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Everything You Wanted To Know About Black Powder Rocket Motors
Article by Paul Nortness
Alright, we have built our first rocket and we have learned how to build launch pads. Heck, we even talked about all the different ways to get your rocket back on the ground safely. But how on earth do we get our rocket up in the air? What do all those codes mean on the rocket motor? When should I use a bigger rocket motor? With this article, I will answer all these questions so you too can be a rocket motor expert!
First, let's learn a little about how hobby rocket motors started. Way back in 1957, a couple of guys created a company called "Model Missiles Incorporated" in Denver, Colorado. They turned to a small family business who specialized in making fireworks. The son, Vern, took up the challenge and built himself a machine that could pack black powder into a cardboard tube to make safe model rocket motors. This machine he called Mabel. Mabel could produce a rocket motor in 5 and a half seconds using compressed air. After a couple years, Model Missiles went out of business, so Vern again took the challenge and in 1961 created a company using his last name. Estes Rockets was born.
Now that the history lesson is over, let's talk rockets! Pictured above, we have three different rocket motors. On the right, we have a 13 millimeter "mini" motor. These motors are perfect for small fields like baseball fields. The motor in the middle is an 18 millimeter "standard" motor. These are the most common motors available in "low-power" rocketry. They are great for slightly larger fields like a football field. Finally, the largest black powder motors available. The Estes "E" series is a 24 millimeter motor that works well on larger fields. In order to properly select an engine to use, consult the table below. It will outline the proper space needed to recover a rocket using a specific motor.
The biggest factor in choosing a rocket motor is "How much space do I have for recovery?" Find a rocket that is appropriate to your area. If you are blessed with large open fields, then you can fly the bigger rockets that use "D" and "E" power. If you have a few schools around with empty soccer fields on the weekends, maybe something with a standard 18 millimeter motor is your cup of tea. Think about the field you will be flying at and select a rocket accordingly.
OK, let's talk about all those goofy codes on the motor. Let's use our friend here, the E9-6 to demonstrate.
The first letter in the code is the "impulse" range of a motor. This range is calculated in Newton Seconds. This motor is an "E" impulse range motor, indicating that it is rated between 20.01 to 40.00 impulse. Each successive letter has twice the impulse as the letter before it, so a "C" will have double the impulse of a "B" and so forth.
The second number indicates the motor's average thrust. Again, this figure is calculated in Newtons. The bigger the number will result in higher liftoff acceleration. This is perfect for larger models. However, a higher number will typically not burn as long. For example, a C6 will lift off the pad slowly but burn longer than the C11 that ripped off the pad in a blink of an eye.
Finally, the third number is the delay in seconds. While a motor is in its delay stage, it will emit "tracking smoke" allowing the rocket to be seen easily from the ground. After the delay has burned through, the ejection charge fires which deploys the recovery device. It is also possible to see a "0" or a "P". Motors with a zero delay are used for multi staged boosters. Once the propellant has burnt, the ejection charge fires immediately. In the case of a booster, the hot gases will do two things. The hot gases expelled from the ejection charge will ignite the second stage while at the same time causing the booster to separate and fall back to earth. A motor with a "P" for its delay is called a "plugged" motor. These are motors that do not contain an ejection charge at all. Common uses for plugged motors are R/C gliders or smaller tumble recovered rockets.
To recap, our E9-6 from above has an "E" class impulse range of 20.01-40.00, while it's average thrust curve is 9 newtons. It has a delay of 6 seconds, after which time it will eject the recovery device.
The above picture shows the internal parts of a rocket motor. The launch sequence begins with the push of a button. That opens the circuit allowing electrical current to flow through the igniter wire. This wire is coated with nichrome to cause it to burn super hot. The igniter is inserted into the nozzle so it comes in contact with the propellant inside the motor case. Once the motor is lit, the propellant burns off creating hot gases. These gases shoot out of the nozzle, creating thrust. After the propellant has burned off, the delay stage burns. This section burns a thick white smoke so you can track the rocket. And finally the ejection charge fires, breaking through the clay end cap and pushing out the nose cone which in turn pulls out the parachute and allows your rocket to come home safely.
Pretty cool, huh? So now you know how rocket motors work and how to select the proper motor for your rockets. Now what is left to do? Oh yeah! Go to the field and launch some rockets!
Have fun and happy flying!