Rocketry Planet

Welcome to the Rocketry Planet How-To Classroom!
In this edition of the How-To Classroom, we are focusing on building a hardened version of the new Estes D-Region Tomahawk (#2037). This kit is 38.8" long, 1.8" in diameter and comes with a 24mm motor mount. I got my Tomahawk from Belleville Wholesale Hobby for just $24.49. Features of the kit include a very detailed blow-molded nosecone, a very detailed fin and fin canister section, a twist-lock motor retainer and a rip-stop nylon parachute.

The blow-molded plastic nosecone is a very nicely done piece and is an attribute to the scale details of the kit, featuring simulated capscrews as well as connector details, and alone is nearly a foot long. It will be used as-is with the possibility of adding nose weight, if needed.

The fin can is made of styrene, featuring a two piece design with individually mounted styrene fins. Details of the fin canister features simulated attachment capscrews and bolts as well as simulated rivets along with very accurately scaled fluting. The fluting is usually one area that gets discarded by scale enthusiasts when modeling the D-Region Tomahawk. The challenge with this project will be to integrate the styrene parts into the hardened version in a way that will create a strong model where the plastic parts aren't ripped off through energetic motors and hard landings, preserving the scale-like qualities of the kit while reducing flight damage.

Contents of the Estes D-Region Tomahawk kit #2037.

Originally designed to fly on 24mm motors, the kit comes with a long 24mm motor mount that doubles as a stuffer tube, which mates with the airframe coupler tube at the mid-point of the airframe. Estes carved the airframe up into two tubes to make packaging easier, but the end result of using a stuffer tube makes for a stronger airframe and simplifies the modifications we are to make in this class. For the purposes of our classroom project, the 24mm motor mount will be replaced by a 29mm motor mount that also will serve as a stuffer tube, while still utilizing the stock upper centering ring.

The picture of the kit parts shows all of the individual components as the kit is received. The parts we will not be using are the 24mm motor mount and related twist-lock motor retainer (although it is a very neat looking piece and I suspect was a carry-over from some other kit in the Estes lineup), the aft centering ring, the engine block and spacer, and the elastic shock cord. Optional are the launch lugs, although if you plan to launch your Tomahawk on any high powered motors, you may want to take the class recommendation and switch to something a little stronger. For the purposes of this class, they will be replaced.

Testor 3501 Plastic Cement

Let me say a word about adhesives before discussing the replacement parts, since we are dealing with several pieces of styrene plastic as well as cardboard. There are some considerations to be made about how to best adhere the plastic, both to itself and to the cardboard. Plus, there are normal places where basic hardening is better achieved with the right adhesive as well.

Typically, model building with styrene has been done with plastic cement. Testors is a popular brand of plastic cement and has been around for decades, considered a mainstay in plastic model building. The gel-style tube-delivered thick cement melts into the surrounding styrene and when it dries, if forms a good bond. Testors No. 3501 (pictured at left) is typically the type of plastic cement you will find in your local hobby shop but any similar brand designed for use in styrene plastic model building is suitable.

A disadvantage of using this type of cement on styrene plastic is that it can soften and melt the surrounding styrene parts, often distorting thinner pieces. It is beneficial for general plastic model building and will be used on this project in areas where a thicker bonding joint is available so that melting isn't a consideration and where there will be styrene-to-wood and styrene-to-cardboard joints, as the plastic cement is capable of penetrating into porous materials and developing a good joint.

Tenax 7R Plastic Welder

For styrene-to-styrene joints, such as the fin canister shell halves and the attachment of the fins, a different adhesive will be used. In this case, the product is Tenax 7R Plastic Welder by Hebco of Hohenwald, Tennessee. Tenax 7R Plastic Welder is a very thin, solvent-type plastic model cement that actually welds parts together. Very fast drying and very strong once dried, it has been said that when you put it together with Tenax 7R, it isn't coming apart.

