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ASTRA and Rocketry

ASTRA has been involved in the activity of amateur rocketry, building and flying models of varying size and design for many years. Rocketry is a medium which can be used as a research tool, flying prototype designs of space vehicle and launch configuration to test viability. It is also a low cost educational method of giving young people hands-on experience in designing and building their own rockets. experience has also shown us that it is a good way of advertising the society, through social interaction at public events such as rocket workshops and open flying sessions. It is also a source of great fun for all participants.

Types of Rockets

There are two types of rocket used in modelling. The first type is called a PYROTECHNIC. These are available in kit form from all good toy shops and are normally of American manufacture. They can also be made at home from cardboard tubing and balsa wood. They are powered by a range of rocket motors which are similar to fireworks, but made to deliver a set thrust, which can be as much as 72 Newtons (16LBs). Ignited by battery, these rockets can fly as high as 1500 feet. The second type is a much more eco-friendly design, made by converting an empty plastic pop bottle, and powering it with water and compressed. It is this type which the AIRDRIE branch has much expertise on, and it is called an AQUAJET (sometimes known as a Water Rocket).

Aquajets

AQUAJETS are a direct descendant of an American toy, patented in 1956, called the PARK rocket. This toy was a small plastic bodied aeroplane with a hollow fuselage, which was half filled with water, pressurised by a mini bicycle pump through a nozzle hole at the rear, and launched when the internal pressure overcame the friction of the nozzle against the pump. Flying pressures were very low, perhaps 15 - 20 psi. Various other designs soon followed including a V2 model, and a space shuttle model can be bought today. Early attempts at rocket modelling used washing-up liquid bottles, which were bound with tape to strengthen them, and then flown at very low pressures with a modicum of success. The advances of plastics technology in the packaging industry have now given us very strong, lightweight, disposable bottles, which make good robust rocket bodies.

Building an Aquajet

The average Aquajet starts off as an empty 1.5 litre plastic pop bottle to which is added a set of fins, which can be either taped or glued to it. Then a soft nose-cone is fitted, and the rocket is tested for aerodynamic stability. The first test involves placing a noose around the rocket at its centre of gravity, then swinging it around ones head at quite a speed, on 3 or 4 metres of twine. This will establish whether it is stable along its longitudinal axis or not. From experience you can tell if the nose needs ballasted, the fins are too small of too large, or its back to the drawing board. A smooth horizontal flight will result if it is of stable design. This is called the SWING test. The model is then suspended through its longitudinal axis and spun rapidly to test its centrifugal stability. This test is essential as some models can be made to spin in flight by setting the fins at an angle of up to 5 degrees. This is called the SPIN TEST. All models are then flown at low pressure, and the flight analysed, in case minor modifications may be required.

Launching Aquajets

The launch system is a pipe of the same internal diameter as the neck of the bottle, fitted with a set of mechanical clamps to hold the model until launch pressure is reached, which is supplied by using either 2 car foot pumps linked in tandem or a small compressor. The model is filled with a measured amount of water up to a maximum of 50% by volume, but it is normally 500cc, then fitted to the launcher, pressurised and released. Flying pressures vary from 50psi for demonstration flights to 80psi for competition events, although bottles have been tested to 150psi without incident. Indeed, we were all astonished by the performance of some German models at last years Rocket Weekend, until we discovered they were flying at a pressure of 200psi. When flying at lower pressure, they were much more competitive, but you must remember that the German recycling laws are very strict, which results in an extremely robust, reusable plastic bottle

The launcher should have a staked bottom so that it can be forced into the ground at the required angle. The Airdrie launcher is of tripod design, with three staked legs, but a friend at KEELE University has a hand held launcher variant to ensure that he gets really wet in the process of launching.

Safety

At all workshops and flying events every stage of construction is supervised by an experienced rocketeer, on a 1 to 1 or 1 to 2 basis, and all launches are carried out by a team of two launch men. All flying field activities are carried out under the supervision of a designated Range Safety Officer, whose decision is final.

All ASTRA rocketry is done under the watchful eye of our Curator Oscar Schwiglhofer, himself a former student of one of the founding fathers of rocketry, the great Herman Oberth, so one can trace ASTRA's history back to the vary dawning of the science.