Lock ring and Engine Mount assembly for 40mm rocket tubes. Use super glue to hold it to the rocket body. This was designed for the Estes 'Color the Sky' kit, but may work on others. Tested with various B-engines. Bring some pliers to remove/reinstall engines for multiple flights -- it can be a tight fit. Print with a high temperature material. PLA will work, but may warp after a few launches. I'd suggest PETG or ABS. Orient the model horizontally in your slicer and print with supports to the bed.