have flown several rockets built from off-the-shelf kits,
you may decide to start building your own scratch built
rockets. Designing and building your own rocket can not only
be a learning experience, but it can be fun and challenging.
There are three major things to consider when designing your
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.
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.
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.
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 (1ž4A
& 1ž2A), 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.
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.
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.
rockets look similar because that basic shape is the most
efficient. Long and sleek 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
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.
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
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
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.