high flame temperature) but usually a fuel-rich mixture containing a large portion of low-molecular-mass reaction products, as shown in Chapter 5.
If very small metallic fuel particles of beryllium or aluminum are suspended in the liquid fuel, it is theoretically possible to increase the specific impulse by between 9 and 18%, depending on the particular propellant combination, its mixture ratio and the metal powder additive. Gelled propellants with suspended solid particles have been tested successfully with storable fuels. For gelled propellants, see Section 7.5.
The chemical propellant combination that has the highest potential specific impulse (approximately 480 sec at 1000 psia chamber pressure and expansion to sea level atmosphere, and 565 sec in a vacuum with a nozzle area ratio of 50) uses a toxic liquid fluorine oxidizer with hydrogen fuel plus suspended toxic solid particles of beryllium; as yet a practical means for storing these propellants and a practical rocket engine have not been developed.
Vapor pressure, atm
Common Physical Hazards
Although the several categories of hazards are described below, they do not all apply to every propellant. The hazards are different for each specific propellant and must be carefully understood before working with that propellant. The consequences of unsafe operation or unsafe design are usually also unique to several propellants.
Corrosion. Various propellants, such as nitrogen tetroxide or hydrogen peroxide, have to be handled in containers and pipelines of special materials. If the propellant were permitted to become contaminated with corrosion products, its physical and chemical properties could change sufficiently to make it unsuitable for rocket operation. The corrosion of the gaseous reaction products is important in applications in which the reaction products are likely to damage structure and parts of the vehicle or affect communities and housing near a test facility or launch site.
Explosion Hazard. Some propellants, such as hydrogen peroxide and nitro-methane, are unstable and tend to detonate under certain conditions of impurities, temperature, and shock. If liquid oxidizers (e.g., liquid oxygen) and fuels are mixed together they can be detonated. Unusual, rare flight vehicle launch or transport accidents have caused such mixing to occur (see Refs. 7-2 and 7-3).
Fire Hazard. Many oxidizers will start chemical reactions with a large variety of organic compounds. Nitric acid, nitrogen tetroxide, fluorine, or hydrogen peroxide react spontaneously with many organic substances. Most of the fuels are readily ignitable when exposed to air and heat.
Accidental Spills. Unforeseen mishaps during engine operation and traffic accidents on highways or railroads while transporting hazardous materials, including propellants, have on occasion caused spills, which expose people to unexpected fires, or potential health hazards. The U.S. Department of Transportation has rules for marking and containing hazardous materials during transport and also guidelines for emergency action (see Ref. 7—4).
Health Hazards. Many propellants are toxic or poisonous, and special precautions have to be taken to protect personnel. Fluorine, for example, is very poisonous. Toxic propellant chemicals or poisonous exhaust species can enter the human body in several ways. The resulting health disorders are propellant specific. Nitric acid can cause severe skin burn and tissue disintegration. Skin contact with aniline or hydrazine can cause nausea and other adverse health effects. Hydrazine and its derivatives, such as dimethylhydrazine or hydrazine hydrate, are known carcinogens (cancer-causing substances). Many propellant vapors cause eye irritation, even in very small concentration. Inadvertent swallowing of many propellants can also cause severe health degradation.
The inhalation of certain toxic exhaust gases or gaseous or vaporized propellants is perhaps the most common health hazard. It can cause severe damage if the exposure is for long duration or in concentrations that exceed established maximum threshold values. In the United States the Occupational Safety and Health Administration (OSHA) has established limits or thresholds on the allowable exposure and concentration for most propellant chemicals. Several of these propellant gas threshold limits are mentioned later in this chapter. Toxic gases in the exhaust could include hydrofluoric acid (HF) gas; its OSHA 8-hr personnel exposure limit is 3 ppm (volumetric parts per million) and its short-term (typically, 15 min) exposure limit is 6 ppm. A concentration of 3000 ppm or 0.3% can be fatal within a few seconds. Pentaborane, which is very toxic and has been used in experimental engines, has an 8-hr personnel exposure limit at a threshold of 0.005 ppm. References 7-2 and 7-5 give more information on toxic effects.
The corrosion, explosion, and fire hazards of many propellants put severe limitations on the materials, the handling, and the design of rocket-propelled vehicles and their engine compartments. Not only is the rocket system itself exposed to the hazardous propellant, but adjacent personnel, structural parts, electrical and other vehicle equipment, and test and launch facilities have to be properly protected against the effects of possible leaks, fumes, and fires or explosions from propellant accumulations.
Material Compatibility. Many liquid propellants have only a limited number of truly compatible materials, both metals and nonmetals, such as gaskets or O-rings. There have been unfortunate failures (causing fires, leakage, corrosion, or malfunctions) when an improper or incompatible material was used in the hardware of a rocket engine. Depending on the specific component and loading conditions, these structural materials have to withstand high stresses, stress corrosion, high temperatures, or abrasion. Several specific material limitations are mentioned in the next section. Certain materials catalyze a self-decomposition of stored hydrogen peroxide into water and oxygen, making long-term storage difficult and, if confined, causing its container to explode. Many structural materials, when exposed to cold, cryogenic propellants, can become very brittle.
Low Freezing Point. This permits operation of rockets in cold weather. The addition of small amounts of special chemicals has been found to help depress the freezing point of some liquid propellants which solidify readily at relatively high temperature.
