Thrust-decay time

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Thrust-decay time is a term used in rocket engine technology. It is the time (usually in seconds) between engine cut-off as ordered by human or computer guidance input, and the actual complete loss of thrust. Usually engine cut-off is achieved by closing valves that supply propellant to the engine combustion chamber or feed pumps. However, in all practical designs the combustion chamber will still have an amount of propellant in it undergoing the chemical process (or thermal process in the case of nuclear rocket engines) that creates thrust. Thrust decays to zero when chamber pressure is equal to atmospheric pressure (essentially zero in vacuum). Thrust-decay time is not very predictable and can usually only be measured by firing tests of the particular engine.

Rocket engine jet engine using stored propellant to produce jet propulsion

A rocket engine uses stored rocket propellants as reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust in accordance with Newton's third law. Most rocket engines use the combustion of reactive chemicals to supply the necessary energy, but non-combusting forms such as cold gas thrusters and nuclear thermal rockets also exist. Vehicles propelled by rocket engines are commonly called rockets. Rocket vehicles carry their own oxidizer, unlike most combustion engines, so rocket engines can be used in a vacuum to propel spacecraft and ballistic missiles.

Thrust reaction force which expels or accelerates mass in one direction. which is physical possible

Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system. The force applied on a surface in a direction perpendicular or normal to the surface is also called thrust. Force, and thus thrust, is measured using the International System of Units (SI) in newtons, and represents the amount needed to accelerate 1 kilogram of mass at the rate of 1 meter per second per second. In mechanical engineering, force orthogonal to the main load is referred to as thrust.

A propellant or propellent is a chemical substance used in the production of energy or pressurized gas that is subsequently used to create movement of a fluid or to generate propulsion of a vehicle, projectile, or other object. Common propellants are energetic materials and consist of a fuel like gasoline, jet fuel, rocket fuel, and an oxidizer. Propellants are burned or otherwise decomposed to produce the propellant gas. Other propellants are simply liquids that can readily be vaporized.

Thrust-decay time is of some importance in rocket staging, because long thrust-decay times can cause catastrophic collisions between stages if the next-stage rocket engine experiences thrust variations.

However, thrust-decay time is a critical variable in vertical landings of rocket-powered spacecraft (like the Lunar Module or the DC-X) or aircraft, because in the time between actual touchdown and complete loss of thrust the vehicle is highly unstable and may topple over.

Apollo Lunar Module A lander used in the Apollo program.

The Apollo Lunar Module, or simply lunar module, originally designated the Lunar Excursion Module (LEM), was the lander spacecraft that was flown from lunar orbit to the Moon's surface during the U.S. Apollo program. It was the first crewed spacecraft to operate exclusively in the airless vacuum of space, and remains the only crewed vehicle to land anywhere beyond Earth.

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