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Rocket Motors


When I talk with people about the rocketry hobby, the most often asked questions I get are about rocket motors. So I
thought I would add a page on the site dedicated to the topic of propulsion and rocket motors.

Rocket motors, no matter what they use for fuel, operate according to Sir Isaac Newton's 3rd Law of Motion:

For every action, there is an equal and opposite reaction.

The most basic example of this law of motion would be a person standing in a rowboat floating in a lake. Let's say
our boater has a stack of bricks in the boat and he picks one up and throws it as hard as possible off the back of
the boat. What happens ? According to Newton's 3rd law, the boat will travel in the direction oppsite of the direction
in which the brick was thrown. Now imagine our boater repeats the process over and over until the bricks are gone.
The boat has now traveled some distance away form the starting point and will slow to a stop. This is exactly how
a rocket motor pushes a rocket into the sky. Instead of bricks, the motor burns some type of fuel in a confined space
inside the motor. As the fuel is burned it creates high energy rapidly expanding gas. This gas is directed out the aft
end of the motor through a specially shaped opening called a nozzle. The nozzle accelerates the gas into a very high
velocity stream. If the rocket is in launch position, the stream flows down and the opposite reaction pushes the rocket
up until all the fuel is consumed. Solid fuel motors are ignited remotely using electrical current through a special
piece of wire dipped in a pyrogen that burns at very high temperatures when current is passed through the wire.

There are two common types of rocket motors in use today. Solid fueled and liquid fueled. Liquid fueled rocket motors
are very complex and are not commonly used in our hobby. The modern solid fueled motors that we use are commercially
manufactured, very safe and reliable. They are divided into 3 catagories according to the fuel they use.
1. Black powder motors are used in the smaller low power rockets. They use a fuel made from basically gunpowder. These
motors produce thrust in the A through E power range (more about motor classification a little later). They are self
contained, single use motors that contain propellant, tracking smoke and an ejection charge to deploy the parachute.
You can see in this cutaway diagram how they are made.



2. Composite propellant motors are used in mid-power rockets and up. The propellant is made up of a slurry of ammonium
perchlorate, powdered aluminum and a elastomer binder. There are additional chemicals used to control burn rate and
to help the slurry cure properly. The ammonium perchlorate is the oxidizer and the aluminum powder and binder material
serve as the fuel. By weight, this type propellant is roughly 3 times more powerful than the black powder propellant.
These motors are available as either single use or reloadable. I use the reloadable type which means that I purchased
the outer metal casing plus forward and aft closures. These are the most costly parts of an assembled motor but you only
have to buy it once. After that, you buy reload kits which include propellant grains, seals, o-rings and nozzle.
After the flight you throw away the used parts, clean up the metal casing and closures and your're ready for the next
flight. It's much more economical this way. Here is a cutaway diagram of a reloadable motor made by Aerotech.



3. Hybrid Motors are also high power motors. These motors use fuel grains that contain no oxidizer. The oxidizer in
these motors is pressurized, liquid nitrous oxide. It's carried in a seperate tank attached to the front of the fuel
grain casing. At launch, a special launch system ignites the fuel grain and opens a port allowing the N2O to enter
fuel grain. I have seen a few flights using these motors but have never used one myself. The ground support equipment
is complicated and a bit expensive.

Regardless of the fuel used, all rocket motors are identified by a 3 character code. The first character is a letter
and indicates the impulse class of the motor. Impulse is the ammount of total energy contained within the motor and
is measured in Newton-Seconds. The smallest is A and the largest certified motor is currently an O motor. Each letter
is approximately twice as powerful as the preceeding letter.
Click Here to download a chart showing all the classes with the impulse range for each.
The second character is a number that indicates the average thrust of the motor in Newtons (1 Newton is approximately
.225 pounds) and indicates how quickly the motor uses up the total impulse. In general lower numbers have a smaller
average thrust but burn longer. High numbers have a high average thrust but burn out quicker. You can approximate
the burn time of a motor by dividing the impulse by average thrust.
The last number is the delay, in seconds, between the time the motor burns out and the ejection charge fires. At
motor burn out the rocket is travelling very quickly. If the parachute was deployed at these high speeds it would
be destroyed. So there is a delay buit into the motors that allow the rocket to slow way down before parachute
deployment.

Here is an example of one of my favorite motors: the Aerotech G64-7.
G indicates a motor with an impulse range of up to 160Ns. The G64 has an impulse of 112 Ns
64 is the average thrust in Newtons, about 14.4 pounds (64 times .225)
7 indicates that approximately 7 seconds after the motor burns out the ejection charge will fire

There are regulations in place that govern the purchase and use of rocket motors. In the USA we currently have 2
national rocketry organizantions. The National Association of Rocketry (which I belong to) and the Tripoli Rocketry
Association regulate motors above H Impulse. G and below do not require certification however in certain states
you must be 18 years old to purchase and use E, F and G motors. For the high power motors there are 3 levels of
certification.

Level 1 - H and I motors.
Level 2 - J, K and L motors (I am currently certified Level 2)
Level 3 - M, N and O motors

The National Association of Rocketry's website is a great resource to learn more about motors.
Click Here to visit and click on the Model, High Power or Motor Information links