Brand's Expendable Extendable Motor Retainer ("BEEMR")

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Larry Brand

Brief:
There are a number of very good HPR motor retainer designs available, and I have used some of them, including the famous Kaplow Clip (homemade) and the beautifully machined (and pricey!) Aero Pack products. But in getting involved with very stubby large rockets (my recent tubefin research), I found an unmet need in the retainer arena: a need for an adjustable retainer that holds in big motor casings that stick out 2 to 6 inches or more behind the rocket airframe and motor tube. Usually, doing this is considered poor practice, but there is no balance problem with tubefins (far, far aft center-of-pressure and CG), and its an easy way to get motor to fit in a cramped "stubby" rocket. So I invented what I modestly call "Brand's Expendable Extendable Motor Retainer (BEEMR). It costs about a buck to make.

Construction:
I have to admit that my BEEMR design looks like a piece of burned trash that fell off a garbage truck. It is made from a small, 2-serving size (8 1/2oz) tin can from a Del Monte canned fruit or vegetable product (pull-off top), and 6 x 9" pieces of heavy duty (50 lb rated) picture hanging wire. Thats it. Looks like hell, works great. I suppose I could have painted it.

The photos are pretty self-explanatory. I cleaned up the empty can, and poked 6 holes (about 1/8" in diameter) around the perimeter with an awl and hammer (a heavy nail would work fine). Next, I cut a 1.5" hole in the bottom of the can, to accommodate the nozzle of the biggest motor I intended to use, a CTI 54mm K445. There are various ways to do this I suppose. I simply traced the circle I wanted, and poked awl holes around the circle until the dots were connected, giving me a jagged hole that I smoothed by going out and buying a $16 Dremel tungsten carbide grinder tip which I clamped in an old power drill and ground away until I was satisfied. Do wear goggles. Like I said, there's probably a better way. The BEEMR was finished by tying a knot in one end of each piece of picture wire (use pliers to pull it tight), and threading the other end through one of the holes from the inside. Done. To use the BEEMR, you must first build your rocket with 6 small eye-bolts (3/32" size) installed in the aft centering ring around the motor tube. I suppose you could "retro-fit" a rocket for use by screwing in eye-screws in the same manner. Six is probably overkill, and 4 or even 3 would be fine.

Flight:
For flight, the motor is first friction fitted in the usual way, but not as tight as you might do typically. I built my BEEMR for a series of big 5.5" tubefin tests after my fellow rocketeers politely told me that friction fitting K-motors and above with only masking tape, which I had been doing, was kind of an unsafe practice. With the motor in place and friction-fit to protrude the amount needed so the rest would fit inside, the BEEMR is placed over it, and the wire retainers are strung through the eye-bolts and securely tied off and the excess twisted around. I wrapped the whole affair with a little tape to keep the wires from vibrating in flight. Maiden flight was with a CTI J330 and was flawless. BEEMR was a little scorched but no worse for the wear. To change motors, I untie 5 of the 6 wires, and pull the BEEMR off the motor casing so I can remove it and install a new one. BEEMR has now also flown on CTI I212SS, I285, and K445 motors. The extended motor and retainer also does a pretty good job of preventing landing dings on my tube fins. There will be those who question the wisdom using $120 worth of 54mm CTI casing and platinum-plated aft closure (at least its priced that way) to protect a $1 tubefin. So far, neither the BEEMR nor the motor casings it retains have been dented or damaged on landings in the SoCal desert. Launch photos (my amateurish blow-ups from beautiful shots by Rocketography at ROCSTOCK) show the BEEMR mounted on a 5.5" tubefin fin I was testing ("TeaBird 54", mentioned in my "Teabird" article on EMRR).

Summary:
PRO: Low-cost (one buck), infinitely adjustable motor retainer for 38-54mm casings, easily replaced, surprisingly sturdy, good for at least 5-10 launches each. Only one around for situations where motors will stick out beyond the motor tube a variable amount.

CON: OK, its ugly. Maybe painting it would help. You do need to install the retaining eye-bolts in the aft centering ring when the rocket is built, but eye-screws might be possible for a rocket that has been built already. BEEMR is really meant for tubefin rockets, If motor is to extend this far (6" or more) behind a 4FNC rocket, balance is critical, and should be checked before using this method.

GUEST's OPINION:
"" (X.X.)

1. Motors
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