Building A Model Rocket - part 6

We're ready to play with paint!

If you want to, you can spend a little time with some extra-fine sandpaper to smooth the glue fillets we applied along the fins. Like anything other paint job, prep is 90% of the way to getting a great finish.

First step is the primer. Use Krylon or Rustoleum, grey, white, or ruddy brown, whatever you can find as long as it's sandable.

You'll need to make a simple tool to hold the rocket while spraying paint. The easiest way is to roll up a sheet of newspaper into a tight cone, and then slip it into the motor mount. You can use a couple strips of masking tape to hold it together. Then you hold the 'wand' with one hand while spraying paint with the other.

The key to getting a nice finish using spray paint is to use light coats. By light, I mean you should still be able to see through the paint until after your third coat.

The primer dries quickly, so this doesn't take a long time. You want to spray many light coats instead of one or two heavy ones. Once you've got complete coverage (up to 4 or 5 coats), let it dry - read the can, it's usually less than an hour - and then lightly sand the entire rocket with that extra-fine sandpaper. Let the rocket sit and dry for at least 24 hours.

It's possible to get near-professional results with this method, but you're going to put in plenty of sandpaper time and effort. If you wanted to, you can repeat the entire priming/sanding process two or more times, running a tack cloth over the surface in between each sanding session.

Next comes the color coat. Once again, I tend to stay with Krylon or Rustoleum. Since the Fat Boy has white decals, I'm going to use a dark color so they show up well. If I were going to paint the rocket a lighter color and I'd used a dark gray or brown primer, I'd then spray a couple of light coats of white first, just to lighten up the final finish coats.

The same spray techniques apply with the color coat. Spray multiple light coats to prevent runs and drips. Read the can carefully and follow their directions for drying times. Sometimes you have to wait a minimum of time between coats, or put the next coat on within a certain time frame.

I suppose I should mention that you should always paint outside. Even if it's cold out, you can go out, spray the coat of paint, and then bring the rocket inside until it's time for the next coat of paint.

Once you get the rocket completely painted with the color coat, let it dry. Because of the many coats used (or if you got impatiant, the thick cover coat), the rocket needs to sit for at least 24-48 hours, and longer is better.

Now, specifically for this rocket, here's what I did: I used Rustoleum sandable primer, and Krylon gloss white, gloss banner red and gloss regal blue, all in spray cans.

Step one was applying four light coats of primer, sanding with 320 grit sandpaper between coats. Then I let it sit for several days.

Next up, an all-over coat of blue, applied in 4-5 light coats, followed by one fairly heavy 'finish' coat. I let the rocket sit in the sun for about ten minutes between coats. The nice thing about Krylon is you can recoat anytime.

If I were going to be masking this rocket off to paint different colors, then I would have let it sit for several days for the blue paint to fully dry. Instead, I decided to 'fade' the colors together. Starting at the top, I sprayed several coats of white over the blue, making sure I never went as far down as the joint where the nose cone meets the body tube. I concentrated more paint towards the top of the rocket to completely cover the tip of the nose. The nice thing about this 'fade' technique is that you can just do it by eye and stop when it looks good to you.

I also did a light fade of white in a band near the top of the fins so that the red and blue would contrast better and to brighten up the red a little. Once that had dried a few minutes, I sprayed the red in the same manner as the white. Concentrate the color more towards the ends of the fins and bottom of the rocket to create the 'fade' into blue. I was careful to not completely cover the white band.

The best tool for painting a rocket like this is a 5/8" dowel about 18" long (or you can use a rolled-up newspaper like described above). Slide it up into the motor mount and you have a wand to hold and manipulate the positioning of the rocket while you spray.

Several hours later (I got impatient, you should wait a day or two), I cut out the decal and put it on the side of the rocket. I didn't use the fin decals. I normally don't like the newer self-adhesive kind, but this worked ok.

I also thought about cutting out the "FAT BOY" letters to write something like "OY BATF", but it's been done before, so I stuck with the original.

So that's it! We now have a completed rocket. If you build and fly your own, I'd love to hear about your experiences and the maiden flight!

Building A Model Rocket - part 5

Parachutes.

Technically, what comes with your typical model rocket are parasheets, and real parachutes aren't measured in diameter, but in square inches (or feet) of canopy. Model rockets have been doing things their own way since the beginning, and it works just fine. The Fat Boy has a purple and white chute, which is 18" across. For reference, Estes 12" chutes are orange and white, and 24" chutes are red and white.

Lately, Estes has been including pre-assembled parachutes in its kits. If you have one, all you really need to do is make sure that the knots are tight. If you want to make the parachute better, follow along as I explain the steps to construct one of the Estes chutes, and re-do a couple of simple things.

Lay out the plastic sheet and, using an x-acto knife and metal straightedge, cut it out on the outside lines. It's a hexagonal shape, and the parachute shrouds will be tied into each corner.

At this point I always stick a binder reinforcement onto each corner. These little self-adhesive paper rings are available in the stationery section of most stores, and keep the strings from tearing through the plastic. Alternatively, you can use a small square of duct tape (about 1/4" square). Whatever you use (if you use anything) make sure it lays flat so the strings can't catch on it.

Stretch the string out and fold it back on itself twice. You're going to cut it into three equal lengths. While we're at it, we'll start calling them "shroud lines" too instead of the "strings".

Using a sharp pencil point or thick needle, punch a hole in each corner of the chute, inside the reinforcement ring or tape square. Thread an end of a shroud line through, then tie a double knot and pull it tight. Tie the other end of the shroud line to the corner immediately to either side. Do all three shroud lines in the same way, so that each corner has one line attached and you have three loops of line coming off of the chute.

