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11.2 How do you keep in a high power motor in its mount, but still alow for the numerous lengths in which HPR motors are sold?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

11.2 How do you keep in a high power motor in its mount, but still alow for the numerous lengths in which HPR motors are sold?

From: billn@hpcvaac.cv.hp.com (Bill Nelson)
I make a clip similar to the ones used on model rockets - however, I do
not pierce the motor mount tube - I place the front end of the retainer
over the front of the tube. It is epoxied/taped in place, just like with
a model rocket. I do not rely on spring pressure to hold the clip over
the end of the engine. I use several turns of strapping tape - wrapped
around the engine or motor mount and the retainer clip. So far, I have
never had a problem with an ejected engine.

From: JCook@Epoch.COM (Jim Cook)
Some folks at NARAM 33 suggested drilling a small hole in the side of
the flange of the rear nozzle retaining ring [of an ISP reloadable motor
casing] to tie the casing to the model. Some might claim this to be
"modification of rocket motors not approved by the mfg." I had though I
heard Aerotech was going to start doing this themselves, but I haven't
seen anything yet.

From: neil@boi.hp.com (Neil Pyke)
I've built #8-32 "t-nuts" into my last couple of rockets and then made
sheet metal brackets to hold the motor in. I drill two holes, 180
degrees apart, in the aft centering ring and then press and glue the
t-nut into the hole. The screw holds the bracket to the centering ring
and I bend the bracket so it hooks over the end of the motor. The t-nut
works great but I've made my brackets too wimpy. Those that saw
me wandering around just past the flight line at LDRS a couple weeks
ago, looking for my ejected motor, will know that I have not perfected
my application of this design.

From: Roger.Wilfong@umich.edu (A. Roger Wilfong)
I've used a similar technique with t-nuts and had no problems - yet.
I've also tried a coarse thread sheet metal type screw (I'm not sure
what they're really called - the threading is about twice as coarse as
a regular sheet metal screw) screwed into the rear centering ring at
three locations. The centering ring needs to be plywood and you need to
carefully drill the correct sized pilot hole for the screw. After
'tapping' the screw into the hole, I took it out and ran a small amount
of thin CA into the hole for reinforcement - let the CA set before you
put the screw back in the hole or you won't get it out again. This has
worked on RMS-29 and while it is not as strong as the T-nuts, so far it
has been more reliable than masking tape.

From: soc1070@vx.cis.umn.edu (Tim Harincar)
On the 2 29mm birds I've constructed, I use a clip and a thurst ring.
It works like this:

         ---:|     |
       =====:|     |====== <- Centering Ring
            :|     |
            :|     | <- motor tube
            :|     |
            :|     |             --:
            :|     |               :  <-Clip made from steel rod
       =====:|     |======         :--
            :
            :--

The steel rod has two opposite 90 degree bends, and is run through the
centerings and along the motor tube. The idea is to spread the force of
ejecting along the top centering ring and to the rest of the motor
mount, instead of making the clip do the work. Also, on larger tubes,
you can design this so that the clip swivels into place, instead of
using spring tension.

The clip then extends 1/4" to 1/2" beyond the end of the tube. You then
use this space for the motor thrust ring. The thrust ring is then added
to the end of motor. I just usually wind the end of my motor with a
bunch of turns of masking tape, but I've heard of people epoxying some
other type of ring to the end of the motor.

From: waltr@netcom.com (Walt Rosenberg)
You use a "thrust ring" - several wraps of masking tape on the nozzel
end of the motor. This prevents the motor from going up the mount.

Of course, if you use re-loadables (ISP, AeroTech), the nozzel enclosure
is latger than the O.D. of the motor mount - in this case, just the tape
to keep it from coming out. Of course there are several methods used to
keep the motor from kicking - screws and washers, screws and hooks,
retaining rings, etc. placed over the ridge on the nozzel end of the
motor.

From: pstemari@well.sf.ca.us (Paul J. Ste. Marie)
Typically what you do is wind a ring of masking tape at the end of the
whoosh generator of the same thickness as the engine mount tube.
This serves as a block to keep the engine from sliding up into the
rocket under thrust. Typical widths of tape to use are:

              .25"      1/4A-B
              .5"       C-E
              .75"      F-H
              1.0"      H-I
              1.5"      I-J
              2.0"      J-K

From: waltr@netcom.com (Walt Rosenberg)
[Referring to the use of different tape widths, above]
1.5" for I-J and 2.0" for J-K may be too wide. You are now going to move
the center of gravity further back. You may introduce instability. I've
never used more than 3/4" for all my high power launches (H-K).

From: kaplowro@hccompare.com (Bob Kaplow)
My [retainer] hooks look like this:

                   ----
                   |  |  <<- this end slips over lip of bottom reload
                   |         closure
                   |
                   |
                   |
               ____|     <<- this end screwed/bolted onto rear bulkhead
  
                 ^hole drilled here for cap screw

The top of the hook wraps over and around the reload closure lip, and
can't push out like an Estes clip. Hooks ARE brass. I use stainless cap
screws to hold the clips in place - cap screws stay on the end of the
tool, unlike other screws. I use T-nuts installed on the back side of the
rear centering ring, or threaded brass inserts to retro-fit older rockets.

 

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