Propellant

A propellant is a mass that is expelled or expanded to create thrust. Thrust is a motive force that moves an object forward. This process follows Newton's third law of motion. This law states that every action has an equal and opposite reaction. To move a vehicle forward, the engine must eject the propellant backward.

This concept applies to many different things. It can move a large rocket or a small projectile. It can even move a fluid payload. In many vehicles, the engine that performs this expulsion is called a reaction engine.

It is important to distinguish between fuel and propellant. Fuel is the substance used to produce energy. Propellant is the reaction mass that is actually pushed out to create movement. In chemical rocket design, people often use the word "propellant" to describe a substance that contains both. However, these are two distinct scientific concepts. The fuel provides the energy, while the propellant provides the mass for the reaction. Even the byproducts of burning fuel can act as a propellant mass. For example, burning hydrogen and oxygen creates H2O, or water. This steam can then be expelled to provide thrust.

There are several ways to power a propellant. Chemical rockets use chemical reactions to create energetic gases. These gases are directed through a nozzle to produce thrust. The resulting exhaust can be a gas, a liquid, a solid, or even a plasma. In aircraft like jets, the propellant is often the product of burning fuel with oxygen from the air. This means the propellant has more mass than the fuel originally carried. Other systems use compressed fluids for movement. A water rocket uses compressed air as fuel to expel water as the propellant.

Spacecraft also use electrical energy to accelerate propellant. One method uses an electrostatic force to expel positive ions. Another method uses the Lorentz force to expel negative ions and electrons. Some engines, called electrothermal engines, use electromagnetic force. These engines heat low molecular weight gases like hydrogen or helium into a plasma. A resistojet engine works differently. It simply uses resistive heating to heat compressed propellant as it is expelled. This increases the thrust produced by the engine.

Solid propellants are often stored in shapes called grains. A grain is any individual particle of fuel or propellant. The size and shape of a grain are very important. They determine the burn time and the rate of energy produced. This affects the thrust profile over time. There are three main types of burns. A progressive burn uses grains with star shapes or perforations to increase surface area. A degressive burn uses simple shapes like cylinders or spheres. A neutral burn uses a single perforation so the surface area stays constant.

Scientists categorize solid propellants into four different compositions. Single-based propellants use nitrocellulose as the main ingredient. Double-based propellants add nitroglycerin to the nitrocellulose to increase energy. Triple-based propellants use nitrocellulose, nitroguanidine, and nitroglycerin, which are often used in cannons. Finally, composite propellants do not use organic nitrates as the primary part. Instead, they use a fuel like aluminum and an oxidizer like ammonium perchlorate. These composites are frequently used in large rocket motors.

Most propellants are stored as solids or liquids to keep them dense. This makes them easier to carry in a vehicle. Some advanced ideas involve using light as a propellant. A proposed photon rocket would use the relativistic momentum of photons. Photons do not have mass, but they move at the speed of light. Because they move so fast, they can still create thrust. To understand this, scientists must use relativistic physics instead of Newton's laws. This shows how the study of propellants connects to the most complex parts of physics.