Chapter 3. Juno: A Solid Propellant Rocket

The rocket we will build in this chapter is called Juno. See What’s in a Name? to learn a bit about where the name comes from. Juno is a workhorse, easy to build but very stable. It’s also capable of reaching altitudes of 1,300 feet—over a quarter of a mile!

There are a lot of rocket motors out there. Table 3-1 shows the recommended motors for flying Juno, along with the projected altitudes. The altitude attained can vary a bit depending on the altitude you fly from, how you finish the rocket, how much glue you use, and even small variations in the motors themselves, but this should give you some idea of what to expect when you fly the rocket. If you are going out to buy parts now, buy a package of A8-3 motors. That’s a great motor for the first flight.

Table 3-1. Recommended motors

Motor	Approximate altitude
1/2A6-2	60 ft
A8-3	200 ft
B4-4	520 ft
B6-4	550 ft
C6-7	1,280 ft

Parts and Tools

Table 3-2 lists the parts you will need to build Juno. A list of additional tools and supplies is found in Table 3-3. Read through the construction description before you go shopping, though; there are lots of places where you can make substitutions, and some of the substitutions are a lot less expensive than the items shown in the tables.

Table 3-2. Parts list

Part	Description
BT-50 body tube, 12”	The main body tube for the rocket. This will be cut from a longer tube; they usually come in 15- or 18-inch lengths.
BT-20 body tube, 2 3/4”	The motor mount. This is precut in the Estes Designer’s Special. It can easily be cut from a longer BT-20 body tube.
BT-50 size nose cone, 2 3/4”	Any BT-50 nose cone of balsa or plastic that is at least 2 1/2 inches long will do. Stick with balsa wood if you turn your own. Harder woods can turn a fun model rocket into a potentially dangerous projectile.
Screw eye (balsa nose cones only)	You will need a screw eye if the nose cone is balsa wood. This is screwed into the base of the nose cone to provide an attachment point for parachutes and shock cords. Plastic nose cones have an equivalent molded into the base of the nose cone.
3/32” balsa fin stock	The balsa wood will be used for the fins. See Fins if you intend to buy your own balsa at the local hobby store.
CR-20/50 centering ring (2)	Two spiral-wound centering rings are used to hold the motor tube inside the main body tube. There are lots of substitutes, from balsa wood to cardboard, but the commercial version is cheap and very easy to work with.
2 3/4” engine hook	This is a small metal hook. They are too cheap to make it worthwhile manufacturing your own.
1/8” launch lug	It’s actually a little thicker, but the launch lug we will use is designed for a 1/8” launch rail, so it’s generally called a 1/8” launch lug. Get the commercial version. You will need a 1 1/2"-long piece.
1/8” shock cord, 18”	Made from rubber or elastic. This will come in the Estes Designer’s Special, or you can buy some 1/8"-wide elastic from the fabric store. You will need a piece 18 inches long.
12” plastic parachute	These come in kit form. If you’re buying parts individually, though, skip this one. Buy some crochet string and some duck tape and get it in a colorful plastic sack from the store, instead.
Snap swivel (2, optional)	I like to attach parachutes and shock cords with a snap swivel, making it easier to move a parachute or payload bay from one rocket to another.

Table 3-3. Tools and supplies

Tool or supply	Description
Hobby knife	A hobby knife with several extra blades.
Cutting board	A cutting board or mat, approximately 18 x 24 inches.
Glue	Yellow wood glue and a small tube of plastic glue. The plastic glue is only needed if your plastic nose cone comes in two parts.
Sandpaper	Medium grit and extra fine grit sandpaper.
Sanding block	A small block of wood, about 0.75 x 1.5 x 2 inches, will do nicely.
Wood primer	Filling, sandable primer, or sanding sealer.
Paint	Enamel spray paint, acrylic paint, or model airplane dope. Practically anything will work as long as you don’t mix paint kinds.
Paper	Used for fin guides, cutting guides, and shock cord mounts. Normal notebook paper or printer paper is fine.
Metal ruler	A metal-edge ruler is not essential, but is very handy. Get one about 15 inches long.
Pencil	Not a pen, but a pencil. You’ll use it to mark body tubes.
Wax paper	Glue does not stick to wax paper. Use it to cover surfaces where glue might drip while parts dry.
Tape	Several kinds of tape are handy. At the very least, get painter’s tape, used to mask areas while spray painting.

Where to Buy Parts

Most of the solid propellant rockets in this book can be built from the parts in the Estes Designer’s Special, a collection of rocket components available by mail order from Amazon.com and many other retailers. You will need a few additional odd parts for some of the projects.

Figure 3-2. The Estes Designer’s Special

All of the parts can also be purchased from mail-order houses and hobby stores. A quick Internet search will turn up a long list of hobby stores and model rocket companies with an online presence. Here are a few that I have dealt with over the years:

http://www.estesrockets.com: The 800-pound gorilla of the low-power model rocket world, Estes Industries has been around since the dawn of model rocketry. It makes and sells the Estes Designer’s Special, as well as various replacement parts, motors, and kits.
http://www.questaerospace.com: One of the few competitors for Estes in the low-power model rocket arena, Quest has a nice online store featuring motors, parts, and kits.
http://www.amazon.com: You can find the Estes Designer’s Special, as well as motors and kits from many manufacturers, on Amazon.