What you have to remember is that you aren't cementing or gluing with Tenax, you are welding. Actually, you are dissolving the plastic, which, when the solvent evaporates, the plastic reformulates together into a very strong bond. You use it by holding the parts together and then flowing small amounts into the joint. Capillary action draws the thin liquid into the joint, melting the plastic together and forming the bond. The Touch-N-Flow applicator is a good investment, as you want to control the amount of liquid you put onto the joint and flow applicators are the best way to do this, although you could use a very thin, very finely pointed paintbrush to do it as well.

Insta-Cure CA

The other two types of adhesive we will use in our construction are basic hobby staples: two-part hobby epoxy in the 12-15 minute variety and thin cyanoacrylate (CA) glue. I patronize local hobby shops for these products, and usually end up with the Bob Smith products. For this particular classroom project, I am using 12 minute Bob Smith brand two-part epoxy and Insta-Cure CA in the 2 ounce bottle. As with any epoxy and cyanoacrylate, proper storage of these product will lengthen their shelf life. Cyanoacrylate can be stored in the freezer section of your refrigerator for a long time. When you aren't using your epoxy or cyanoacrylate, replace the caps and store them airtight.

Getting to the replacement parts, let's begin with the last item we discussed possibly replacing and that is the stock styrene plastic launch lugs. It is in my judgment a good idea to update the launch lugs to something a little more suitable for use with high power rocket motors to insure the safe, straight launch of your Tomahawk. I decided to replace my launch lugs with ACME's 1.64/RX aluminum conformal launch lugs.

ACME's conformal rail lugs.

The 1.64/RX is designed to fit a 1.64 inch nominal diameter airframe and features an extended rail lug that is offset to allow for use on rockets with fin cans and motor retainers that otherwise would cause contact problems. With the fluted fin canister of the D-Region Tomahawk, this lug offset is needed to insure that there are no clearance issues between the rail itself and the extremities of the of rocket's fin canister. The 1.64 inch nominal airframe size is a perfect fit for the 1.8 inch Estes airframe. Using the enclosed self-adhesive tape strips, the lugs will be an easy addition when the project has been completed and painted.

In the place of the standard Estes elastic shock cord, I ordered six feet of 1/4" tubular Kevlar®. This choice allows me a much longer, and certainly more robust, shock cord than the standard kit. This should be considered a no-brainer as the elastic shock cord is probably the most probable point of failure built into the original Estes kit. Both the tubular Kevlar and the ACME extended rail lugs were ordered from our good friends at Giant Leap Rocketry.

Giant Leap was also my choice for replacement of the 24mm motor mount tube that came in the stock Tomahawk kit. The 24mm motor mount tube is being replaced in this project with a 29mm motor mount tube, so I also ordered a length of 29mm phenolic tubing from Giant Leap as well. In this case, a 15-1/4" length of 29mm phenolic tubing was cut from the tubing I ordered and would form the basis of the D-Region Tomahawk's motor mount assembly.

Giant Leap's tubular Kevlar.

This 15-1/4" measurement is what I came up with to work with my choice of motor retention, the Aeropack 29mm motor retainer. Some students might balk at spending more on a motor retainer than the cost of the rocket kit, but for me, positive motor retention is something I do on all of my rockets since the cost of replacing a reloadable motor's hardware is certainly more than the cost of the rocket kit, so an adequate motor retention device is cheap insurance.

That is not to say that you couldn't devise another method of motor retention, just be aware that the 15-1/4" measurement is the 29mm tube length that needs to be cut to allow the proper amount of motor tube to stick out the back end of the rocket to attach the Aeropack retainer. Because of the way the airframe tube coupler is utilized and the termination of the motor tube at the airframe mid-point, there is some "fudge room" in that area where the chosen motor retention's affect on the motor mount tube length doesn't have to be exact.

Aeropack's 29mm motor retainer.

Beyond those basic components, a student will need basic hobbyist skills as well as a few basic tools to complete the classroom project. The only complex manufacturing aspect of the project is the creation of two plywood centering rings and a recovery attachment point. Because of the nature of this, if you don't have basic tools to create centering rings of custom sizes, you may be better off ordering them from someone like Patrick Waite of CNC Rings N Fins (http://www.cnc-rings-fins.com/). The centering rings have an OD of 1-5/8" with a 29mm hole for the motor mount. For the recovery attachment point, a 1-3/4" length of carbon fiber arrow shaft is utilized.

