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I watched NASA's YT transmission (TL code 04:06:30) of the launch and I noticed a bumpy surface on the middle section while it deployed the Solar arrays and shortly before separation of the mid-section. Can you explain, what happened to the surface of the spacecraft? Was it burned mildly like when you try to peel of coating with a heat gun?

Image shows: YT Transmission Detail enhanced with GIMP and using Filter Unsharp Mask.

YT Transmission Detail enhanced with GIMP and using Filter Unsharp Mask

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TL;DR:

Can you explain, what happened to the surface of the spacecraft?

Foam insulation, applied during construction, similar to the orange TPS on the main core stage.

Was it burned mildly like when you try to peel of coating with a heat gun?

No

Partial info:

You are looking at the interim cryogenic propulsion stage ICPS:

enter image description here

enter image description here

The ICPS is based on the five-meter-diameter version of ULA's Delta Cryogenic Second Stage (DCSS) that has flown 24 times on Delta IV missions since 2004 with 100 percent mission success.

ICPS features a slightly larger liquid hydrogen tank as compared to the Delta IV second stage, as well as electrical and mechanical interfaces specific to attaching and supporting the Orion spacecraft, and a second hydrazine bottle for additional attitude control propellant.

https://blog.ulalaunch.com/blog/icps-2-ula-built-upper-stage-arrives-for-artemis-ii-launch

The DCSS cryogenic tanks are insulated with a combination of spray-on and bond-on insulation, and helium-purged insulation blankets.

The above information likely covers the internal insulation, the outside insulation looks a lot like a trimmed or shaved version of the TPS that covers the core stage and skirt but is painted white, and not left the orange color seen on that stage.

Overall TPS foam information:

https://www.nasaspaceflight.com/2017/12/protecting-sls-fire-ice-tps-foam-application-proceeding-maf/

enter image description here

enter image description here

There are three different types of foam applied to different parts

The “net spray” leaves a textured surface on top called the “rind”.

“Net spray is ‘as applied,’ so it’s got that rind material on it,” “That bumpy surface, that’s the rind and so ‘net spray’ just means that rind is still left on. And either trimmed or shaved would be where we take that rind and we remove that rough section of it.”

https://www.nasa.gov/exploration/systems/sls/multimedia/tps-on-lvsa

https://www.nasa.gov/exploration/systems/sls/insulation-protects-sls-from-fire-and-ice

https://www.boeing.com/features/2021/07/shields-up-spray-foam-evolving-to-protect-nasa

You can see the texture was there before launch:

enter image description here

https://www.ulalaunch.com/interim-cryogenic-propulsion-stage-(icps)

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    $\begingroup$ Thank you for the good sources and answer. $\endgroup$
    – Semo
    Nov 16 at 12:21
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    $\begingroup$ Does the rind not contribute to increased friction in the atmosphere? Even competition bicyclists smooth every surface to reduce friction - and I presume that they are traveling much slower than the rocket. $\endgroup$
    – dotancohen
    Nov 17 at 7:44
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    $\begingroup$ @dotancohen It may have the opposite effect, like with golf balls: The indentations cause vortices to form which act a bit like ball bearings. $\endgroup$ Nov 17 at 9:13
  • $\begingroup$ Let's prove and build a rocket with a shark's skin properties surface. Maybe that'll reduce the friction. Perhaps the foam is a good starter bc it's 'cheap'. $\endgroup$
    – Semo
    Nov 17 at 11:29

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