Today's article was going to be about the completion of the CPR3000, but there are still a couple more steps, so that will have to wait until tomorrow. In the meantime I started working on the fin cans. The 38mm fin can dry fit perfectly and was ready for glue right away, so I put the easy stuff together. The 54mm motor mount did not dry fit so nicely.

Problem - The Motor Tube is Too Big!

The 54mm motor mount was ordered as an addition to the kit so I could launch the newly two stage recovery Spitfire on some larger motors.

This isn't the first time I've gotten a motor tube which came a little to big, my Public Missiles Quasar came that was as well. At that time I broke the fragile 2.1 in to 3 in centering ring while trying to sand the inside of it. This was a major pain and lead to some extra creative gluing which took about 5 times the amount of glue and time as it would have taken had it fit in the first place.

On the current rocket I remembered this experience from the last rocket and decided to be a little more careful and not try to force pieces in place. To let you know how much it was off, I couldn't even fit the HAMR motor retainer on the motor tube, which has never happened before.

Research

There are essentially 2 ways to fix a motor tube that is too big around. You can either sand the outside of the motor tube or sand the inside of the centering rings to fit.

If the motor tube is very large, or is a large way off of the needed diameter, sanding the inside of the centering rings requires less work. If your centering rings are small, typical of many near minimum diameter rockets, they can be fragile and extra care should be taken to prevent breaking them when sanding.

Solution - Sand Inside of Centering Rings, With Care

Last time I ran into this problem, and broke the centering ring, it was because I tried to sand the inside of the centering ring with fine grit sandpaper and it kept ripping. This frustrated me and I tried to sand it harder to make it go faster.

This time I realized that a more coarse grit sand paper would make the sanding go faster and it was going to be glued over anyway so looks were not important. I got a hold of some 60 grit sand paper, but I was still running into the same problem with it tearing often because flat sand paper is hard to mold to the inside of a centering ring. Then I ran to Home Depot to look for a solution and came across the marvel that is sand paper sponges!

The technique here is to hold your fingers out so that you can spin the sponge without running into your fingers constantly. When my hands got tired (there were 3 centering rings to do), I switched it up and spun the centering ring around the sponge. Whichever technique you are using, be sure to switch directions when it gets smooth and it goes much faster. This is a way to be very gentle with the rings while at the same time getting the work done in a timely fashion.

After a few short minutes, the centering ring finally fit on the tube.

Once I had finished the 3 centering rings, I was very happy, but the motor retainer still did not fit smoothly on the tube, so in the end I had to sand the tube as well. That didn't take too long and then I was ready for a good dry fit.

Next time: the CPR3000 build and recap (seriously this time).