Liquid Bipropellant Systems

The requirements for selecting a fuel and oxidizer as a liquid bipropellant system are usually a compromise between the demands of the vehicle system, the propulsion system, and the propellants themselves. The vehicle and propulsion system will determine performance levels, physical property requirements, thermal requirements, auxiliary combustion requirements, degree of storability and package-ability, hypergolicity, etc. The final propellant selection must not only satisfy such requirements but is also dictated by thermochemical demands which the fuel and oxidizer make on each other. Frequently, specifically required properties are achieved through the use of chemical additives and/or propellant blending. 

As a result of this constant evaluation and compromise between the demands of the vehicle and propulsion systems and the current propellant technology, various liquid propellant systems have been developed and are being applied in current vehicle systems (Table IV). In Table IV thrust level is used to demonstrate the size of the propulsion system. Some of the systems in this table have been phased out, while others are still in development. However, Table IV does represent the current status of operational liquid bipropellant systems. 

Nitroparaffins. Nitro-organics as a class of compounds are of greater interest in solid than liquid propellants. However, some nitroparaffins have been utilized in liquid systems. Nitromethane has been used as a monopropellant, while tetranitromethane, hexanitroethane, nitroform, and nitroform salts have been considered as oxidizing components in bipropellant systems. The high melting points of the latter and their sensitivity restrict their use to solutes in lower freezing, insensitive liquids. A solution of 70 wt. % C(N02)4 in N204 is an example of such a system and has been proposed as an oxidizer (3). 

The use of a heterogeneous fuel, in which the metal compound is suspended in a liquid fuel, avoids a third storage vessel because it is used with the oxidizer as in a conventional bipropellant system. The technical problems are then associated only with the stabilization of the suspension, with the rheological properties of the stabilized fuel, and with the reactivity of the suspended solid with its carrier. 

Applications. To date, the liquid propellant systems used in chemical propulsion range from a small trajectory control thruster with only 0.2 lbf (0.89 N) thrust for orbital station-keeping to large booster rocket engines with over l. 0 million lbf (4.44 MN) thrust. Bipropellant propulsion systems are the most extensively used type today for... 

Fig. 8.34 The two generic types of bipropellant liquid propulsion systems used to launch spacecraft where liquid hydrogen is the fuel and liquid oxygen is the oxidizer.

Traditionally the liquid systems are considered to be either bipropellants or mono-propellants although as discussed in later sections multicomponent systems are feasible. The most common liquid systems are the bipropellant ones in which the fuel and oxidizer are introduced separately into the rocket combustion chamber. 

We have seen that different ionic liquids are proposed for the different propulsion systems (monopropellants, hypergolic bipropellants, ion electrospray, hydrid engines). The current and future challenges that can be drawn from this review are summarized as follows (i) to find new and safe EILs whose thermal or catalytic decomposition avoid the formation of nitric acid as a primary product (ii) to develop new catalysts able to decompose EILs at low temperature and stable at high temperature (iii) to find new fuels associated to IL oxidizers but with no inhibiting properties for the catalysts and (iv) to develop cheaper and safer synthetic method to prepare ILs. 

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