What
is Model Rocketry 
What
is Model Rocketry
Most people these days have probably seen a model rocket kit in a hobby or toy store, but really have no idea what the hobby is about or whether it is even safe or not. So to answer any of the un-asked questions and calm any fears, I thought that I would provide a short history of the hobby and a description of what parts go into a model rocket.
How Model Rocketry Came About
The model rocket, and perhaps more
importantly, the model rocket motor, were both designed in 1954
by Orville Carlisle, a licensed pyrotechnics expert, and his
brother Robert, a model airplane builder. They originally
designed the engine and rocket for Robert to use in lectures on
the principles of rocket powered flight. But then Orville read
articles written in Popular Mechanics by G. Harry Stine about the
safety problems associated with young people trying to make their
own rocket engines.
During the late 1950's, many people, excited about the idea of
space travel, tried to design and build their own flying rockets,
just as decades earlier people thought of designing and building
their own models of the new airplanes that were taking to the
skies. Unfortunately, designing and building a working rocket was
not as simple or safe as building a model airplane. Most tried to
build their models entirely out of metal parts, and mixed
dangerous chemicals to make motors. The results were disastrous.
Most of these rockets blew up like bombs, injuring and killing
their builders and spectators. Some began to call for making the
activity illegal, or at least restricting the availability of the
chemicals used.
Orville realized that his designs could solve these problems and
sent samples of his rockets and motors to Mr. Stine in January
1957. Stine, a range safety officer at White Sands Missile range,
built and flew the models, and then devised a safety code for the
activity based on his experience at the range. That humble
beginning was the start of model rocketry as we know it today.
What Makes a Model Rocket Safe
Model rocketry is, and has been
since it began with Carlisle and Stine, one of the safest hobbies
available for youths and adults. This is due to the Model
Rocketry Safety Code, which specifies the two parts that make a
safe flying model rocket: the pre-manufactured motor and the
light-weight model rocket itself.
The Pre-manufactured Model Rocket Motor
The engine type code printed on the case has three
parts:
A - Total Impulse
The letter part of the code indicates the total impulse (or "push") produced by the engine. Each succeeding letter has twice the total impulse of the previous letter (i.e. a "B" has twice the total impulse of an "A", etc).
8 - Average Impulse
The first number of the code is the engine's average of thrust divided by the burn time in Newton's-seconds (4.45 Newton's=1lb of force)
3 - Delay Time
The second number of the code is the amount of time between the end of the propellant burn and the ejection charge in seconds.
The inside of a model rocket engine looks like this:
Literally hundreds of
millions of model rocket motors have been produced and safely
flown in the last 38 years, a testimony to their safe design and
ease of use.
A Typical Model Rocket
The other half of what makes
model rocketry so safe is the basic design of, and materials used
in the model itself. The following picture shows the parts that
go into a typical model rocket.
NOSE CONE
Usually made of balsa wood or plastic, helps to direct airflow smoothly around rocket.
PAYLOAD SECTION
Area for electronics, cameras, eggs, or any other load allowed by the safety code.
BODY TUBE
Usually made of paper/ plastic or any other crushable material, it's the basic airframe to which all other parts are attached.
RECOVERY SYSTEM
A parachute, streamer or any other device or system that will safely return the model.
LAUNCH LUG
Small tube that fits over a rod to help keep the model stable during the first few feet of flight.
RECOVERY WADDING
Keeps hot ejection gasses from damaging the recovery system.
FINS
Usually made of balsa wood or plastic, and located at the rear of the rocket, fins are what keep a model rocket flying straight up.
ENGINE MOUNT
Holds the engine securely.
Rocket Engine Operation 
The diagram to the right depicts the operation
of a rocket engine during a typical flight. There are four phases
to rocket operation and these are:
Ignition and Lift-Off
Ignition for a rocket engine takes place
when an electric current is passed through a small device called
an igniter. The igniter is a small piece of wire and powder and
is inserted into the nozzle of the rocket engine. When the
electric current is applied, the wire heats up, ignites the
powder and in-turn ignites the propellant in the engine. Once
ignition happens, thrust will commence and the rocket will lift
off.
Thrust Phase
The thrust phase of rocket engine
operation is where combustion of the propellant takes place and
the exhaust gasses are expelled through the nozzle of the engine.
This expulsion of gasses creates the thrust.
Coast Phase
The coast phase of rocket engine
operation is the period where no thrust is produced, only smoke,
and it is the delay time between thrust and ejection. One does
not want the parachute to be ejected until the rocket has slowed
down and reached its maximum altitude.
Ejection
The ejection phase of rocket engine
operation is when a charge is emitted out of the top
of the rocket engine. This creates a positive pressure inside the
rocket body and will force
the cone to part company with the tube, pulling the parachute or
streamer out with it for recovery.