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Aviation History
1964
1964 - 0154.PDF
fLlCHT International, 16 January 1964 107 Hybrid Propulsion By D. D. Ordahl and W. A. Rains* ONE of the most promising products of current propulsionresearch is the hybrid concept. It uses a combination ofsolid and liquid propellants, and is a cross product of the practical experience and research on both all-liquid and all-solid systems. The development of hybrid engines is the most promising means for meeting the complex propulsion requirements being en- countered in many advanced military and space missions. This is because, as in most hybrid products, proper selection enables the designer to emphasize the strong and suppress the weak features of each element in the combination. Typically, the features of con- trollable thrust, high-energy performance, and low propellant cost that are normally associated with liquids can be combined with the structural simplicity, high density, and wide-range storability of solids. Engines designed in this manner can provide optimum per- formance where combinations of such features as outer-space storability, high-impulse performance and intermittent operation with demand thrust are required. A significant extra advantage of some of the preferred hybrid combinations is that they can provide the greatest safety in manufacturing, handling, and operation of all possible high-energy chemical propulsion systems. Although most of the potentials of hybrids have been recognized and widely discussed for at least ten years, no substantial effort was made to exploit the concept for ordnance or space systems until the Navy started a small exploratory investigation in 1958. Progress in the last two years has been sufficiently encouraging to warrant the redoubled effort that is being expended, and hybrids are being given serious attention by the National Aeronautics and Space Administration as well as each of the defence agencies. No prob- lems have been identified that cannot be solved by known engineer- ing methods, and the use of hybrid propulsion is fully expected to appear in developments of the near future. To facilitate an understanding of the operation of a hybrid engine it is convenient to refer to typical examples of the basic technologies from which the concept has been derived. In a typical storable liquid-propellant engine (Fig 1) liquid fuel and oxidizer are packaged in separate tanks and the rate of flow into the combus- tion chamber is controlled by appropriate valves on both feed lines. Driving forces that cause the liquids to flow are provided by pres- surizing the tanks with a gas generator or an auxiliary gas bottle. The ratio of the oxidizer and fuel (as well as the total mass flow) is controlled by adjusting the valves on the propellant lines. Igni- tion is usually accomplished by hypergolic reaction. Where the propellants are not themselves hypergolic, a small quantity of a third material is injected with the first portion of fuel or oxidizer. The other technology which makes up the second half of a typical hybrid propulsion system is the mechanically simple solid engine * United Technology Center (a Division of United Aircraft Corporation,Sunnyvale, California). Test-firing of a l0,0O0lb thrust exptrimental hybrid rocket engine at United Technology Corporation's development centre near Morgan Hill, California, last summer. The UTC hybrid uses a solid fuel and a liquid oxidizer • (Fig 2). A normal solid propellant contains all of the fuel and oxidizer required for complete reaction in a continuous solid matrix. Chemically, the solid propellant is considerably more complex than the usual liquid combination, because the mixture must contain chemical agents to accomplish the function provided in the liquid engine by the control valves. In other words, the mass flow is con- trolled by catalysts and ballistic modifiers and by surface geometry rather than by mechanical means. These propellants are ignited by means of either a pyrotechnic or a hypergolic igniter, and no modu- lation of thrust is generally possible; however, for some propellants thrust can be terminated by a suitable drop in pressure. Hybrid systems classically employ the same oxidizer delivery as the storable liquids, and the same motor construction as the solids. The simple concept illustrated by Fig 3 depicts an engine in which liquid oxidizer such as nitrogen tetroxide is pressure pumped into a chamber containing a typical hybrid fuel such as rubbery polymer Fig I Liquid bi-propellant system • PRESSURiZATION TANK DUAL PROPEUANT VALVE
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