Looking through photos of Apollo CM capsules after reentry, I was trying to have a feel for how the heat was distributed on the sides of the capsule. By the look of surviving patches of Mylar tape and not melted aluminum EVA handles the reentry temperature on the side of the capsule doesn't seem to be too high. Certainly not high enough to charr the ablative material on the side surface and melt the handles. But then I paid attention to interesting pattern of charred ablative material around the RCS thrusters (appeared to be the roll thrusters that would be used to "steer" and manipulate lift during reentry).

This one from Apollo 11: image from http://www.collectspace.com/review/apollo11_cm_radiationlabels01-lg.jpg enter image description here

Apollo 13: image from https://www.nasa.gov/sites/default/files/thumbnails/image/capsule_.jpg


Apollo 12: image from https://www.nasa.gov/sites/default/files/thumbnails/image/s69-22271.jpg

enter image description here]([![https://www.nasa.gov/sites/default/files/thumbnails/image/s69-22271.jpg

Apollo 8: image from https://en.wikipedia.org/wiki/Apollo_8#/media/File:Ap8-S68-56310.jpg

enter image description here

Question: Why this shape of the pattern?

Given it is only around the thrusters (and not elsewhere on circumference of the conical part) it probably means some interesting reaction of thruster exit gas with the air flow (and/or the shock wave) around circumference of the main shield at the bottom. But why is it on both sides (above and below) of the thrusters? One would think that velocity of incoming air would be way higher than what comes out of the thrusters and push the exhaust upwards, that's why I can't really understand why the shield is charred immediately below the thrusters as well.

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    $\begingroup$ I've fixed your image links which were not displaying. You can insert them properly by clicking the image icon above the text box that you type the question into, then click "paste", and paste the image url into the box that appears. You may have been using the mobile app; I've never been able to get it to properly insert pictures or do many other simple, normal tasks; too bad Stack Exchange management is focused on improving other things besides the app. $\endgroup$ Commented Nov 26, 2019 at 15:27
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    $\begingroup$ @OrganicMarble yes, you are correct, I'm posting from mobile (not from app though). I did use the insert image icon, but unfortunately it doesn't work sometimes. Thanks for fixing this! $\endgroup$ Commented Nov 26, 2019 at 15:54

1 Answer 1


Reaction control system jet interactions with a vehicle's aerodynamic flowfield can be counterintuitive. Here's a capsule simulation results graphic showing similar spreading effects below the jet as well as laterally (from the first link below).

enter image description here

This paper states in reference to Apollo:

Interference heating in the case of the yaw and roll jets covered significant acreage of the backshell. In particular, forward-firing roll jets produced the most energetic interaction with the shear layer, coming off of the capsule’s shoulder, and this yielded the greatest heating augmentation over the largest area.

(emphasis mine)

This Apollo-era diagram is quoted in the second link below.

enter image description here

This is from Analysis of Aeroheating Augmentation due to Reaction Control System Jets on Orion Crew Exploration Vehicle. Other papers of interest are Experimental Measurement of RCS Jet Interaction Effects on a Capsule Entry Vehicle and the canonical Effects of Cavities, Protuberances, and Reaction-Control Jets on Heat Transfer to the Apollo Command Module which sadly doesn't seem to be on NTRS.


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