For my centering rings, I used standard 1/8" aircraft plywood, cut with an adjustable fly cutter. I purchased the General Tools adjustable circle cutter (General No. 55) online for my hobby use. It features a 1/2" chuck, 1/4" pilot hole size and cuts holes from 1-3/4" to 7-7/8" in diameter. By flipping the cutter bit, you can make inside and outside cuts.

When cutting centering rings of such small dimensions, there are some tricks you need to utilize in order to get your motor hole centered directly in the small centering ring — the subject of which is probably a class all to itself, but I will cover my tricks in hopes that it will benefit you.

I start by cutting the centering ring outer dimension (1-5/8") by carefully adjusting and testing the cut dimension on a piece of scrap luaun door skin. The 1/8" luaun sheets are perfect for testing because they are cheap and thin. Once I have the fly cutter adjusted to the proper size (sometimes this takes 3-4 adjustments), I start my first cut using the aircraft plywood, making the first cut halfway through the wood. I then flip it over to finish the cut. It makes a much smoother finished product and with centering rings as small as we are making, it helps to avoid splitting and splintering.

I make twice as many as I will need, because you can never tell when you are going to screw one up (translation: break one because of the small size). The pilot hole size of the fly cutter is 1/4", so when I am finished with cutting them all out, I place all of the plywood disks on a 1/4" bolt long enough that will go up through the middle and allow me to put a washer and a nut on to tighten the plywood disks up into a single unit. I can then chuck this in my drill press and sand the outside smooth.

General Tools No. 55 Circle Cutter.

Before I remove them from the 1/4" bolt, I drill a small nail hole through all of the plywood disks inside of where the 29mm motor mount cutout will be. I use this hole to place a small finishing nail all the way through the plywood disks. Before I remove the 1/4" bolt, but after I've removed the 1/4" nut and washer, I place the grouping of plywood disks flat onto a piece of scrap wood and drive the finishing nail down into the scrap to securely hold the disks to the scrap. This is to prevent the disks from turning while drilling out their centers.

To finish, I drive another finishing nail outside of plywood disks, but adjacent to and touching the disks, into the scrap to further prevent them from turning when the 29mm centers cut through. So I have a finishing nail inside of the 29mm cutout area and a finishing nail outside of the disks, abutting and touching the disks. I want to push the stack of disks against this second finishing nail "stop" so there is no chance of rotation. At this point you can remove the 1/4" bolt by slowing unthreading it from the arbor hole in the center of the plywood disks while holding them firmly.

For the final 29mm center hole, I have a 1-1/4" carbon hole saw I bought from ACE Hardware that I will use to cut the motor mount holes with. This hole saw also has a 1/4" pilot hole size, which is important so the hole saw centers itself properly in the hole already in the plywood disks. The 1-1/4" hole saw without modification is a tad large for 29mm motor mounts, so I chucked it in my drill press and ran a file against the outside cutting edge of the saw to remove the sharp tips from the outer cutting edge. This is a trial-and-error approach and the luaun door skin scraps come in handy here as well to let you know when you have the right fit. Once you get there, it'll be there forever, or until you wear it out.

When ready to cut the 29mm hole, I placed the scrap of wood with the stack of plywood disks on it and s-l-o-w-l-y, with light pressure, started cutting through stack of disks, to insure that the stack didn't move and end up destroying my work so far. Light pressure is your friend, as well as patience. It's not easy but it's not difficult either. Just take your time. As each ring completes the center hole cut, you can stop and remove it to prevent it from being damaged.

When your centering rings are completed, check their fit on your 29mm motor mount tube. Do not force them, or you could break the fragile ring. Sand them as needed to fit without having to force them but not so much as to make the fit sloppy. With the centering rings finished, we are now ready to assemble the motor mount.