The Liftoff! Beginners Guide to Model Rockets

With acknowledgement to Estes Industries.
Note: this Beginners Guide is contained within the Liftoff! Teachers Guide.

Model Rocket Safety

Always read the Safety Information, such as the NAR Rocket Safety Code, that is supplied with your rocket kit. In addition there are some British guidelines that apply to model rocket use in this country. This Safety Code is also reprinted on the Liftoff! website.

Safety Disclaimer: Whilst we have made every effort to provide accurate and comprehensive safety information as part of this model rocket programme, the University of Surrey cannot be held responsible for your rocket launching activities. In particular, it is your responsibility to carry out any necessary risk assessments and to implement appropriate safety measures.

The Model Rocket

All model rockets have the following parts: (1) One or more body tubes, the long round tube that holds the engine and payload, (2) a nose cone to reduce air resistance, (3) fins to provide stability in flight, (4) an engine to power it, (5) a launch lug, and a (6) a recovery system. The more complex rockets may have several body tubes connected by tube couplers.

Rocket Engine

Igniters

The heart of the rocket is its engine. It contains a propellant for lift-off, tracking smoke for the coasting phase, and an ejection charge to activate the recovery system. The engine is activated through the use of an electrically controlled ignitor. This is an important safety device as it lets the operator be a significant distance away from the rocket when the engine begins to burn.

The Launch Pad

The launch pad provides stability for lift off. It consists of a long launch rod to provide direction. (Shown left is the Estes Porta-Pad; its launch rod is 32 inches long.) and a deflector plate. Both are mounted on a tripod support.

The rocket is placed on the launch pad by sliding the rocket’s launch lugs down the launch rod.

The Launch Controller

The Controller is a simple battery powered switch. A wire runs from the controller to the ignitor placed in the engine. Controllers have a continuity light that comes on to indicate that the circuit is complete and a safety key to prevent the switch from being accidentally pushed.

Launching the Rocket

The completed rocket is taken to the launch site, the ignitor is placed in the engine, and the rocket is placed on the launch pad. The operator moves a safe distance away from the rocket. He alerts spectators and participants that a launch is about to take place. He double checks the controller's continuity light to insure the circuit is OK, then removes the safety interlock key, and counts down to launch.

There are several phases to the rocket's flight. The first is the thrust phase during which the impulse section of the engine burns. Once this is exhausted, the coasting phase begins. The engine is still active, but it is burning smoke, permitting you to follow the flight. The rocket is still climbing at this point. Then the ejection charge ignites, which actives the recovery system. This is usually a parachute to permit the slow descent of the rocket, or it can be a streamer so that you can follow its path. The rocket can then be recovered, a new engine installed, and it is ready to be launched again.

Model Rocket Engine Information

A model rocket engine, such as those manufactured by Estes and several other companies, consists of a cardboard casing with a clay nozzle at one end a clay retaining cap at the other. The engine contains three type of charges:

• The Propellant provides the power for the model to lift off and climb. When the propellant is exhausted the rocket is travelling with its maximum velocity, and it continues to climb in height.

• During this ‘coasting’ phase a Delay charge is burned which emits smoke to help visually follow the model.

• Finally the Ejection charge is ignited. This charge fires in the forward direction up inside the rocket body to eject the parachute, or other recovery system. At this point the forward motion of the rocket is brought to a sudden halt.

The specification of a model rocket engine is denoted in the form of a three-digit identification code. On the above engine the code is: A8-3.

The three digits represent the following:

• Total Impulse Power. The letter indicates the range of total impulse of the engine. The total impulse is a measure of the power the engine produces, determined by the mass of propellant it contains.

Total impulse is measure in Newton seconds. One Newton second is the amount of impulse by one Newton of thrust for a duration of one second. For example a "B" type engine has a maximum total impulse rating of five Newton seconds. All Estes engines have the maximum total impulse permitted for a given size.

*Note: only engine sizes up to D are available over the counter in the Britain. In the US sizes up to double-G are possible!

Time Thrust Profile for a B6-4 Engine

• Average Thrust. The first number states the average thrust in Newtons that the engine delivers during the thrust phase. The actual thrust varies, and is shown on the time thrust curve. For a particular engine size (for example a "B" engine), the propellant may be burned quickly, giving high thrust for a short time, or slowly, giving lower thrust for a longer time. A higher average thrust engine is best for heavier models while lower average thrust and longer burn engine is more efficient for lighter models.

• Time Delay. The second number shows the time delay. This is the number of seconds between the end of the thrust phase (propellant burned) and activation of the ejection charge. The time delay allows the model to coast to its peak altitude before the recovery system is deployed.

Thus the "A8-3" pictured above delivers 2.5 Newton seconds of power at an average thrust of 8N for a duration of approximately 0.3s. There is then a 3s delay before the ejection charge is activated.

Similarly a "B6-4" has 5.0 total Newton seconds of power, but delivers an average thrust of 6N for a duration of approximately 0.8s.There is a four second delay before the ejection charge is activated.

In addition to the engine’s ‘letter rating’, rocket engines are grouped into different physical sizes (of which only 3 sizes are relevant in Britain): mini, standard and large. The table below shows the dimensions of each type of Estes rocket engines:

Estes rocket engine dimensions

Note that a D engine has a larger diameter than a C engine – if you want to swap between these two engine sizes for a particular model you will need to use a cardboard tube insert to accommodate the smaller diameter C engine.

• Engine Performance & Weight. Estes Model Rocket Engines have colour coded labels:

  Green labels - single stage rockets.

  Purple labels - for upper stage rockets, but these may also be used to light single stage rockets.

  Red labels - "0" delay engines, for use in booster stage or special projects only. Contain no delay or ejection charge.

  If you have a choice of engines for a particular rocket, then you can experiment with the different performance of each. The various trade-offs can be quite subtle, but here are a few general tips:

• Choose a lower power engine for a first flight, eg. a B engine instead of a C engine. When you have got familiar with the power/weight ratio of your rocket, the wind speed, and the size of your landing area you can use a more powerful engine.
• To stop a rocket going up too high, choose an engine with a shorter delay time. This is particularly useful if you want to restrict the possible drift distance as the rocket comes back down to the ground.
• You need a larger power engine if your rocket is either heavy, or large diameter, or both. Don’t be tempted to fix a very large engine onto a very light rocket as it will go up like a firework and probably never be seen again!
• For a given engine letter (eg. a B engine) a smaller average thrust will give a longer burn time. This will tend to make the launch speed slower and arguably more impressive. However if your rocket is too heavy or too wide it may barely get off the ground.

Finally, always remember to follow the sensible safety guidelines, such as the Rocket Safety Code produced by the US National Association of Rocketry (NAR), that are reprinted on the Liftoff! website.