Propellant density considerations

If the criterion of propellant density is added to that of performance yet another optimum mixture will result. The importance of density depends upon the propulsion system and mission to be performed and is not reducible to a unique, well-defined performance parameter or figure of merit. It suffices to comment that the optimum mixture ratio will be influenced by the desirability of a high propellant bulk density. Since oxidizers are generally more dense than fuels, with notable exceptions, the optimum mixture ratio which includes the effect of propellant density is generally to the oxidizer side of the optimum mixture ratio based on maximum specific impulse alone. The effect of consideration of propellant density, then, in general, results in a shift toward the stoichiometric mixture ratio. 

The effect of chamber pressure has been considered in relation to the role of chamber pressure in determining the optimum mixture ratio, as was discussed in the previous section. An important effect of increasing chamber pressure is to elevate the heat of reaction and adiabatic flame temperature through inhibition of endothermic decompositions. The undesirable increase in product molecular weight is not of sufficient importance to overcome the advantages associated with decreasing 

For a fixed vehicle diameter and thus a fixed maximum nozzle exit area, higher area ratios are obtained at higher pressures simply because the throat area decreases for fixed mass flow. This is the major effect of pressure and, indeed. contributes to increase the performance for both atmospheric and space operation. 

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Although modern chemistry allows development of even more effective rocket propellants, energy efficiency is not the only consideration factor. For example, fluorine and its derivatives arc better oxidizers than oxygen, but their extreme toxicity make them environmentally dangerous. The same concerns prevent the use of beryllium hydride—an excellent fuel that combines high density with the energy efficiency comparable to liquid hydrogen. 

Ammonium dinitramide [ADN, NH4(N02)2] was first synthesized in Russia in 1972 (Luk yanov) and in the U.S. in 1989 (Bottaro, SRI) [132], ADN has been considered as a propellant ingredient due to its calculated performance and lack of chlorine, but its poor density somewhat offsets these advantages (Table 4). A considerable effort has been undertaken to solve its physical problems (poor morphology, high hygroscopicity, low thermal stability) and improve its synthesis [133-138]. In 1991, the first-bench scale synthesis produced material at 4000/lb. By 1997 Bofors had patented a aqueous solution preparation which afforded ADN at 525/lb. ADN is much less thermally stable than AN, probably due to a substantially lower melting point (94°C) and light sensitivity. However, like AN, addition... 

Propellant selection then should consider density, as well as specific impulse. For pure terrestrial weapon systems other considerations such as reliability, readibili-ty, cost, are also of importance. Other desirable properties of propellants are detailed below. 

In volume limited applications, high density propellant combinations are favored and some appropriate trade-off between performance and density is established. In a truly volume limited system as shown in section IV. A. 1., the appropriate performance parameter is the product of the specific impulse and the propellant bulk density, a quantity usually labeled the density impulse. Conceivably, mixture ratio may be determined by yet other vehicle system considerations. If a new propellant combination is to be utilized in an existing vehicle, the optimum mixture ratio may be influenced by such considerations as existing pump flow rate capacities, tank volumes, and structure load carrying capacities. Even other system considerations, such as the desirability of operating at equal fuel and oxidizer volume flow rates to allow interchange of fuel and oxidizer flow hardware, may determine the propellant mixture ratio. 

At pressures up to 40 tons/in2, corresponding to density of 0.35 g/cc, only the first 3 terms in the equation need be kept. Thus the pressure dependence of the thermodynamic props can be evaluated from a knowledge of the 2nd 3rd virial coeffs of the various gaseous products. Tables are presented which cover the range 1600° - 4000° K, and which have found considerable application in internal ballistics. These tables give covolumes of propellants with a systematic error of less than 5%- The basis of Corner s theory is the expression of the 2nd virial coefficient of a gas as a simple function of the parameters of the intermolecular field... 

The theory of initiation has mainly been expounded in the field of solid-propellant ignition. A recent effort for pyrotechnics has been published by Johnson.The calculations are difficult since they involve calories transferred to the surface of the initiated column at certain temperatures and over a time interval heat absorption and flow in the main item, heat developed in the main item from the incipient final reaction, and, of course, heat losses. Johnson advocates on the basis of theoretical considerations a small area on which lire transfer is concentrated low thermal conductivity, density, and specific heat of the first fire and use of high-energy, flare-type first fire mixtures in such a way that radiant heat transfer is optimal. 

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