Liquid Propellant

Liquid Propellant refers to any combustible liquid that is fed to the combustion chamber of a rocket engine. The petroleum that is commonly used as a rocket fuel is a type of highly refined kerosene, called RP-1 (for refined Petroleum-1). Usually it is burned with liquid oxygen (LOX) in rocket engines to provide thrust. Cryogenic propellants are very cold liquid propellants. The most commonly used are LOX, which serves as an oxidizer, and liquid hydrogen (LH₂), which serves as the fuel.

Cooling and compressing gaseous hydrogen and oxygen into liquids vastly increases their density, making it possible to store them (as cryogenic liquids) in smaller tanks. Unfortunately, the tendency of cryogenic propellants to return to their gaseous form unless kept super-cool makes them difficult to store for long periods of time. Therefore, cryogenic propellants are less satisfactory for use in strategic nuclear missiles or tactical military rockets, which must be kept launch-ready for months to years at a time. However, cryogenic propellants are favored for use in space boosters because of the relatively high thrust achieved per unit mass of chemical propellant consumed. For example, the space shuttle’s main engines burn LH₂ and LOX.

Hypergolic propellants are liquid fuels and oxidizers that ignite on contact with each other and need no ignition source. This easy start and restart capability makes these liquid propellants attractive for both crewed and un-crewed spacecraft maneuvering systems. Another positive feature of hypergolic propellants is the fact that, unlike cryogenic propellants, these liquids do not have to be stored at extremely low temperatures. One favored hypergolic combination is monomethyl hydrazine (MMH) as the fuel and nitrogen tetroxide (N2O4) as the oxidizer. However, both of these fluids are highly toxic and must be handled under the most stringent safety conditions. Hypergolic propellants have been mostly used in the core liquid propellant stages of the Titan family of launch vehicles.

Some of the desirable properties of a liquid rocket propellant are:

• high specific gravity (to minimize tank space)
• low freezing point (to permit cold-weather operation)
• stability (to minimize chemical deterioration during storage)
• Favorable thermo-physical properties (e.g., a high boiling point and high thermal conductivity—especially valuable in support of regenerative cooling of the combustion chamber and nozzle throat)
• Low vapor pressure (to support more efficient propellant pumping operations).

Some of the physical hazards encountered in the use of liquid propellants include toxicity, corrosion, ignitability (when exposed to air or heat), and explosive potential (when the propellant deteriorates, is shocked, or is excessively heated)

Questions to Ponder:

• Why are liquid propellants generally preferred over solid propellants?
• What provides the most thrust – Liquid, solid or gas propellants?
• Name the first fuel used as a liquid propellant.