Chemical systems are still best for short-term power requirements of two or three days, in some cases up to two weeks, and there are prospects for even longer duration. Theoretically fuel cells-devices that convert chem Fuel cell. A device fed by gases or other chemical fuels that converts chemical energy directly into electricity through the process of electrolysis. Most of the fuel cells presently under development follow the hydrogen-oxygen cycle. Fuel and oxidizer are introduced at the electrodes and combine chemically with the aid of a catalyst, generating an electric current in the external circuit and forming exhaust products such as water for the hydrogen-oxygen case. Fuel and oxidizer are supplied continuously to meet the external electrical load placed upon the cell. ical energy directly into electrical energy-can prove to be the lightest and most efficient of all chemical systems, for a wide range of output and duration. Most space systems require power for durations achievable only through the use of solar or nuclear-reactor sources. In such systems-at least for the next few years-power below 500-1000 watts will be produced by static conversion. As it is now visualized, higher power will be produced by the dynamic systems-i.e., systems utilizing moving masses, such as turbines, in the conversion process. Solar dynamic systems seem to have a weight advantage over all other long-duration systems in the range from about 0.5-1.0 kilowatts to about 25 kilowatts. The lower end of this range will move upward in three to five years if we make anticipated improvements in the conversion efficiencies of static devices. The range from 0.5 to 25 kw is important because it encom passes most of the anticipated nonpropulsive power requirements of longduration satellites of the next decade. But we need to know more about component design and performance factors. Applied research is needed on radiator Solar dynamic heat engine. A mechanical engine operated by sun heat. Solar radiation is concentrated by a reflector into a boiler, where it is tranferred to a working fluid which drives a rotating turbogenerator. Diagram shows working of the Air Force 15-kilowatt solar mechanical engine now under development for power applications in space. A potential payload package is at left of the mirror. collectors, boilers, and other zerogravity heat exchangers, radiators, higherefficiency turbines, generators, thermal energy storage, voltage and speed controls, and orientation. Even so, we can begin now to make use of partially developed hardware. Future power requirements above 100 kw can now best be met by nuclearreactor turboelectric systems. Sizes of most interest in the immediate future Nuclear dynamic heat engine. Heat from a nuclear reactor is transferred to a suitable working fluid to drive a rotating turbine coupled to an electric generator or alternator. Exhaust working fluid is recycled through a radiator (condenser). Diagram shows the working scheme of the Air Force nuclear-reactor power-conversion system now under development. It is designed to produce 300 to 1000 kilowatts of electrical power for spacecraft, as for use in energizing ion-drive propulsion. range from 30 to 300 kw. Studies and development of high-voltage electrical generating equipment are needed, particularly of electrostatic generator concepts and designs. Currently achievable conversion efficiencies of 10 per cent and estimated system weights of 10 watts per pound make practicable now-although at high cost-the use of photovoltaic systems with up to about 500 watts of power Photovoltaic converter. The photovoltaic converter, or solar cell, absorbs solar radiation and converts it directly into electricity through bombardment of a semiconductor material, such as silicon, by solar photons. A continuous shift of electrons is caused within the cell and through the electrical load. This electron flow is made possible by an N-type silicon wafer which has an excess electron, the result of doping it with material having five electrons in its valence orbit, and a P-type silicon layer which lacks an electron, the result of diffusing it with a material having three electrons. The N-type wafer is usually made up of “stacked" layers, each of which is sensitive to a different wave length, in order to intercept the maximum amount of light and attain maximum conversion efficiency. output. But we need to improve cell conversion efficiency and to reduce system cost and cell weight. For the low power ranges, photovoltaic systems are preferred over all other long-duration solar energy conversion systems, at least until either thermoelectric or thermionic conversion efficiencies become competitive. Thermoelectric systems are attractive from the standpoint of simplicity in low-power applications, but their ultimate utility depends upon obtaining higher conversion efficiencies at the elevated temperatures at which space power equipment must operate. Thermionic systems promise potentially high conversion efficiencies of above 25 per cent, with either a solar or nuclear energy source. We can justify development of the system if we can achieve 10.8 per cent efficiency with close-spaced diodes. propulsion for space vehicles Much of our present work in development of auxiliary power systems heat rejection cooler heat exchanger thermoelectric semiconductor heat input heat collector and/or Thermocouple. An electrical potential develops at the interface between dissimilar metals that are subjected to temperature differences. This long-known thermoelectric phenomenon is the basis for present-day instrumentation thermocouples. Thermionic converter. Application of heat, as from a nuclear reactor or solar radiation, to a specially coated cathode causes electrons to escape to the cold anode by virtue of their kinetic and potential energy. Useful work is done by the flow of the electrons through an external resistance load back to the cathode. heat source electrical cathode heat sump anode for satellites is in laying the foundation for propulsion systems that can be used in future space vehicles. We may still be able to make some use of chemical propulsion, although it does present formidable staging requirements. But the requirements for the utmost in efficiency, durability, and reliability all suggest that we look elsewhere for power and propulsion sources. In space we will no longer have some of the problems that face us in atmospheric flight. We will not need large amounts of thrust, as we will no longer have to overcome the effects of gravity and atmospheric drag. Our vehicles can take a variety of configurations, as we do not have to be concerned about wind resistance. For these reasons nuclear propulsion promises even greater benefits in space than it does in the atmosphere. The power plant and cabin of a space vehicle can be widely separated, thus eliminating much of the weight and bulk of shielding, and radiation products can be dissipated harmlessly in space. Electric propulsion systems offer even higher specific impulses than nuclear propulsion, although their low thrust-to-weight ratio makes them suitable only for use in space. Electric propulsion requires an electrical Thermal arc jet. Thermal energy, added to the propellant fluid in an electric arc, is converted to directed kinetic energy in the nozzle, thereby producing thrust. power source, a supply of propellant, and an engine in which the electrical power is used to add energy to the propellant so as to produce a high-velocity exhaust jet. Three general types of electric propulsion engine are now in research and development. They are the thermal arc jet, the ion engine, and the magnetohydrodynamic (MHD) plasma engine. In the thermal arc jet, the propellant is heated by an electric arc, which produces higher temperatures than can be produced by chemical combustion. It can use hydrogen alone as the propellant, thereby achieving the least. possible molecular weight in the exhaust jet. The higher temperature and |