January 14, 2004

Build It - 7

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. You can find the rest of the series here. To learn more about what model rocketry is about, see this Q&A.

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

Parachutes. Technically, these 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. 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 ¼” 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, since you’re removing a part of the canopy. On real chutes, adding a spill hole can actually increase the efficiency of a canopy, which will increase its descent rate.

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 here. 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. :) Where the CP and CG become important is when you design 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.

Posted by Ted at January 14, 2004 06:24 AM
Category: Build It
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