Model Forum / General / Rockets / November 2004

Here's how I retain motors in most of my mid-power rockets, a
technique especially useful for those rockets whose motor tubes
are barely smaller than their airframes.

After assembling the motor tube, including centering rings, I
drilled two 1/8" holes through the rear centering ring.  I
drilled the holes on opposite sides of the motor tube, kind of
like this (sorry for the crudeness of the picture):
  ____
 / __ \
/ /  \ \

\ \__/ /
 \____/

Then I got some 1/8" threaded rod (any hardware store should
stock this; I bought a 12" length; it was quite inexpensive).
You need to cut off a piece long enough for a washer, a locking
washer, and a nut, plus the depth of your motor's retaining ring
(for me, it's a ring of masking tape), plus the width of the
centering ring, plus another 3/4".  I cut mine with a hack saw,
and then filed the cut end a little to allow a nut to thread onto
it.

Once the piece is cut, carefully (slowly) bend it into an "L"
shape, bending just the last 1/2" or so.  If it breaks instead of
bending, try it again.  The trick is to go slow, and don't bend
it past a right angle.  If you manage to do it without breaking
the rod, you'll end up with a piece of rod like this:
_______________________

                     | |
                     |_|

Then carefully bend the base of the "L" shape to round it a bit,
so that looking at the base of the "L" from the end, it kind of
looks like this:
_
/ /

\_\

This is because the base of the "L" needs to fit between the
motor tube and the body tube, and there's not much room, so you
have to make the base of the "L" a little rounded to follow the
contour of the motor tube.

Finally, push the long part of the "L" through one of the holes
in the centering ring from the motor tube side, until the base of
the "L" butts up against the centering ring.  All said and done,
it will look something like this:
                  __
                _|1 |______________
               | |  |
_______________|_|  |__

               | |  || |   3
               | |  ||_|
               |_|  |______________
                 |__|

In the diagram above, (1) is the rear centering ring.  (2) is the
1/8" threaded rod bent into an "L" shape, and (3) is the motor
tube.

Once you've done this on both sides, epoxy the bases of the "L"s
to the motor tube and centering ring.  Once complete, you'll have
a motor tube with two threaded rods sticking out around where the
motor is inserted.  Then finish assembling the kit as per the
instructions.

When I fly, I insert my motor, then I put a large washer, a
locking washer and a nut on each threaded rod.  Assembled for
flight, it looks like this:
            _
      _  _ | |
_____|_|| || |_____----------------

     |_||_|| ||   |
           |_||   |
              |   |
  1   2  3  4 | 5 |       6
            _ |   |
      _  _ | ||   |
_____|_|| || ||___|

     |_||_|| |     ----------------
           |_|

In the diagram above, (1) indicates the threaded rods, (2)
indicates the nuts screwed onto the rods, (3) indicates the lock
washers, (4) indicates the bigger washers holding the motor in,
(5) indicates the motor retaining ring (built into an RMS motor,
or a masking tape ring on a single-use motor) and (6) indicates
the rocket's body tube.

On my LOC Legacy, I actually recessed the motor tube to that the
motor's retaining ring would be flush with the rear of the body
tube, thus giving the washer support on the retaining ring on one
side and the body tube on the other, so the washer remains level
rather than canted.  Either way, the system will work.

And that's how I did PMR on the LOC Legacy.  Writing it down, it
sounds way more complicated than it really is.  I have used this
technique, or a variant of it, for all of my mid-power and some
of my high-power rockets, and it's worked fine every time.

Have fun...Rick

No prob.  Most intersting thing I did all day :)

Doug
Engineer trapped in marketing...

If you're not already sold on the Binder Design retainer, let me tell you about
a method used when confronted with the same problem when using a 29mm MMT in a
BT-60 (it transitioned to a BT-70 in the top half). It's kinda similar to the
"Estes hook on steroids", but doesn't use spring action or flat metal......I
used a length of coat hanger bent at right angles on each end that rotates
within a slightly larger diameter brass tube. The brass tube is epoxied to the
MMT & you cut or drill a small piece out of the bottom centering ring to allow
the piece to stick out. I use masking tape wrapped around coat hanger & motor
to assure hook doesn't rotate when I don't want it to.
I discovered there are flimsier & stronger coat hangers. You could pick up thin
rods at a hobby shop instead. I've flown that rocket over a dozen times on G35s
& G64s with no problems.
I did a variation on that when retrofitting a rocket with motor retention when
I started using reloads. I epoxied the brass tube on the outside of the rocket
where the fin & body tube meet, similar to how we sometimes glue a launch lug
there. It's not that noticable, especially when painted the same color as that
portion of the rocket.
 --  Richard "even got one bent to go down a 3 or 4" boat tail" Hickok