While it's flat, decide whether you'd like to cut a spill hole. This is a hole in the apex of the canopy that lets the air out from underneath. The reason for it is that without it, a chute will tend to oscillate in the air as the air spills out from the edges of the canopy. If you remove enough, it's also a good way to increase the speed that the rocket comes down beacuse you're removing a part of the canopy. On real chutes, adding a spill hole can actually increase the efficiency of a canopy, which can decrease its descent rate (make it come down slower). Parachutes are subject to the same laws of aerodynamics as rockets, airplanes and birds.

Estes chutes have the optional spill hole already marked. Just use your xacto knife to cut out the dotted lines around the center logo. I do recommend doing this for the Fat Boy, because that 18" chute is awfully big for the weight of the rocket.

If you ever want to make your own model rocket parachute, it's easy to do. Any plastic bag material will work, or you can use the heavier plastic from those rolls of picnic tablecovers. Whatever you use, add some color if needed with permanent markers or hilighter marker because a clear plastic chute will be invisible at altitude.

For shroud lines, you can use heavy carpet thread, braided nylon, dacron or kevlar, or a brand of dental floss called Glide. The Glide is made of teflon and is fire resistant, which is a good thing for our purposes.

To attach the parachute, gather all of the shroud lines and thread them through the plastic loop on the nosecone. Pull the lines through and open them enough to slip the canopy through. Keep tightening the lines by lightly pulling on the canopy until the shroud lines snug up against the nosecone loop.

Alternately, you can attach the chute to a fishing swivel using the same steps. This way, you can move the chute from one rocket to another just by opening the swivel and reattaching it to another nosecone loop. You might need to use needlenose pliers for this. There's a picture of fishing swivels in part 2 of this series. The shroud lines go through the small loop at one end, and the big end opens like a safety pin so you can attach it to the nose cone.

Now a little bit about aerodynamics and what makes these rockets safe to fly. For the Fat Boy kit, it should be perfectly stable as built, assuming you didn't add a bunch of weight at the aft end. Not all kits are naturally stable, so if it comes with a chunk of clay in the kit, you'll need to put it inside the nosecone as the kit instructions direct. In any event, you should at least do a quick check on a completed kit. The following tells how and why.

On standard rockets - fins at one end, nose cone at the other, nothing really odd going on in between - there are two places on the rocket that are critical to stability. First is the Center of Gravity (CG) and it's the point where the rocket weighs the same in either direction, like a fulcrum of a teeter-totter, or perfectly balanced scales. In the exact same way as a teeter-totter, you determine the CG by balancing the rocket on a pencil or some such (I use my finger - it's close enough). The point where it balances is the CG. Put a little pencil mark there.

I talked a little bit about the CG here without naming it (the bit about the hand out the window). The CG is the point that the rocket will rotate around as the fins correct the flight path.

The second place is called the Center of Pressure (CP). This one is a little harder to explain, but just like the Center of Gravity is where all the weight of a rocket balances, the CP is where all the various aerodynamic forces balance. These forces include thrust, drag and gravity, as well as the roll, pitch and yaw of the flying rocket.

To determine the CP, the easiest way is to make a cardboard cutout of the rocket outline, then balance it on something like you did for the CG. The difference here being that the cardboard is only two dimensional. It also represents the rocket flying through the air sideways (90 degree angle of attack), since it's presenting the largest possible cross-section to view. What this does is give the most conservative CP of the airframe. This CP will be farther forward - toward the nose - than any other angle of attack.

Your rocket will be stable if the Center of Gravity (CG) is in front of the Center of Pressure (CP) by at least one diameter of the main body (caliber). So if the CG is twice as far in front of the CP as the body diameter, then the rocket has two calibers of stability.

All this is great for regular rockets, but the Fat Boy is rather short and squatty, so the margin for stability is shortened a bit, and you'll find you probably have around 3/4 of a caliber stability, which is fine for that kit.

To move the CG forward, you can add weight to the front of the rocket, or add length. To move the CP backwards, you can either add length to the rocket, or increase the size of the fins, or the number of fins, or sweep them backwards.

Having the CG too far ahead of the CP is called 'overstable', and can cause the rocket to be overly sensitive to wind gusts. It can behave like a weathervane and cock sharply into a breeze, just like a... uh, weathervane.

One last thing, you should measure the CG when the rocket is prepared to fly - motor, chute and the works, because that's how the rocket will actually fly. Sounds dumb, but it's not. The motor can shift the CG significantly backwards.

A simple test for stability is called the 'swing test'. Find the rocket's CG (remember, ready to fly configuration), and tie a long piece of string around it at that point - use a spot of tape to hold it in place. Then take the string and swing the rocket around your head like you were using a rope lasso. The rocket should settle into place and look like it's flying horizontally around you. Sometimes it will settle in tail first, that's ok. And for certain weird cases, a rocket will tumble as unstable, even though in actual flight it'll be fine. But for 99% of the time, this is a good test, and even scale models of real rockets have been checked this way by engineers in informal testing.

Or you can trust the kit. :) Knowing where the CP and CG are become critical when you design and build your own rockets.

The math to determine the CP isn't that difficult, and was worked out in general form by Jim Barrowman in 1966. Known as the 'Barrowman Equations' (duh - and the link is a .pdf document), they simplify the process by making several assumptions about the rocket and aerodynamic environment. They're still a useful approximation and are still frequently used.

So what kinds of practical use is all this CG and CP hocus-pocus?

Well, for our rockets, we want them to be stable so that they fly straight and safe, especially since model rockets are unguided, and rely on fins to keep it going straight up.

In general, an airplane (real or model), wants the CG and CP to be closer together, so that they're neutrally stable. That way, the plane is easy to steer because the airframe isn't fighting to keep itself pointing in the same direction. A military fighter is going to be closer to unstable, and thus more nimble, than a passenger jet.

Military missiles, especially air-to-air versions like the Sidewinder, are purposely designed to be unstable. They can turn-on-a-dime, figuratively speaking, and the only thing that allows them to fly straight at all is the onboard guidance computer, and controls like fins that rotate, tiny steering rockets along the sides, or thrust deflection. Larger missiles without fins steer by changing the direction that the engine bell is pointing, using the rocket thrust itself to steer.