Local hobby stores frequently carry model rocket kits and supplies. Most will have starter sets, kits, and motors. Some will have individual parts like body tubes and motors, too. There’s nothing quite like the convenience of dropping into a local store and seeing all of the parts firsthand, or picking up an igniter or motor for a launch the same day. Try to support your local retailers so they are there to support you when you need it.

You will also find an extensive list of retailers, most with an online presence, in Appendix B.

A Detailed Walk-Through of the Parts List

Since this is our first solid propellant rocket beyond the match head rocket, it’s appropriate to take some time for a thorough look at the materials used to build model rockets.

Body tubes

Body tubes are the main structural element in the model rocket. They are used for the exterior body of most rockets, as well as for payload bays and engine mounts, and sometimes for decorative elements.

Figure 3-3. Body tubes are generally made from spiral-wound paper. Seen here are the BT-5, BT-20, BT-50, BT-55, and BT-60. The clear payload tube is the same size as a BT-50.

Body tubes are almost universally made from spiral-wound paper, usually brown but occasionally white or yellow. Some payload bay tubes are made from clear plastic, and a few rockets are built around other components entirely, but the paper body tube is the mainstay of model rocketry.

Body tubes come in a variety of diameters and lengths. The most common lengths are 18 inches and 15 inches. Tube diameters are most commonly labeled with BT- followed by a number. The BT- numbers originated as stock numbers for Estes Industries body tube sizes, but became a de facto standard. Table 3-4 lists the most common body tube sizes.

Table 3-4. Common body tube sizes

Size	Inside diameter (in)	Outside diameter (in)	Inside diameter (mm)	Outside diameter (mm)
BT-5	0.518	0.544	13.2	13.8
BT-20	0.710	0.736	18.0	18.7
BT-50	0.950	0.976	24.1	24.8
BT-55	1.283	1.325	32.6	33.7
BT-60	1.595	1.637	40.5	41.6
BT-70	2.180	2.217	55.4	56.3
BT-80	2.578	2.600	65.7	66.0

The BT-20 body tube is exactly the right size to hold a typical A, B, or C model rocket motor. While the diameter varies a bit, most rocket motors are 0.7 inches in diameter and 2.75 inches long. Juno uses a short section of BT-20 for the motor mount.

Juno uses a BT-50 for the main body tube. This makes the rocket a little bigger than the motor mount, giving room for a motor retention clip that is internal to the rocket. The BT-50 is also the right size for motor mounts for D and E motors.

We will use several of the other body tube sizes in other rockets. The BT-5 is perfect for tiny motors and decorative trim, the BT-55 is just right for the main body tube on a D or E rocket, and the BT-60 is great for large, impressive-looking rockets.

Nose cones

Nose cones are usually made from plastic or balsa wood. They are never made from metal. I prefer working with balsa wood, which is easier to modify. Plastic nose cones are becoming more and more common, though, because they are cheap to make in large quantity.

Figure 3-4. Nose cones come in a variety of sizes, shapes, and materials. Most are plastic or balsa wood.

Juno is designed for a plastic nose cone from the Estes Designer’s Special. It’s the one that fits the BT-50 body tube with a length of 2 3/4”. That’s the length of the part that is exposed when the nose cone is in the body tube.

You might get a package of nose cones from another manufacturer, or Estes may change the Designer’s Special. Is it safe to make changes?

Within certain limits, yes. We’ll go over rocket design in Chapter 7, where you’ll learn all of the things you need to know to evaluate a rocket design for stability. For now, these rules of thumb will keep your rockets stable if you vary from the design of the rockets in this book:

You can always use a longer, heavier nose cone. That increases the weight at the front of the rocket, bringing the center of gravity forward, making the rocket more stable.
You can always lengthen the body tube. That also moves the center of gravity forward.
You can add more fins or make them larger.
You can change the shape of the fins as long as you keep the surface area at least as big as that of the fins in the design. You can move them toward the base of a rocket, but not toward the nose cone. You can sweep the fins more, but only toward the base. All of these changes move the center of pressure toward the base of the rocket (or leave it unchanged), making the rocket more stable.

Screw eyes

You need some way to attach a parachute and shock cord to the nose cone of a rocket. A small screw eye is a perfect attachment point for a balsa nose cone. Plastic nose cones come with a premolded loop for this purpose, so you don’t need a screw eye if you are using a plastic nose cone like the ones in the Estes Designer’s Special.

Screw eyes are available at hardware stores. Look for small ones, about 1/2” to 3/4” long.

Figure 3-5. Screw eyes are mounted in balsa wood nose cones to provide a place to attach shock cords and parachutes.

Fins

Kits sometimes come with plastic fins, but almost all scratch-built rockets use balsa wood sheets for fins. Balsa wood is light, strong, and easy to work with, so it is likely to remain the material of choice.

Figure 3-6. Fins are usually cut from sheets of balsa wood, although preformed plastic fins are fairly common.

The fins for Juno are cut from 3/32"-thick balsa wood sheets. It’s a great general-purpose thickness for small- to medium-size rockets flying on A, B, or C motors. Other common thicknesses are 1/16”, which we will use for a high-performance rocket in Chapter 15; and 1/8”, which we will use for larger rockets and rockets with D and E motors. Most sheets are three or four inches wide. Hobby stores generally carry sheets that are 36 inches long, but this varies a bit.

