Liquid Oxidizers

Many different types of storable and cryogenic liquid oxidizer propellants have been used, synthesized, or proposed. For high specific impulse this includes boron-oxygen-fluorine compounds, oxygen-fluorine compounds, nitrogen-fluorine formulations, and fluorinated hydrocarbons; however, they all have some undesirable characteristics and these synthetic oxidizers have not been proven to be practical. Oxidizer liquids that have been used in experimental liquid rocket engines include mixtures of liquid oxygen and liquid fluorine, oxygen difluoride (OF2), chlorine trifluoride (C1F3), or chlorine pentafluoride (C1F5). All of these are highly toxic and very corrosive. Several commonly used oxidizers are listed below.

Liquid Oxygen (02)

Liquid oxygen, often abbreviated as LOX, boils at 90 K at atmospheric pressure; at these conditions it has a specific gravity of 1.14 and a heat of vaporization of 213 kJ/kg. It is widely used as an oxidizer and burns with a bright white-yellow flame with most hydrocarbon fuels. It has been used in combination with alcohols, jet fuels (kerosene-type), gasoline, and hydrogen. As shown in Table 5-5, the attainable performance is relatively high, and liquid oxygen is therefore a desirable and commonly used propellant in large rocket engines. The following missiles and space launch vehicles use oxygen: (1) with jet fuel— Atlas, Thor, Jupiter, Titan I, Saturn booster; (2) with hydrogen—Space Shuttle and Centaur upper stage; (3) with alcohol—V-2 and Redstone. Figures 1-4 and 6-1 show units that use oxygen. Figures 5-1 to 5-6 give theoretical performance data for liquid oxygen with a kerosene-type fuel.

Although it usually does not burn spontaneously with organic materials at ambient pressures, combustion or explosions can occur when a confined mixture of oxygen and organic matter is suddenly pressurized. Impact tests show that mixtures of liquid oxygen with many commercial oils or organic materials will detonate. Liquid oxygen supports and accelerates the combustion of other materials. Handling and storage are safe when contact materials are clean. Liquid oxygen is a noncorrosive and nontoxic liquid and will not cause the deterioration of clean container walls. When in prolonged contact with human skin, the cryogenic propellant causes severe burns. Because liquid oxygen evaporates rapidly, it cannot be stored readily for any great length of time. If liquid oxygen is used in large quantities, it is often produced very close to its geographical point of application. Liquid oxygen can be obtained in several ways, such as by boiling liquid nitrogen out of liquid air.

It is necessary to insulate all lines, tanks, valves, and so on, that contain liquid oxygen in order to reduce the evaporation loss. Rocket propulsion systems which remain filled with liquid oxygen for several hours and liquid oxygen storage systems have to be well insulated against absorbing heat from the surroundings. External drainage provisions have to be made on all liquid oxygen tanks and lines to eliminate the water that condenses on the walls.

Example 7-1. Estimate the approximate temperature and volume change of liquid oxygen if an oxygen tank is pressurized to 8.0 atmospheres for a long time before engine start. Assume the tank is 60% full and the evaporated oxygen is refrigerated and recondensed (constant mass).

SOLUTION. Using Table 7-1 and Figs. 7-1 and 7-2, the vapor pressure goes from 1.0 atm (0.1 MPa) to 8 atm (about 0.8 MPa) and the equilibrium temperature goes from the boiling point of 90 K at 1.0 atm to about 133 K. The corresponding specific gravities are 1.14 and 0.88 respectively. This is an increase of 1.14/0.88 = 1.29 or about 77% full (29% more volume).

In tanks with turbopump feed systems the actual tank pressures are lower (typically 2 to 4 atm) and the evaporated oxygen is vented, causing a cooling effect on the liquid surface. So the numbers calculated above are too large (8 atm was selected to clearly show the effect). The warming occurs when there is a long hold period of a pressurized cryogenic propellant tank and is most pronounced when the final portion of the pro-pellant is being emptied. Nevertheless the higher temperature, higher vapor pressure, and lower density can cause changes in mixture ratio, required tank volume, and pump suction condition (see Section 10.1). Therefore tanks with cryogenic propellant are insulated (to minimize heat transfer and density changes) and are pressurized only shortly before engine start, so as to keep the propellant at its lowest possible temperature.

