Lesson 1: The Motor Mount

In this lesson, we're going to build up the motor mount and install it into the booster body tube section. See lesson 0 for the introduction and preparations.

The first thing the instructions for my Tomahawk say to do is build the motor mount tube (MMT) and centering rings (CRs). Right off the bat, there are a few things to think about.

Motor Retention

The instructions don't mention motor retention until the end of the construction phase, but you need to plan for motor retention before building the motor mount. If you're planning to use AeroTech reloadable motors, you have many choices. I like the Aero Pack retainers, but many other styles are possible. See also the Motor Retention article in INFOcentral.

I have chosen to use the Aero Pack retainers because they are the slickest thing going. However they are a bit pricey. To use these retainers, the MMT must stick out 9/16" from the aft centering ring (for the 54mm size). This means we have to adjust the position of the centering rings a bit from the instructions.

If you choose to use other retention systems or a home-made system (Kaplow clips), you should install the T-nuts into the aft centering ring at this time.

Interior Fillets

I like to use good interior fin fillets in rockets with through-the-wall fins. The Tomahawk instructions (like most kits) tell you to bond all the centering rings to the motor mount tube at the same time. I like to modify this by leaving the aft one loose until after the fins are mounted.

My kit already had three CRs, but if yours has only two, I suggest you add a third one for additional support and to make alignment easier. If your MMT is short and you have plenty of space in the booster, you might also want to use a longer MMT to provide more support for long motors.

When we bond the motor mount into the booster airframe, we will bond only the two forward CRs. This way, we can add fillets inside the airframe and make a really solid fin bond. This also allows the forward and aft CRs to tightly match the fin tabs making a solid structure.

Bonding the CRs

Measure the aft edge position of the CRs on the MMT and mark them with pencil. Another key tip that is usually left out of instructions is that you need to scuff up and clean phenolic surfaces before bonding. Once you have the marks drawn, sand the tube from the aft end up past the center CR location with 60 grit sand paper. Also sand the forward end of the MMT (where the forward CR will bond). Wipe off the MMT with alcohol.

It's important to get the center CR on perpendicular to the axis of the MMT. This allows all four fins to butt against this CR and still be aligned. Draw a line around the MMT at the mark. A neat trick to make the line even along the MMT is to take a piece of paper and wrap it around the tube. When you get one edge of the paper to touch the mark and wrap back around itself in a straight line, you have a good edge. Draw a line here.

To support the CRs while the epoxy is curing, I like to wrap a band of masking tape around the tube. Put down a wrap (or more if the CRs are loose) with one edge of the tape at the center CR line. Then slide the CR down onto the tape and proceed to the forward CR. Now you can apply the epoxy without worrying about the CR moving. Only apply the fillet on the side opposite the tape. Once the epoxy gels, you can take off the tape and apply a fillet to the other side (for the forward CR only this time).

Note that at this time, we will only apply a fillet to the forward end of the center CR. Apply fillets to both sides of the forward CR and don't bond the aft CR at all.

If you are using an epoxy system where you have to add fillers, paint some unthickened epoxy on the wood parts before adding the filler. For example, I use West System products, so I mix up an ounce of 105/205 and stir it thoroughly. Then paint a little on the CRs at the joint with the tube. (Try not to get it on the outside of the CRs.) Then, thicken the epoxy with your chosen filler (I use 404 for most bonding) and apply your fillet as usual.

If you are using hobby shop epoxy, squeeze out an equal blob from both bottles onto your pad of Post-It notes and mix thoroughly. Then apply the epoxy to the joint and make an even fillet. The Post-It notes are nice because you can just throw away the top one each time you mix epoxy and have a fresh surface.

Install the Eye Bolt

My kit included eye bolts for the recovery system. Install the MMT eye bolt per the kit instructions. However, I suggest you use thread locker (such as LocTite®) to make sure the nuts don't come off. (My Tomahawk kit was missing one of the two nuts for each of the eye bolts, which is why it's a good idea to check your kit contents before you start building.)

My instructions said to epoxy the parachute bridle to itself through the eye bolt. This is messy and too permanent for Nylon (which wears out over time). It's better to use a quick link here and sew or tie a loop in the Nylon strap.

One key tip: When you install the eye bolt, make sure it won't get in the way when you try to install the motor mount into the airframe. Also turn it so that you will have an easy time getting the quick link through the eye bolt when you rig the recovery system. If you have the space, align the eyebolt so that the hole is at right angles to the inside of the airframe.

Install the Motor Mount

Once the fillets on the two CRs have cured, it's time to install the motor mount into the booster airframe. Sand the inside of the booster airframe with 60 grit sandpaper around the fin slots and where the CRs will bond. (Don't sand the all the way up to the very forward end of the tube.) Wipe off the inside with alcohol. These are important joints so take time and make sure they are right.

Mix up epoxy for bonding the two CRs into the airframe. Often, it is almost impossible to get epoxy on the forward edge of the center CR. Slide the motor mount into the airframe until the center CR is just below the top of the fin slots. Drip/push as much epoxy as you can into the top of the fin slots and try to get it to run around the body tube against the forward end of the center CR. Then push the MMT into the airframe until the aft edge of the center CR is at the top of the fin slots.

If the airframe is slotted correctly and you got the CR on properly, it should line up exactly at all fin slots. Double-check your work at the aft end of the MMT. Lay a ruler across the aft end of the airframe tube; it should touch the mark you made on the MMT for the aft CR.

Since we left the aft CR loose, but will use it for alignment, we need to be careful not to bond it in place when things get messy. Make sure the aft end of the MMT and body tube are free of epoxy and your hands are clean. (Wipe the tubes clean with alcohol if necessary and take off your gloves.) Now carefully slide the aft CR onto the MMT until it just barely touches the aft end airframe. Don't slide the CR all the way inside the airframe because you will need to pull it out again. This will help keep the MMT aligned properly inside the airframe while the epoxy cures.

Now bond the forward CR to the airframe. From the forward end of the airframe, apply some epoxy with a long stick to the outside edge of the forward CR. Get a good fillet here and smooth it with your finger. Be careful not to drip epoxy on the inside of the airframe and if you do, clean it off with alcohol. Be very careful not to get epoxy inside the MMT or you will be very sorry when you try to install a motor!

Let the booster section sit until the epoxy is fully cured. You can set the airframe on a roll of tape with the MMT through the hole in the center if you think the weight of the airframe might change the position of the motor mount while curing.