Like any natural material, balsa wood varies from one sheet to the next. For rocket fins, pick the densest, hardest balsa sheet in the bin. It is also important that the sheet not have twists or warps. Sight along the length of the sheet to see any bends and warps. A sharp, localized bend is not as bad as a long, gentle twist—you can always cut around a short imperfection.

Check balsa sheets for twists and bends by sighting along the length of the sheet. The sheet on the left is clearly not good fin material. The one on the right will work well.

Figure 3-7. Check balsa sheets for twists and bends by sighting along the length of the sheet. The sheet on the left is clearly not good fin material. The one on the right will work well.

Don’t plan to share a cutting board with the fabric artists in the house, though. The model rocket cutting board will get a lot of abuse, collecting deep cuts, glue, and paint—all things that will make you very unpopular with fabric artists!

You can substitute an old piece of Masonite, a flattened cardboard box, or even an old piece of wood, but a cutting board is best. It is smooth, provides good support, and doesn’t dull the knife blade as quickly as some of the alternatives.

Glue

Model rockets can be built almost exclusively with yellow wood glue. White glue will also work, but wood glue is quite a bit stronger—strong enough that it’s the wood or paper that gives way on a properly glued fin, not the glue.

Figure 3-16. Use yellow wood glue for almost all construction. You will need an occasional dab of plastic cement for fastening pieces of plastic nose cones.

Model rocketeers are passionate about their choice of glue. Personally, I don’t like a single glue. I use several glues and cements, depending on the material being glued. Still, if you have to pick one glue, use yellow wood glue.

Wood glue works great on porous surfaces, but not on plastics. Use plastic cement, superglue, epoxy, or household glue to glue plastic pieces together.

Cyanoacrylate (CA) glue, also called superglue, is a great choice in many situations. Some people use it as their primary glue for building rockets. There are several thicknesses of CA glue. Consider adding a small bottle of thin CA glue and another of thick CA glue, often called gap-filling CA glue, to your toolbox. Don’t get a big bottle unless you plan to use it for a big project, though. The shelf life for CA glue drops once you open it.

Thin CA glue is great for strengthening parts. We’ll use it later in the book to strengthen body tubes and thin balsa parts. Thick CA glue is best for gluing parts together, especially porous parts like paper and balsa wood.

Some of the components in later chapters will also use epoxy glue, but it’s not needed for Juno. Epoxy is usually the strongest adhesive available, and unlike the other glues in this section, it will set quickly even if it’s locked in areas that don’t get good airflow.

Sandpaper

There is a wide variety of sandpaper. It’s rated by grit, which indicates the size of the particles used in the sandpaper. The bigger the number, the smaller the particles and the smoother the sandpaper will be.

Figure 3-17. Get at least one medium grit sandpaper and one fine grit sandpaper.

Medium grit sandpaper is used to shape fins, remove burrs from plastic parts, and rough-sand balsa parts before finishing. A grit of 120 to 150 is about right.

The wood filler will be sanded between coats. A fine sandpaper is best for this job. I like 320 or 400 grit sandpaper for sanding the wood filler.

Sanding block

A lot of the surfaces on model rockets are large, flat areas that need to stay that way. Sanding with just finger pressure can easily etch small grooves into a flat fin. A sanding block holds the sandpaper flat, applying even pressure across the area being sanded.

Figure 3-18. Use a sanding block to sand flat surfaces.

Sanding blocks don’t have to be fancy. I’ve used a piece of scrap wood that measures 3/4” x 1 1/2” x 2 1/4” for years. It’s the perfect size for 1/16th of a sheet of sandpaper.

Wood primer

Balsa wood is very porous. Paint applied directly to the wood will be quite rough. There are a number of wood fillers available that prepare the wood for paint, creating a satin or glassy surface.

Figure 3-19. Use some form of filler to prepare the wood for paint.

While lots of products will do, my favorite when working with a brush is sanding sealer. It’s getting a little hard to find, but is usually available at hobby stores that cater to model airplanes. There will be a section of paint called “Butyrate Dope” or “Hot Fuel Proof Dope,” and somewhere in the rack will be a creamy, translucent jar labeled sanding sealer. If you can’t find it locally, check online: you can order it directly from SIG Mfg. Co.

Dry-sandable filling primer works very well for spray-on applications. This is also a little hard to find. Be sure and get a filling primer, not just a standard primer. While wet-sandable primers will do, I prefer a dry-sandable primer like Rust-Oleum 2 in 1 Filler & Sandable Primer. I occasionally find this at Lowe’s hardware store, but it’s a specialty item and often out of stock. It’s available from Amazon.com, too.

Some people skip this step. That works particularly well on rockets that have plastic fins. Sure, you can still see a little of the paper winding through the paint, but it doesn’t have much effect on the way the rocket flies.

You can also use paint as a primer. It doesn’t work quite as well, but you can paint a wooden part, let the paint dry, and sand the paint. Depending on the wood, you might have to repeat this process several times to get a smooth finish—or maybe you don’t care if the finish is perfectly smooth. The rocket will fly either way.

Paint

Rockets should look cool. The only way to do that is with a good paint job. Almost any kind of paint that won’t dissolve in water once it is dry will do.

Enamel spray paint works well, and it’s what I use most of the time.