Next up: Paint!

Cloud Appreciation

If you take the time to really look at them, clouds can be incredibly cool. You might see shapes in the fluffy ones. They can evoke emotion, such as dread in a darkened sky just before a big storm breaks. With experience you can read them to get clues about the coming weather, and scientists create man-made clouds with sounding rockets to study the upper-atmospheric winds.

Here's a website devoted to all the various types of clouds: The Cloud Appreciation Society. Click that link, look at their photo gallery and be prepared to be amazed.

Building A Model Rocket - part 4

Cardboard tubes, balsa, paper and plywood all fall into the category of wood products, and the best adhesive for these materials is wood glue. A well-fit joint will be stronger with wood glue than even epoxy or industrial grade cyano-acrylate (CA) - aka 'superglue'. The technique to make these indestructable glue joins is called the 'double-glue method'.

That 'indestructable' claim isn't exageration either, because the materials being joined will break before the glue bond fails. It's not unheard of to have a fin break just beyond the glue.

So what's this secret way to glue rockets together? It's simple.

1. Lay a light bead of glue along the root of the fin.
2. Put the fin into place so the glue gets onto both parts to be joined. In this case, the fin root and the body tube.
3. Pull the pieces apart. There should be a light coat of glue on both pieces. Let it dry almost completely.
4. Put another bead of glue on the fin root, then press the two pieces together for good.

The reason this works is because the first coat of glue penetrates the materials to be joined, and the second coat chemically bonds with the first, locking eveything together. This makes for an incredibly strong joint.

For the Fat Boy, I suggest just sliding the motor mount assembly into position without glue (the metal motor hook will be sticking out the bottom), then gluing the fins into position. Use plenty of glue on the fin tabs which go into the slots of the body, and less on the parts of the fin root that don't fit into the slots.

It's fairly important to get the fins straight, but vertical alignment is more important that being perfectly spaced or perpendicular to the body tube. Since the Fat Boy has slots that the fins fit into, this is taken care of for you. When gluing the fins into place, take the nose cone off and set the rocket body upside down on your work surface so that you can look down at it and better gauge alignment. Looking from above, all three fins should point to an imaginary spot directly in the middle of the motor mount tube. Wipe excess glue away with your finger, smoothing it into the joint where the fin meets the body tube.



Tip: If you've already bought motors, put one in the motor mount because it's easier to 'aim' the fins at the small nozzle than it is at the imaginary point in the empty tube.

Leave that be and let's assemble the shock cord mount. Hopefully you've taken my suggestion and picked up a package of 1/8" sewing elastic, because the length supplied with the kit is just too short.

In the instructions is a diagram for the standard Estes shock cord mount, sometimes called a 'paper sandwich'. If you're building a different kit then follow whatever directions you've got, or use the following diagram to make one like ours (click it and it gets bigger). At the end, you should have a truncated pyramid shape folded twice with one end of the elastic embedded inside. Here, you should be generous with the glue, yet squeeze it out so that it's as flat as possible.



Let everything dry. Be patient, give it a couple of hours.

For each side of each fin, run a small bead of glue along the edge where the fin meets the body tube. Then take your finger and smooth the glue into the crease. Don't wipe too much glue away, just try to leave a smooth rounded fillet. If you're using brown carpenters glue, the gel formula will keep the glue from running and you can do all the fin fillets at one time. Otherwise, just do one or two at a time and let it dry before moving on to the next. These glue fillets add lots of strength to the fin joint and you should always do them.

We didn't glue in the motor mount before attaching the fins, so lets do that now as well. Apply the glue fillet to the seam where the centering rings meet the body tube, just like you did with the fins. Smooth it with your finger, and since we're using wood glue I recommend putting a second coat on after the first is dry. Do this for both ends of the motor mount. The top fillet is deep inside the body tube, so what you can do is take a long scrap of the balsa that the fins came from, and use that to apply the glue. Don't worry about being perfectly neat, the important thing is getting the joint glued.



The instructions tell you to mark a line between the fins to help you align the launch lug. Instead of that, I usually install the lug in the corner where the fin meets the body tube. This way the lug is automatically lined up vertically (the pre-cut fin slots help), plus it's stronger for the extra surface to glue against.

Finally, it's time to glue the shock cord mount into place. You should have a "paper sandwich" which has the elastic coming out of one end. Use a good bit of glue, and attach the mount to the inside of the top of the body tube, with the elastic pointing up towards the nose cone. Make sure you get it far enough down inside the body so that it doesn't interfere with the shoulder of the nose cone.



You can trim the elastic to a length of about 24" or so before or after gluing the mount into place.

The reason for making the shock cord longer is a phenomenon known as the 'estes dent'. What happens is that during the flight, the nose cone is propelled forward by the ejection charge. If the shock cord is too short, then the nose cone stretches the elastic until it zings right back at the rocket, crunching the top of the body tube. Using a longer shock cord prevents this from happening. A good rule of thumb is to make the shock cord 2-3 times the length of the body tube.

Once the shock cord mount is dry, use more glue to make sure it's firmly glued into place. This part is going to keep your rocket attached to the parachute and nose cone, so use some care here. You also want to make sure it's as flat as possible, so that there's nothing to snag the parachute on it's way out.

When everything is dry, tie the end of the elastic shock cord to the plastic loop of the nose cone. Use a double knot and make sure it's tight.

At this point, the rocket is ready to fly except for the parachute. Next Friday we'll put up the next part talking about the parachute and discuss flight stability a little more. We'll also get ready to paint the rocket.

Building A Model Rocket - part 3

It's time to spread some glue! I know, finally, right?

This is a series of posts where we're building a basic model rocket online. Each post shows part of the process step by step, including pictures and passing along tips and tricks I've learned along the way. To learn more about what model rocketry is about, see this Q&A.

