It is not generally realised that the ground effect principle on which the air cushion vehicle is based was first expounded by a Scandinavian scientist some hundred years before Sir George Cayley grasped the basic principles of flight and two hundred and fifty years before the hovercraft went into commercial service. The basic principle of the air cushion vehicle is extremely simple, but, as in many branches of engineering, simplicity of principle and ease of achievement do not go hand in hand!

How does a hovercraft hover? Let us take a vehicle with a weight of 1000 lb (454 kg) and a flat undersurface of 100 sq ft (9m2) in area. If we increase the air pressure under the machine by 10 lb/sq ft (478 N/m2), the total reaction pushing upward from the undersurface of the machine will be 10 x 100 = 1 000 lb (454 kg). This, being equal to the weight of the machine, will cause it to leave the ground.

It is only in fairly recent years that the idea of sending an object to another planet has become a practical reality. This has led to the rapid development of celestial mechanics, although the subject is by no means new. The purpose of the paper is to introduce the subject and to see how well-established results and equations may be utilised to find out about the motion of an object on its way to another planet. Once the path is determined, various deductions may be made about the rocket vehicle which must place the object or spacecraft along that path.

The concept at “launch window” will be introduced and a method of determining it will be presented. Once it is known, the number of available trajectories (although theoretically infinite) are narrowed down by selecting desired values of some parameters.

Although an interest in electric propulsion has existed in the UK for a considerable time, it is only in the past few years that it has been considered worthwhile to undertake the development of hardware and produce detailed systems studies. The practical developments can be divided into two categories: —

1. 1. The development of comparatively large thrusters (in the region of ½ kW power, giving 10-20 milli Newtons of thrust), for producing major movements in a satellite's position.

2. 2. The development of micro-thruster systems for the maintenance of a satellite's attitude and position against the small disturbing forces and torques it normally experiences in orbit.