You might also want small (1/4 ounce) bottles of enamel paint in a few colors to paint details.

Figure 3-20. Use almost any kind of non-water-based paint.

You will need some paintbrushes. A nice wide brush, 1/4” to 3/16” wide, works really well. Brush cleaner is also a must. Paint thinner for the kind of paint you are using works well. Nail polish remover works fine for enamel paint.

Acrylic paint is my favorite if I’m painting the entire rocket with a brush. You can also use acrylic paint with an air brush. An air brush is a little more expensive to buy, and it will take a little time to get used to it, but it does save money compared to spray paint in the long run. You can also mix your own colors if you use acrylic paint and an air brush.

Model airplane paint, also known as butyrate dope, also works fairly well. It’s a bit smelly, though, and needs to be used in a well-ventilated area. It also tends to shrink as it dries, which is good and bad. Shrinking on most surfaces smooths out small imperfections. Shrinking near the joint of a fin can cause the paint to pull away from the fillet.

Paper

Normal-weight writing paper is used for marking body tubes for cutting, for making some kinds of fin guides, and for shock cord mounts. Any scrap will do. Larger sheets are useful for making parachute patterns. You can paste several sheets of writing paper together, but a more practical idea is to use some of that leftover wrapping paper from the last holiday or birthday.

Pencil

There are several situations where we will mark guide lines on the rocket during construction. A normal pencil works best. Ballpoint pens tend to make indentations in the surface of balsa wood and body tubes, and often don’t work on plastic nose cones. Felt tip markers work well, but tend to bleed through the paint.

Wax paper

Glue occasionally drips, but it will not stick to wax paper. Any time you are working over a surface you don’t want to get glue on, especially while a part dries, cover it in wax paper. This is particularly helpful when gluing fins over a fin guide.

Metal-edge ruler

A metal-edge ruler is a great addition to your tool kit. It’s really nice when you are cutting balsa wood fins or paper. A 15-inch ruler is a good length; they are available from office supply stores.

Tape

Several kinds of tape are useful. At a minimum, get a roll of painter’s tape. It’s available from the paint sections of hardware stores, and is usually blue or green. This stuff works a lot better than traditional masking tape for masking areas when you spray paint. Clear cellophane tape is also handy.

Building the Rocket

With parts and tools assembled, it’s time to build a rocket!

Read all of the instructions before you start. I know. You hate directions. There is a reason, though. If you follow through step by step, you will build a perfectly good rocket, then discover that there are some cool options you could have used—but it’s too late. Read through so you know what options you want before you start.

Cutting Body Tubes

Juno uses a 12-inch-long section of BT-50 body tube. Most body tubes are 18 inches long, while a few are 15 inches long. That means we’ll need to cut the tube.

Measure 12 inches from the base of the tube and make a small guide mark with a pencil. Wrap a piece of paper around the body tube and match the edges. The paper forms a perfectly straight edge that is exactly perpendicular to the length of the body tube. Mark this line carefully with the pencil.

Figure 3-21. Measure the location to cut, then use a piece of paper to extend the line around the body tube.

Put something hard or thick into the tube, directly under the location to be cut. The long orange motor adapter from the Estes Designer’s Special works well, as does a tube coupler, or a used D or E motor (not an unused one!). This gives a cleaner cut by supporting the tube as the hobby knife slices through.

Take your time, starting with a light cut that barely scores the body tube. Take extra care to create a nice, straight groove for the deeper cuts. I like to use the flat part of the blade for this cut—it makes it easier to get a straight cut.

Figure 3-22. Place a support under the cut location. Start with a shallow cut using the flat part of the knife blade.

Switch to the point of the blade and increase the pressure after the guide cut is complete. Take your time. It should take three to five complete turns of the tube to make the cut. If you cut too deep, you could end up with a jagged edge.

Figure 3-23. Take your time, making the cut in three to five turns of the tube.

If you do mess up, ending up with a body tube that is not perfectly flat or one that has some ragged edges, sand the body tube to flatten the end. Place a very fine grit sandpaper on a flat surface. Hold the tube perpendicular to the sandpaper and sand in a circular motion. You can check to make sure the end is still square by inserting a nose cone. Does it fit flush with the tube? If not, sand off the high spots.

The Estes Designer’s Special comes with a precut BT-20 body tube for the motor mount. It’s white, about 3/4” in diameter, and 2 3/4” long. If you are working from a longer piece of BT-20 body tube, repeat the above process to cut off a 2 3/4” piece.

Marking the Body Tube for Fins

The fins and launch lug must be straight for the rocket to slide off of the pad and fly straight. Marking the body tube is the first step in a perfect flight.

Figures 3-24 and 3-25 are fin marking guides. Use Figure 3-24 as a fin marking guide for Juno. Place the body tube on the guide, using the concentric circles to position the body tube in the exact center of the guide. Work around the rocket, making small marks at the base of the body tube to mark the locations of the three fins and launch lug.

Figure 3-24. Fin marking guide for three-finned rockets. Use this one for Juno.

Figure 3-25. Fin marking guide for four-finned rockets. Don’t forget to mark the location for the launch lug.

The next step is to extend the lines up the body tube. Find a convenient door jamb or the edge of a drawer, placing the body tube so it sits in the V shape formed by the wood. Rotate the tube until the guide mark is just next to the wood, then use your pencil to extend the guide lines about three inches for each fin and about six inches for the launch lug. Don’t press hard-—just hard enough to make a mark you can see clearly. Be careful not to mark the door jamb!

