Quick Tip: Engine Blocks and Hooks

Always use engine blocks when using engine hooks.

I have never felt comfortable with the idea of hook-only motor retention, but I didn’t have clear rationale behind my concerns until now.  On launch day of rocket camp 2014, one of our students inadvertently pushed his engine (along with the hook!) too far into his Alpha III while installing his igniter plug into the nozzle.

At first I had assumed that the front end of the hook had ripped a slot through the thin-walled BT-20 engine mount tube, allowing it to slide back and forth.  But after examining the model more closely at home, it appears that the body tube was only damaged slightly.  What seems to have happened (although it’s hard to be absolutely sure by only examining the mount from inside) is that the flexible hook, after pushing open its mounting slit just a little, threaded its way into the inside of the mount.  Whether ripped tube or sliding hook, if there had been an engine block in place, no problem would have occured.

The good news is that I was able to fix the student’s Alpha III by gluing a modified engine block in from the top (using a long 1/2″ dowel) and reinforcing the damaged tube from the bottom using thin CA on the end of a narrow stick.  His rocket is now stronger than the others, but next time we teach a rocket camp there will be engine blocks for everyone.