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A Mid-power or High-power rocket needs to be able to accomodate motors of different lengths. Therefore a motor block in the motor mount tube is not used to keep the motor from moving up through the rocket like in low powered models. Most reloadable motors are manufactured with a lip on the aft (nozzle) closure as a thrust ring to prevent the motor from moving up through the rocket. Others employ the use of an exterior snap-ring to act as the thrust ring. Additionally, whether you use motor ejection, or your motor mount opens to your ejection chamber, when your ejection charge goes off, it has a propensity to test just how well your motor is retained. You need to make sure that this force is greater than the grip your couplers have on the other end of the tube (shear-pins included), or you will deploy your motor, and not your parachute!
You might imagine that there are several ways to accomplish the task of retainiing motor casings. They range from very simple, to quite complex. Many of them work equally well, but some are better left to making up for an afterthought, rather than a well designed solution. Examine the following examples as possible methods for motor retention:
A Panel clip is a simple stamped piece of metal, usually brass or steel that is used on screen doors. Sometimes referred to as a "Kaplow Klip" within the hobby, these inexpensive clips can be found in your local hardware store. However, prior to using one, you must install a "T-nut", or some other threaded insert in the aft centering ring, which means that you will typically have to decide on this method at least before you install your aft centering ring. If you use T-nuts, you should make sure that the base of the T-nut is firmly secured so that over the life of your rocket, it can't be poked into the sealed chamber on the other side of the aft centering ring, because you'll never get it out without damaging anything. You can usually do this with epoxy if you're careful about not getting any adhesive into the threads. I have seen some people assist this with a well-placed staple. During motor installation, an appropriate screw is uesd with the T-nut and the panel clamp to keep the motor in place.
Much like the Panel Clip method, a washer can be used in place of the panel clip. You must still first install the T-nut or other threaded insert, and it is a good idea to have a particular washer in mind (and a decent stock of them) because you need to space your T-nut the right distance from the motor mount in order for this to work properly. You want the edge of the screw to cover just enough of the motor to be able to hold it in place, but you still need the screw to clear the thrust ring of every motor that you can think of wanting to fly in your rocket.
For clusters that are placed close enough together, the washer method can be quite effective, but the process is just a little bit different. Sometimes, the proximity of the motors will prevent the use of a T-nut, because there isn't enough space in between them. Instead of fixing the t-nut to the aft centering ring, you epoxy the screw in place. Because the motor mounts are co slose together, it is usually easier to use allthread (a long screw thread with no head) instead of an actual screw because there is no head to get in the way either. After the installation of your motors, you place the washer on the allthread, and use a nut to snug it against the motors to keep them in place.
T-nuts cannot be added to an already constructed rocket. However, expansion bolts and other devices available in any hardware store can be also be used to add positive motor retention to an assembled rocket. Some companies have screw-on retainers that can be added to existing rockets.
It is always best to plan ahead, and consider all points before you build a rocket, but sometimes you forget. It happens to the best of us. And it is from those who have forgotten before that you can learn, there's always a way. Sometimes, a blank centering ring is just begging for a good wood-screw, and some bailing or 'safety' wire. This is thin, steel wire that is easily bent, but has a high tensile strength. Wrapping some around a screw, then around the nozzle of a single-use, or certain reloadable motors, and back around the screw, or around another screw makes for a decent last-minute method of retaining your motor. Be aware though, that this wire can only be used once because of the heat and stress placed on it during a flight, and it must be replaced with every flight.
Legacy Info-Central info submitted by Dean Roth, updated by Tim Scott |
