As we can see in the figure below, the interstage between the escape module and the command module is vented. Why did they go for vented interstage?

Wouldn't it increases the serodynamic loadings and hence structural loading to the interstage trusses?

How did they manage the flexibility of these interstage trusses because of the engine vibrations and aerodynamic loadings?

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    $\begingroup$ Are you talking about the truss that that the LES is mounted on? $\endgroup$ Commented Sep 20, 2018 at 13:40
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    $\begingroup$ @OrganicMarble, yes. I have mentioned it I guess. $\endgroup$
    – Amar
    Commented Sep 20, 2018 at 13:47
  • $\begingroup$ There should be an escape for the exhaust of the launch escape system. How should a launch escape work if there is no vented interstage? $\endgroup$
    – Uwe
    Commented Sep 22, 2018 at 10:20
  • $\begingroup$ It is a launch escape system but not a final stage. The launch escape system is never used in a sucessful launch, only in case of an emergency to separate the command module from an exploding Saturn to rescue the astronauts within the command module. $\endgroup$
    – Uwe
    Commented Sep 22, 2018 at 10:27

1 Answer 1


The LES support truss doesn’t have the same function as an ordinary interstage.

Unlike normal rocket stages, the LES may have to take the command module below it along when it fires. Thus its rocket motors have to be pointed outwards to avoid burning the CM. The further outward they’re turned, the more of their impulse is wasted.

Mounting the LES further forward lets the outward angle be minimized. The support truss exists to do exactly that, and the open triangular grid structure minimizes the weight penalty for it. Presumably the cost incurred in aerodynamic drag is less than the weight of a closed shell around the truss would be.

  • $\begingroup$ So why not mount the escape rockets on the sides of the capsule (allowing them to point more nearly downward and avoiding blowing superheated exhaust gasses over the command module) instead of on top? $\endgroup$
    – Vikki
    Commented May 18, 2019 at 22:54
  • $\begingroup$ @Vikki Given the size of the LES, I can't see a good way to place an equivalent volume of motors on the sides of the CM, but I'm not an aeronautical engineer. There might also be reliability/complexity concerns since you'd have to use at least two motors for symmetry. $\endgroup$ Commented Jun 22, 2023 at 21:42

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