D. P. MISHRA 6.1 INTRODUCTION
6.5 LIQUID PROPELLANTS
Liquid-propellant rocket engines in spite of inherent complexities are pre- ferred over the solid-propellant engines due to the added advantages (see Table 6.4) of liquid propellants. They have higher specific impulse and are capable of being throttled, shut down, and restarted easily. Liquid pro- pellants consist of liquid fuel and liquid oxidizer and certain liquid addi- tives. Several types of liquid propellants have been devised over the last six decades. Liquid hydrocarbons, liquid hydrogen, alcohols, and so on are examples of liquid propellants. Some of the examples of liquid oxidizers are liquid oxygen, nitric acid, and liquid fluorine. Liquid propellants can be classified based on the fuel–oxidizer arrangement, energy content, ignitabil- ity, and storability. Liquid propellants can be divided broadly into mono- propellants and bipropellants. In liquid monopropellants, both fuel and oxidizer elements are located in the same molecular structure. Examples of monopropellants are hydrogen peroxide (H2O2) and hydrazine (N2H4). The monopropellant can be decomposed in the presence of a suitable catalyst into high-temperature and high-pressure gases. Monopropellants can be further divided into (1) simple and (2) composite. In simple monopropel- lant, fuel and oxidizers are contained in the same molecule. For example, methyl nitrate (CH3NO3) can be decomposed into CH3O and NO2. But the composite monopropellant consists of a mixture of oxidizer and fuel. For example, nitric acid and amyl acetate can undergo exothermic reactions to be used as composite monopropellant. In case of liquid bipropellant, fuel and oxidizer are mixed separately to have exothermic reactions. Liquid hydrogen and liquid oxygen are examples of liquid bipropellants. Based on the nature of ignitability, liquid propellants can be broadly divided into two categories: (1) hypergolic and (2) nonhypergolic. In case of hypergolic pro- pellant, fuel and oxidizer when brought in contact will ignite spontaneously
without any external ignition energy. Some hypergolic propellants are hydrogen–fluorine (H2/F2), hydrazine–nitric acid (N2H4/HNO3), unsym- metrical dimethyl hydrazine–nitric acid (UDMH/HNO3), and ammonia–
fluorine (NH3/F2). Based on the energy contents, liquid propellants can be broadly classified into three categories: (1) low-energy, (2) medium- energy, and (3) high-energy propellants. Although the energy content of a propellant is dependent on the heat of combustion, in practice, this clas- sification is based on the level of specific impulse. Let us now discuss the physical and chemical properties of certain liquid propellants.
6.5.1 Liquid Fuels
Several kinds of liquid fuels that can be used as propellant in rocket engines have been devised and developed. These fuel compounds contain mainly some atoms of carbon, hydrogen, nitrogen, boron, metal hydrides, organo- metallics, and so on. Some examples of liquid fuels are kerosene, furfurly/
ethyl alcohols, hydrazine, aniline, amines, dimethyle hydrazine, xylidine, hydrogen, and ammonia whose properties are given Table 6.5. Some of these fuels are not being used in recent times. We will discuss in the following some of the commonly used liquid fuels in liquid-propellant rocket engine.
6.5.1.1 Hydrocarbon Fuels
Hydrocarbon fuels are a mixture of complex hydrocarbon chemicals that are basically refined from crude oil. Some of the fuels used for other engines, namely, kerosene, gasoline, jet fuels, can be used as propellants in rocket engines. For the rocket engine, a type of highly refined kerosene known as RP1 is devised, which is a mixture of saturated and unsaturated hydro- carbons with a narrow range of densities and vapor pressure. Generally, petroleum fuels are usually used in combination with liquid oxygen as the oxidizer. Note that RP1 and liquid oxygen are used as the propellant in the first-stage boosters of the Atlas, Titan, Delta II, and Saturn launch vehicles. Recently, Indian space research organization (ISRO) has devel- oped a rocket petroleum fuel, which is known as ISROsene, with certain appropriate ratio of olefins and aromatics to avoid coking during its opera- tion. Although petroleum fuels provide a specific impulse considerably less than cryogenic fuels, they are preferred due to their simplicity and cost- effectiveness. Besides, they are far superior to the hypergolic propellants.
