a rocket motor retainer used for keeping a rocket motor in the MMT, fits 38mm LOC tubing. I plan to release more sizes later. 

I printed the retainer bit in PLA as that's what I had, the cap should be printed in PETG as its going to be a pain to remove/ replace it if melted. In my experience the semi knurl will not need support. if you have any questions you can reach me on TRF with the same user name.