"Motor clip-rip" prevention
 

Hello All,

Although it has not (yet) happened to me, I have read on YORF and EMRR about several instances of "motor clip-rip." This is a type of rocket damage that occurs when the kick-back from the motor's ejection charge causes the forward end of the motor clip to rip rearward through its insertion slit in the motor mount's motor tube (or the slit in the body tube, in minimum-diameter rockets).

I have observed what may be the "enabling factor" for "motor clip-rip"; if so, there are three possible solutions. I noticed that in my Quest Starhawk, Quest and Estes 18 mm motors have noticeable longitudinal "play" in the motor mount tube. Estes motors have ~2 mm of fore-and-aft freedom of movement, while Quest motors (the German-made ones) can slide almost 4 mm back and forth. This allows the motor to slam against the ends of the motor clip like a hammer at both ignition and ejection.

Although the motor can slide forward at ignition, the thrust ring prevents the forward end of the motor clip from moving forward and ripping its insertion slit in the motor mount tube or body tube. At ejection, however, there is nothing other than the slit in the thin kraft paper tube itself to resist the rearward kick of the motor against the motor clip, and the tube can rip at the slit under the stresses of either repeated ejections *or* just one particularly energetic "shotgun" ejection. Besides reinforcing the tube around the slit with glue while building the rocket, there are three ways to prevent the motor from slamming against the ends of the motor clip. These are:

[1] Inserting a "shim ring" (made of cardboard, pasteboard, fiber fin material, basswood, or model airplane plywood) into the motor mount tube or rocket body tube ahead of the motor to ensure that the motor fits snugly between the ends of the motor clip;

[2] Inserting a smaller shim (not a complete ring) between the rear end of the motor and the rear end of the motor clip (taping the shim in place so that it doesn't come loose after launch), and;

[3] Applying a narrow wrap of masking tape around the protruding rear end of the motor *and* the motor clip; if desired, this wrap of tape can be made thick enough to act as an external thrust ring (against the rear edge of the motor mount tube or the body tube) to prevent the motor from moving forward at ignition.

I hope this information will be helpful.

IIRC, Dr. Zooch instructions recommend gluing a small strip of paper behind the slit cut into the motor tube for the hook, so at least one manufacturer is aware of the problem.

I wonder whether this type of damage only happens when there is no thrust ring so that movement of the hook in both directions causes the tube to tear.

I always install a thrust ring whether the kit comes with one or not. And I apply a liberal amount of glue on and around the forward end of the hook between the skit and the ring or tape holding its middle in place. Hopefully, that will be strong enough.


Bill

That is very considerate of Wes to include the paper reinforcing strip suggestion in his kit instructions. While I have never experienced a tube-ripping anomaly, I *have* had the motor clips in Gnome kits (which don't include thrust rings) become loose. In each of those rockets, the slit in the body tube had become enlarged after a few flights, sometimes with the beginnings of creases radiating from the slits. That convinced me to always use thrust rings, and no subsequent Gnome that I built had its motor clip come loose or enlarge the slit.

You can glue one of these into the motor tube just behind the forward end of the hook. Test fit it first with a motor casing in the tube. If it is a little too thick (long), you can rub one end of it on a piece of flat sandpaper to decrease the height slightly.

If you are scratch building, you can use sturdier motor tubes. Semroc ST-7 (for 18mm motors) and ST-9 (for 24mm) are both appreciably more sturdy than their Estes equivalents. Estes tubing became the standard back when ejection charges were not as strong. I also use a small amount of slow curing epoxy to firmly bond the upper end of the motor hook to the tube and I also add a sturdy clip retaining ring (often made from a few wraps of a narrow strip of cardstock) around the ring and glue it down firmly, with fillets and everything. I have not used the internal block behind the hook very often, and yet I have not had any problems with the hooks zippering my motor tubes on anything that I have ever built, either from scratch or from a kit.

If I was building a kit, and it came with a thin, toilet paper core tube for the motor mount, I would probably chuck it and substitute my own stronger tube. But you can still use the stock tube and avoid the problem by wrapping the tube with a layer or two of cardstock in between the centering rings, and then firmly bonding the non-flexing part of the hook to the tube and reinforcing it with another strip of cardstock glued over the hook.

MarkII

I wish he sold those for BT-50


Bill

Don't they? (Spiral-wound CR-2050 Centering Rings double as 24 mm motor thrust rings.)

I've always cut my own thrust rings using engine casings that have been cleaned out and cut with a razor saw or a dremel. works really well, cheap (IE free after motor usage), and you know it will stand up to the heat of the ejection charge. You can also make them any size you can cut properly.......

Don't see why you couldn't make an "oversized" thrust ring then cut a slit for the top of the engine hook then glue that in place.

Well, it's been awhile since I've built a motor mount using the instructions :o so maybe something has changed over the years (I'll check some instructions when I get home).

The Estes instructions used to have you set the forward portion of the motor hook in a line of glue on the motor tube and then wrap tape (I prefer 1/2 inch wide masking) around the motor hook approximately one inch forward of the aft end of the motor tube. When a motor block isn't supplied with the kit and I don't happen to have any laying around (not a problem now), I've always coated the inside of the motor tube, between the end of the clip and the forward end of the motor tube, with glue. I've always built mine this way and have never had any problem with "motor clip-rip". But, I have had some motor mounts come loose while taking core samples...... :(

I have only had the hook get shoved forward due to landing on the motor/hook. This has only happened with kits designed without a thrust ring, such as the Generic E2X kits we use at school. We use streamers instead of chutes to keep the rockets on the playground, so the landings are a bit harder. I've remedied the problem by cutting up used motors to make thrust rings for the build sessions. I haven't had the problem since.

I second what MarkII said about substituting the ST-7 for the BT-20 and ST- 9 for BT-50s.

This is especially true for friction fitting engines in small diameter tubes. You have less chance of the tube buckling and bending when inserting or removing engines held in place with friction wraps of masking tape.

I tend to do both reinforcements - an engine block ahead of the engine hook and the reinforcing strip of cardstock like in the Dr. Zooch kits.