This positive motor retention (PMR) system is designed for use with the Aerotech 24mm RMS hobbyline reload rocket motor. It can also be used with Estes and Aerotech single use model rocket engines. Especially when it comes to reload hardware, losing your motor is something to be avoided. While a regular engine hook will likely be all you need, why take the risk? When installed and used correctly, the CDI motor retainer should keep the rocket engine inside the motor tube under all circumstances.

And as a bonus, you can actually use the plywood plate as an aft closure RMS wrench. Simply slide it onto the aft closure and twist/turn as you would a regular RMS wrench. Works like a charm.

Parts List

This PMR system includes the items listed below, as shown in the picture above.

- 2 brass wood inserts
- plywood plate

To install the motor retainer, you'll need two 4-40 machine screws and a drill with an 11/64 drill bit. Epoxy is used to glue the brass inserts in place.

Installation

This CDI motor retainer is available in 24 and 29mm versions and is quick and easy to install. Having to buy screw seperatly is unfortunate. Having them included would have prevented a trip to the hardware store.

The PRM system can be installed on an existing rocket or during constructions, which is the easiest way to install it. Start by drilling two holes into the aft centering ring with a 11/64 drill bit. For new rockets, you can do this before gluing the centering ring to the motor mount. Put the brass inserts into the holes and glue them in place with some epoxy. The image on the right shows the inserts in the centering ring. The aft centering ring has been glued to the motor mount and a 24mm RMS motor has been inserted.

I usually put the brass insert on a 4-40 screw with a nut on top of the insert. Turn the machine screw to install the insert into the hole. Continue until the nut touches the centering ring. Next, use a pair of needle-nose pliers to keep the nut in place and remove the 4-40 screw out of the insert. Doing this will keep the insert in place as you remove the screw.

When using through-the-wall fins, make sure the inserts don't get in the way of the fins.

Field Testing

To use the motor retainer, place the motor into the motor tube, fit the plywood plate over the motor and use two machine screws to attach the plate to the brass inserts in the aft centering ring. If you wish, you can paint the plate to match the color of the bottom of the rocket.

This PMR system was tested on the modified Estes Fat Boy.