Technical Publication #1 Why Apogee Rocket Motors Make Models Fly
Higher
If two engines made by different manufacturers have the same
total power (called total impulse) shouldn't they both launch
similar rockets to the same altitude? The answer to this question is
Yes and No. It depends on where you launch the rocket.
A model rocket is affected by three forces during its flight.
These are the thrust of the rocket motor, the force of gravity
trying to pull the model back down to the ground, and air resistance
which is called drag. So there are two forces trying to hold the
rocket down, while one force  thrust  must overcome these two
other forces in order for the model to take to the air.
It would seem logical that if the total thrust is the same for
two different rocket engines that the models should travel to the
same altitude (with all other things being equal). But the reason
one rocket may travel higher is because differences in the drag
force.
Aerodynamic drag or simply, drag, is the resistance
or friction force experienced by any object moving through air. All
rockets have a drag force and, although it cannot be eliminated, it
can be reduced. Knowing what conditions or factors cause an increase
in drag allow us to make rocket designs that will minimize this
undesirable force so that the rocket can travel higher.
Factors that affect the total drag force on a model are: the
density of air, the frontal area of the model exposed to the
oncoming air, a unitless factor called the coefficient of
drag, and the velocity of the model.
Mathematically the drag force experienced by a rocket can be
expressed by the following equation:
where:
D="Drag" force on the rocket S ="Frontal" area of the
rocket V ="Velocity" of the rocket = Density of air Cd = Drag
coefficient of the rocket
We should look at each of these factors to find out how we can
make our rockets fly higher, and find out why Apogee model rocket
motors push our rockets to higher altitudes.
Frontal Area
The frontal area is the area you would see if you were looking
directly down on a rocket as it was sitting in a vertical position.
From the drag equation, it can be seen that if this area were
decreased, the drag of the model would be decreased.
It stands to reason that a smaller diameter rocket should fly
higher than one with a larger diameter. This reason alone should
allow Apogee rocket motors to fly higher. Apogee model rocket motors
have a smaller diameter than other rocket engines, so they can be
used in rockets with smaller diameter tubes.
But this is only one reason why Apogee rocket motors push models
higher into the air. What happens when the diameters of the two
rockets being compared are equal? You'll have to read on to find
another reason Apogee rocket motors still have a better chance of
pushing your rockets higher into the air.
Density of Air
The number of air molecules per unit volume is what is termed as
density. In the drag formula, it can be seen that as the density
increases, the total drag force increases. Likewise, when the
density of the air is reduced, the drag force decreases. Can the
density of air be controlled by the modeler? Unfortunately, no. The
only way you can decrease the density of air is to launch from the
top of a tall mountain, or on a hot day (with low humidity).
But, look what happens when the density of air is zero  the
total drag force is also zero, so the rocket thrust is only fighting
against the force of gravity. Where can you find nearly zero air
density? In outer space! Where there is no air, you have no air
density. If you could perform your comparison test of two rockets
from the surface of the moon, then they would both fly to the same
altitude if the rocket motors had the same total impulse.
Since we usually launch our rockets from the surface of the
earth, we'll always be dealing with air density, so we'll always
have a drag force working against us.
Drag Coefficient
The drag coefficient takes into account the shape of the rocket.
It is a unitless number, so it may be hard to understand. This
number is a combination of several factors, which are: form drag,
induced drag, and skin friction drag.
Form drag is due to the shape of the rocket. For example,
a rounded shape has less drag than a blunt shape, because the air
flows over the surface easier. This is the reason why airplanes and
rockets have rounded noses. The act of making the air flow easier
over a rocket is called streamlining. This is why fins should
be sanded into a streamlined or teardrop shape; making them round at
the leading edge, and tapering them to a sharp edge at the trailing
edge.
Skin friction drag is caused by the air particles in the
airstream being slowed down by the microscopic bumps on the surface
of the rocket. This TYPE of drag can be reduced by making the
surface of the model as slick and smooth as possible.