Follow along and when we're done you'll have built and flown your first model rocket. Questions asked from previous posts are answered too, so if you have questions, please leave them in the comments or email me.

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.

While's it's drying, I'll explain 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.

Isaac Newton's third law of motion 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.

Building A Model Rocket - part 2

This is a series of posts where we're building a basic model rocket online. Each post shows part of the process step by step, including pictures and passing along tips and tricks I've learned along the way. To learn more about what model rocketry is about, see this Q&A.

I hope you follow along because when we get done you'll have built and flown your first model rocket. Questions asked from previous posts are answered too, so if you have questions, please leave them in the comments or email me.

The instructions for most model rocket kits are wonderful. Estes has been doing this for years, and their experience shows. Let me stress one point right up front: always, Always, ALWAYS follow their suggestions for glues to use. You can sometimes use an alternate (I have almost 20 different kinds of adhesives for various situations), but their recommended glue will give you the strongest bond.

Looking at the Parts

Almost everything that needs assembly tells you to lay out the parts and make sure you have everything, and also to read through the directions first to understand things. This is a simple kit, so do it if you'd like, but it won't be a problem if you don't. For more complicated kits, I do recommend doing it.

Lets look at the various parts, most of which are obvious. The biggest tube with the slots cut in one end is the body tube. In a simple rocket like this, it's main purpose is to hold all of the important bits in their correct places. The nose cone is straightforward, as are the fins. The parachute is the plastic sheet with the strings attached. So much for the obvious bits.

The tiniest tube (it looks like a drinking straw) is the launch lug and it's used to steady the rocket on the launch rod. The length of elastic is the shock cord, remember I recommended replacing it with a longer piece bought at the store. The two cardboard disks, the medium sized tube and black ring will be put together with that little metal strip and become the motor mount.

Pre-Assembly

Using your x-acto knife, carefully cut the fins out of the balsa sheet. They're die cut and held in by just a few short bits of wood. If you want to, you can gently sand the fins (with the grain) with the fine sandpaper before freeing them. Make the same kinds of cuts to remove the smaller middle circles from the cardboard disks. Finally, you may need to open the inside of the squared-off loop at the bottom of the nose cone (see the picture). Do all of these carefully, and watch for the sharp knife.



Make sure the fins fit the slots in the body tube. Sand them lightly if needed to ensure a smooth fit.

The following steps are completely optional.

Using the sandpaper, sand the seam on the plastic nose cone until it disappears. This isn't a quick process, but it does make for a much nicer looking end result.

Lightly sand the entire body tube until you've scuffed the shine off. Don't sand too much, the purpose here is to remove the glassine layer, which will make for a stronger bond between the glue and the paper tube underneath.

Take some of the Fill'n'Finish and thin it with water until it's about the consistency of pancake batter. Slather it on the body tube (I use my finger) and work it into the spiral groove. You won't need much, and most of what you use will be sanded away. Let it dry (it's pretty quick) and then lightly sand. The Fill'n'Finish sands easily, and when you get done there should be no spiral groove left. Repeat if you need to.

Use the same thinned Fill'n'Finish to fill the grain of the balsa fins. Keep the coats very very light, and sand between coats when dry. When you do the fins, do both sides at once, because the balsa will warp slightly and this will help even it out. The warp will straighten out when both sides are dry.

The reason for all this filling and sanding is because the smoother the surface, the less drag which makes for a higher flying rocket. I don't do it for every rocket, but I do take the time for most of them. The paint job looks much nicer on the smooth finished surface too.

Questions Answered (from previous posts)

What is a "fishing swivel?" Also known as snap swivels, they're used to prevent the fishing line from twisting. They have a small loop on one end and a large loop on the other end that opens like an old fashioned safety pin. Here's a (not great) picture of a few, and like I said, you'll only need one, and it's optional. Also in the picture you can see the package of sewing elastic, the glue and Fill'n'Finish, and an x-acto knife.



This is our rocket so far, after sanding the nosecone seams smooth and filling the spiral grooves on the body tube. Total sanding time was maybe 20 minutes. If you did those steps, you'll notice that the tube is a little fuzzy. Don't worry about that, because we'll smooth it out when we spray primer. I also had to spend a few minutes sanding the tabs on the fins so that they would slide easily into the slots. Nothing is glued together yet.



You may have noticed that the fin tabs have a small slice trimmed out at the bottom. This shallow notch fits over the black ring of the motor mount. This particular rocket boasts a nice bit of engineering because everything fits together and reinforces itself, making for robust construction. In fact, although we'll be flying this bird stock on B and C motors, I've seen the same kit strengthened and modified to fly on I motors (128 times more powerful)!

Now, I'd like to talk about what actually happens during the flight, and some of the basic aerodynamics involved.

Model rockets are set up on a 'launch rod', which ensures that the rocket stays straight until the rocket is moving fast enough for the fins to keep it stable. A good way to picture how the fins work is to compare it to a weathervane, and how it always points into the wind. When a rocket is moving through the air, its flight through the air provides the 'wind' that the fins work with.

Everyone has stuck their hand outside a car window at speed and felt the rush of air. When you keep your hand flat to the ground, the air moves smoothly past it, but if you try to cup your hand against the wind, then the wind pushes against it. The fins work in the exact same way, and it's this push that causes the rocket to stay straight.

The main effect of this is that the straighter the flight, the less drag the rocket has to overcome and the higher it will go. I'll go into other aspects of this as we go.

Next time, we start gluing things together!

And as always, leave questions in the comments. Thanks!

Building A Model Rocket - part 1

We're going to start a series of posts where I'll build a basic model rocket kit online. The idea is to show the process step by step, including pictures and passing along tips and tricks that I've learned along the way, while you follow along and build your own rocket.

Please feel free to ask questions as we progress, and hopefully we'll see some pictures as folks build and launch their own rockets.

