Article: 1355
By: David Randall, Nov 2009

Motor Retention – getting started …

So, you opened up your spiffy new kit, bought your epoxy and paint, and you’re reading the instructions… “Requires motor retention (not included)”.   ACK!  So, now you’re trying to figure out what type of motor retention you need…  For those of you who’ve built a zillion rockets and have your favorite retention method, don’t expect to learn something completely new.  For those of you getting started, read on…

For purposes of this article, there are basically two types of rockets.  Those that are built and don’t have motor retention (yet) and those that haven’t yet been built.    This short article is going to sound a bit like an advertisement, but rest assured, I have no stock in Aeropack.  

Link to Aeropack’s motor retention information is here.  You can find them from a number of vendors.  Take a look on our links page for vendors.

If your rocket is not yet built – get the Aeropack retainer for your motor mount tube type.  If you’re rocket is built already, you may be able to use the Aeropack Universal retainers.  These bolt on to your centering ring and give you the same great quick change ability. 

Yep, you’ll probably wince a bit at the price.  Save a few pennies if you need to wait to buy the retainer, but buy the retainer for your motor mount size and tubing type.   It’s worth it.   When you get your retainer, install, admire and be glad you made the right choice…

That’s it.    Not particularly exciting, but that’s my story and I’m sticking to it.  

Here’s why I like them.  1)  They are very easily installed on your rocket.  2) It takes all of 3 seconds to secure your motor. 3) They wipe off cleanly and easily in the field – both from dirt and from exhaust gases.  4) They fully adhere to the KISS principle.  5) You can easily step down in size with Aeropack’s adapters.

While I don’t have first hand knowledge of using the “Slimline Kwik-Lok”, it appears functionally equivalent to the Aeropack – although it has finer threading.

Ok, Ok, Ok… you’re craving more information – after all, this is an article… Here’s my experience with various types of motor retention, and for the most part, why I don’t like them.  I’m not going to call out specific products here, or post links.  If you spend some time looking at other rockets at your next launch, or perusing the internet, you’ll run across most of these methods or devices.  And, I’m not saying that these other methods don’t work, I’m simply saying that for me, they don’t work nearly as well as the Aeropack retainers do.

1. Snap-ring based retention rings.  These are similar to the Aeropack – its  a section of aluminum tubing with a groove machined out.  You epoxy it onto your motor tube and use a snap ring that fits in the groove to hold the motor in.  Why don’t I like them?  A) Snap rings are easier to lose.  B) Snap rings require a tool to remove.    I’ve also had experience with one type that was a basic aluminum retention tube and it got eaten up by the exhaust gases and corroded within a few days of not cleaning the retention ring.

2. Internally threaded retention ring.  Yet another tool required, a nifty little –specialty– spanner wrench.  While the snap ring solution requires a screwdriver to get my snap ring out, I just don’t want to worry about losing my single-purpose tool and then trying to jury-rig something to get the ring in/out or find one from another flyer at the launch.

3. Bolts, Washers and Blind nuts.  This method is the quick and easy way to get motor retention done – all at a low, low price from your local hardware store.  And, I’m guilty of indulging in this method on several of my rockets.  Here’s why I don’t like it long term.  1) Inevitably the washers get re-used for another project, and I’ll be missing one – the critical one – while trying to get my rocket prepped.  2) Especially on larger motors, it’s easy to bend the bolt on a hard landing.  That may render the blind nut useless depending on how hard it hits by popping out the blind nut inside your fin can assembly space.   3) Probably the biggest problem – you can’t always get the washers to hold the motor in place. 

4.  Screw-in clips – These are small pieces of steel or aluminum, usually in a “Z”, “C” or “S” shape.  One end is bolted or screwed to the aft centering ring, the other end hangs over the edge of the motor.   They are usually good for retrofitting on a rocket, but I wouldn’t recommend them for any rocket you’re building from the start.   Here’s why – 1) They often bend under stress and eventually require replacement.  2) They can easily swing to the side and flop around, having no retention benefit at all. 

5. Screw on retention plate.  This is a piece of aluminum or steel that has a couple of holes in it around the sides.  You use threaded inserts in your aft centering ring or possibly use blind nuts, and hold the entire motor in place with the retention plate.   I had one of these on my very first 29mm rocket – and it was cool.  Until 1) It took forever to screw in the fine threaded screws, 2) I couldn’t find the one screwdriver in my toolbox that fit the screws best, 3) the screws got bent and I couldn’t retain the motor with only 2 of the three.

6. Spring Clips or Wires – While I’ve never used these beyond Estes kits, I’ve seen some that have been put together, especially on minimum diameter rockets.  These work reasonable well for 18, 24 and 29mm rockets, but above that, I wouldn’t trust the strength. 

7.  Blue Tape (aka Friction Fit).  Wrap your motor in tape, stick it in, twist, press, twist some more, tape some more and voila! … motor retention and its simplest.   Works great for your 18, 24 and 29mm motors.  Personally, I would only do this for single use motors. 

So, there you go.  I hope this helps you get on your way to great success in flying mid and high power rockets!

– Dave Randall

NAR 84939 L3