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While researching the purpose of some rocket engine components, I came across several images of perforated shrouds surrounding the combustion chambers of rocket engines. I was highly curious about what purpose they serve, but was unable to find clear evidence.

The LR87-AJ-11, a late version of the LR-87, which was designed and build by Aerojet-General features such shrouds, for example, as seen here and here (all rights reserved to Jack Snell).

It should be noted that earlier variants of the LR-87 do not feature such a shroud as far as I am able to tell.

Another engine which would have featured such a shroud is the AJ-1200, which was an engine proposed for a pressure-fed space shuttle booster which was never built. The proposed schematics of the AJ-1200 can be found here, and the shroud can be spotted on pg. 5, 9 and 11.

The only thing these engines appear to have in common is that they were both designed (or updated) by the Aerojet-General Corp. in the late 60s or early 70s (1968 - 1972).

Perhaps someone of the designers from back then is aware of the function of the shrouds? Or does someone have a clue as to what it could be?

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    $\begingroup$ I think the shroud should bear a part of the thrust force to reduce the strain of the combustion chamber. $\endgroup$
    – Uwe
    May 7, 2022 at 13:25
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    $\begingroup$ @Uwe agreed, and with holes cut out for weight reduction. $\endgroup$
    – Organic Marble
    May 7, 2022 at 15:06
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    $\begingroup$ Thanks for the link to the AJ-1200 document, very interesting. $\endgroup$
    – Organic Marble
    May 7, 2022 at 15:15
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    $\begingroup$ @Woody are you talking about the cylinder with oval slots cut in it above where the nozzle flares out? Because that's what we are talking about. $\endgroup$
    – Organic Marble
    May 7, 2022 at 15:35
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    $\begingroup$ @Woody There is no bellows. These are the tubes to cool the nozzle wall. $\endgroup$
    – Uwe
    May 7, 2022 at 17:55

1 Answer 1

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They are structural supports for the combustion chamber.

I didn't find a specific reference for those engines, but the Space Shuttle Main Engine (SSME) has a similar support, although it isn't perforated. On the SSME it's called the "throat ring".

enter image description here

The chamber includes a liner, jacket, throat ring, coolant inlet manifold, and coolant outlet manifold. The liner outer surface has 430 vertical milled slots that are closed out by electrodeposited nickel. The jacket halves are placed around the liner and welded. The coolant manifolds are welded to the jacket and the liner. The throat ring is welded to the jacket to add strength to the chamber.

Presumably the cutout holes are for weight reduction, this is common in aerospace structures.

References -

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    $\begingroup$ I am curious about the contents of the volume under the throat ring, if it isn't ventilated. Do they draw a vacuum? Flush it with something? Leave in whatever welding gas was used (if any)? Let in air? I would have assumed they put a vent hole in, so that the pressure differential to the outside doesn't fluctuate during cool down and ascent. $\endgroup$
    – Rikki-Tikki-Tavi
    May 7, 2022 at 18:01
  • $\begingroup$ @Rikki-Tikki-Tavi Interesting, and I don't know the answer. I took this picture of an SSME main combustion chamber in a museum...it doesn't look to have any vents. i.imgur.com/F7t7Ga0.jpg $\endgroup$
    – Organic Marble
    May 7, 2022 at 18:20
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    $\begingroup$ @OrganicMarble Very interresting. Thank you very much for your answer! Perhaps the volume under the throat ring is connected to the accoustic cavities somehow? $\endgroup$
    – Angel Sachse
    May 7, 2022 at 19:39
  • $\begingroup$ @Darkcoucou There is a decent cross-section of the acoustic cavities here space.stackexchange.com/a/28142/6944 and while there is some flow through them, from this, I can't confirm a connection. $\endgroup$
    – Organic Marble
    May 7, 2022 at 20:20
  • $\begingroup$ @OrganicMarble For various reasons, I think this was probably welded by electron beam and it therefore contains a vacuum, if there is no vent. I could also see it being filled with helium. $\endgroup$
    – Rikki-Tikki-Tavi
    Jun 13, 2022 at 8:14

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