Out of this world

27th April 2001 at 01:00
Explore new frontiers in your science teaching by making a real rocket launch the culmination of activities investigating speed, force and gravity and much more, says Paul Sellin.

Have you been looking around recently for that special attention-grabbing activity that will help get your students enthusiastic about science? Last summer 12 groups of Year 9 pupils were invited to the University of Surrey for a model rocket competition called Liftoff! to launch rockets constructed from cardboard and balsa wood kits. On the launch day each model was fitted with a solid-fuel rocket engine, and even with this type of simple rocket, heights of up to 200 metres were easily achieved.

Following the success of the Liftoff! launch day, the University of Surrey has now produced a Teacher's Guide to Model Rocketry, which is available free to schools. For teachers, model rocketry is an activity that has many attractions, and can be extremely effective in both motivating and interesting pupils of all ability ranges in science. And it is very easy to get started with model rockets - despite the common misconception, rocket science really is not that difficult.

Model rockets can be tackled at many different levels, from simple water and air-powered "pump-up" rockets, through to more sophisticated models powered by solid-fuel rocket engines. Clearly there are some important safety guidelines that need to be followed by anyone involved in model rocketry, and there is a widely available code of conduct for flying model rockets, produced by the National Association of Rocketry (NAR) based in the United States.

A wide range of classroom teaching activities can accompany the rocket launch, although most of these have been developed in the US. The "Liftoff!" model rocket programme, funded by the Particle Physics and Astronomy Research Council, is specifically tailored for British students in Year 9 of all abilities.

At key stage 3 several simple classroom experiments can demonstrate some of the scientific principles behind rocket flight. For example, a "rocket pinwheel" can be constructed, consisting of a slightly inflated balloon attached to a drinking straw of the type that contains a crimped right-angle bend at one end. A pin is used to fix the centre of the straw loosely to the top of a pencil, and as the balloon deflates the straw spins by "jet propulsion" (see Box A). This type of activity can teach students about action and reaction of forces, and that the speed of an object varies with the size of the force.

There are other classroom topics that can be brought out of studying model rockets, linking to different aspects of the school curriculum in science, mathematics and design and technology. For example the ideas of Newton's First Law, that an object's motion is unchanged if acted on by balanced forces, can be discussed in terms of rocket flight, where there is a constant change in the balance of forces. A rocket is acted on by range of forces during flight: from the launch pad, during the thrust phase, and at the instantaneous moment of highest flight.

Similarly, the concept of equal and opposite action and reaction of forces is well demonstrated by a solid fuel rocket motor, where the action is produced by expelling gas from the motor and the reaction is the movement of the rocket in the opposite direction.

For more advanced students the ideas of force, mass and acceleration, terminal velocity and the force due to gravitational acceleration can all be discussed. Trigonometry can also be introduced by getting students to measure the height of their rockets using an altitude tracker, with varying levels of mathematical sophistication depending on the number of tracking positions used (see Box B).

But however enthusiastic your classroom activities might be, the point will soon arrive when nothing but a launch will keep your students happy. Here there is a basic choice to be made between the air or water-propelled rocket, or the solid-fuel type. The use of air or water-powered rockets is an excellent way to introduce the topic to a younger set of pupils, and they can even be launched indoors in a large room or sports hall.

The most popular commercially available air-powered rocket is the Stomp rocket; simply stamp on the footpad and the small plastic rocket can travel distances up to 100m. The Stomp rocket is made in the US and is widely available in this country from hobby and toy shops. A number of commercial water-powered rockets are also available, typically consisting of an attachment to allow a hand-held bicycle pump to pressurise a plastic bottle. The bottle is first filled about one third full of water and then mounted vertically on a stand. The pump pressurises the bottle, eventually causing the stopper to be ejected from the bottle, with the resulting water jet giving the bottle an impressive thrust. There is considerable scope for experimentation with home-made variants of the water-powered bottle rocket, such as adding stabilising fins or streamers, or even ballast in the rocket nose.

The art of building and flying "real" model rockets with solid-fuel engines is simplified by the availability of ready-to-fly complete rocket kits. The leading brand of model rocket kits is Estes, readily available in hobby and model shops. If you are starting from scratch with powered rockets, the easiest way to begin is with an Estes starter kit. This contains everything you need to fly a rocket, except for the engines. For about pound;20, the kit provides a pre-built small cardboard rocket, a launch tripod, and an electrical ignition box to fire the engine. A pack of three rocket engines will cost about pound;5.

Once you have bought a starter kit and obtained your launch tripod and ignition box, you can use these items to launch other rockets made from more sophisticated kits, or you can build home-made rockets to your own design. Essentially, all model rockets consist of a body tube, normally of cardboard, with fins at the base that are either made from pre-moulded plastic or balsa wood. A parachute connected to the detachable nose cone of the rocket is placed inside the top of the body tube.

When the rocket reaches the top of its flight, a secondary charge in the engine fires and detaches the nose cone from the body, so stopping the rocket's motion and releasing the parachute. The rocket floats on the parachute to the ground, where ideally it is recovered and reused. However, if there is a significant wind, the rocket can blow a considerable distance on the parachute and a large open space is essential for safe recovery.

