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When including an image in an answer to this question: Was New Horizons launched on a copper rocket? I noticed that for the larger diameter fairings (Atlas 501 and up) the Centaur upper stage is submerged in the fairing.


This makes the fairing a lot longer than it would be if it was mounted only to the top of the Centaur as it is on the smaller models.

The answers to this question Why does Atlas V have so many different types of payload fairings? don't address this issue.

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    $\begingroup$ Hahaha, I almost wrote the same question, then I decided I knew the answer. $\endgroup$ Jan 2 '19 at 0:48

A document from NASA Safety & Mission Assurance suggests the reason is structural:

Thin-­skinned Centaur cannot easily support the giant 5.4 meter diameter payload fairing, so the Contraves composite fairing also encloses Centaur.

My first guess, however, was aerodynamics, particularly the Whitcomb area rule: for transonic flight, abrupt changes of cross-sectional area produce lots of drag.

The Centaur upper stage is about 3m in diameter, and the Atlas V first stage is 4.1 in diameter.

The 4xx configurations use a 4.2 meter payload fairing, so the profile is 4.2m-3.05m-3.8m; the upper stage cross section is 53% of the fairing cross section, which is a pretty significant drop.

If the 5xx configurations left the Centaur un-faired, the profile would be 5.4m-3.05m-4.1m; the upper stage cross section would be only 32% of the fairing cross section. Instead, it's faired; there's only one cross-section transition instead of two (on the 501, at least), and the first stage is 58% of the fairing cross section.

There might also be airflow/turbulence issues necking down further behind the 5m fairing that would cause structural problems; ISTR something like that related to Orion on Delta IV, but I could be making that up.

This thread on nasaspaceflight.com discusses both structural and aerodynamic possibilities, as well as thermal considerations on the Centaur (though it doesn't explain why the 4m version wouldn't have the same thermal requirements).

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    $\begingroup$ Would you like to pick one? $\endgroup$ Jan 2 '19 at 1:26
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    $\begingroup$ No, I would not like to pick one. $\endgroup$ Jan 2 '19 at 1:34
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    $\begingroup$ The comment about it being done for Titan 3E and 4 sort of undermines the area rule argument, doesn't it? $\endgroup$ Jan 2 '19 at 2:00
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    $\begingroup$ Somewhat, yeah. $\endgroup$ Jan 2 '19 at 2:10
  • $\begingroup$ The structural reason could well also be aerodynamic. A wider fairing is draggier than a narrower fairing, and a fairing that necks down abruptly (like a Centaur-mounted 5-meter Atlas V fairing would be) is draggier than one that doesn't. The increased aerodynamic drag on the fairing translates directly to increased structural loads for whatever's supporting the fairing; if the 5-meter fairing were mounted atop the Centaur, this entire (rather large) load would have to be borne by the Centaur's thin balloon-tank skin, potentially causing it to buckle or collapse during first-stage flight. $\endgroup$
    – Vikki
    Jan 15 '20 at 21:55

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