While I understand the screamingly obvious reason for having that somewhat unsightly metal clip sticking out of the bottom of a rocket, is there a workable alternative for mounting motors besides clips?

I've built a couple of little BT-20 tube rockets and all they had was the thrust ring glued up inside. Granted, when one of them went up and ejected, the motor came shooting out the back end to fall to the ground, which didn't hurt the flight a bit.

Besides the bad mojo for leaving spent motor casings out in the field, are there other good reasons for not relying on such a system more often?

Possibility of fires? Beaning somebody on the noggin with a little piece of lightweight cardboard?

If they can or will come out at ejection - and there doesn't seem to be any horrible outcry over it - why don't more designs utilize it? I've read of another method or two for securing motors (masking tape the obvious one), but not having any experience with them leaves me - well, inexperienced...

I'd like to be able to stand all my rockets up, too.

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For minimum diameter designs, like your BT-20 example, I can think of a couple of alternatives to both motor clips and friction-fitting.

The first one involves moving the fins 3/8" to 1/2" forward from the aft end of the body tube. See the Estes Wizard for an example. After installing the motor, wrap 2 or 3 layers of masking tape around the motor right where it emerges from the body tube, and do it so that the tape overlaps the end of the body tube. Make sure that the tape is well-adhered all around. Essentially you are taping the motor to the end of the tube. This is a very effective method of motor retention. After the flight, all you have to do is unwrap the tape and pull out your motor. You just have to check to be sure that the rocket is still stable with the fins moved slightly forward.

The second method is a technique called the Lariat Loop. When you build the rocket, assemble the shock cord but do not anchor it to the body tube. Now take a length of thin but strong Kevlar cord (such as 100 lb. microbraid Kevlar) and tie a simple noose on one end (like a cowboy's lariat.) (You can also just make a slip knot in it.) Tie the other end of the Kevlar to your shock cord, and then tie on the nose cone and parachute or streamer.

When you prep the rocket for flight, drop the noose end of the Kevlar/shock cord combination down through the body tube from the top so that the entire noose is just out of the motor mount. Now install the motor, pinching the Kevlar between it and the body tube. (That's why you want to use a thin but strong cord.) Slip the noose over the end of the motor that extends out of the rocket and cinch it down. If it won't stay tight, or if you just want some insurance, put a couple of wraps of masking tape around it. Now carefully pull on the shock cord from the top to eliminate any slack. Go ahead and finish preparing your rocket.

When the ejection charge fires and deploys the recovery device and shock cord out of the front of the rocket, the noose around the motor will hold it in and keep it from being ejected. Because the shock cord is being forcefully shot out of the front of the rocket, as it extends and becomes taut it will keep the motor from popping out of the back end. It only has to hold onto it for a half a second, because once the nose cone is off and the device deployed, the pressure instantly drops inside the tube, so the motor is no longer at risk of being ejected. But in practice the noose or lariat holds the motor in quite firmly. And then because the shock cord is anchored to the motor, the recovery system stays connected to the rocket.

One of the additional benefits of this method is that you can inspect the condition of the shock cord/Kevlar combination along its entire length between flights, and replace part of it or all of it as needed when it appears to be worn or weakened. I am currently building a 29mm minimum diameter rocket and I'm using this method to retain the motor.

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SAM # 0011

If the motor ejects there is a good chance that the recovery system will not eject and the model comes in ballistic. In addition to the techniques Mark describes the simplest and oldest is to friction fit the motor - wrap tape around the casing until it is very hard to get in therefore hard to eject

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Best advice I ever got for friction fitting a motor:

apply enough tape to the front end of the motor that it is hard to insert. Let about 1/8" of tape overlap the front of the motor and roll it over the front edge. This keeps the tape from peeling back from the front edge while inserting it and rolling up while you are trying to push it in. If you put the tape in the middle of the motor, it can swell there from the pressures and what was hard to put in is now impossible to pull out. The front of the motor doesn't tend to swell and it will take about the same amount of force to remove the motor as it did to insert it.

Greg

Right. The methods that I described are alternative to using either clips or friction. I use friction-fitting in rockets with thicker, heavier airframe tubing. I am, however, quite reluctant to use it in rockets that are built from thin-walled LPR tubing. If you make the motor fit tight enough to require some force to get it into and out of the rocket, then you run the risk of damaging your rocket whenever you install a motor. I have tweaked or kinked more than a few airframes that way.

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SAM # 0011

Here are detailed descriptions (with photographs) of this improved tape-based friction-fitting method (see: http://www.oldrocketforum.com/showt...1238#post101238 ). In that thread, pantherjon posted that since the tape strip(s) is/are applied to the motor at an angle (forming a "loosely wound" cylindrical helix, like screw threads), the motor can be literally threaded into and out of the rocket (or motor mount) like a screw or a bolt.

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NAR #54895 SR

I also like the Lariat Loop motor retention method that you described. One small modification of this method would allow ordinary "thicker" Kevlar shock cords to be used. The section that loops around the protruding rear end of the motor and extends along (and up past) the motor casing could be made of thin stainless steel wire with an "eye loop" at its front end, to which the lower end of the shock cord would be tied. If the piece of stainless steel wire was 4" - 6" long, the shock cord tie point would be located farther away from the initially very hot, erosive blast of the ejection charge. The loop at the rear end of the wire could be taped down to the motor with a wrap of masking tape.

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NAR #54895 SR

Another, although expensive option, is to use the Rocketarium or the AeroPack retainers. They look pretty, are absolutely guaranteed to keep the motor in, but won't work on minimum-diameter rockets.

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