The Universal Motor Retention System

In the Summer of 1997, Estes started shipping their new line of Dark Star single-use 29mm motors.  These, the F62 and (yet to be shipped) G70, have a molded thrust ring structure formed as part of the rear of the case.  With the advent of this new type, there are now four separate, only partially compatible styles of motor, all nominally 29mm, all capable of fitting standard 29mm motor mount tube -- but which aren't compatible with each other's retention methods.  The Gorilla Lock (TM) retainer supplied with Estes North Coast (TM) kits like the Phantom 4000 (TM) won't hold any kind of motor other than a Dark Star without modification of either the retainer (a significant pain) or the motor (a Safety Code no-no).  The old standby methods like the Kaplow Klip require significant modification, or a set of subtly different retention clips, in order to accommodate Dark Stars, other (flangeless) single-use 29mm motors with tape thrust rings, the Aerotech RMS (TM) 29/40-120 consumer reload system, and the Aerotech HPR 29mm RMS comprising the 29/60, 29/100, etc. up to the 29/240.  An adapter for 24mm RMS or single-use motors adds yet another dimension.  And of course neither Dark Stars nor RMS are really all that secure with a plain tape wrap, unless the motor is allowed to protrude an extra distance from the mount to accommodate a ring of masking tape - which can move the CG to the rear far enough to render a stable rocket unstable.

Fortunately, there's now a better way.  If you like to fly 29mm model and High Power motors in the same rocket, and might even occasionally use an Estes D12 or RMS 24/40 as well (when field size limitations enter the equation or you just want a low, slow, visible flight), you'll like the Universal Motor Retention System.  This one system can be constructed for a parts cost of under $2 plus adhesive.  It requires only simple woodworking tools to build; a fine-toothed saw and a 3/4" spade bit or (preferably) twist drill for an electric drill or drill press are all you'll need.  It can be retrofitted to many existing mounts (if they were built with enough protruding mount tube to accommodate the tape-ring method of securing single-use motors), and you can find the parts in any large hardware store such as Home Depot, Lowe's, or any place that sells plumbing supplies.  It will work with any airframe at least 1.9" in diameter, and it only weighs about an ounce -- which means, for most rockets, it will require only a small amount of nose weight.
Figure 1: Here are the parts needed for the UMRS.  The pipe cap and nipple should cost less than two dollars.
To make the Universal Motor Retention System, take yourself to the nearest supplier of plumbing supplies, and purchase these parts:  First, you need what plumbers call a "nipple" -- a short length of pipe with male threads on both ends.  The size you need is 1 1/4" Schedule 80, in PVC, and you'll want the shortest length that isn't called a "close nipple," about two inches long.  You'll also need a 1 1/4" Schedule 40 PVC pipe cap -- it should screw onto the nipple a couple turns by hand.  If you don't already have some left over from home repairs, you'll also want to pick up a small can of plastic pipe cement -- I used the gray variety, but the important thing is that it be the kind thickened with PVC, for use on interference fit joints.  You'll probably have everything else you need on your workbench or in your scrap bin.