Hydrogen Peroxide (H202)

In rocket application, hydrogen peroxide has been used in a highly concentrated form of 70 to 99%; the remainder is mostly water. Commercial peroxide is approximately 30% concentrated. Concentrated hydrogen peroxide was used in gas generator and rocket applications between 1938 and 1965 (X-l and X-15 research aircraft).

In the combustion chamber, the propellant decomposes according to the following chemical reaction, forming superheated steam and gaseous oxygen:

This decomposition is brought about by the action of catalysts such as various liquid permanganates, solid manganese dioxide, platinum, and iron oxide. In fact, most impurities act as a catalyst. H202 is hypergolic with hydrazine and will burn well with kerosene. The theoretical specific impulse of 90% hydrogen peroxide is 154 sec, when used as a monopropellant with a solid catalyst bed.

Even under favorable conditions H202 will often decompose at a slow rate during storage, about one percent per year for 95%, and gas will bubble out of the liquid. Contaminated liquid peroxide must be disposed of before it reaches a danger point of about 448 K, when an explosion usually occurs. Concentrated peroxide causes severe burns when in contact with human skin and may ignite and cause fires when in contact with wood, oils, and many other organic materials. In the past rocket engines with hydrogen peroxide oxidizer have been used for aircraft boost (German Me 163, and U.S. F 104) and a missile (Britain: Black Knight). It has not been used for a long time, partly because of its long-term storage stability. However, there has been some improvement and some renewed interest in this dense oxidizer, which produces a nontoxic exhaust.

Nitric Acid (HN03)

There are several types of nitric acid mixtures that have been used as oxidizers between 1940 and 1965; they are not used extensively today in the United States. The most common type, red fuming nitric acid (RFNA), consists of concentrated nitric acid (HN03) that contains between 5 and 20% dissolved nitrogen dioxide. The evaporating red-brown fumes are exceedingly annoying and poisonous. Compared to concentrated nitric acid (also called white fuming nitric acid), RFNA is more energetic, more stable in storage, and less corrosive to many tank materials.

Nitric acid is highly corrosive. Only certain types of stainless steel, gold, and a few other materials are satisfactory as storage containers or pipeline materials. A small addition of fluorine ion (less than 1 % of HF) inhibits the nitric acid, causes a fluoride layer to form on the wall, and greatly reduces the corrosion with many metals. It is called inhibited red fuming nitric acid (IRFNA). In case of accident of spilling, the acid should be diluted with water or chemically deactivated. Lime and alkali metal hydroxides and carbonates are common neutralizing agents. However, nitrates formed by the neutralization are also oxidizing agents and must be handled accordingly.

Nitric acid has been used with gasoline, various amines, hydrazine, dimethylhydrazine, and alcohols. It ignites spontaneously with hydrazine, fur-furyl alcohol, aniline, and other amines. The specific gravity of nitric acid varies from 1.5 to 1.6, depending on the percentages of nitric oxide, water, and impurities. This high density permits compact vehicle construction.

Vapors from nitric acid or red fuming nitric acid have an OSHA 8-hr personnel exposure limit or a threshold work allowance of 2 ppm (parts per million or about 5 mg/m3) and a short-term exposure limit of 10 ppm. Droplets on the skin cause burns and sores which do not heal readily.

Nitrogen Tetroxide (N204)

This is a high-density yellow-brown liquid (specific gravity of 1.44). Although it is the most common storable oxidizer used in the United States today, its liquid temperature range is narrow and it is easily frozen or vaporized. It is only mildly corrosive when pure, but forms strong acids when moist or allowed to mix with water. It readily absorbs moisture from the air. It can be stored indefinitely in sealed containers made of compatible material. It is hypergolic with many fuels and can cause spontaneous ignition with many common materials, such as paper, leather, and wood. The fumes are reddish brown and are extremely toxic. Because of its high vapor pressure it must be kept in relatively heavy tanks. The freezing point of N204 can be lowered (by adding a small amount of nitric oxide or NO) but at the penalty of a higher vapor pressure. This mixture of NO and N204 is called mixed oxides of nitrogen (MON) and different grades have been 2 and 30% NO content.

Nitrogen tetroxide is a storable propellant oxidizer and is used in the Titan missile together with a fuel mixture consisting of hydrazine and unsymmetrical dimethylhydrazine. It is also used with monomethylhydrazine fuel in the Space Shuttle orbital maneuver system and reaction control system and in many spacecraft propulsion systems. In many of these applications care must be taken to avoid freezing this propellant. The OSHA 8-hr personnel exposure limit is 5 ppm or 9 mg/m3.

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