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11.9 All these high power motors are different sizes. How do I hold them in? What do I use for a motor block and where should I put it?




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

11.9 All these high power motors are different sizes. How do I hold them in? What do I use for a motor block and where should I put it?

From: jackson@sn3.jsc.nasa.gov (Al Jackson)
For mounting and retaining HPR motors I have this suggestion, especially
with PML models. See if you can let a good one inch of motor mount
protrude from bottom of model. Then when using a reload motor, besides
using a tape friction fit, put a wrapping of strapping tape around the
end enclosure and wrapped also around the piece of motor mount sticking
out.

From: jjirvine@aol.com (Jerry Irvine)
Perhaps i'm just tired of seeing people reinvent the wheel to
non-round shapes, but I have found that:
1. There is no need for thrust rings inside rockets of any power or
weight. The application of a masking tape thrust ring on the nozzle
end of the motor of adequate width for motor thrust is always
adequate, to the point where a fiberglass or metal one is better.
a. 1/4" wide masking tape is often used for 1/4A-F motors with
thrust levels under 40 newtons.
b. 1/2" wide masking tape is often used for 1/4A-J motors with
thrust under 200 newtons.
c. 3/4" wide masking tape is often used for F-K motors with thrust
under 600 newtons.
d. 1" up to 1000 newtons, 1.5" up to 2000 newtons, then above that
a structural ring at the rear of the motor.
2. With the above system one can add an external motor hook with NO
protruding rear thrust block, extended out the rear the exact width
of the masking tape you most prefer. The hook should typically be
metallic and bonded to the outside with epoxy for maximum strength
and instead of protruding hooks, they can fan out to the side for
better bonding strength.

From: kaplowro@hccompare.com (Bob Kaplow):
You want to install blind nuts on the BACK side of the aft centering ring,
before the mount is installed in the rocket. That way it can't pull thru.
[Epoxy a little around them] to hold them in place when not bolted in. The
screws go into these threaded holes, and hold in whatever clip you are
using. I personally prefer cap screws and an allen wrench to machine screws
and a flat blade screwdriver. The allen wrench holds the screw while I'm
installing it at a funny angle.

From: bmcdermo@ix.netcom.com (Buzz mcDermott):
You can use blind nuts (also called T-nuts), available from many hobby
shops and most hardware stores. Two or three size 4-40 work fine for
up to 38mm motor mounts. For anything bigger I would use two or three
size 6-32 nutes. For three and four motor clusters that don't have a
central motor you can expoxy a balsa or spruce strip into the central
gap between the motors. Drill a 1 inch deep hole in the exposed end of
the strip appropriate for epoxying in a 2.5 inch length of 1/8" threaded
rod. Use a washer and nut to retain all three or four motors from a
central point.

 

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previous page: 11.8 Is there any way to retrofit my existing rockets to have some type of positive retention system?
  
page up: Model Rockets FAQ
  
next page: 12.1 Copperhead, squib, electric match, thermalite, flash bulb. What are all these types of igniters, how much current do they require, and when are they used?