Propellant has two uses. In one, as a noun, it is a synonym for propellent - a substance or object which, when expended, provides thrust or propels something. An example of the former includes rocket fuel as opposed to oxidizer, if it is not a monopropellant). For the latter, compressed gas or pyrotechnics, or the base materials to create those on demand in order to provide impulse, count. For example, the compressed gas (or liquified gas, in extreme cases) in an aerosol can is placed there not as a payload, but as a propellant for the active ingredient.

In its second form, propellant is an adjective which describes something which provides propulsion. Example: "The oxygen leaking from the rear of the spacecraft had a propellant effect, exerting several pounds of thrust."

Types of Propellant (noun)

Rocket fuels fall into many types. One axis of variance is the number of substances which must be combined in order to produce the reaction desired to produce thrust. Thus, we have monopropellants - substances which either provide thrust without combustion (such as compressed gas thrusters) or which are unstable enough to combust without a separate oxidizer. Black powder can be considered a monopropellant, as it does not require ambient oxygen to burn.

Bipropellants require the mixing of two distinct substances in order to achieve the desired reaction. Typically these consist of a fuel and an oxidizer to allow the reaction to continue in a vacuum. For example, the Space Shuttle Main Engines burn hydrogen and oxygen - and are thus bipropellant engines. Early boosters (and some modern ones) burn kerosene with a liquid oxygen oxidizer; these, too, are bipropellant. Note that the definition of 'substance' here does not necessarily mean chemically distinct; it means a substance which is stored and handled as one component. Black Powder, for example, is composed of a variety of materials, but is considered a monopropellant because it is handled as one substance.

Some propellants are considered hypergolic. These propellants are chosen and formulated such that no ignition procedure is required to start the reaction; simply mixing them will cause them to spontaneously react. Many maneuvering thrusters use hypergolics, as engines which burn hypergolics are mechanically much simpler. A common example of a hypergolic bipropellant setup is one that utilizes nitrogren tetroxide as an oxidizer and monomethyl hydrazine as a fuel. Note that hypergolics are almost always horrifically toxic, so don't try this at home! This is why the Space Shuttle sits by itself for so long after landing - not only to cool off, but to make sure that no hypergolics from the maneuvering thrusters are present. The hypergolic storage tanks have bleed-off valves; as the shuttle enters the atmosphere and heats up, the hypergolics will vent to relieve pressure. You'll see ground crew who first approach the shuttle wearing chemical protection gear - until the thrusters are defueled and made safe, it is dangerous to approach the orbiter.

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