In a comment from Boosted Bertha build Post #2, Openroad asked:
"I’m curious about how the engine retainer holds up on the sustainer - how many flights will it take for the booster motor’s ignition charge to “cook” the screw on retainer ring?"
Bernard Cawley answered and sent a picture via email:
"I have five flights on my Boosted Bertha. The sustainer's motor retainer ring is still serviceable - I'd guess it has 3-5 more flights in it before it won't fit well enough to be trusted. I have some of the 18mm retainers that are sold separately on hand, so I do have spares."
This is pretty much what I expected. It's interesting that the longer booster BT-20 tube and engine block at the top has directed the booster "flame" towards the center, there isn't any charring around the sides.
Estes does have replacement 18mm retainers on their website: CLICK HERE
Bernard Cawley gave additional information about the engine retainers in the comments below -