Gaseous Propellants

Cold gas propellants have been used successfully for reaction control systems (RCS) for perhaps 50 years. The engine system is simple, consisting of one or more high-pressure gas tanks, multiple simple metal nozzles (often aluminum or plastic), an electrical control valve with each nozzle, a pressure regulator, and provisions for filling and venting the gas. The tank size will be smaller if the tank pressures are high. Pressures are typically between 300 and 1000 MPa (about 300 to 10,000 psi). The mass of spherical storage tanks is essentially independent of pressure if they contain the same mass of gas.

Typical cold gas propellants and some of their properties and characteristics are listed in Table 7-3. Nitrogen, argon, dry air, krypton and Freon 14 have been employed in spacecraft RCSs. With high-pressure hydrogen or helium as cold gas, the specific impulse is much higher, but the densities of these gases are much lower. This requires a much larger gas storage volume and heavier high-pressure tanks. In most applications the extra inert mass outweighs the advantage of better performance. In a few applications the gas (and its storage tank) are heated electrically or chemically. This improves the specific impulse and allows a smaller tank, but it also introduces complexity.

The selection of the gas propellant, the storage tanks, and RCS design depend on many factors, such as volume and mass of the storage tanks, the maximum thrust and total impulse, the gas density, required maneuvers, duty cycle, and flight duration. Cold gas systems are used for total impulses of perhaps 1200 N-sec or 5000 lbf-sec. Higher values usually employ liquid propellants.

TABLE 7-3. Properties of Gaseous Propellants Used for Auxiliary Propulsion

Propellant

Molecular Mass

Density'

Theoretical Specific Impulse' (sec)

Hydrogen

Helium

Methane

Nitrogen

Argon Krypton

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