Figure 3-26. Mark the position of the fins and launch lug using the fin guide, then extend the marks up the tube using a door jamb as a guide.

Motor Mount Construction

Figure 3-27. The completed motor mount.

The motor mount will slide into the body tube, holding the rocket motor in place during flight. It needs to be strong.

Start by measuring 1/4” from one end of the motor mount tube and making a mark 1/8” long, perpendicular to the length of the tube. Puncture the tube at this location with a hobby knife. Insert the flatter end of the engine hook into the slot.

The engine hook protrudes into the motor mount. It may not look like much, but this little tab of metal will easily hold a rocket motor in place during the thrust phase of flight.

Figure 3-28. Cut a notch in the motor mount and insert the engine hook.

Cut a notch in one of the centering rings. The notch should be wide enough for the engine hook and about half of the depth of the centering ring.

Figure 3-29. Cut a notch in one of the centering rings.

Test fit all of the pieces. Use your fingernail to score the edges of the centering rings if they don’t slide easily over the motor tube. Use sandpaper if you must.

Figure 3-30. Score the edge of the centering ring to make it easier to slide it onto the motor tube.

Insert the engine hook. Place a bead of wood glue around the end of the motor mount, just above the slot for the engine hook, then slide the uncut centering ring onto the end of the motor mount. Wipe away excess glue with your finger, forming a nice fillet where the centering ring meets the motor mount tube.

Figure 3-31. Glue the uncut centering ring onto the end of the tube. Form a fillet using your finger.

The notch in the remaining centering ring gives a little more room for the engine hook to bend. Without some support, though, the engine hook will eventually work loose from the slot at the end of the motor tube. The Estes Designer’s Special comes with a Mylar ring to support the engine hook.

Place glue under the engine hook, from the end that is inserted in the motor mount tube to about halfway down the tube. Glue the Mylar ring about halfway down the tube, then apply a liberal amount of glue over the engine hook between that ring and the forward centering ring. Don’t apply glue below the ring.

Figure 3-32. Secure the engine hook with glue and a band for stability.

There are plenty of adequate substitutes for the Mylar ring, so don’t bother ordering one if you are buying individual parts. A couple of turns of masking tape works well, as does a couple of turns of 1/2"-wide paper coated in wood glue.

Apply a bead of wood glue in a ring near the base of the motor tube. The glue should extend from 1/4” to 1/2” from the base of the motor tube. Slide the remaining centering ring onto the motor mount and form generous fillets on either side of the centering ring.

Figure 3-33. Glue the remaining ring 1/4” from the end of the motor mount. Apply generous fillets of wood glue.

Set the motor mount aside to dry. It’s best to prop it up on its end so the glue doesn’t drip off. Placing it on a small scrap of wax paper will make cleanup a lot easier if any glue does drip. The glue will set pretty quickly, but check it occasionally as the glue dries to make sure none has dripped. If any glue does drip, clean the drip with a damp paper towel or, if it is firm enough, with the hobby knife.

Cutting Fins

Take a look at a piece of balsa wood. You can clearly see the grain in the wood—long, parallel lines that were formed as the tree grew new rings. The wood is much stronger going across the grain than parallel to the grain (Figure 3-34).

Balsa wood is stronger perpendicular to the grain of the wood. It breaks easily and cleanly with the grain, but is harder to break against the grain.

Figure 3-34. Balsa wood is stronger perpendicular to the grain of the wood. It breaks easily and cleanly with the grain, but is harder to break against the grain.

Cut two pieces of balsa from a 3/32"-thick plank, both about 1/2” wide and 1 1/2” long. Cut one with the grain parallel to the long axis, and one with the grain perpendicular to the long axis. Bend each piece until it breaks. It’s not hard to break the piece with the grain running perpendicular to the direction of the bend, but the other piece is both more rigid and much harder to break.

If you think about it a bit, you’ll understand why fins need to be cut so the grain of the wood extends out from the body tube, not running parallel to it. The general rule is to cut the fin so the grain runs parallel to the leading edge of the fin. Fins should be designed so there is never a line of grain that does not lead back to the body tube or a glue joint. If you need to cut a really fancy design, perhaps a concave curve, use multiple pieces of wood so the edges don’t break off.

Take a look at the fins in Figure 3-35. Root indicates the part of the fin that is glued to the body tube, and in all cases, the nose cone is up. The lines with arrows indicate the direction of the wood’s grain.

The fins for Juno are on the left. While the grain direction follows the general rule of cutting the fin so the grain is parallel to the leading edge of the fin, it would work just as well if the grain were perpendicular to the fin root. The middle fin shows a case where this is a bad idea, though. The bottom tip of the fin is very fragile, and is likely to break off even on a mild landing.

The rightmost fin presents an interesting case. There is no way to orient the grain of the wood so there is no fragile section. The normal rule of placing the grain parallel to the leading edge leaves a piece at the tip of the fin that is still very prone to breaking. The solution is to build the fin from two pieces of wood, as shown at the top. The second piece has the grain running vertically, where it is connected with glue to the rest of the fin. Wood glue is quite strong; if properly glued, the joint will be stronger than the surrounding balsa wood.

Figure 3-35. Some dos and don’ts for fin grains.

