Once you've built a few model rockets, you'll want to design your own rocket and you should definitely go ahead and do it. One of the best things about model rocketry is that you can experiment with rocket designs cheaply and easily.

There are three major things to consider when designing your own rocket:

• motors The motors you want to use with the rocket will to some extent dictate the weight of the rocket and the materials to be used.
• stability Rockets will not necessarily fly straight just because they have fins. To insure that your rocket will fly correctly, you need to verify its stability.
• materials Hobby rockets are built from light and strong materials. Paper tubing, plastic and wood are used for model rockets and mid-power rockets. High-power and amateur rockets generally used composite materials and occasionally aluminum.

Motors affect the design of the rocket in two ways: their physical size dictates some elements of construction and their average thrust dictates the weight limit they can lift. Pre-manufactured rocket motors come in several standard diameters: 13mm (¼A & ½A), 18mm (A-C), 24mm (D-F), 29mm (E-G), 38mm (H-J), 54mm (I-K), 76mm (K-M) and 98mm (L-O). Motor lengths vary quite a bit, depending on how much propellant they contain. A through D engines are typically 3" long, and larger ones can range up to 8 feet.

Motors are categorized in many ways, but the two most important ones are total impulse and average thrust. Total impulse refers to total amount of productive energy the rocket develops during its burn. Average thrust refers to the average amount of "push" the motor has. Total impulse defines the motors letter category (see the Motors section) and is most useful for determining the altitude the rocket will reach. Average impulse determines how much rocket the motor can lift. The rule of thumb is that the motor can left a rocket which weighs less than 5 times its average thrust.

A rocket is stable if it will fly straight up when launched. Stability is affected by many things: rocket design, motor selection, the launcher, and even the weather! If you follow the rule of thumb described above when selecting your motor, fly when there is little or no wind and use an appropriate launcher, the only remaining major factor is rocket design.

Most model rockets look similar because that basic shape is the most efficient. Long and sleep with medium-sized fins at the aft end is the shape allows the rocket to be stable without special arrangements. (Commercial and military rockets use "active guidance" systems which control the flight in real time through onboard sensors, computers and adjustable fins or motor nozzles.) You can actually make fairly unusual shapes fly, as long as you understand the forces which act on a rocket in flight.

For a basic understanding of stability, you need to understand the centers of gravity and pressure. The center of gravity (C.G.) is the point along the length of the rocket at which it balances. The finished rocket, with the chosen motor installed, is balanced like a see-saw. The balance point is the C.G. The significance of the C.G. is that this is the point that the rocket will tend to rotate around. A tube with no fins would spin around this point instead of flying in a straight line.

The center of pressure (C.P.) is the point, also along the length, at which the aerodynamic forces are equal. Imagine a cross-section of your rocket along the length; most of the body is a thin tube and at the aft end, the fins stick out. The C.P. is the point along this length at which the area of the cross section is the same forward and aft. In fact, this method (called the "cardboard cutout method") is a simple way to determine the C.P. The cross section cutout of the rocket is balanced, with the weight of the cutout representing the surface area, and the balance point determines the C.P. The more common way to calculate the C.P. is through software. The best all-around program for rocket design is Apogee's RockSim (also see the ThrustCurve.org simulators page for more references).

Now you know the C.G. and C.P. of your rocket, you can determine whether it will be stable. The rule of thumb is: the rocket is stable if the C.G. is one to two calibers forward of the C.P. Why is this? Remember when we talked about the rocket having a tendency to rotate around the C.G.? Well, for the fins to be effective, the C.P. must be significantly behind the C.G or the affect of wind pressure will not be enough to overcome the tendency to rotate. (Fins work by using the air rushing past them to correct the rocket's tendency to rotate.) The C.P. can be changed by changing the size of the fins and the C.G. can be changed by adding weight forward or aft

Materials and building techniques are also an important consideration. Model rocket materials (paper tubes, plastic or balsa fins and glue) are not appropriate for more powerful rockets. By the same token, high-power materials are not appropriate for model rockets because they weigh too much for model rocket engines to lift. Steel is not used for airframes or fins because it's too heavy for its strength. Large high-power and amateur rockets generally use composite materials like fiberglass. Follow the same techniques you find in kits of rockets using the same motors you plan to use in your designs.