Here's a photo of that 7 motor cluster mount, by the way.

The retaining hardware consists of three 4-40 threaded rods, three #4 hex nuts, and three #4 fender washers. The three threaded rods are held in place at the other end of the mount by three 4-40 brass acorn nuts and three more #4 fender washers.

Mark \\.

Attached Thumbnails

 

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SAM # 0011

One of my problems with friction fit is the reverse of what many of you are describing. It's not that I need too much tape, but that by the time I'm done building, the motor is too tight even before I think about putting tape on it. I have a couple models that have a motor hook that I have to sand the tubes and the motors just to get the thing in far enough for the hook to catch. There would certainly be no room for tape.

On the other hand, I am intrigued by the idea of applying tape to the forward edge of the motor. I may try that for my few rockets with friction fit.

I actually saw this system used at my second HPR launch, when reloads were first introduced. My first thought was that it was a clever way to retain larger motors. But then I thought "what would happen if that rocket with those metal things sticking out the back came down on my car, my tent, or my head?". No RSO would let you fly a rocket with a metal spike in the nose, so why would they allow a metal spike in the landing end?

So it was off to re-engineer the system so there wouldn't be pointy metal thingies sticking out the back end of the rocket. The end result was the Kaplow Klip.

My current biggest cluster rocket is a "Super Ranger" BT-80 with 4x24mm MMT. What I did there was to imbed a threaded brass insert in the center of the cluster. A single #6 x 1" cap screw holds a single fender washer which holds all 4 motors in place.

Unless on one flight, instead of using 2 D11-P and 2 D12-7, you forget 2 of the MMTs are plugged and fly it with 4 D12-7s...

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I fought the law, and the law LOST!

I should point out another circumstance where friction fit is needed - competition models that use a piston launcher. You HAVE to use friction fit because any kind of hook would interfere with the piston tube slipping over the motor. As I said in an earlier post, try taping the OUTSIDE of the motor and rear of the model. If you use the right tape and burnish it down so it gets a really good grab, the motor will not come out. To remove the spent motor, just remove the tape holding it in and slip the spent motor out - it doesn't have to be tight in the motor tube when using external tape. I design all my contest models with a 1/4 inch gap between the aft end of the body tube and the back of the fins just for this reason. The tape also acts as a shim to seal the motor into the piston tube nicely. Doesn't work quite as well with the German made Quest motors because they are slightly smaller (takes several layers of tape) but one layer is perfect with Estes motors. As I said - I've had a lot of luck with this method using Tamiya brand masking tape. Regular masking tape does work but I have had it let go once or twice.

Greg

cool discussion...

Like many older BAR's, I learned how to friction fit because there was no other choice back in the early 60's. Like clustering, it's a mix of experience and art

Here are a couple of tips I've learned over the years:

• I test fit the motor before adding any tape to see if the fit is already tight enough.
• NOTE: Motor tubes are pretty much standard in dimensions. However, motor diamter can be all over the spectrum due to many, many issues (manufacturing, humidity, etc)
• When adding tape, I use a spiral method where I go around the motor case, top to bottom, in a spiral fashion (think old fashion barber pole or candy cane)
• Also, I always start the tape ABOVE the top of the motor and fold the tap over the top. This prevents the tape from peeling and rolling over edges while inserting
• One piece of tape at a time, test fit, add more if needed.
• Once you have a tight fit, insert the motor and add one last piece of tape around the bottom of the motor and outside of the motor tube
• Use your finger nail to press the tape down tightly against the motor and motor tube.
• Now, after flight you should remove the motor as quickly as possible (I usually do this out at the recovery spot even before repacking the chute
• If the motor doesn't come out easialy, STOP and try one of these methods:
• 1) Use a wood dowel that will fit inside the motor casing. Insert this from the nose cone end, into the casing and then rap the dowel on the ground sharply a few times till the motor comes loose
• 2) Use pliers to firmly grasp the nozzle end of the motor (NOTE: You can even squeeze this end to remove the nozzle giving you more casing to grab with the pliers)
• 3) Bring your model home and (assuming it is small enough) freeze it then pull the motor out
• 4) Knock out the nozzle and peel the motor casing out from the inside out (this can be accomplished by starting the inside of the casing to peel then gluing a wood dowel to the paper and turning it inside the casing)

Another thought came to me. If this is a skill that you would really like to learn but are concerned about the risk to your valued rockets, then use one of our paper download rockets (Click here ) as your test subjects. If you spit the motor all you've lost is some time which is something you *always* loose while gaining experience

Hope this helps!
jim

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I've never had a single problem with friction fitting motors. It's not that hard to master. All of the techniques have been covered, no sense in repeating any of them. I prefer a hook in most cases, much quicker to prep. But on clusters or scale, friction fit really is the way to go. And in somes cases it's nice to be able to simply stand the model on it's fins without the hook getting in the way.

You must be crazy, asking me to learn new skills!


Actually, I built and flew one of the Semroc V-2's at NARAM-50, and the motor is still stuck in it. There was so much going on at the time that I never got or borrowed a dowel to try to knock it out, but it's in there really tight. I clearly used too much masking tape.

I do like Greg Poehlein's method of taping around the outside of the aft end of the motor and the motor mount. That has worked well for me when I've done it.

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John Thro, NAR #84553 SR
I was too old when I started! Now I'll *never* become a BAR!

Many of my older rockets, built during my early BAR'dom, have that problem. I was using liberal amounts of yellow glue on the motor tubes, including healthy fillets around the rings.

The problem is that yellow glue shrinks. And the more you use, the more shrinkage you get (so never, never, ever carry glue in your underwear ) Finally, one day I figured it out and began using minimal yellow glue, and sometimes using other, non-shrinking glues (CA, epoxy). The problem went away.

If you stop and think about it, how strongly do the 20-50 rings on an Alpha motor mount need to be affixed? If they need more than a dab to withstand an anomalous event, there's gonna be a whole lot of other stuff wrong with the rocket besides a dislodged motor mount They need to handle the kick from a nuclear (Estes) ejection charge. But surviving a lanwdart from 2000' isn't necessary All those uber fillets I have in my fleet provide little benefit, but cost me an extra 10 minutes of prep time sanding motor cases every time I fly them


Doug

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YORF member #11

Seems I stirred up a hornet's nest. Good! Anyway, the input is valuable. My problem is I don't like kinking body tubes puttting them in or out, and also, I'm launching with young, excited kids, so they are almost as impatient as me to get the next one in the air with minimum amount of fiddling!

By the way, I'm guessing that a BAR is a Born Again Rocketeer, am I close? Also, what is 3FNC? Finally, I've only used Estes engines A-D. What are the other ones mentioned in this thread, (A10-3T, A3-4T?) Thanks!

There is NOTHING easier for goodness sake! Friction fitted BP motor have been in use since the very beginning of our hobby. it's a perfectly OK method of motor retention once you "learn" to feel the proper fit. Motor hooks, clips and other methods are not always workable into the design. and Motor hooks are notorious for NOT holding the motor in the model without external taping anyway.
Long story short. LEARN to use each and every method available, you can then make your own decision on how you'll build each model to retain the motor(s).

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Keep em flyin Micronized
John