Figure 3-36 shows a fin-cutting guide for Juno. As in most fin guides, the direction of the grain is clearly marked. Juno fins fit together snugly on a sheet of balsa.

Figure 3-36. A fin guide and one layout for efficiently cutting the fins from a plank of balsa.

Trace the fin guide on a sheet of paper, cut it out, and use a pencil to draw the outlines of the fins on a 3/32"-thick plank of balsa wood. This is the middle-thickness plank from the Estes Designer’s Special. Cut the fins using a hobby knife. Just like when cutting the body tube, start with a light guide cut, then make three to five additional cuts to cut all the way through the wood. While not essential, a metal ruler helps make nice straight cuts.

Figure 3-37. Transfer the fin guide to the balsa wood, then cut out all three fins.

The fins are not likely to be perfect, and that’s OK. Press them together and sand them on a sheet of medium grit sandpaper, being careful to keep the stack of fins perpendicular to the sandpaper.

Sand until all fins are even.

Figure 3-38. Use sandpaper to even out the fins.

There is nothing wrong with leaving the forward and trailing edges of the fin flat, but it will reduce the drag a lot if you round the edges. There are lots of alternatives to a simple semicircular rounding on the leading and trailing edges, but rounded fins are easy to create and work pretty well. Fin cross section is a topic we’ll return to later.

Start by roughing out the rounded edge by sanding the fin at about a 45° angle from the sandpaper. Finish the shape by sanding by hand; your hand has some give and will help the sandpaper form a gentle curve. Be sure to round the leading and trailing edges and the fin tip, but do not round the fin root (the part that will be glued to the body tube). Figure 3-39 illustrates.

Figure 3-39. Sand a rounded edge on the leading edge, trailing edge, and fin tip, but not the fin root.

Now that the fins have their rough shape, it’s time to smooth out the surfaces. Using a sanding block, as illustrated in Figure 3-40, sand the surfaces of each fin with the grain using medium grit sandpaper until the fin feels smooth.

Figure 3-40. Use a sanding block to smooth the top and bottom surfaces of the fin.

Close your eyes and feel the surface of the wood, then run your fingers along the rounded edges. Is everything smooth? If not, carefully touch up the roughness. Remember, it doesn’t matter what the wood looks like—it’s going to get painted soon. All that matters is what it feels like. Any roughness or irregularities will show up clearly once the surface is painted.

Gluing the Fins

Fins are glued to the body tube in two steps. Start by placing the cutting board on a flat surface that is low enough to allow you to sight down along the length of the tube when it is standing on end. Tape the fin guide under a piece of wax paper and center the body tube over the fin guide with the marked end down. Put a bead of glue onto the root of a fin, make contact with the base of the rocket, and press the fin onto the rocket, being careful to make sure the fin aligns exactly with the guide line (Figure 3-41).

Glue each fin in place, then sight along the length of the body tube from above to make sure each fin is perfectly straight and exactly follows the lines on the fin guide. Let the fins dry thoroughly, checking them occasionally to make sure they don’t drift out of alignment.

Figure 3-41. Glue the fins over the fin guide to make sure they are aligned properly.

Once the fins are completely dry, apply a fillet along each of the six edges where a fin meets the body tube. Hold and roll the rocket between your palms for a minute or two while the glue sets, then place it vertically on the fin guide to dry.

Figure 3-42. Place fillets at all joints between the fin and body tube.

Attaching the Launch Lug

Cut about a 1 1/2” piece from a 1/8” launch lug. They are generally three inches long, so you are cutting it in half. The exact length isn’t really critical; anything from about an inch long to a full three inches will work, but cutting the launch lug in half gives you another launch lug for your next rocket.

Use a sharp hobby knife to saw through the launch lug. Sand the cut end on fine sandpaper if needed to remove any roughness.

Use a generous amount of glue to attach the launch lug three inches from the base of the rocket. Carefully sight along the length of the rocket to make sure the launch lug is perfectly aligned with the body of the rocket, as seen in Figure 3-43.

You can do all of this while the fin fillets are setting. Moving the rocket around as the fillets set helps prevent sags in the glue. Check the glue on the launch lug as the glue on the fin fillets sets, too, making sure there are no sags in the glue on the launch lug.

One final check is in order. As the glue starts to get tacky, but before it is completely set, carefully slide a launch rod through the launch lug to check the alignment. Straighten the launch lug if necessary, and then carefully remove the launch rod, making sure the launch lug does not twist as the launch rod is removed.

Figure 3-43. Cut a 1 1/2” piece of launch lug and attach it 3” from the base of the rocket. Check the alignment carefully.

Installing the Motor Mount

Test fit the motor mount in the rocket once the fins and motor mount are completely dry. Gently score the edge of the body tube or lightly sand the centering rings if the motor mount does not slip easily into the body tube.

The motor mount must be glued securely in the body tube. You will need to apply a generous amount of glue to the inside of the body tube, using your finger or a small stick, so that when you slide the motor mount into the tube, the glue gets smeared around underneath and in front of it.

Measure your finger against the motor mount, holding your thumb as a guide, so the tip of your finger almost reaches the end of the motor mount (Figure 3-44). This tells you how far into the body tube your finger needs to be when you apply the glue. Place a large dab of glue on the end of your finger and smooth this onto the inside of the body tube. Repeat as often as needed to get a thick ring of glue for the forward motor mount ring. Repeat this process with a thinner ring of glue about 1/4” into the body tube, and then slide the motor mount into the tube.

