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The Northrop Grumman video James Webb Space Telescope Launch and Deployment describes the trajectory from launch to orbital insertion and illustrates several key maneuvers and incremental steps in the deployment of the telescope.

After about 03:00 just after 2nd stage ignition the narrator says:

The exposed telescope is delicate, and must be protected from the Sun's fierce heat. To avoid overheating and damage, a carefully designed series of oscillations are performed that provide the necessary protection.

Question: Why does JWST need "a carefully designed series of oscillations" to avoid overheating during the 2nd stage burn? Why not rotate "rotisserie style"?

From Spaceflight Now's Europe’s spaceport, flagship launcher ready for historic Webb mission:

The rocket’s French-made core stage will perform roll maneuvers to keep the observatory in thermal balance, ensuring parts of the telescope don’t get too hot or too cold. Webb’s control team at the Space Telescope Science Institute in Baltimore, Maryland, expects to receive the first signals from the spacecraft as soon as the fairing separates from the rocket.

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  • $\begingroup$ Mabe it's not the sun, e.g. air? $\endgroup$ Dec 25, 2021 at 7:36
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    $\begingroup$ That looks like they are rotating forth and back by about 60° to distribute the heat as equally as possible, but without letting the Sun shine directly on the inner parts that are not behind the folded sun screen. $\endgroup$
    – asdfex
    Dec 25, 2021 at 10:07

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@asdfex comment is correct. It was explained during the live broadcast by Daniel de Chambure, who was in charge of the modifications made to Ariane 5 to accommodate the JWST. Basically, they want to keep certain parts of the telescope away from the sun at all time, so a complete roll is not allowed. Hence, they're rolling back and forth to avoid overexposure while at the same time keeping certain parts shielded.

First we have to recall that Webb is the largest payload ever to be accommodated on an Airane V. It fully occupies the fairing with gaps as small as 10 centimeters for a diameter of 5 meters. It is the reason why we had to develop a specific integration procedure to ensure there is no contact with them.

During separation, the venting system of the launcher has been improved in order to balance as much as possible the inside and the outside pressure of the fairing, and this is due to the fear of depressurization shock, which could damage the delicate layers of Web's sunshield.

After separation, due to the fact that Webb - some equipment of Webb are very sensitive to Sun exposure, the roll control of the launcher will be tuned in order to make sure that Webb is always oriented with only one face to the Sun. In addition, in order to avoid hot spots, the launcher will create some oscillating movements, like a barbecue mode, to avoid overheating. And after Webb separation, there will be a specific end-of-life maneuver to be applied to the superstage to X liberation orbit around the Sun, in order to avoid collision in the long term.

There is the video of the live stream at 59:41 just before where the explanation of roll maneuvers starts:

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    $\begingroup$ Thanks for your answer! I included some transcription because there is so much interesting information in this interview. Good find! So while this is not "rotisserie style", it seems we can call it "restricted barbecue mode". :-) $\endgroup$
    – uhoh
    Dec 27, 2021 at 20:13

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