Introduction

The rocket we're going to build is the Estes Fat Boy (see below). You used to be able to get this rocket in the toy section of WalMart, look for model rocket kits near the car model kits. You can also find it in some craft stores and hobby shops, sometimes in other packaging like a bag or box instead of the plastic bubble-package pictured, but it will probably cost a little more. If you'd like to build along and can't find the Fat Boy, you can get something similar because the basic steps will be the same. The Baby Bertha or Alpha would also be good choices, although most any Estes rocket kit will do.



Materials

Now is also the time to gather building materials. If I mention a specific brand, it's because I've used it and know it works. There are all kinds of products out there that'll work just as well.

You're going to need an x-acto knife (or equivalent, you could get by with a single-edge razor blade). You'll also need some yellow or white glue. I recommend Elmer's carpenters glue, if you get the exterior stuff it's gelled and doesn't run and drip nearly as much (it's also brownish). You can also use Eileen's Tacky Glue, TiteBond, white school glue, or anything similar.

The only other must-have will be a pencil.

The following things aren't strictly necessary, but if you use any or all of them you'll have a nicer looking and better flying rocket. They're completely optional, and I'll note when to use them if you want to.

I highly recommend that you get a pack of sewing elastic. You want to get the flat 1/8" wide stuff, and it'll probably be 3 yards long. WalMart sells it for about a dollar, back in the sewing department.

Super-fine sandpaper, at least 220 grit (the higher the number, the finer the grit). You can find this in the hardware department in sheets, or small pads of it in the craft section. WalMart sells an assortment from 3M called 'wet or dry' sanding pack that contains two sheets of 220, two of 320 and a sheet of 400 grit.

Elmer's Fill'n'Finish. Also found in the hardware department, get the smallest tub of this. If they have more than one kind with similar names, hold a tub of each in either hand and pick the lightest weight one. We'll use this to fill the grain in the balsa wood fins and the spirals in the rocket body. You could use a lightweight spackle too.

Fishing swivel. This makes attaching the parachute easier. Don't buy a package of these, but use one if you can borrow it or already have one in your tackle box.

You can wait to get primer and paint, and I'll talk more about it later. Here's a little about it up front though.

Spray primer. I use Rustoleum sandable primer, it comes in white, gray, or even black. Get whatever they have.

Spray paint. Rustoleum or Krylon is what I use. Get whatever colors you want to use. The little cans of Testors paint near the models are cool colors, but very expensive for their size.

Masking tape. You'll need a roll of 1/2" tape if you want to paint your rocket with more than one color.

Next up, we'll take a look at the various parts of the kit and do some pre-assembly work.

The Space Race

Fifty years ago today, the Space Race was won, but the Soviet Union. The USSR launched Sputnik, a small, simple satellite whose continuous beeping announced to the world their victory.





Three and a half years later, on 12 April 1961, the USSR would take the next step into space, when Senior Lieutenant Yuri Gagarin travelled into space aboard Vostok 1 (while in orbit, he was promoted to the rank of Major).




(Please ignore the obnoxious soundtrack in this video.)

The United States would put Alan Shepard into space less than a month later.

Bernard Smith: An Amazing Life

Bernard Smith was born in 1910 in New York City. By age 22, he had dropped out of high school, working odd jobs and spending most of his time at the libraries and museums of the city.

One night, he attended a meeting of the American Rocket Society and while looking over one of their failed rockets, made some suggestions on how to improve the device. The president of the Society, Edward Pendray, handed him the pieces of the rocket and invited him to create the next version.

According to Smith, his motivation was simple. America was in the midst of the Great Depression, and "It was a lousy planet. The rocket ship was the only way to get off it."

After making major modifications to the original rocket design, including several weight-saving changes, the new rocket was ready to fly in early 1933. It wasn't entirely successful, but proved that the basic concepts were sound.

Bernard Smith never did graduate from high school, but he did earn a degree in Physics. In fact, after World War II he started his career working for the US Navy, eventually heading up the Weapons Development Department at the Naval Ordinance Test Station. Among the projects that he worked on or managed are the ASROC, Sidewinder and Shrike. He also spearheaded Project Pilot, which placed at least one small satellite into orbit via air-launched rockets in 1958.

So that's the "rocket" side of Bernard Smith. But there's more, for there's the "sailboat" side of Bernard Smith.

Smith also spent over 40 years pursuing his dream of creating the perfect sailboat. He tried many unconventional designs and met some notable successes. In fact, one of his early efforts, the Aerohydrofoil, could make 20 knots in a 12 knot wind. He also designed craft he called Monomarans, Fliptackers, and experimented with a concept called the Sailloon, which was a gas-filled sail that would help provide lift, and thus speed, to a sailboat.

Follow that link above for a fascinating look at the creations of a mind who saw radically different, and sometimes better, ways to accomplish his goals, both through the air and on the water.

How High Is That?

If you want to figure out how tall something is (like a house or tree), or how high it goes (like a rocket or balloon), there’s a simple and inexpensive way to get a fair estimate.

You’ll need a couple things for this, but they’re easy to find and you probably already have them around the house. Find a protractor and a rectangular piece of cardboard bigger than the protractor. You’ll also need a push pin, some string and a weight of some kind (I used a fishing sinker).

First, let’s make a simple theodolite, which is a tool used to measure vertical angles. Take the cardboard and using the push pin, fasten the protractor to it so that the flat edge of the protractor runs along the top of the cardboard. Tie the weight to one end of the string and the other end to the pin. This way, when you tilt the cardboard you can read the angle by seeing where the string hangs past the protractor.

The other thing you’ll need is a tangent table, which can be found in any trigonometry textbook. That’s right, you’re using trigonometry for this! Use the one below, or find one to your liking, they're all the same (click it and it gets bigger).