I like to align the engine hook with the launch lug; it makes it easier to connect the igniter wires to the rocket later when it is prepared for launch. In truth, though, it doesn’t matter all that much.

Apply a generous amount of glue so the top centering ring will hit it when slid into the body tube. Apply a thinner ring of glue where the lower centering ring will sit.

Figure 3-44. Apply a generous amount of glue so the top centering ring will hit it when slid into the body tube. Apply a thinner ring of glue where the lower centering ring will sit.

Slide the motor mount tube in until its end is even with the end of the body tube.

Hold the rocket upright while the glue sets. It will just take a couple of minutes.

Figure 3-45. Slide the motor mount in until the end of the motor mount tube is flush with the end of the body tube, then stand the rocket up to dry.

Once the glue is firmly set, turn the rocket over and dribble a glue fillet into the slot between the body tube and motor tube. The glue should cover the centering ring everywhere except where the engine hook pokes through; make sure there is no glue there, clearing away any drops that fall into this area. Stand the rocket so the fins are up while this fillet dries.

Shock Cord Installation

The rocket is popped apart with an explosive charge as it coasts to the top of its flight. This explosion forces the parachute out of the body tube and flings the nose cone away. The shock cord is an elastic cord connecting the nose cone, parachute, and body tube. It absorbs the shock of the explosion, keeping everything in one nice, neat package.

Figure 3-46. Shock cord mount for 1/8” and 1/4” shock cords. Trace and cut from printer paper.

The shock cord is fastened to the body tube with a trifold paper and glue mount. The Estes Designer’s Special comes with some of these drawn on a sheet of stiff paper, but you can also cut one from normal writing paper.

Figure 3-47. Mounting the shock cord.

Using either the Estes shock cord mount or one you cut yourself, apply wood glue to the smallest tab, lay the shock cord in the glue, and fold the paper over (Figure 3-47). Don’t mash it down too much, or the glue will squirt out, leaving a dry gap.

Repeat this process with a second fold. Finally, apply glue to the inside edge of the paper and glue it inside the body tube, about one inch from the end of the tube. Make sure there is room for the nose cone to be inserted with at least a quarter-inch gap between the nose cone and the shock cord mount.

I generally have the end with the dangling shock cord pointed toward the nose of the rocket. Press down just enough for some of the glue to squirt out, and then smear this across the top of the paper to form a complete seal. Hold the mount in place until the glue begins to set.

Once the shock cord is dry, apply another coat of wood glue over the entire mount and let that dry, too.

Making a Parachute

The Estes Designer’s Special comes with several prebuilt plastic parachutes. It’s a nice little package, and you should use one of the 12-inch parachutes if you bought that package. Don’t buy a parachute if you are using individual parts, though.

A bag from a retail store makes a great parachute, as does the metallic Mylar used for helium party balloons. Some of the plastic bags are pretty flimsy and made from plastic that sticks together. Use one of the slightly thicker ones that seems a little less like plastic food wrap.

You can make a nice parachute template using a protractor and a few sheets of printer paper taped together, or the back of a piece of wrapping paper. Use the protractor to draw six equally spaced lines radiating outward from a central point, as seen in Figure 3-49. These will be 60° apart.

Extend a nice, long line—about 8” long for a 12” parachute—from the center of the circle through each of the six points.

Measure 6 15/16” from the center of the circle along each line. Connect these lines around the outside edge of the parachute to form a hexagon. This is the outline for the finished parachute.

Why 6 15/16”? And how long should this line be for other parachute sizes?

Finding the correct length is pretty simple with a little dab of trigonometry. Figure 3-48 shows a sketch of one of the six sections of the parachute. The line h is the one that is 6 15/16” long. We want a 12-inch parachute, so the line d is half that—-6 inches. The angle between the lines d and h is 30°, or half of the 60° you marked off to form each of the six sections.

From trigonometry, we know that the length of the line d (the adjacent line in trig-speak) divided by the length of the line h (the hypotenuse in trig-speak) is the cosine of the angle. You can get the value of the cosine of 30° from just about any calculator; it’s about 0.866. All of this makes it easy to find the length of the line h, which is the one we marked to find the points that form the hexagon for the parachute:

Figure 3-48. Parachute trigonometry.

You can use the same formula to find the length of the guide line for any size parachute.

Use a protractor and ruler to form a parachute template from an old piece of wrapping paper or several sheets of printer paper taped together.

Figure 3-49. Use a protractor and ruler to form a parachute template from an old piece of wrapping paper or several sheets of printer paper taped together.

Use the template and a hobby knife to cut one or two layers of your chosen parachute material.

Figure 3-50. Cutting the parachute.

Cut three parachute shroud lines that are twice as long as the parachute’s diameter—24”, in this case—from crochet string. Any string that is thick enough not to get tangled in itself is fine; normal sewing thread is too thin.

Attach the shroud lines to the corners of the parachute with 1/2” square or round pieces of a sticky tape like duck tape. Form a loop under the tape to hold the string firmly in place as shown in Figure 3-51.

Figure 3-51. Fasten the shroud lines to the parachute at the corners.

