Constriction Bands

 The Thrifty Rocketeer blog continues...

If you've ever built low power kits, you're familiar with motor tubes, motor retaining hooks or clips, as well as how they are mounted and the function they serve.

But let's review just a little.

It's my understanding that the NAR (National Association of Rocketry) organization have a line in the safety code about not using metal parts.  This always confused me, because in every kit I've assembled, there's at least a metal spring (a motor retaining hook or clip) as well as a metal screw eye into the nosecone. 

Now, that might not seem like much, but to my mind, those are definitely metal parts.  Perhaps I mis-understand the prohibition against metal parts, so let's move on.

As I understand it, the metal spring motor retention clip is used specifically to keep the low power motor from kicking back and exiting the motor mount tube when the exhaust charge is blown or fired.  (Now, there ARE some very small gliders and similar rockets that expect and plan on this kick-back ejection to lighten the load... but that's not what we're here for.)

The metal spring is bowed just a little, and one end pierces the motor mount tube, anchoring itself either before or just after a thrust block. (The thrust block is a ring glued inside, that the motor itself pushes against with the forward thrust, and propels the rocket.)

The lower end of the metal motor retention spring is sometimes a simple 90 degree bend, to hold the bottom edge of the motor in place, and sometimes it's a more elaborate "handle" that can easily be gripped or thumbed to pull back and release the spent motor.

But in order to maintain the spring in place, it has to be held against the motor mount.  And that brings us to the topic of today's entry.

Now, I admit that I don't know in which order these variations were developed, so bare with me here.

In early kits, the instructions talk about marking various distances along the motor mount tube, and using them to position centering rings, the thrust block, the motor retention spring, or other features.

In my case, there was a middle mark that was the point where a wrap or two of masking tape was to be added.  This helped to hold the metal retention spring snuggly against the motor tube until needed.  However, this wrap ALSO shortened the lever action of the spring clip, making it much tighter, and less flexible. 

A second variation involves a frequently clear or white plastic ring that is just big enough to slip over the motor tube and slides down over the metal spring clip to that same point.  Frequently, this ring is then glued in place, usually with wood glue, and again, holds the clip and increases tension.  Some refer to this as a cigar band, or a constriction band.

A third possibility is that a third centering ring is slid down over the motor tube, to some predetermined mark, and is glued in place.  This ring served double duty to hold the motor tube the correct distance in the center of the body tube (much like the forward and rear centering ring do) but also serves as a constriction point, increasing tension of the metal spring clip as above.

Recently, a born-again rocketeer asked what type of tape should be used to wrap around the tube and clip... electrical or masking tape.   Almost everyone who responded sided with masking tape, but a few said it didn't matter.  

I disagree.  I think with the burning of the propellant, most motors develop significant heat... warm enough to soften any adhesive and allow it to slide or flex.  It's my feeling that a wrap of electrical tape, while secure enough at construction, will dramatically soften and slide when the motor burns.  This will release whatever tension has been achieved by the constriction band.

Masking tape, on the other hand, is also designed to have a flexible, releasable adhesive, allowing the tape to be removed.  But as anyone who has an older project with masking tape involved, you know this adhesive is known to dry out, grip permanently, or flake off, over time.  My feeling is that if you wrap the spring and motor tube twice, overlapping a complete turn, the tape is more likely to stay and fairly tight.  However, the problem of heat from the burn again rears its head...and is not certainty.

This became an issue for me recently, as during a rocket kit build on the dining room table, my wife decided that she had had enough, and wanted her table back.  She scooped up the various parts, instructions, packaging, stock card, glue bottle, tape and everything else into a small basket and tucked it into the adjacent bookcase.

Of course, when I came home from work, I was appalled to see that my rocket assembly area had been swept clean.  But I kept my mouth shut, cause 1) it was too late, and 2) I don't want to sleep on the couch.

As I continued the build, which I admit had started a bit out of order, with fins and balsa sealing and sanding BEFORE the motor mount was built, I got to the point of gluing the assembly in place and discovered there was no constriction band.  I was certain that it wasn't missing when I unpacked the kit, as I usually always keep everything inside the smaller pre-pack plastic sacks that they come from.  However, for some reason, this constriction band was missing.  I looked everywhere for it...on the table, below the table, under the couch, under the bookcase, anywhere that I thought it could have rolled.

No dice. No ring.

So, I fell back on my earlier experience and simply broke out the small masking tape, and made two wraps around the tube, trapping the spring in place, and finished the installation.  I figured while it may not have been as permanent as the plastic ring would have been, at least the rocket could be completed.

All this was several rockets ago, and I had forgotten it all until just last night.

For some reason, I was hunting through the several small plastic baskets that my wife has taken to stuffing things into before hiding them in the bookcase or elsewhere in the house, when I spotted it.

There, on the bottom of the basket, was the lone, translucent plastic band or ring.  I recognized it immediately, and taking it over to the table, confirmed that it was just the right size for a standard low power 18 mm motor tube.

I was pleased to have found it, but the motor mount assembly was long since sealed up and inside a finished rocket.  I had no further need for it.  It must have dropped out of the opened mini package of parts when my wife had swept it into the basket.  I must have overlooked it, or it was caught on something else and I just didn't see it.

So, what is the point of this long rambling tale?

The Thrifty Rocketeer reminds you, don't leave your rocket parts out on the dining room table unless you want to sleep on the couch.   Keep your parts inside a sealable ziplock bag until you need them.