This paper relies on the simple relations between energy and momentum which are fundamental to the various possible space propulsion systems. Energy transmission by new techniques, such as laser power station, and the various limitations on its conversion to momentum are covered. Momentum storage and direct momentum transfer are also treated. Casual examination of any 50-year period since the invention of the airplane shows clearly the futility of attempting any meaningful prediction over such a time span. Consequently, this paper attempts to define not what will occur in cis-lunar transportation in the next 50 years, but rather what it might be possible to achieve. The possibilities are shown to be spectacular. The actualities remain to be seen. SUPPLYING ENERGY Conventional rockets have proven, so far, to be very awkward devices for space transportation. The fuel loads they must carry are so great that it is believed by an increasing number of people that we will never have space transportation good enough to open up the solar system for human use until we develop some system other than pure rockets. Before examining these nonrocket systems, it is instructive to examine the basic problems of rockets and see if they could be patched up to suit our purposes. After all, they are our sole means of space transportation existing today. They must have something to offer. The problem with rockets is not that they must carry all fuel and reaction mass onboard, but that the fuel and reaction mass required for current rockets is too high. If it were only a small fraction of launch weight, it would not be much bother. Vehicles move in space in accordance with Newton's second law. Pure rockets utilize an energy release within the rocket combustion chamber to create an exhaust stream. The momentum created is partitioned equally between that of the outgoing exhaust and that of the accelerating payload. The amount of reaction mass required in the exhaust stream decreases with increasing exhaust velocity. This requires, unfortunately, an increased energy concentration per unit of exhaust mass expelled. The basic question in creating an improved rocket, i.e., one with lower fuel consumption, is the question of the concentration of energy per unit mass which can be achieved in the outgoing exhaust. Chemical combustion reactions, unfortunately, are limited in achievable energy concentration. The modern hydrogen-oxygen engine operates quite close to the maximum achievable exhaust velocity. Hydrogen-fluorine is slightly better and would result in substantially better overall rocket designs. Hydrogen figures prominently in all high exhaust velocity rocket propellant combinations. It is the lightest element, and the highest exhaust velocity is achieved for a given combustion temperature with the lightest exhaust products. Hydrogen-fluorine engines can be operated at very high mixture ratios of fluorine to hydrogen with the result that the volume of the hydrogen tank is only about 1.65 times the volume of the fluorine tank. The rockets can thus be very compact. Particularly with high velocity multistage rockets, the structural savings can lead to surprisingly small launch weights. The volume of the hydrogen portion of the shuttle external tank, on the other hand, is 2.85 times the volume of the oxygen tank. There are some triple propellant combinations which can be made to produce even higher exhaust velocities (although not much more than 1090. hut all of them require even greater
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