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Back to Basics - Motors Print
Written by Joe Pfeiffer   
Sunday, 11 July 2010 10:42


Back to Basics - Motors

The key to a flying model rocket is the motor. After all, with no motor, it wouldn't be a flying model rocket! This first installment of "BACK TO BASICS" will discuss the motors we use in our rockets. Here's a picture of a model rocket motor, like we'd buy from a company like Estes or Quest:

Common Black Powder Motor

A model rocket motor has five parts: a propellant charge, a clay nozzle, a delay, an ejection charge, and a cardboard casing. The outside of the casing is stamped with a code, which describes the motor (the motor in the picture is an A8-3).

The propellant charge provides the thrust for the rocket. As the propellant charge burns, hot gasses are ejected out the back of the motor. This creates an opposite force, which makes the rocket take off. The letter and the first number in the code describe the propellant charge.

The letter tells the motor's total thrust. A B motor has twice the thrust of an A motor; a C motor has twice the thrust of a B, and so forth. The number describes the motor's average thrust - it's defined as the total thrust for the motor, divided by the time taken by the propellant charge. A larger number means a faster takeoff, but for a shorter time.

The clay nozzle forces the hot gasses from the propellant charge through a small opening. This increases the speed of the gasses, which increases th e thrust. The size of the nozzle, the shape of the cavity in the propellant (you can see it in the picture), and the amount and type of propellant detennine the motor's total and average thrust.

After the propellant charge has burned out, the delay charge gives the rocket time to coast to the top of its flight. It also produces a lot of smoke, to make the rocket's flight easier to see. The last number in the motor code tells how many seconds of delay the motor has. A shorter delay will be used for a heavier rocket, and a longer delay for a lighter one.

The ejection charge is fired after the delay charge has finished burning. When the ejection charge fires, there's nothing blocking the gasses from the front of the motor any more, so the force is forward instead of back. This blows the nose cone off of the front of the rocket, and ejects the parachute.

The example motor, an A8-3, is a small motor which will only have thrust for a fraction of a second, and will then let the rocket coast. It's a good choice for a fairly small rocket such as the Estes Athena.

All model rocket kits give recommendations for the motors to be used in them. Use the "First Flight" motor recommendation for the rocket's first flight, and then go to bigger motors to get higher flights. lf your rocket's parachute ejects before the top of the flight, use a longer delay; if it ejects too long after the top, use a shorter delay.

In More Detail: Catos

'Cato" is a term used to describe a "catastrophic failure" of a rocket flight. Most of the time, it refers to a flight that fails due to the motor exploding. How dangerous is it when a model rocket motor explodes? What causes it to happen? And how can we avoid it?

If a model rocket is built and flown according to the safety code established by the National Association for Rocketry (the NAR), a cato is simply not dangerous. The motor casing and nozzle are made of cardboard and clay, and are deliberately made to be weak. If pressure inside the motor gets too high, either the casing or the nozzle fails and the pressure is vented (the motor blows up!). If any pieces are thrown by the explosion, they are light enough that they won't go far enough to hit anybody. It's loud, it's impressive, and it ruins the rocket, but nobody gets hurt.

There are several possible causes of a cato. The most common is for cracks to form in the propellant. The propellant charge is a single, large piece of gunpowder (this is called a grain). The speed it burns is determined by how much of the grain is exposed to the fire at any given time (this is how the cavity in the propellant helps to determine the average thrust - it controls the burning area). If there is a crack in the grian, more surface is exposed to the fire - this increases the speed of the burn, which increases the pressure past the point where the casing fails. The result: a cato.

There are two main reasons for cracks to form in the grain of the motor: factory defects and temperature fluctuations.

There isn't much we can do about factory defects. If we have a failure, it's a good idea to look at the motor that failed and see if you have any others with the same "lot number." If you do, don't use the others! It can't hurt to give the manufacturer a call - I've never tried it, but I hear they're  pretty good aboot replacing kits that are destroyed when the motor blows up.

Letting a motor get hot, and then cooling it off. will cause cracks to forn in the grain, too. Try to avoid this! Don't let your motors sit in a parked car during the summer, for instance.


Last Updated on Sunday, 11 July 2010 11:15

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