Saturday, April 12, 2014

Extending a motor mount

First some background. A couple of flying seasons ago, I lost two 24/40 RMS casings and, so far, haven't replaced them. That grounded a few of my rockets that required more thrust than a D12 or E12. I have a 29/60 case and, while the main body is the same length as an E12, the charge well extends further. And, unfortunately, several of my rockets use spring motor retainers and/or engine blocks. The 29/60 doesn't fit in these. (I never used blocks on HPR and ditched them a long time ago on my 29mm MPR builds. However, I continued to use them on 18mm/24mm kits and some scratchers.)

I decided to figure out a way to adapt one of my earliest post-BAR 24mm birds, the BNC-R1, for the 29/60. I first Dremeled-off the spring retainer. I found that a BT-70/24mm plywood centering ring was about the same diameter as the body tube. Thus, it would work well as both a thrust plate and a mounting point for an Estes PVC retainer. I mounted that retainer to a short piece of BT-50 and the ring, just as if it were the end of any other rocket. I then sectioned another piece of BT-50 and slid it over that tube. This sectioned piece was measured to it would overlap the existing motor mount. I also found that the packing tube from 38mm CTI reloads was close to the ID of the BNC-R1's body. I added a short section to provide more 'bite' between the new bulkhead/centering ring and the old one.  All these parts were epoxied together and, tadah, the BNC-R1 will accept the 29/60 and anything shorter.

Luckily, the old rocket was stable enough to accommodate the extra parts, larger  motor, etc.