December 10, 2003

Build It - 4

This is a series where we build a model rocket step-by-step. You can find the rest of the series here.

The main part of the post is in the extended entry so you don’t have to deal with it if you don’t want to, but I hope you follow along because when we get done you’ll have built and flown your first model rocket. Questions asked from before are answered too.

This time we're going to put together the motor mount. It's a simple process. If you're building the Fat Boy, then the motor mount consists of the motor tube, two cardboard centering rings, a metal engine hook, and the black engine holder ring. Every model rocket has this setup, with minor variations. I'll talk about that after assembly.

Test fit the centering rings on the motor tube first. I had to widen the inner holes a little bit by reaming it out with a pair of scissors. The rings should slide on easily, don't force it.

Mark the motor tube (it's not quite 3" long) according to the instructions. Carefully push the tip of your x-acto knife into the tube at the proper mark to make a small slit. The slit only has to be wide enough to accept the width of the motor hook.

Push the "L" shaped end of the motor hook into that slit, so that the motor hook lays flat along the length of the tube. Then slide the black engine holder ring onto the tube and over the motor hook. Don't glue anything yet.

Now slide the rings onto the tube. If one ring has a notch in the inner cutout, then that notch fits over the squiggly end of the motor hook. The idea here is to allow you to lift the overhanging end of the motor hook out of the way to insert and remove the rocket motors.

I recommend putting a couple of wraps of masking tape around the motor tube and hook right where the hook goes into that slit you cut. It's not strictly necessary, but it's simple insurance to prevent a potential problem later.

Now it all looks like this. Nothing is glued yet, but we're ready to go.


Put a bead of glue all the way around the place where the motor tube goes through the centering ring. If you're using the gelled stuff that won't run, do both sides of both rings all at once, otherwise just set the motor mount on end and do the 'top' surfaces. When dry, flip it over and do the other sides.

You don't need a ton of glue here, but use enough to completely circle the tube. Use your finger to lightly smooth it into the corner of the joint and then straighten out the centering ring again if needed.

Set it aside to dry.

Here's how this whole assembly works. The rocket motor goes into the motor tube and rests against the hook (the one through the slit). When the motor ignites it pushes against that hook, which is secured to the motor tube, which is glued to the centering rings, which will be glued to the airframe. Simply put, the motor takes off, and everything else goes along for the ride. That's the reason for the wraps of masking tape I recommended earlier - to keep the hook end in place. If the hook slips out of the slot, then the motor will just thrust straight up through the rocket and blast off by itself, knocking the nosecone out of the way on it's way through. Entertaining, but not in any way a successful flight.

Some kits use a 'thrust ring' to prevent this instead of, or in addition to the motor hook. It's just a cardboard ring that is glued inside the motor tube where the hook enters, to give the motor something substantial to push against.

The other end of the motor hook (the squiggly bit), has an important function as well. Besides letting you move the hook out of the way to extract an expended motor, it also keeps the motor in place when the ejection charge goes off, which deploys the parachute.

Newton's third law states that for each action there is an equal and opposite reaction. The ejection charge of a model rocket motor fires forward (towards the nose, which means that the body of the motor is forced backwards. Without the motor hook in the way, the motor would eject out the back of the rocket and the nosecone would stay in place (meaning no chute). Lawn dart.

If your rocket doesn't have a motor hook, then you can do a couple of things. First off is what they call friction fit. This is simple and wonderfully effective. Use pieces of masking tape (I use enough for about a half-wrap) around the end of the motor case closest to the nozzle end, until the motor is a very snug fit in the motor mount. The idea is to make it easier for the nosecone to come off than it is to expel the motor, 'path of least resistance' style. Another method that I've used is to put the motor into place, and then use a couple wraps of masking tape around the motor and motor mount tube. You can also do both, but that's usually overkill.

Next up will be the shock cord mount, and putting the motor mount into the body tube. Maybe a little bit about the chute too.

Posted by Ted at December 10, 2003 11:09 AM
Category: Build It
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