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The Boeing Starliner has a black, perforated skirt that Spans the circumference of the service module. It is unclear what the purpose of this is and have not found anything mentioning it specifically.

I wasn’t sure how to describe it, hence ‘chicken wire’.

enter image description here

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    $\begingroup$ I'm still looking for references to confirm my recollection, but I think this was introduced at the same time the extended skirt extending around the Atlas, and helps to manage the energy in the boundary layer around the skirt and the service module. All that was to manage aeroacoustic issues. Older depictions of the service module lack these parts. $\endgroup$
    – Erin Anne
    Feb 23, 2023 at 21:00
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    $\begingroup$ That's where they stow the chickens for flight, obvs ! $\endgroup$
    – Criggie
    Feb 24, 2023 at 19:48

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Not primary sources, but the most consistent term I've seen to refer to these is "lattice structure," which unfortunately is a generic structural engineering term.

Everyday Astronaut's writeup is the longest:

Another design consideration is due to the blunt nose of the Starliner you’ll see these little lattice structures. The Starliner was designed to be as stable as possible for reentry, which means a short and stout design. The lattice structure helps diffuse the airflow over the vehicle, helping to make sure there are no shock waves or any inadvertent pressure areas on the lower portion of the vehicle on the scent [sic, probably ascent]. Especially since the rocket actually tapers down to the skinny centaur upper stage. They also added an aerodynamic skirt to ensure smooth airflow.

This Spaceflight Insider page includes an infographic, credited to Derek Richardson, Spaceflight Insider, and Orbital Velocity, that mentions the lattice structure as well and credits it with diffusing "inadvertent pressure waves." They credit this Boeing launch press kit as their primary source, but I haven't found a version in The Wayback Machine that includes any information about the lattice structure. I'm going to reproduce the image here in case of link rot. An infographic showing dimensions of, and features of, the CST-100 Starliner and its service module, presented before the 2021 launch of OFT-2, specifically calling out the lattice structure

My understanding, based on these sources and a compressible flow course I took 15 years ago, is that these parts are for managing shockwaves during the trans-sonic regime. Once the vehicle is fully supersonic the taper from the Starliner's diameter down to the Centaur's diameter is protected by the bow shock, but just before the entire stack reaches Mach 1 there's a chance shocks will form there and cause pressure spikes, leading to vibration and stress. This perforated lattice structure trips the shocks upstream, next to the large flat surface of the service module.

Searching for "Starliner shock management" got me this excellent Twitter thread by Chris Combs which digs into the history of aerodynamic studies of "Hammerhead launch vehicles," including more nuanced/correct dissection of the aerodyanmics than mine:

At transonic speeds (M ~0.7-1.3) you get a shock structure that can form just downstream of your capsule. This shock interacts with the growing boundary layer/separated flow & oscillates rapidly. This produces large pressure loads

Combs also links to a tweet from Tory Bruno, which is frequently one of the best publicly-available primary sources on Starliner or older ULA matters:

Those are for Aerodynamic management. Helps to control the flow around and past the skirt. The skirt is there to manage the location of the shock attach point on Centaur which would otherwise be high up and asymmetric

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    $\begingroup$ The term everyone here uses for that structure is the "perf ring" $\endgroup$
    – Tristan
    Feb 24, 2023 at 15:25
  • $\begingroup$ @Tristan cool! unfortunately the term doesn't seem to have gotten out to the public much; all I see is two mentions on the Nasaspaceflight forums, both from the same person. $\endgroup$
    – Erin Anne
    Feb 24, 2023 at 19:31
  • $\begingroup$ "Perforated ring" does turn up some more of the discussion I remember, but really only tertiary sources, not secondary or primary ones. $\endgroup$
    – Erin Anne
    Feb 24, 2023 at 19:36
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    $\begingroup$ @Tristan I love the "perf" abbreviation, it could mean either perforated or performance. $\endgroup$ Feb 25, 2023 at 3:00

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