FIGURE 7-3. Operating parameters for decomposed hydrazine at the exit of a catalytic reactor as a function of the ammonia dissociation fraction. Adapted with permission from Ref. 7-8.
The catalytic decomposition of hydrazine can be described ideally as a two-step process; this ignores other steps and intermediate products. First, hydrazine (N2H4) decomposes into gaseous ammonia (NH3) and nitrogen (N2); this reaction is highly exothermic, i.e., it releases heat. Secondly, the ammonia decomposes further into nitrogen and hydrogen gases, but this reaction is endothermic and absorbs heat. These simplified reactions can be written as
Here x is the degree of ammonia dissociation; it is a function of the catalyst type, size, and geometry, the chamber pressure, and the dwell time within the catalyst bed. Figure 7-3 shows several ideal rocket engine parameters for hydrazine monopropellant as a function of x, the fraction of ammonia that is decomposed. The values are for an ideal thruster at 1000 psia chamber pressure with an area ratio of 50 expanding at high altitude. The best specific impulse is attained when little ammonia is allowed to dissociate.
Hydrazine is manufactured in several grades of purity. The standard commercial hydrazine has about 1.5% maximum by weight of water, less than 1.0% aniline, and no more than 10 mg/1 of particulates, including carbon. Monopropellant-grade hydrazine has less than 1% water, less than 0.5% aniline (whch is a material commonly used in the manufacture of hydrazine), and traces of ammonia, carbon dioxide, chlorides, and iron- or carbon-containing materials such as UDMH or MMH. Aniline and other organic impurities can poison the catalyst used to decompose monopropellant hydrazine; as mentioned in Chapter 10, this can cause operating problems. There is also a highly purified grade of hydrazine that has less water, less than 0.005% aniline, and less than 0.003% carbon materials; it does not contaminate the catalyst and is used now in many monopropellant applications.
Hydroxyl Ammonium Nitrate (NH2OH+N03)
This is a relatively new, synthetic, propellant material rich in oxygen, but with combined hydrogen and nitrogen (fuel ingredients), it is abbreviated as HAN. It is an opaque hygroscopic solid when pure, and a clear colorless odorless liquid in aqueous solutions. The solid HAN (specific gravity of 1.84) is a potential solid propellant ingredient and the liquid HAN solution is a potential monopropellant (a 13 molar solution has a specific gravity of 1.523). Both can be made to burn smoothly and several catalysts have been effective in obtaining controlled decomposition. The boiling point (110 to 145°C) and the freezing point (-15 to -44°C) vary with the water content. HAN becomes more viscous as the percentage of water is reduced. The liquid is corrosive, toxic, denser than hydrazine monopropellant, and does not seem to be carcinogenic. The liquid is incompatible with alkali materials, many metals, and other materials. Even with relatively very compatible materials HAN solutions decompose slowly in storage; a satisfactory stabilizer has yet to be found. The monopropellant's specific impulse is between 200 and 265 sec, depending on the water content and the mixing of the aqueous HAN with one of several possible compatible organic fuel liquids. The HAN propellant formulation, its rocket engines, and solid motors are still in their research and development phase, as shown in Refs. 7-9 and 7-10.
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