Sunday, March 4, 2012

Estes Pro Series II motor retainers - final review

After finishing up the level 2 rocket, I used a bit of JB weld to install rail guides, reinforce the airframe joint in the 38mm Thunderbolt, and mount two of these motor retainers. All in one mix! That is one benefit to using a slow epoxy like JB weld; no rush to finish projects as you use it.

Here are both ends of the retainer on a 29 40-120 case before installation in the rocket. This image does not provide much information but it looks cool.

The retainers were installed on this kit, an unpainted Wildman Dragon, and a Loc Graduator. JB weld was used, but I bet 30 minute epoxy would be fine. Consider the thermal exposure, and also the ability of an epoxy to bond with plastic. JB weld works very well on both counts so it was my choice for this use. I would not use 5 minute epoxy because it is rated to only 200 degrees.

Because these rockets had been built and used long before the retainers were installed, the fit was too tight. Epoxy residue was a factor, and I also gather that fiberglass will always be a tight fit. I did sand the motor tubes a bit, but spent most of my time sanding the plastic tube (threaded portion that is attached to rocket) because it was easy to sand. I made each loose enough to fit, but tight enough to require a tap with the end of a screwdriver on installation.

Here you see the 29mm motor case dropped into the rocket. (Including the obligatory rocketry feet.) The fit will be perfect as long as you keep any epoxy residue out of the tube. Keep paper towels and alcohol on hand for this. My Loc MMA1 (29mm to 24mm) did not fit. It has too much electrical tape on the back to fit vertically (easy to fix) but also seems to be a bit too wide. I can also fix this by removing the outer paper layer. The biggest test of the retainer will be flying this rocket on a 24mm CTI motor with the MMA1 and an F240.

After dropping the motor in, it is quite easy to screw the cap in place. I did not crank down on it as the JB weld is only 12 hours old and I prefer to go 24 hours before any stress is put on a part, just to be on the safe side. Even after the epoxy has cured, I am concerned that screwing too hard could crack the bond (this is plastic after all) and weaken it. So users should decide how hard to screw the unit down and not exceed that amount of torque.

Overall my impression is that these are simple, inexpensive units. If I had unlimited money, I would use slimline snap-ring retainers for all of these kits. But the cost is a bit much for smaller rockets. $20 for a retainer on a $60 rocket... In the past, I simply left the rockets without any retainer and used the time-tested electrical tape method. There is nothing wrong with electrical tape, but this is a bit neater and faster, and certainly looks more professional when you walk up to the RSO table.

If you are interested in purchasing these, I expect they will come online soon.


Dick said...

I too wouldn't stress it until the JB was set but wouldn't worry afterward. On my home grown 38mm version, I didn't crank it hard anyway - it wasn't going anywhere. I have not flown that rocket much. The real acid tests will be 1) flying a lot with repeated temperature cycling and 2) tail end landings on hard surfaces. I think either will be more abusive than a hard tightening.

This is a great product. The idea has been around for many years and I can't believe it took an old-school hobby company on their first entry into mid power to produce them commercially!

High Power Rocketry said...

Yes just about any rocketry company could have started to sell PVC units like this, but they all seem to have gone with aluminum. For HPR I get it, but for mid power this seems like a good alternative.

I will have to fly one on an H54 to test heat exposure.