Text 16 c Application of electric-propulsion system

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An electrically powered spacecraft will probably be used for a round trip to some distant planet. A comparison between an electric and a conventional system for a proposed trip to Mars will show definite advantages of an electric system. For an eight-man crew to go on a 500-day trip to Mars, the weight of the electric and conventional system would be 450,000 and 8,000,000 pounds respectively. Both systems would have to be assembled in an Earth orbit. It would, however, take only two boosters to lift the material for the electric system while forty boosters would be needed for the conventional one. Electric power propulsion and all other needs would be generated by a nuclear-fission turbo-electric system.

There is, however, one problem that has not yet been discussed - the radiator equipment. Vapour exhausted from the turbine must be cooled and condensed before it returns to heat exchanger and the cycle is repeated. The cooling is accomplished with a radiator. This creates a weight problem, since a great deal of surface area is required for efficient heat exchange. Besides, to make the electric system practical, a large number of engines would be required because present designs are for engines generating only a small amount of thrust.

Much research has been conducted on electric propulsion systems as they can produce such low thrusts and can run for long periods. This means a high degree of reliability will have to be attained for such systems. Even with the large amount of research already accomplished, the electric system is still in a stage of development. Lighter and more powerful units must be developed if we want such systems to fulfil the promise they offer for interplanetary travel.

Unit 17

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Text 17 a an efficiency of nearly 100 per cent

Perhaps the most interesting thing about semiconductor lasers is that they can transform electrical energy directly into light wave energy. They do this with an efficiency approaching one hundred per cent as compared with a maximum of about one per cent of other lasers.

Semiconductor lasers are sure to open up great prospects for solving various scientific and technical problems. Calculations and experiments show that already super-hard substances (diamonds, rubies and so on) and hard alloys can be worked profitably by ruby lasers, for example. The development of powerful-efficient semiconductor lasers is certain to considerably raise the power efficiency of a number of technological processes.

The high frequency radiation of optical generators makes it possible to transmit an enormous flow of information. This is of great significance for the advancement of communication techniques. The small dimensions of the semiconductor laser make it especially suitable for use in super-speed computers.

Ruby crystals about ten centimetres long can intensify light ten times. The same results can be obtained from semiconductor crystals only a few microns long. Thus it is possible to develop quantum light intensifiers with maximum sensitivity.

Theoretical calculations have shown that devices similar to semiconductor lasers can also transform the energy of light waves into electrical energy with an efficiency of close to 100 per cent. This means that electric power may be transmitted over considerable distances with negligible losses without the use of transmission lines.

The high efficiency of semiconductor lasers opens up possibilities of developing extremely economical sources of light. Luminescent lamps were instrumental in increasing the efficiency of light sources from two to ten per cent. Semiconductor lasers will increase this efficiency several times over. In addition, semiconductor devices will greatly increase the efficiency of luminescent crystal lasers. Semiconductor lasers are sure to have a great future.

The diversity of available lasers is now determined by the fact that all possible states of matter are used as their active media – gases, liquids, solids and plasma. The use of each new form of matter as an active medium is associated with definite stage in the development of quantum electronics. The research for various active media may be regarded as the method of quantum electronics at an early stage of its development. Over the past decades high-power lasers have found a place in manufacturing processes: welding, heat–treating of parts to improve their surface properties, cutting, drilling of small holes and in electronic devices and medical instruments. In all these operations laser systems have made productive lines more efficient and have reduced costs.