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Detail of the darker-colored ablator at the center the APS engine thrust chamber. Compare with the hot-fired section on display at the Cradle of Aviation.

This is the second APS thrust chamber (i.e., the one which was inaccessible when I took the previous batch of photos); once tipped over, the light available in the interior of the engine was more favorable than the other.

The line going down the side at about the 8:00 position appears to be a water stains, rather than any sort of structural damage. I understand that the thrust chambers were acquired by the former Michigan Space & Science Center from a federal surplus action at Johnson Space Center, where they'd been located in a back storage area, so it's possible that the discoloration happened long ago.