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After ignition the walls of the SRB are separated from the "reaction chamber" by solid rocket fuel. But as the burn progresses, more and more fuel is used up and so the isolation from the solid fuel is reduced until the shell of the booster itself is basically the reaction chamber.

Question:

Why are the walls of the solid rocket boosters not glowing red hot (or even brighter) at the end of the burn?

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    $\begingroup$ If the shell of the booster itself is the reaction chamber, there no solid fuel left to heat that shell to glow red hot. Chamber pressure is falling rapidly at the end of the burn when the shell is no longer fully covered with solid fuel. The solid fuel requires a minimum pressure to burn and stops burning below that pressure. $\endgroup$
    – Uwe
    Commented Oct 27, 2022 at 8:35
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    $\begingroup$ Just to clarify the way an SRB burns in most cases the propellant burns around the surface of a cylinder in the centre of the SRB as the burn continues this surface eats into the propellant expanding the cylinder until the expansion reaches the edge of the rocket casing or lining. They don't normally burn as a circular area that works its way up the SRB from bottom to top. $\endgroup$
    – Slarty
    Commented Oct 27, 2022 at 15:11
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    $\begingroup$ While I was looking for an answer last night I stumbled across a Northrop-Grumman press packet about the 5-segment SRBs which contain the quote "During operation, the temperature of the five-segment booster motor chamber gases reach 5,600°F. At this temperature, steel does not melt – it boils." Good thing there isn't efficient thermal transfer to the casing! $\endgroup$
    – Erin Anne
    Commented Oct 27, 2022 at 20:22
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    $\begingroup$ Is that not because the fuel burns too quickly to heat the walls that much? $\endgroup$
    – Robbie Goodwin
    Commented Oct 30, 2022 at 0:17

2 Answers 2

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Solid fuel does not burn unless it has been vaporized by heat from combustion. The boundary between solid and gaseous fuel is located at the point where the temperature equals the fuel vaporization temperature. When this boundary penetrates to the SRB wall, the fuel extinguishes.

This means the burn can heat the wall up to, but not beyond, the vaporization temperature of the fuel grain. The walls could never get red hot despite the 5000F flame nearby.

This is true only for core burner designs, which include all large SRBs http://what-when-how.com/rocket-motor/cylindrical-core-burners-rocket-motor

enter image description here

It is not true for end-burners. http://what-when-how.com/rocket-motor/end-burners-rocket-motor-2

enter image description here

The lower part of the casing is exposed to flames from the upper portion of the fuel. But end-burners are limited to smaller rockets.

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    $\begingroup$ So it's a combination of physics (basically evaporation cooling and isolation by the fuel) and some kind of liner inside the steel casing in some cases? So building a SRB in segments seems like asking for desaster to happen in light of this information. $\endgroup$
    – TrySCE2AUX
    Commented Oct 28, 2022 at 4:49
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    $\begingroup$ @TrySCE2AUX ... Building a SRB with a one piece casing is more structurally sound than in sections. But that would require a one-piece grain as well. With paired SRBs, it is imperative the two grains burn (and burn out ! ) the same. Maybe there is a safety trade-off here. $\endgroup$
    – Woody
    Commented Oct 28, 2022 at 5:31
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    $\begingroup$ My impression was that it was a question of transportability... Each one of them was almost 600 metric tons. Transporting that from Utha to florida looks like a hard task to me, so splitting them up would have made transporation slightly easier. But I have no sources, maybe that's another question waiting to be answered here :) $\endgroup$
    – TrySCE2AUX
    Commented Oct 28, 2022 at 6:14
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    $\begingroup$ @TrySCE2AUX somewhere on on spaceSE is a diagram showing how the joins worked, which included an ablative layer trying to avoid this happening. $\endgroup$
    – GremlinWranger
    Commented Oct 28, 2022 at 6:37
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Simply - even a very thin layer of fuel is enough to isolate the heat. So only in the very last seconds will there be not enough fuel to isolate the heat flux.

Then there is a liner between the fuel and the casing further increasing the thermal resistance.

The residual thermal flow will not be enough to get the casing to glow red in the short time there are both flames inside and little to no fuel to isolate the casing from the flames.

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    $\begingroup$ The shuttle SRBs had a case liner made of "NBR", a kind of rubber. There is some information about it in this paper ntrs.nasa.gov/api/citations/19870011601/downloads/… $\endgroup$
    – Organic Marble
    Commented Oct 28, 2022 at 0:17
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    $\begingroup$ @OrganicMarble apparently it contains no asbestos, unlike almost every NASA building $\endgroup$
    – Erin Anne
    Commented Oct 28, 2022 at 8:51
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    $\begingroup$ @ErinAnne I'm sure you've seen it where there are people are working on the facilities completely enclosed in protective suits, and management says that the office workers are fine to be there. $\endgroup$
    – Organic Marble
    Commented Oct 28, 2022 at 20:02
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    $\begingroup$ @OrganicMarble: Office workers can safely inhabit a building containing asbestos unless the asbestos is disturbed. There are a number of things that can cause such a disturbance, including remodeling. If that happens, the building is evacuated and an abatement is performed. That's when you see the people in the protective suits. No one is allowed in the isolated area (the area containing free asbestos) while the abatement takes place. See en.wikipedia.org/wiki/Asbestos_abatement $\endgroup$
    – Robert Harvey
    Commented Oct 28, 2022 at 23:55
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    $\begingroup$ @RobertHarvey we know. The "joke" is that the people in bunny suits tape off the area immediately around the ladder while the people in the office work a couple yards away. Presumably the asbestos knows not to cross the tape. $\endgroup$
    – Erin Anne
    Commented Oct 31, 2022 at 5:48

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