Motor Mount and Retention

This page covers the mounting and retention of the rocket motor. For information on the transfer of force from the motor to the rocket (which sometimes can be down through the motor mount), see the Thrust Plate page.

The motor must be constrained in all 3 axes of movement inside the rocket. Usually, this is accomplished with a tube that stops the motor from moving side-to-side in the rocket, a thrust plate that stops the motor from moving up, and a retainer that stops the motor from moving down.

Motor Mount Tube

The motor mount tube, sometimes shortened to just the motor tube, is the tube that constrains the motor in 2 axes of movement inside the rocket. Tubes in rocketry are commonly sold to match the outer diameter of the motor casing. 

The motor mount tube should be as long, if not longer, than the motor to ensure full enclosure.

It is connected to the airframe through the motor centering rings (or if it is a minimum-diameter rocket, the motor tube is the airframe). Motor centering rings keep the motor mount tube centered, as the name implies, and are usually made of 1/4" plywood that is epoxied to the outside of the motor mount tube and the inside of the airframe. Depending on the thrust plate configuration, these can sometimes transmit the load of the motor into the airframe, so they may require reinforcement or special considerations for application. There is no set rule for the number of centering rings, but usually, you will see between 2 and 6 depending on the size of the motor.

In some applications, you can remove the motor mount tube altogether and have a "tubeless motor mount" where the ID of the centering rings is in direct contact with the OD of the motor casing. This is generally not recommended, since it increases complications during assembly and can lead to odd stress concentrations on the outside of the motor casing. 

Motor Retainer

The motor retainer keeps the motor from moving in the direction opposite to flight. It is especially important during events like ejection, where there will be a sudden jerk of the motor backward. There are two types of motor retainers; passive and active. 

Passive motor retainers rely on friction or some other sliding contact force to stop the motor from coming out. They are rarely used (usually only seen on minimum-diameter rockets) since they have a higher chance of failure and are outright banned at many launch sites. The most common approach to passive retainers is to wrap the base of the motor casing in aluminum foil tape, which makes it very difficult to slide in/our of the motor tube, The Rocketry Forum has a few threads discussing the technique. On the team, we refer to this technique as "Crazy Jim's Motor Retention" in honor of the original forum page where we learned the practice, which it seems has been lost to time now.

Active motor retainers are the better choice for most applications and are split into two categories. 

Base retainers are the most common and generally regarded as the most reliable option. A base retainer goes on after the motor is installed, and presses it into the thrust plate. To come out of the rocket, the base retainer would somehow have to overcome the (usually metal) lip of the material holding it in place. The most common base retainer is the Z clip, which is just a metal clip that slides over the edge of the motor. Recently, there has been a renaissance of base retainers that fully encircle the motor and provide better retention force all across the motor flange. 

Top retainers are the second type of active retainers and are usually used in minimum diameter rockets, or as a backup to base retainers. A top retainer involves securing the top of the motor with a threaded piece to a bulkhead inside the airframe. Many casings have a section at the top that can accept a thread (or there are aftermarket adapters that allow them to). It is common practice to use the bulkhead immediately above the motor and an eyebolt to hold the top retainer into the rocket. One notable restriction on the use of top retainers is IREC, whereas of 2019, they can only be used as a backup to base retainers.

When using a mechanical motor retention technique, it is important to consider flight forces. The forces of flight can sometimes cause things to unscrew or unbind, leading to the retainer failing. To avoid this, make sure to use some sort of thread-locking mechanism for parts that screw together.