The engines used in model rockets are carefully graded with a letter designating their total impulse. For example, a type A engine has a total impulse of up to 2.5 Newton seconds, whereas a type D engine has a total impulse of up to 20 Newton seconds. In this country, type D engines are the largest that can be bought over the counter, whereas in the US, engines up to double-G are available. When choosing your engine you can also select the duration of both the thrust phase and the coasting time before the parachute is deployed. If you are limited for landing space and want to limit the range of your rocket, it is useful to choose an engine with a short coasting period.

Estes also supplies a large range of more complex rocket kits, which require type D engines. A large rocket powered by a D engine gives a more impressive launch, with a slower initial take-off velocity and considerably more noise and smoke. The final height reached by a large rocket can be up to 150m, depending on the wind speed and type of engine. At the other end of the scale you can easily make tiny paper rockets rolled around a pencil, powered by a mini half-A size engine. These go up so fast that it can be difficult pinpointing them in the sky.

Whatever size or type of rocket you make, whether bought commercially or home-made from a kit, you must always launch your rocket using a suitable launch tripod and electrical ignition box. You should also respect British restrictions on model rocket use that forbid launching within 8 kilometres of a major airport.

For our launch competition last summer we used a large open space close to the university that was free from trees. All the rockets built by the students launched successfully, although there were a few honourable losses in the nearby lake, and adjoining woodland, during "re-entry" . Prizes were awarded for the categories of best overall flight, best decorated and the most novel rocket payload. Overall the day was judged to be a great success by both teachers and pupils.The range of resources and materials for model rockets available has never been greater, so now is an excellent time to use this type of activity, which may inspire in pupils a more general enthusiasm for science and technology.

* Free copies of the University of Surrey "Liftoff!" Model Rocket guide for teachers are available from the website at www.ph.surrey.ac.ukliftoff The website also contains practical information about model rocketry, contact details and addresses of model rocket stockists * Rocket models can be bought at good hobby and toy stores thoughout the UK or contact manufacturers for a local supplier Air-powered: Stomp, TKC Sales and Marketing, 20 Wansdyke Business Centre, Oldfield Lane, Bath BA2 3LY. Tel: 01225 466661.

Water-powered: Rokit, Hinterland Ltd, Stanstead Road, Hertford SG13 7HY. Tel: 01992 501377.

Ready to fly kits: Estes (www.estesrockets.com) UK importer: Ripmax, Green Street, Enfield EN3 7SJ. Tel: 020 8282 7500 for details of local stockists. Web: www.ripmax.com * Safety information, including the National Association of Rocketry (NAR) Safety Code, is also available on the "Liftoff!" website Dr Paul Sellin is a lecturer in the Department of Physics at the University of Surrey, and is the department's schools liaison officerE-mail: p.sellin@surrey.ac.uk

* BOX A: Jet propulsion

The gravitational attraction between the Earthand a rocket gives the rocket its weight. If a rocket is going to take off it must be going fast enough to escape from the Earth's gravity. How can we observe how propulsion affects speed? Build a rocket pinwheel with:

* wooden pencil with a rubber on the end

* small pin

* flexible drinking straw

* round balloon

* sticky tape

Inflate the balloon so that it is slightly stretched and easy to work with. Insert the straw into the neck of the balloon and seal it to the balloon using sticky tape. You should be able to inflate the balloon by blowing into the straw.

Bend the straw into a right-angle at the flexible section.

Find the balance point of the straw and balloon by balancing the straw on your outstretched finger. Push the pin through the straw at the balance point.

Push the pin into the rubber at the end of the pencil and spin it a few times to loosen up the hole.

Blow into the straw to inflate the balloon and watch what happens when you let the balloon deflate.

Find out: What happens to the pinwheel when you allow the balloon to deflate?

Does it make a difference whether the straw is bent or straight?

Does the air coming out of the straw have to push against something, such as a piece of card, for the pinwheel to work?

Does it make a difference if the balloon is inflated more? * BOX B: Altitude tracking

Using simple trigonometry it is possible to determine the altitude a rocket reaches in flight. The Teachers' Guide on the "Liftoff!" website explains how students can construct simple devices (www.ph.surrey.ac.ukliftoff). The basic assumption of the activity is that the rocket travels straight up from the launch site. If the rocket flies away at an angle other than 90 degrees, the accuracy of the procedure is diminished: if the rocket flies toward a tracking station as it climbs upward, the altitude calculation will give an answer higher than the actual altitude reached; if the rocket flies away from the station the altitude measurement will be lower than the actual value. Tracking accuracy can be increased by using two or three tracking stations positioned in different directions. Average the altitude measurements.

The altitude tracker is aimed at the highest point the rocket reaches in the sky, and a reading of the angle is taken. If we know the angle and the distance from the launch site, we can work out the rocket's altitude.

Remember to add the height of the person holding the tracker to the measured rocket's altitude.

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