Still with me? Good! Believe me, this is simple. In fact, this explanation takes longer than the process. The figure below shows the basic concept of determining height or altitude:

Take the theodolite and stand a known distance from what you’re trying to measure. In the diagram, it's where the black and blue lines meet. This distance is the baseline, and the farther the better (as long as you can see the top of the thing you’re measuring). For instance, say you’re going to measure the altitude of a model rocket, and you’re launching from a football field. The tracker is on one goal line, 300 feet (100 yards) away from the launch pad on the other goal line. When the rocket launches, the tracker follows the rocket with the theodolite until the rocket reaches apogee (it's highest point). The angle is read (where the string marks it on the protractor), and this angle is written down.

Time for some simple math. The formula is on the diagram. Look up the tangent for the angle on the table, multiply that number by the baseline, and that is the altitude in feet. Simple!!!
An example: baseline is 300 feet and your measured angle is 40 degrees. The tangent for 40 degrees is .839, so 300 * .839 = 251.7 feet.

This technique works great for things that stand still or go straight up, but the measurement will be off if there’s any horizontal movement. Using our model rocket example again, if the rocket curves towards you on the way up, then your measured angle will increase and the calculated altitude will be too high. One way to compensate for this is to have two people with theodolites standing at 90 degrees from each other (imagine a rocket launching from home plate on a baseball diamond and trackers standing on first and third bases). You can average their measurements and get a pretty good estimate of the correct height.

You can also make a sturdier theodolite by replacing the cardboard with a length of wood or broomstick. Screw the protractor into the side of the wood, hold the theodolite like a rifle and sight along the length of it to get your measurement. You can drive a couple of finishing nails into the top of the wood to help with your sighting if you want, but it's not strictly necessary.

So, how high is the tallest tree in your neighborhood?

Liftoff!

Liftoff!

For those of you that have read Ted's Model Rocketry Q & A for Beginners post, this is what you'll see when you launch your very own rocket.

Within moments of engine ignition, a model rocket is moving at a speed of several hundred miles an hour...so you'd better have a quick trigger finger! Not even a shutter speed of 1/1000th of a second can completely "freeze" the motion of the rocket as it surges toward the sky.

The Most Incredible and Fantastic Thing in Human History

Thirty-eight years ago today, for the first time in history, a human being walked on the surface of another world.

Model Rocketry Q & A for Beginners

Several years ago I put together a web site devoted strictly to model rocketry. One of the most popular pages was an introduction set up in question and answer format. Looking back on it, I can see that we've come a long way since those early days. I've copied that page below and added links where I could.

Q: Why do you think rockets are such a great thing to do with your kids?
A: If I sit down to play video games with the kids, or we watch sports together, or read in the same room, we might be spending time together, but it's not necessarily 'together' time. Model Rocketry is more interactive for us, there is a give and take, and an exchange of ideas. It's not just spending time together, it's spending time with each other.
My kids have picked up some very good habits from rocketry; setting goals, planning, following directions, working together, teamwork, and keeping records.
They've also felt satisfaction. Imagine the look on 9 year old Rachaels' face as her rocket, designed, built, and launched all on her own, roared off the pad for a perfect flight. As it drifted down on its pink streamers, everyone was cheering and congratulating her. I don't know who was more proud at that moment, her or me.
And they've learned how to deal with the minor tragedies of life. The lost rockets, and the ones dinged when the parachute didn't deploy (because Dad forgot the baby powder).
Flying rockets teaches about science too. You'll see practical demonstrations of aerodynamics, physics, chemistry, and more. The kids become engineers, meteorologists, photographers, and journalists, without any pain, and possibly without even realizing it.
One thing we've discovered about rocketry is that the only way to get bored with it is to quit dreaming. We've yet to launch anything bigger than a 'C' motor [we have now], but that's ok. We've still got clustered rockets to try, and staged rockets, and 'gap' staging. We haven't done near enough glider or helicopter recovery. How about night launches, how can we make these smaller rockets visible in the dark?
My kids have a million ideas, to go along with my one or two.
I probably should also mention that model rockets are fun.

Q: Isn't model rocketry like launching fireworks?
A: There are some basic differences between rocketry and fireworks.
To start with, model rockets are never launched by lighting a fuse. The ignition is electrical, with the power supplied by batteries. This lets you stand back a ways from the rocket when it is launched. Much safer.
A second difference is that model rockets are designed to be recovered. This means that you can reuse a rocket over and over. There are various ways of recovering a rocket, such as parachutes, streamers, gliding, and more (there's more about recovery later).
Another difference is the use of a launch rod. This is simply a guide for the rocket to follow for it's first few feet of flight, keeping it straight up until it's going fast enough to be stable on its own. Once again, it's a safety thing.

Q: Is this really safe enough for kids?
A: Model rocketry is an amazingly safe hobby, provided you follow the Safety Code. When you read it over, you'll find most of it is just common sense. Over the years, there have been literally millions of rocket engines fired safely.
As for kids doing rockets, if you insist on following the safety code, and have adult supervision, it's almost impossible to get hurt. Explain that each and every one of them is responsible for safety when launching rockets.
I have normal kids, they get into their share of mischief. But when we launch, they know what is expected of them, because it's been that way since day one. A brief example that really happened:

My youngest, Rachael, was doing the countdown. When she got to '3', her brother TJ yelled 'STOP' from where he was standing (about 100 feet away). Rachael immediately pulled the safety key and put the launch controller down. Then we saw a mom chasing a toddler, who was running full steam towards the rocket.
After mom corraled her child (he never even got within 20' of the rocket), we made sure the area was clear again, and started the countdown over. It was a perfect launch.

Some rules we use:

The countdown is LOUD.
ANYONE can stop a countdown at any time, for any reason.
When someone yells 'stop', that's it. No exceptions.
The only time the safety key is in the launch controller is during a countdown.
We don't resume a countdown from where it stopped. We start over.
Before a countdown starts, everyone has to give an 'OK', meaning they're in position, ready, and the area is clear.