Figure 2: The nipple has been cut to length, 3/16 longer than the motor tube will extend beyond the centering ring.One of the most important points with the 1 1/4" Schedule 80 PVC nipple is the inside diameter -- it's just about exactly 32 mm, a loose fit for the 29mm motor mount tube.  Depending on the brand and type of tube, you'll probably have to build it up a bit with a layer of card stock, paper, or 1/32" balsa (grain parallel to the tube) to get a snug fit into the pipe.  On the tubes I've used so far, shirt cardboard was perfect.  Shim as needed, gluing the shim securely to the motor mount with an appropriate adhesive and sanding if needed, so that the pipe nipple is a snug slide fit onto the motor mount.  Now cut the Figure 3: The shim for the motor tube has been cut to size and is ready to glue into place.  I've put the coated side in, to allow the back side to soak up more PVC cement when I attach the nipple.pipe nipple to length so that the motor mount tube will be recessed about 3/16" inside the end of the pipe when the pipe is butted hard against the rear centering ring or, if you're building the UMRS into a new rocket, you can adjust the position of the motor mount to achieve the same recess depth.  Once the recess depth is established, use the plastic pipe cement to glue the nipple to the shimmed motor mount tube and rear centering ring (if you have a phenolic tube, you may want to rough up the inside of the pipe and use epoxy instead) and let the cement dry.  Be sure you don't leave any cement on the inside of the pipe aft of the end of the motor mount, as it may interfere with mounting the motor later on.
Figure 4: The shim has been glued, using the nipple as a clamp to ensure a good fit.  Now it's ready for the pipe cement.
Figure 5: Here's the complete Universal Motor Retention System, built into the Binder Designs Thug that will be my Level 1 bird.  To view in stereo, cross your eyes and let them relax back into focus while keeping the images fused.While the plastic pipe cement is drying, you can drill the hole in the pipe cap for the nozzle extension.  Locate the center of the pipe cap, preferably on the inside, and drill a 1/16" pilot hole, then use that hole to guide the point of the 3/4" diameter bit.  Drill the 3/4" hole cleanly through the cap (use low speed to prevent melting the plastic), then clean out any burrs on the inside of the cap and make the inside surface smooth -- but don't worry about removing the molding flash outside a 1" diameter.  Last thing for the cap is to use the plastic pipe cement to glue an AR-2050 centering ring or equivalent to the inside of the cap, centered on the hole.  This will be the retaining shoulder to keep the motor against the end of the mount, and will also serve to keep a 24mm motor in the mount if you use an adapter.

The last thing you'll need to do is to cut a scrap of motor mount tube about 9/16" long, for use with the RMS -- this length, combined with the closure flange on the RMS, should be about 3/4".  The two 29mm RMS rear closures are close enough to the same size that one piece of tube will work for both, if you fly both in the same rocket.  You'll get better durability if you soak thin cyanoacrylate glue into the ends of this tube section to harden the material.

To use the Universal Motor Retention System, prep your motor normally.  For flangeless single-use motors, wrap 3/4" wide masking tape around the rear end of the casing flush with the rear shoulder to a thickness of 1 to 1.5 mm to form a thrust ring -- if you fly these motors, you're probably already using a variation of this technique.  For RMS, after completing motor assembly, slide the motor mount tube ring onto the casing from the front (ejection) end, so it rests against the rear closure flange.  For Dark Stars, with their molded-in thrust flange, you won't need any adjustment.  For an RMS 24/40 in an adapter, put a tape thrust ring on the adapter, then slip the motor into the adapter and let rear closure act as a thrust ring -- it's designed for this.  For single-use 24mm composites and Estes D motors, you should have a thrust block in the adapter that lets no more than 1/4" of the motor protrude from the adapter -- I set mine up so the D12 is flush with the adapter and use the RMS 24/40 in the same adapter.  Hint: for the Estes D12, I find it easier to install the igniter before inserting the motor, and thread the igniter leads through the cap, but you have to fold the paper tape back around the lead wires and bend the igniter to parallel the motor after you install the plastic plug, in order to let it fit through the hole.

Now slide the front end of the motor or adapter into the mount, adjusting the diameter with tape if needed to get a neat slide fit -- the motor should slide easily, but it shouldn't rattle.  It is okay, though, if the motor slides out when you turn the rocket nose up; that'll make it easier to extract after the flight.  There's no need for a motor latch, friction fit, or other retainer in a 24mm adapter; the AR-2050 bears against the rear closure of the RMS 24/40, and against the case of an Estes D12 or other 24mm single-use.  Finally, screw the pipe cap onto the nipple until it snugs against the rear closure, flange, or case.  It doesn't need to be very tight, just enough so it's firmly in contact.  The cap will hold snugly on 24mm RMS and single-use motors, because of the AR-2050, but easily clears the nozzle extension on any 29mm single use or RMS motor I've tried.  In addition, the 3/4" exit diameter of the cap is wide enough not to interfere with the exhaust even from an Estes D12, the motor that seats with the nozzle exit plane furthest forward relative to the UMRS. Once the glue is fully dry, the Universal Motor Retention System is at least as secure as anything screwed into the rear centering ring, and much faster to mount and dismount the motor and/or adapter.

One word of warning: don't over tighten the cap; once there's a little crud on the threads, it's not all that hard to tighten it to the point where you have trouble getting it off, and if you need pliers to loosen it, you're likely to damage the rocket in the process.  You might even want to lube the threads with a little of the same grease you use for assembling your RMS, to simplify assembly and disassembly.


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