After about 20 minutes of searching I didn't find anything relating to the problem of tubes loosing length after pressing. Apologies if I missed it. i'm not sure what the correct term for this phenomenon is.
I am working on a 3/4" 1lb motor design after purchasing some tools from Caleb at Woody's. I live in the UK but just can not find decent tooling over here so spent a fortune on some quality stuff.
Anyways, my tubes are 7.5" long but lose close to 1/4" in length after pressing the whole thing using a 3 ton arbor. The nozzle presses in one increment, fuel grain in about 14 increments, delay takes about 5 and the bulkhead takes 2. I'm using a clam-shell support and when the finished motor is extracted it looks fine. No tube wall crimping or rippling and the things take off like lightening.
However, there is this shrinkage thing going on. What is causing this?
Tubes are convolute from Stell. Had 500 made up. They tell me they are made from the same materials their pyro clients use.
I am fairly new to pressing rockets. I have a PtoF gauge but I find just pulling by feel works well so far. It's a 3 tonnes so maybe I'm exerting way too much force without realising?
Anybody else experience this sort of thing before?