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According to NASA's Deployment Explorer tool, the solar array was expected to deploy at T+33 minutes.

In the NASA livestream, the solar array deploys at T+29:08 (the clock isn't on stream at that moment, but at T+27:08 is exactly 2 minutes before that and the clock is visible), almost exactly 2 minutes after separating from the upper stage.

Before the solar array deployment, the narrator was talking about how the mission operations manager would be "calling out the procedures that will lead to the deployment of Webb's solar array". However, immediately after he says this, the solar array deploys. Then, he says, "a bit earlier than planned, but there is the solar array having been deployed".

So, it's my understanding that the solar array deployment is autonomous. The tool mentioned above specifically says:

After shedding its fairing and booster rocket, the first two deployments are 'automatic,' meaning they happen without the ground giving commands.

So, is the timeline above wrong, that the array was supposed to deploy at T+29 minutes? Or was there some other reason why it seems to have deployed 4 minutes before it is supposed to have done so, especially seeing as how the procedure was supposed to be "automatic"?

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    $\begingroup$ Fascinating question! $\endgroup$ Dec 26, 2021 at 5:38
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    $\begingroup$ I doubt there's a verifiable answer to this. According to this video, the arrays were supposed to deploy at T+31 minutes; other sources say T+33. NASA has dumbed down the information on its public-facing websites to the extent that launch providers and spacecraft developers just don't bother providing PAO with details. In a way, this question is similar to the now-deleted question on how much the JWST weighed at launch. Some NASA and ESA sites stated 6200 kg, others 6500 kg. $\endgroup$ Dec 26, 2021 at 16:04
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    $\begingroup$ I suspect the T+33 minutes was a "no later than" time. If deployment hadn't occurred by then it would have been time for key personnel to update their resumes, ASAP. The actual time would have depended on the time of separation and attaining a proper attitude, and the sooner the better given that the JWST was running on batteries up until solar array deployment. The time also depends on what is meant by "solar array deployment". Was that the time at which deployment started or the time at which latching occurred (and was confirmed)? $\endgroup$ Dec 26, 2021 at 16:06
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    $\begingroup$ @DavidHammen the reason I had decided to ask was because (as I mentioned above), even the person narrating what was occurring was surprised by the timing of deployment. They were about to do some pre-deployment procedures, but it deployed before they even got a chance to. And even if the expected was at T+31, they deployed 2 full minutes before that. It was mainly notable to me because the people who (in theory) who should have the best inside line on things (outside the people actively running the mission) were surprised. And it seems like we may have actually gotten an answer, after all! $\endgroup$ Dec 27, 2021 at 0:28
  • $\begingroup$ @fyrepenguin We have one answer that is a good guess -- but it's just a guess. Those who know the exact algorithm that was used are most likely forbidden from answering thanks to NDAs (non-disclosure agreements) whose violation could cost them not only their jobs but also losing their accumulated wealth. $\endgroup$ Dec 27, 2021 at 16:40

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JWST solar panels have limits on what angular rate of rotation they can be deployed at. After separation, there is rotation introduced into the spacecraft, which it has to dampen with thrusters until the angular rate is within acceptable limits. In this case the tip off (rotation after separation) from the Ariane 5 rocket was basically perfect (compared to the conservative estimate) which meant the spacecraft had to spend less time burning RCS to get within limits and thus solar panel deployment was sooner.

This is confirmed on the JWST blog:

That [solar array] deployment was executed automatically after separation from the Ariane 5 based on a stored command to deploy either when Webb reached a certain attitude toward the Sun ideal for capturing sunlight to power the observatory – or automatically at 33 minutes after launch. Because Webb was already in the correct attitude after separation from the Ariane 5 second stage, the solar array was able to deploy about a minute and a half after separation, approximately 29 minutes after launch.

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  • $\begingroup$ Citation needed. Unless you know of the algorithm the JWST flight software used to decide when to deploy its solar array, this is speculation. And given how restrictive NASA has become regarding release of information, I suspect you do not know that this is correct. $\endgroup$ Dec 27, 2021 at 16:37
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    $\begingroup$ @DavidHammen So... you dispute the sources quoted in this answer? Funny, i vaguely expected "Thomas Zurbuchen , Associate Administrator, NASA Science Mission Directorate" to be a reliable source? $\endgroup$ Dec 27, 2021 at 17:31
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    $\begingroup$ @CuteKItty_pleaseStopBArking In this case, yes. Zurbuchen's tweet implies that solar array deployment was commanded from the ground. From everything I have read, solar array deployment was one of the few fully automated JWST operations. It's important to keep in mind that rising higher and higher in a larger management hierarchy means getting further and further detached from technical details. (I estimate technical half-life has a five year period, at best.) $\endgroup$ Dec 28, 2021 at 3:28
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    $\begingroup$ @DavidHammen twitter.com/SouthernWave1/status/1475186186247282689 $\endgroup$ Dec 28, 2021 at 3:47
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    $\begingroup$ In fact, that last tweet is the answer. $\endgroup$ Dec 28, 2021 at 22:35
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Such deployment is driven by spacecraft dynamics, meaning that it has to have certain attitude (Sun-pointing in this case) and angular rates below certain threshold to limit mechanical stress of a solar array. When it is being deployed it is most vulnerable vs. the stress. What NASA communicated was either average expected time or latest nominal (assuming highest nominal separation rates) time.

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    $\begingroup$ As a non-space specialist, this answer is basically useless to me. What are "s/c dynamics" and why would they result in something happening sooner rather than later? $\endgroup$
    – Richard
    Dec 26, 2021 at 11:32
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    $\begingroup$ Do you have a source for this information? $\endgroup$ Dec 26, 2021 at 13:16
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    $\begingroup$ Hello, I cannot edit my answer unfortunately. Such deployment is driven by spacecraft dynamics, meaning that it has to have certain attitude (Sun-pointing in this case) and angular rates below certain threshold to limit mechanical stress of a solar array. When it is being deployed it is most vulnerable vs. the stress. What NASA communicated was either average expected time or latest nominal (assuming highest nominal separation rates) time. $\endgroup$ Dec 27, 2021 at 14:48

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