In recent times, liquid methane (111 K) is considered to be a poten- tial cryogenic hydrocarbon fuel due to its highly reproducible physical properties as compared to other petroleum fuels. It can be used along with
TABLE 6.5Properties of Commonly Used Liquid Fuels FuelAmmoniaAnilineEthyl AlcoholFurfuryl AlcoholHydrazineDimethyl HydrazineXylidineTriethyl AmineKeroseneHydrogen FormulaNH3C6H5NH2C2H5OHC5H6O2N2H4(CH3)2N2H2C8H11N(C2H5)3NC10H20H2 Molecular weight (kg/kmol)17.03293.1246.0698.132.0560.1121.17101.191402.016 Density (kg/m)680 (239.7 K) 1012 (298 K) 785 (298 K) 1129 (298 K) 1011 (288 K) 808 (288 K) 980 (288 K) 723 (288 K)
800 (at 298 K)70.9 (at 20.5 K) Melting point (K)195.42279.6158.6241.2274.7215288.7158.223013.96 Boiling point (K)239.8457.6351.7444.2386.7354490.2362.52.039 Specific heat (kJ/kg
. K)
4.42.012.592.433.142.691.892.0934
7.327 (at 14 K) Heat of fusion 5.665.0212.6710.110.117230 (kJ/mol) Heat of vaporization 23.36653044.3438.640.1142.7135.0345.8131.442860.904348 (kJ/mol) Heat of formation −46.234.3−278−142.3550.4547.3163.64−154.912−247.0210 (kJ/mol) (at 298 K) Viscosity (centipoises ) to (kg/m
. s) 255 (239.6 K) 6600 (283 K) 2370 (313 K) 1400 (293 K) 8100 (283 K) 1290 (274 K)
1450 (253 K)
347 (298 K)
1600 (at 288 K)24 (at 14 K) Thermal conductivity (kJ/m
5s . K) × 10
5.017.716.720.920.7618.4912.090.0001558420.000066291
liquid oxygen in liquid rocket launch vehicles in future as it has higher performance than the state-of-the-art storable propellants, of course, with reduction in volume as compared to the LOX/LH2 system. Its overall lower vehicle mass is low as compared to the commonly used hypergolic pro- pellant. It is contemplated that this can be used for future Mars mission, because it can be manufactured partly from Martian in situ resources.
6.5.1.2 Hydrazine (N2H4)
Hydrazine (N2H4) is a colorless flammable but an excellent storable liq- uid with odor similar to ammonia gas. It can be used as a liquid fuel in the rocket engine, but it is quite toxic and unstable in nature to be used as a coolant. Hydrazine provides the best performance as a rocket fuel, but it has a high freezing point. It decomposes easily in the presence of a suitable catalyst and hence can be used as an excellent monopropellant.
Some catalysts that can decompose hydrazine are iridium, iron, nickel, and cobalt. Iridium is found to be suitable for decomposition of hydra- zine even at room temperature. But at higher temperatures beyond 450 K, several catalysts such as iron, nickel, and cobalt can be used to decompose hydrazine. Generally, hydrazine decomposes into gaseous ammonia and nitrogen, undergoing exothermic reactions leading to flame temperature of 1649 K. But ammonia gets decomposed further to form nitrogen and hydrogen with overall endothermic reaction, which results in a lower tem- perature. The global reaction model for the decomposition of hydrazine (N2H4) is given as follows:
N2H4→ xNH3 + (1 − x/2) N2 + (2 − 3x/2) H2 (6.6) where x is the degree of ammonia decomposition, which will be depen- dent on the catalyst type, size, geometry, chamber pressure, and residence time in the catalyst bed. When all ammonia is dissociated to hydrogen and nitrogen, then the flame temperature is around 867 K, which is not desir- able. Hence, it is recommended to have least dissociation of ammonia for achieving higher specific impulse. In practice, 20%–40% ammonia disso- ciation is being preferred as it produces maximum specific impulse (189 s).
Hydrazine can be used as a hypergolic bipropellant along with nitric acid (HNO3) or nitrogen tetroxide (N2O4) as it has short ignition delay and can be ignited easily with these oxidizers.
Two more derivates of hydrazine fuels with similar physical and ther- mochemical properties, namely, monomethyl hydrazine (MMH) and