Monday, January 10, 2011

Review: Lobbin' Bobbin

I've had reading about spool rockets on r.m.r for a long time, and had a big plastic spool stashed for several years. Someone had built a similarly sized spool with a 29mm mount, which only reached an altitude of about 100 ft. I decided I wanted a bit more altitude, so I built mine with a 38mm mount. The spool is one piece plastic, 10.5" tall, with 10.5" plates. No fins, no finishing...this thing is a skill level-0 HPR rocket!


Construction:
one large plastic spool (10.5" tall, with 10.5" plates)
one 38mm tube, 10 7/8" long
two 1/8" ply centering rings (38mm - 4")
one 1/8" ply centering ring (38mm - 3")
two threaded inserts, with bolts and washers
one 1/2" launch lug
1/8" tubular Kevlar® , 5' long
18" RocketChutes drogue

The body of the spool is about 3.5" in diameter and the through hole almost fit a 38mm tube. I used my Dremel to expand this hole by about 1/8". I glued two 4" centering rings together, installed a pair of threaded inserts, and attached them to one end of the motor tube. After it dried, this assembly was inserted into the spool, and the smaller centering ring was glued to the other end of the tube. I used epoxy for these steps.

The ends of the spool have several holes of various sizes. One 1/2" hole was positioned such that a 1/2" LOC launch lug, when abutted to the spool's body, would line up with it. It was just far enough from the body to accommodated the wall of the lug - perfect! Unfortunately, the hole on the other end was offset, so I had to drill my own. I glued the launch lug on the lower end of the spool's body using Liquid Nails.

I thought about several methods of attaching the shock cord. What I ended up doing was adding loops to both ends of the tubular Kevlar® , using the method published by Giant Leap in High Power Rocketry Magazine. You loop the material, wrap the overlapping area with cloth twine, and soak it in epoxy. This method has worked well for me several times. I looped this tether through two existing holes in the top plate and around the body. The chute attaches to the second loop.

Finishing was easy - paint the centering rings and launch lug flat black.

Flight:
I first flew the Lobbin' Bobbin on a H123-S. Prep was simple: install the motor retainer clips, wrap the chute in a protector, and cram it down into the tube. The flight was slow and noisy and went to maybe 200 - 300 feet. Ejection appeared to occur right at apogee. The descent was a bit faster than you would generally like, but the tube bounced and was recovered undamaged.  The second flight was on a Loki H144-5.  The delay was too long and the chute didn't open. The top plate cracked.  I added a plywood support on the lower side but have not flown it again (yet).

Summary:
This was a very quick build, and if you like *very* low and slow flights (similar to a flying saucer style rocket), a spool rocket may be for you!