High Specific Gravity. In order to accommodate a large mass of propellants in a given vehicle tank space, a dense propellant is required. It permits a small vehicle construction and, consequently, a relatively low structural vehicle mass and low aerodynamic drag. Specific gravity, therefore, has an important effect on the maximum flight velocity and range of any rocket-powered vehicle or missile flying within the earth's atmosphere, as explained in Chapter 4. Specific gravities for various propellants are plotted in Fig. 7-1. A variation of the temperature of stored propellant will cause change in liquid level in the tank.
For any given mixture ratio r, the average specific gravity of a propellant combination <Sav can be determined from the specific gravities of the fuel 8f and of the oxidizer 80. The average specific gravity is defined as the mass of the fuel and oxidizer, divided by the sum of their volumes. Here the mixture ratio is defined as the oxidizer mass flow rate divided by the fuel mass flow rate.
Values of <5av for various propellant combinations are listed in Table 5-5. The value of <5av can be increased by adding heavy materials to the propellants, either by solution or colloidal suspension. The identical type of equation can be written for the average density pav in terms of the fuel density and the oxidizer density.
In the SI system of units the specific gravity has the same numerical value as the density expressed in units of grams per cubic centimeter or kg/liter. In some performance comparisons the parameter density specific impulse Id is used. It is defined as the product of the average specific gravity 8 and the specific impulse hr h = (7-3)
Stability. No deterioration and no decomposition with long-term (over 15 years) storage and minimal reaction with the atmosphere have been attained with many propellants. Good chemical stability means no decomposition of the liquid propellant during operation or storage, even at elevated temperature. A good liquid propellant should also have no chemical deterioration when in contact with piping, tank walls, valve seats, and gasket materials, even at relatively high ambient temperatures. No appreciable absorption of moisture and no adverse effects of small amounts of impurities are desirable properties. There should be no chemical deterioration when liquid flows through the hot cooling jacket passages. Some hydrocarbons (e.g., olefins) decompose and form carbonaceous deposits on the hot inside surfaces of the cooling passage. These deposits can be hard, reduce the heat flow, increase the local metal temperatures, and thus can cause the metal to weaken and fail. About 1% per year of stored concentrated hydrogen peroxide decomposes in clean storage tanks. Between 1 and 20% of a cryogenic propellant (stored in a vehicle) evaporates every day in an insulated tank.
Heat Transfer Properties. High specific heat, high thermal conductivity, and a high boiling or decomposition temperature are desirable for propellants that are used for thrust chamber cooling (see Section 8.3).
Pumping Properties. A low vapor pressure permits not only easier handling of the propellants, but also a more effective pump design in applications where the propellant is pumped. This reduces the potential for cavitation, as explained in Chapter 10. If the viscosity of the propellant is too high, then pumping and engine-system calibration become difficult. Propellants with high vapor pressure, such as liquid oxygen, liquid hydrogen, and other liquefied gases, require special design provisions, unusual handling techniques, and special low-temperature materials.
Temperature Variation. The temperature variation of the physical properties of the liquid propellant should be small. For example, a wide temperature variation in vapor pressure and density (thermal coefficient of expansion) or an unduly high change in viscosity with temperature makes it very difficult to accurately calibrate a rocket engine flow system or predict its performance over any reasonable range of operating temperatures.
Ignition, Combustion, and Flame Properties
If the propellant combination is spontaneously ignitable, it does not require an ignition system. This means that burning is initiated as the oxidizer and the fuel come in contact with each other. Spontaneously ignitable propellants are often termed hypergolic propellants. Although an ignition system is not a very objectionable feature, its elimination is usually desirable because it simplifies the propulsion system. All rocket propellants should be readily ignitable and have a small ignition time delay in order to reduce the potential explosion hazard during starting. Starting and ignition problems are discussed further in Section 8.4.
Nonspontaneously ignitable propellants have to be heated by external means before ignition can begin. Igniters are devices that accomplish an initial slight pressurization of the chamber and the initial heating of the propellant mixture to the point where steady flow combustion can be self-sustained. The amount of energy added by the igniter to activate the propellants should be small so that low-power ignition systems can be used. The energy required for satisfac tory ignition usually diminishes for increasing ambient temperature of the propellant.
Certain propellant combinations burn very smoothly without combustion vibration. Other propellant combinations do not demonstrate this combustion stability and, therefore, are less desirable. Combustion is treated in Chapter 9.
Smoke formation is objectionable in many applications because of the smoke deposits on the surrounding equipment and parts. Smoke and brilliantly luminous exhaust flames are objectionable in certain military applications, because they can be easily detected. In some applications the condensed species in the exhaust gas can cause surface contamination on spacecraft windows or optical lenses and the electrons in the flame can cause undesirable interference or attenuation of communications radio signals. See Chapter 18 for information on exhaust plumes.
The propellant properties and quality must not vary, because this can affect engine performance, combustion, and physical or chemical properties. The same propellant must have the same composition, properties, and storage or rocket operating characteristics if manufactured at different times or if made by different manufacturers. For these reasons propellants are purchased against specifications which define ingredients, maximum allowable impurities, packaging methods or compatible materials, allowable tolerances on physical properties (such as density, boiling point, freezing point, viscosity, or vapor pressure), quality control requirements, cleaning procedures for containers, documentation of inspections, laboratory analyses, or test results. A careful chemical analysis of the composition aand impurities is necessary. Reference 76 describes some of these methods of analysis.
Altering and tailoring propellant properties can be achieved with additives. For example, to make a non-hypergolic fuel become hypergolic (readily ignited), a reactive ingredient has been added. To desensitize concentrated hydrogen peroxide and reduce self-decomposition, it is diluted with 3 to 15% water. To increase density or to alleviate certain combustion instabilities, a fine powder of a heavy solid material can be suspended in the propellant.
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