Induced drag is produced by the fins and wings of a rocket
or airplane. Whenever a lift force is produced, induced drag is also
produced. It is a result of air flowing around the tip edge of the
wing or fin from the high pressure side to the low pressure side.
Making the air change direction to flow around the corner takes
energy, and this energy must come from somewhere. The place where it
comes from is the speed of the rocket  the model must slow down by
some small amount. This is drag.
On rockets, the induced drag can be reduced by proper fin shape.
The drawing shows a few fin shapes that have low induced drag.
Induced drag can be eliminated completely if you have no lift. If
the rocket flies completely vertical and doesn't wobble during
flight, there should be no need for the fins to produce a restoring
force (lift) to correct the trajectory of the rocket. Therefore to
make the rocket fly high, keep it stable during its flight.
Since we are comparing similar rockets, we are assuming that the
drag coefficients are identical. So we can ignore this term for our
comparison of different rocket motors.
Velocity of the
Rocket
We have finally got to the factor that allows us to see why a
rocket using an Apogee model rocket motor will fly higher than one
made by other manufacturers.
Many novices think that if you fly very fast, you must be able to
fly higher. But this is not true if you are flying through air.
From the drag equation, you can see the detrimental effect of
having a rocket fly fast  it has more drag. Not only does it have
more drag, but one rocket traveling twice as fast as another doesn't
have twice as much drag, it has four times the drag force. This is
because the velocity term in the equation is squared. The faster you
fly, the more drag you must overcome. So to fly higher, you want to
fly slower.
The velocity of the rocket is controlled by the rocket engine. A
high thrust rocket motor will make the rocket travel at a higher
speed. A low thrust rocket motor will keep the speed of the model
down, therefore keeping the drag lower so the rocket can travel
higher.
How can you determine the level of thrust of the rocket motor?
The simplest way is to read the code printed on the rocket motor
case. The first number (after the letter) tells the average thrust
force of the rocket in Newtons. The smaller this number, the lower
the thrust force.
Apogee model rocket motors are designed to keep the drag on a
rocket low, so that the model can fly higher into the air. They use
the same safe propellants used in model rocketry for the past 35
years, but they burn the propellant in a different way. They burn
the propellant slower, for a longer period of time. This keeps the
total impulse of the motor the same as others, but allows the model
have travel higher into the air.
Can you use Apogee model rocket motors in kits from other
manufacturers? Yes! All that you need to use Apogee model rocket
motors in other manufacturer's kits is an adapter to allow the small
diameter motors to fit into large diameter tubes. Apogee has such an
adapter available that you can order so you can use the high
performance rocket motors in your current models. Just call or write
Apogee Components and order Adapter Kit (#3102). It can be used in
most any kit that has an 18 mm motor mount.
There is one very important factor you must consider when
selecting any rocket motor for your model. That is the liftoff
weight of the rocket. Since Apogee rocket motors allow the rocket to
fly at slower speeds, it is possible to fly too slow. Heavy rockets
need higher thrust levels to reach sufficient speeds where the fins
become effective at controlling the stability of the rocket. If your
model doesn't reach this critical speed, it will become unstable,
which is a very dangerous condition. So always make sure your
rocket's weight is below the maximum liftoff weight for the rocket
motor you are using. These liftoff weights are printed on the
instructions that come with the rocket motors.
You now know why Apogee model rocket motors make your rocket fly
higher than those made by other manufacturers. If you combine a
Apogee rocket motor in a small diameter rocket, the altitudes can be
immense. We believe that every U.S. altitude record (as verified by
the National Association of Rocketry) will be smashed using an
Apogee model rocket motor. We hope that you will be the one to set
the new record!
Apogee customer
comment:
"The only thing better than Apogee's excellent products is
their customer service. You cannot go wrong dealing with
Apogee"  Norman Dziedzic 
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