All right, slight change of plan! A comment exchange with Chris Udy prompted me to check my local hobby shop for #2-56 threaded rod, and indeed they had it, in 1-foot lengths for something like a buck fifty. Unfortunately they were out of stock on #2-56 nuts and #2 washers, but I won’t need them until I launch and if all else fails I can get them cheaply enough on eBay. So I bought the rod and am using that for the motor retention. That means the motor tubes can be in contact, which will make for a stronger mount — also the minimum width of the centering rings won’t be quite so narrow.
There’s an Apogee Rockets video on cutting centering rings which I found fairly helpful, though there’s pretty much nothing non-obvious about it. Except maybe wearing safety goggles — you’re putting some sideways stress on the knife blade when you cut tight curves, and the tip can snap and go flying! It also might not be entirely obvious that you want to cut the interior hole first, then cut the ring out, so you have a large piece of cardboard for easy manipulation right up to the end. And for another reason, coming up.
I used a rather thick piece of cardboard, maybe twice the thickness of cereal box cardboard. Possibly more. I printed out two copies of the pattern and glued them to the cardboard; Milligan suggests using repositionable spray adhesive, but I decided to go with ordinary (permanent) glue stick. Less messy and smelly than spray adhesive, and I see no particular problem with leaving the patterns stuck to the rings.
Actually I printed and glued down three copies of the pattern, because of a problem with the first ring. I cut out the interior hole, cut the ring out, and tried fitting three motor tubes. The interior hole was a little too small, I pushed a little too hard trying to get it over the tubes, and the ring broke at one of the thin spots. I tossed that ring and tried again. This time I test fitted the tubes and sanded out the hole until they fit easily before cutting the ring out of the sheet. Also, I decided to use scissors to cut the ring out; easier than the knife. I intentionally cut the ring a little too big (especially around the places where it’s narrowest) to be trimmed down down after gluing the mount together.
Next steps, much like the Ranger. I cut a piece of #2 threaded rod 2¼” long. Shorter would have been all right, but I decided to make it long. I glued two tubes together, epoxied the rod in the gap, and glued the third tube on top.
I glued one centering ring on with white glue. The other I had more trouble with; despite having checked it before cutting the ring out, it didn’t want to fit over the tube assembly without further adjustments. I wasn’t having much luck trying to sand and scrape the cardboard, so I ended up treating it with thin CA to stiffen it up. That allowed me to sand the interior hole to size, but of course with the cardboard filled with CA, yellow glue would probably not have adhered to it well. So that ring was glued in place with epoxy.
(I downloaded CHDK, a hack that lets you override settings and run scripts on a Canon point and shoot camera last night, and of course one of the first things I had to do was to try a time lapse video:
Which chews up battery power like nobody’s business. I’ll have to buy an AC adapter if I’m going to do more with that.)
This time I glued the thrust rings in after putting the tubes together, a better procedure than doing it before.
This is a dry fit. Glue happens later.