We have never had anyone hurt, or been even remotely close to having an accident. It's not luck, it's designed to be that way. And by the way, that mom and child stayed and watched us for about an hour that day, and still stop by occasionally when we are launching a few.

Q: What's the easiest way to get started?
A: I'd suggest an Estes Starter Set. They start around $20.00, and you can get them at stores like Wal-Mart, K-Mart, Toys-R-Us, hobby shops, and even some craft stores like Michaels or MJDesigns. The starter set includes almost everything you need, except batteries and glue. There are even some 'Ready To Fly' starter sets out now, where the rocket is pre-built for you. Other sets have a variety of rockets (1 or 2) that you have to build yourself. Rockets like the Alpha 3 and Sabre goes together quick and easy. Other sets have 1 simple rocket, plus another that takes a bit more skill to assemble. Another company, Quest, also makes starter sets, but I've never seen one. I hear they're pretty much the same.

Q: Launch controller, recovery wadding... What's all this stuff really do?
A: I'm going to assume that you are looking at a starter set, and I'll just run down the assembled parts.

* Launch pad - Usually has 3 or 4 legs, with a blast deflector and launch rod sticking up from it.
The launch pad holds the launch rod and blast deflector. The wide legs keep it from tipping over in a breeze, and you can adjust the pad to tip the rod a few degrees for launching into the wind. The launch rod is what guides the rocket until it's moving fast enough for the fins to keep it stable. In the starter sets, the launch rod is usually sectional, always use both pieces. The blast deflector keeps the engine exhaust from hitting the pad and ground. Safety again. There is also a rod cap included. Put it over the tip of the upright launch rod, and it helps prevent injuries where someone leans over the top of the rod while preparing a rocket for launch. Make sure you remove the rod cap just before the countdown, and replace it immediately after.

* Launch controller - This is where the batteries go, usually 4 AA size. It has a continuity light or buzzer that tells you when the rocket is set up properly for launch and the safety key is inserted. The safety key must be inserted before pushing the 'fire' button has any effect. In other words, keep the safety key with you when you work around the rocket, and no one can accidently launch it when someone could get hurt. Coming out of the launch controller is a long wire (about 15 feet) that ends in two small microclips. These clips connect to the ignitor, explained below. When you launch, the length of the wire makes it easy to stand back at a safe distance.

* Rocket - A simple rocket is 3 or 4 fins and a nose cone. These are connected to each other by the body tube. On the side of the body tube is the launch lug, a small tube or loop which is slipped over the launch rod prior to igniting the engine. Connecting the nosecone to the body tube is the shock cord. This keeps the pieces of the rocket together as it comes down. Inside the rocket is the recovery system, often a parachute (there is a whole section on recovery later on). The recovery wadding protects the parachute from the ejection charge, which is what deploys the recovery system. Finally, at the bottom of the rocket is the motor mount. This is the place where the engine goes, and it transfers the thrust of the engine to the rocket itself.

* Engine - The 'whoosh generator', also called a motor. This small cardboard cylinder is actually quite complex in design and function. That doesn't mean it's complicated to use. First turn the engine upright so the small hole is facing up. That's the nozzle, the business end of the engine. The ignitor is a small U or V shaped piece of wire. Drop the point of the ignitor into the nozzle, and gently make sure it goes in as deep as possible. There will be two wires sticking out of the nozzle quite a bit. Next take an ignitor plug (color coded, check the directions in the set), and gently push it into the nozzle. This holds the ignitor where it needs to be to fire the engine. Insert the engine into the rocket motor mount and you're almost ready to go!
When ready to launch, connect the controller clips to the ignitor. After everyone is away from the rocket, insert the safety key, and the light should light (or buzzer buzz, depending on your controller). This means that the rocket will be launched when you push the button.

Q: What do the motor numbers and letters mean?
A: This is an easy code to provide complex information. Here's the bare minimum needed to start with.

A sample engine code might be: B6-4

The 'power' range of an engine is indicated by the letter, in this case a 'B'. The codes start with 'A' and keep right on going up the alphabet. So B is twice as powerful as A, C is twice as powerful as B (and 4 times more powerful than A), and so on. This is overly simplified, but you'll absorb the details as you gain experience.
Bigger engines (higher letters) achieve higher altitudes, or lift heavier rockets.

The '6' is the average thrust of the engine. It's measured in 'newtons', but don't worry about it for now. Just keep in mind that a '6' has a higher average thrust than a '4'.

The '-4' is the delay, measured in seconds. This means that 4 seconds (more or less) after the propellant burnout, the ejection charge fires. That deploys your recovery system.

There are '-0' engines. These are booster engines designed for multi-staged rockets. As soon as burnout occurs, the ejection charge fires to ignite the next engine. Don't use these on a single stage rocket. '-P' engines are plugged, and have no ejection charge. They're made for gliders.
Some Estes engines have a 'T' listed after the delay time. This means it's a mini-motor, and has a smaller diameter casing.

Q: Where can I launch a rocket?
A: This is going to depend on your local laws. In some places you will only be allowed to launch with a rocketry club who have already gotten permission. Besides flying with our local club, we launch at the local middle school (Jr. high) field. This is a football field, a baseball diamond, and 2 soccer fields, all bent around an L shape. The bigger the field, the better your chances of recovering the rocket. We've had a few rockets drift away on the wind into, or over the trees. Be aware that it can be calm on the ground, and fairly windy a couple hundred feet up! Because of the L-shape of our regular launch field, we limit ourselves to A and B engines on most rockets. We've got a few heavier birds that fly normally on C's, and on one spectacularly calm day, we launched our little rockets on C's. Straight up well over 1000', and recovered on a parachute less than 30 yards away. For more information, read about rocket clubs below.

