The following is some of what I wrote for classroom use and then adapted for use
original eHobbies website (when they had "Rocketry 101". It's copyrighted by me,
usable by anyone who is using it for educational purposes and not using it for
profit, re-publication for profit.
How the Model Rocket Motor Works:
Model Rocket Motors are real miniature solid propellant rocket motors that are
commercially made and tested for reliability. The consumer does no mixing of
propellant and oxidizer. Model Rocket Motors contain the following parts: a
hold everything inside, a "nozzle" to direct the flow of gasses produced,
(fuel pre-mixed with oxidizer and pressed into a solid "grain") which burns to
huge amounts of gas, a "delay" which burns slowly after the propellant is used
up to allow
the rocket to coast upward and an "ejection charge" to activate the recovery
ejection charge blows the nose cone off of the top of the body tube in a typical
rocket and ejects a recovery system such as a parachute or long streamer. The
is connected to the body tube with a shock absorbing cord (the "shock cord").
recovery wadding is installed for each flight to protect the recovery system from
They are ignited from a safe distance using electrical igniters and a remote
launch controller with a removable safety key. The igniter heats up when enough
electricity passes through it and this in turn ignites the propellant in the
"Booster" motors have no delay or ejection charge. They are designed to
another engine mounted above them for real multi-stage flights! They do this
gasses blow out of the top and up into the nozzle of the engine above.
larger the nozzle of the upper engine, the more reliable it's ignition will be.
"Single Stage" motors have a short to medium delay time and are used for medium
models. The heavier the model, the shorter the delay to insure that the
deploys near the peak altitude ("apogee"). Too long a delay can be hazardous
are deploying too close to the ground!
"Single or Upper Stage" motors have long delay times. They are used in upper
multi-stage models or in skinny lightweight models because those models will go
and require more coasting time to reach peak altitude.
Motor Designation Code:
The motor designation code consists of a letter, a number, a dash, and another
occasionally another letter that indicates special features provided by that
like small size or colored effects). Don't be confused by some motors that also
secret manufacturing code that identifies the day they were produced (it's
much smaller than the motor designation anyway).
Power Class: The letter indicates the total amount of power available in the
Every time the letter increases the power class doubles. Example: a "B" motor
the total power of an "A" motor, and a "C" is twice as powerful as a "B" and 4
powerful as an "A". Model Rocket Motors are classified from "1/4A" through "G"
Most motors are pretty close to the "full" end of their letter class, but not
Always check the motor specifications to be sure if there is a question.
Average Thrust: The number directly after the letter indicates the average
Newtons (the Metric unit of force). The higher the number the more average
newtons (and the shorter the burn time of the propellant). Example: A B4 motor
thrust for twice as long as a B8 motor but it will have half the thrust. Higher
thrust motors are good for heavier models. BUT sometimes a motor has a really
peak thrust that drops down to a long and low thrust ("C5" motors). The motor
has a low
average thrust, but they can lift a heavier model. Always consult the maximum
weight in the engine instructions or catalog! The maximum lift-off weight is
function of delay time.
Delay Time: The final number after the dash is the delay time in seconds. This
amount of time that the delay will burn between propellant burn-out and the
the ejection charge. Heavy or draggy models need short delays. Small,
may use longer delays.
What Makes The Model Rocket Fly Straight?:
NASA type rockets use moveable rocket engines or moveable fins to steer and stay
course. Model Rockets do not use an active guidance system. They are designed
"stable" and keep going in the direction they are pointed. They do this because
"fins" at the back end (like the feathers on an arrow) which guide the fast
through the air. All objects will tend to spin around their "center of gravity"
or balance point. The fins create more surface area behind the c.g. than in
front for the
air rushing by to hit. This forces the rear of the rocket to stay in the rear.
with fins that are too small will not be stable and will fly crazy all over the
You can fix this by adding larger fins or by adding clay to the inside of the
plastic nose cone. <a link to your "Beyond the Basics" section (which hopefully
on doing stability tests) would be nice here.> Because the rocket is not moving
instant of ignition, we have to guide the model during the first few feet of
flight as it
builds up air speed. The launch pad has a "launch rod" or guide and the rocket
"launch lug" that slides along the rod or guide.
(Do the "balloon" demo to show an unstable rocket - no fins....)
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