When you step up from low power to mid power rocketry, one thing you will not find included with your mid power kit is a motor clip or any other form of motor retention (Aerotech kits do come with a clip but you need to be he-man to pull the thing back). Why is it so? Well most mid and high power kits can be flown on motors of various diameters. For example, many kits can be flown using 54mm, 38mm and 29mm motors. If you think about it you can see why a clip will not work in situations such as this. Also, by not providing a clip, you get to choose how you retain the motor. You have three options here: 1) Buy a commercial motor retention system. 2) Design your own 3) Friction fit.

Commercial Motor Retention Systems

There are a few different 'off the shelf' retention systems out there and some are better than others for certain situations. What you go with all boils down to what will work on your kit, what you like and how much you want to spend. I'm just going to provide some links to various commercial retention systems so I don't violate any copy right laws. Links will open in a new window.

Design Your Own

So you want to design or make your own retention system? First go and have a look at the links above to see the various ways it can be done and then try your hand at making your own. You might want to test your design on a bit of scrap wood before you start drilling holes in your centering rings. If you are not overflowing with design and construction skills, or you just want a simple, easy to implement solution, then you might try the method that I use (which is also great for kits that use a boat tail and either don't have an aft centering ring or only a very small one (such as most AGM-12 bullpup's)).

The way I do it:

Overview: Aluminium bar has holes drilled along the length and is epoxied to the motor tube, passing underneath both centering rings. A hole at the end of the bar where it passes out the back of the rocket allows you to put a button head cap screw through the hole, held in place by a nut. The cap screw extends over the motor case (but not as far as the nozzle) and this prevents the motor from being ejected. I'm sure the photos will clarify the concept (click for larger images).

Simple yet effective. Works even when motor adapter's are used.			Good solution for boat tails.

1. Buy some alluminium flat bar - I use 2mm x 10mm bar.

2. Place your RMS motor case in the motor tube and measure the distance from the end of the motor case (gold closure) to the end of the motor tube (nose cone end). Add another 10mm or so to that distance and then cut the bar.

3. Drill (4mm - 6mm dia) holes almost all the way down the bar from the point just behind the aft centering ring. This reduces the weight of the bar and allows for the formation of 'epoxy rivets'.

4. The hole in the very end of the bar (the part that passes through the aft centering ring and is visible) needs to be at the right distance so that when it has a cap screw in it, the screw will pass over the top of the gold closure or locate in the slot in the closure. It doesn't matter if it's a bit far back and there is some play because the ejection charge is not very powerful.

5. Mark the motor tube so that you can epoxy the bar in the right place so that the last hole stays at the correct distance above the motor.

6. Cut slots in the centering rings for the bar to pass beneath and then epoxy the bar to the motor tube. Be sure that it wont foul the fins or piston strap/shock cord (if used). Apply epoxy fillets to the bar and also drop some epoxy into the holes. Make sure you do not put fillets all the way down the bar - you still need to put the centering rings on.

7. Fit the forward centering ring over the bar and epoxy the centering ring in place. Continue construction as per kit instructions.

Now that you have finished your kit and you are ready to fly, simply load your motor as normal and then put the cap screw through the hole and hold it in place with a nut. There is no way your motor is going anywhere! I've used this technique on three of my kits so far with good results. As mentioned, it is very good on boat tails and is far neater than some other methods I've seen.

Friction Fit

A lot of people just use friction fit for motor retention. This is simply a matter of wrapping some tape around the motor and making sure it's tight in the motor tube. This is fine for single use motors but you run the risk of the motor being ejected and the recovery device not being deployed. Also, reloadable motor cases are not exactly cheap and you sure don't want to loose one! I would not risk my $120 rocket with it's $120 motor casing to a bit of sticky tape wrapped around the motor, but you might be a bit more adventurous than me ;-)