Q: How do the recovery systems work?
A: You spend time to get your rocket looking good, and to fly well. You hate to lose them! Recovery is one thing that keeps this hobby from being glorified fireworks (I'm not knocking fireworks hobbyists). There are many ways to recover a rocket. Here's the most common:

Featherweight - for the lightest rockets. The have such a high surface area compared to weight that they 'float' to the ground, like the name says.

Tumble - for very light rockets that are too stable for featherweight recovery. Usually the nose cone is ejected (it's all connected by the shock cord, remember), and the whole thing comes down. If something wasn't done to ruin the stability, it might come down like a dart. At best, hitting the ground like that could damage or destroy the rocket. At worst, it could hit and hurt someone. There are terms for rockets that accidently come down hard, they're called Prangs or Lawn-Darts. No fun, and very hard on the rocket.

Streamer - this is a long, thin piece of plastic or crepe paper. It creates enough drag to bring the rocket down gently. These are good for days when the wind causes too much drift in a parachute.

Parachute - these range in size from 8" up to 24" for model rockets. To minimize drift, you can cut a spill hole in the center of the canopy. This will help the rocket come down faster, but it hits harder when it reaches the ground. If you cut a spill hole, cut it large because too small a hole can actually increase the lift the parachute generates as it descends. Estes parachutes have a spill hole marked with dotted lines, just cut it out if needed. Another technique to minimize time in the air is to 'reef' the shroud lines. Take a piece of masking tape and wrap it around all the parachute lines about halfway between the rocket and the canopy. This prevents the chute from opening fully.

Glider - It goes up like a rocket, and comes down like a glider airplane. Really cool.

Helicopter - Ever see a maple seed fall? Spinning on one wing is one method of helicopter recovery. Another is to have rotors deploy at ejection, causing the whole rocket to rotate.

Q: What about rocket clubs?
A: The National Association of Rocketry (NAR) is America's model rocket organization. Their site can be reached from my links page, and from there you can find a local chapter near you. Flying with a club is a great way to learn from others' experience. The NAR also offers insurance for rocketry activities. Sometimes the deciding factor on whether you can fly in some areas (a public park, for instance) is whether or not you have this insurance. On top of that, you receive the NAR rocketry magazine, full of useful tips, plans, and articles. NAR also offers it's Technical Services division, called NARTS. This is where you can get anything from rocket designs to wind tunnel plans to baseball caps. Check out their site, it's worth it!
Another organization devoted strictly to high power rocketry (HPR) is the Tripoli Rocket Association (TRA). Since this is Q&A for beginners, I'll mention that they're there, and not go into HPR. You can find a link to TRA from Rocketry Online.

Q: Can you recommend a book or something to learn more?
A: Some very good books:

The Handbook of Model Rocketry by G. Harry Stine.
Model Rocket Design and Construction by Tim Van Milligan.
The Art of Scale Rocketry by Peter Alway. [out of print]

At least the first two can be found in your local library, NARTS also offers these books and more for sale. See my links page for Saturn Press, they have the entire collection of Peter Always' rocket books. There's also a link to Apogee Components, where you can find Tim Van Milligan's books. Apogee has a complete line of educational rocketry publications, including 69 Science Fair Projects with Model Rockets: Aeronautics.
The Rocketry Online webpage has all kinds of links to good sites on the web related to rocketry. See my links page for a link to them.
The Rec.Models.Rockets (RMR) newsgroup is a vast source of experience. I've always found the folks there to be willing to answer questions without talking down at you. A great group of people.
The RMR FAQ (frequently asked questions) file will answer many questions you may have. I keep a copy of this handy by my workbench, because it's that useful.

Q: Couldn't I save money by making my own rocket engines?
A: No. When you factor in the cost of the chemicals, equipment you'd need, and materials, the store bought motors are actually a pretty good deal. Also consider that a home-made motor is more likely to malfunction, which could destroy your rocket or, worse yet, hurt someone. The commercial motors are reliable and consistant performers, and you'd have to make literally hundreds of motors yourself to even come close to that kind of reliability.
Now let's talk about safety. It's dangerous to deal with some of these chemicals unless you know what you are doing. Even among experienced rocketeers, there is a surprising amount of 'lore' and common knowledge that is just plain wrong. It's not safe to try to make your own motors, please don't do it.
If you absolutely have to make homemade motors, check out the RMR FAQ (links page) where there is information about a course in making rocket motors. The Rec.Pyrotechnics newsgroup has folks that can help too.
Simply put, Model Rocketry means using commercially available motors. To save money on these, you can mail order them (or order from companies on the internet), or buy them in bulk packs at your local store.

Q: I remember these cool rockets I saw as a kid. Are the old companies still around?
A: Estes is still with us. They absorbed Centuri a while back, and once in a while release an old Centuri design. There are many small companies producing quality rocket kits today, check the Rocketry Online website for links.

Q: I can't believe that white glue is strong enough for rockets. Shouldn't I use model glue or epoxy?
A: For gluing plastic to plastic, model airplane glue is best. There are some times and places where epoxy is handy. But for Estes kits, white or yellow glue is king (yellow is superior). A bond you make between the cardboard body and the balsa or cardstock fin will be so strong that the tube itself will tear before the glue joint breaks. Two secrets to getting even stronger joints; lightly sand the body tube to remove the glasine coating (the glossy stuff), and use the double glue method. The way to double glue is to apply a small amount to the pieces to be joined and press them together. Pull them back apart, and let the glue dry for a few minutes. Apply a little more glue, then join like normal. This technique results in super strong bonds that will easily handle A-D engines. I've heard of rockets built with just yellow glue that fly on G motors. [I've flown H motors this way.]

Q: It goes up, it comes down. What's next?
A: If you look at rocketry webpages out there, you will find a hundred people experiencing rocketry in a hundred ways. I mentioned in passing cluster rockets, staging, scratchbuilding, high power rocketry, scale modeling, gliders, and more. I didn't mention payloads, or contests, or arial photography, or altitude records, or... The list just goes on and on, and you can decide what suits you best.

Do it safely, and have fun!