One shroud line is attached across opposite corners of the parachute, while two others are attached to adjacent corners so they do not cross each other.

Figure 3-52. One shroud line crossed from one corner to the opposite corner. The other two attach to the remaining adjacent corners.

Apply baby powder to the finished parachute to keep the plastic and tape from sticking together when folded in the rocket (Figure 3-53). I like to store my parachutes open on a clothes hanger so they don’t get permanent creases that may impede their snapping open.”)))

Figure 3-53. Apply talcum powder to keep the top side of the tape from sticking when the parachute is folded.

Attaching the Parachute and Shock Cord

You can attach the shock cord and parachute directly to the loop molded into the base of the nose cone, but I prefer attaching them using a snap swivel. The snap swivel attaches the parachute or shock cord to the nose cone. There are two main advantages to using a snap swivel.

First, rockets are made from standardized parts. It’s pretty easy to swap a nose cone on Juno for a payload bay made from a BT-50 body tube. The shock cord has to be untied and retied if it is tied directly to the nose cone, but the change is easy if the shock cord is attached with a snap swivel. The same is true for parachutes. You might want to quickly change parachutes in the field, either due to damage or to change sizes as payload weight changes.

Rockets tend to twist a lot as they descend. A snap swivel can twist, too. The snap swivel generally won’t keep up with the rapid twists of the rocket, but every little bit helps.

Figure 3-54. Snap swivels make it easy to connect and disconnect parachutes and shock cords.

The method for attaching a parachute is the same for both a nose cone and a snap swivel. Poke the shroud lines of the parachute through the opening, then loop the parachute around and pass it through the shroud line loop (Figure 3-55). Tighten the knot to secure the parachute.

Almost any knot will do for tying the shock cord to a snap swivel or nose cone, especially if you place a drop of glue in the middle of the knot. Don’t cover the entire knot, though; just add a small dab where the end pokes through the knot. The glue is pretty rigid and makes a sharp edge, so covering the entire knot can lead to a broken shock cord.

Figure 3-55. Find the center of the shroud lines and loop the parachute through a snap swivel or the loop on a nose cone.

The really classy way to attach a shock cord is to use a buntline hitch. Figure 3-56 shows what this looks like, along with some instructions on tying the knot from Wikipedia.

Figure 3-56. The buntline hitch is a great way to tie the shock cord to a nose cone or snap swivel.

Before You Paint

Your rocket is structurally complete. You can fly it if you like. If you go to a launch where there are lots of rockets, you will probably see several that are not painted. A poor finish does cause a bit more drag, but it’s not that big a deal for a sport rocket. There is only one real problem with not painting the rocket…

It’s ugly.

You’ve spent all of this time building a classy rocket. Take a little more time to make yourself proud!

There are two steps to take before applying the paint. Start with the nose cone. The one I picked out for Juno from the Estes Designer’s Special gave me a pleasant surprise: the nose cone was molded vertically, so there is no flash running the length of the nose cone. There is another nose cone in the kit that does have flash. Take a look at the 6 1/2” nose cone for the B-60; it’s about 1 1/2” in diameter, and there are two ridges running the length of the nose cone. The molding process causes this flash, and it’s the norm, not the exception.

Look at Figure 3-57 carefully—the nose cone shown is a bit big for Juno, but it shows the flash well. You’ll want to remove the flash using a hobby knife. You’re not cutting the flash off; you are dragging the hobby knife backward to scrape it off. If the nose cone is still a bit rough after scraping, sand the entire nose cone with a very fine grit sandpaper. As with the fins, sand by feel, not sight. You will probably still be able to see the flash mark when you finish scraping and sanding, but if you can’t feel it, you’re done.

Figure 3-57. Scrape any flash off of the nose cone, then sand with a very fine grit sandpaper if needed. You’re done when it feels smooth.

The balsa fins will have a very rough surface if they are painted right away. You can paint, sand, and repeat, but there are lots of great products for preparing the wood.

Sanding sealer is the primer of choice if you are using butyrate dope (model airplane paint). Butyrate dope tends to fill and shrink a bit on its own, and does a pretty good job of covering up small grooves left from the spiral winding process used to manufacture body tubes.

First, brush on a coat of sanding sealer. After it dries completely, sand the fins with fine grit sandpaper until they are smooth. You will probably need two or three coats of sanding sealer for a good finish.

Another option is a sandable filling primer, such as Rust-Oleum 2 in 1 Filler & Sandable Primer, which can be sprayed on. Unlike butyrate dope, spray enamel will show up every imperfection in the spiral winding of a body tube. A filler primer also works well on the body tube, filling in these gaps.

Shake the can well before you start to spray. There is a steel ball in the can that will rattle—if you don’t hear it rattling, it’s stuck in the paint. Keep shaking. Shake for at least another minute after the ball starts to rattle.

Spray the primer onto the wood and paper parts of the rocket, as seen in Figure 3-58. You can spray it on the nose cone, but the one used with Juno doesn’t need it. After the primer dries, sand it until you see wood and paper showing through the primer. The intent is not to build up a thick layer of primer; it’s to fill in the low-lying cracks and minor dings in the rocket. Repeat this process until the primer looks smooth when sprayed on, and give that final coat of primer a light sanding to smooth out any rough spots in the primer itself. At this point, you are looking at the finish, not feeling it, to see if it is smooth. Imperfections will show through dramatically.