 10 years now and is still going strong.
  • The Ingenuity Mars helicopter is rated for 3–5 flights and 30–45 days and to not survive the winter, but has been operating for 32 flights and well over 500 days and is still going strong.
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    $\begingroup$ Dont forget the Voyagers with the 40 extra years of service. $\endgroup$
    – masterX244
    Sep 28, 2022 at 7:57
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    $\begingroup$ @masterX244 Their caretakers also performed long past their rated service time! :-) $\endgroup$ Sep 28, 2022 at 9:22
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    $\begingroup$ @IanKemp IIRC Challenger is a big part of why we have such strict rules about flight readiness. The engineers at the time said it wasn't safe to fly, but the suits overruled them and tried to fly it anyway. $\endgroup$ Sep 28, 2022 at 19:40
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    $\begingroup$ Note there's a slight political motive to have your equipment hugely outlast its nominal lifetime: it's easier to get initial one-off funding approved than a long-term support budget, but if you already have a rover working on Mars it's easier to say "hey can we have more money so we can do something with this? would be a waste to let it sit there..." $\endgroup$
    – llama
    Sep 28, 2022 at 20:30
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    $\begingroup$ @IanKemp: The engineers at Morton Thiokol did run the numbers and they concluded that it was not safe to launch. But, they were put under political pressure to change their assessment, and thus company management overruled them. $\endgroup$ Sep 28, 2022 at 23:16
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Partial answer to

What constrains how many rollbacks Artemis can endure?

A big part of it is resonant vibration (due to the crawler) causing fatigue to various and sundry parts of the SLS. You can see some of the analysis done in the paper Operational Modal Analysis of the Space Launch System Mobile Launcher on the Crawler Transporter ISVV-010 Rollout (I was reading that in hopes it would say what the actual weak points were, but it didn't).

Shuttle ran into this problem, or at least became aware of it, in the Agonizing Reappraisal of Everything after the STS-107 failure. The weak points there were multiple odd things like structural connections at the aft payload bay bulkhead, various flex hoses, and especially a bending mode of the vertical tail. They ended up slowing down the rollout speed, which changed the frequency of the vibrations resulting from the crawler, to mitigate the issues.

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    $\begingroup$ 4 miles, 8 hours (and 8 hours back in case of a repeat): OK, that is significant. As an aside, the platform is driven by a combined 16MW power plant... $\endgroup$ Sep 28, 2022 at 9:40
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    $\begingroup$ Another reason why rollout/rollback may be more stressful for SLS is that the vibrations here are "sideways", instead of along the long axis. Launch vehicles are generally built to handle a lot of stress along the long axis, but not so much "sideways". $\endgroup$
    – Strabbi
    Sep 28, 2022 at 12:57
  • $\begingroup$ @Strabbi Why are the vibrations "sideways"? The rocket is standing, and the vibration comes from the track segments hitting the road and the rollers. $\endgroup$ Sep 28, 2022 at 19:18
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    $\begingroup$ @Strabbi Ah, I thought your "more" makes a comparison to e.g. space Shuttles or Saturn Vs, but your argument would apply equally to them all. "More" probably compared to the actual launch which is vertical -- but so much more violent that I'd bet the lateral oscillations are also much stronger. They just take only a few minutes so that fatigue and rubbing/chafing etc. doesn't have so much time to progress. $\endgroup$ Sep 28, 2022 at 19:59
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    $\begingroup$ @Peter-ReinstateMonica 0.5 mph, 0.008 mpg! The latter figure is about the same as a cruise ship. $\endgroup$
    – hobbs
    Sep 28, 2022 at 21:25

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