Rocket engine

Template:TOC-right A rocket motor, also called rocket engine, actually propels a rocket-propelled guided missile or unguided rocket. The most general definition of such a propulsion system simply requires that it be independent of atmosphere, and propels a vehicle by the reaction to an action that releases energy in the opposite direction, one can consider even more exotica, such as ion generator propulsion. In general terms, two factors characterize the efficiency of a reaction-based propulsion system: the specific impulse (energy of the emitted particles) and the mass of the system that produces the particles. With conventional rockets, the relationship with these factors change constantly, because as propellants are turned into exhaust, the mass of the propulsion system decreases; the implications of a changing mass ratio are a matter of overall vehicle design and exterior ballistics, and beyond the scope of this article.

The most common forms,however, use chemical reactions to generate hot, fast-moving gases that are directed through an exhaust nozzle, and, as a reaction to the action of the moving gases, the motor develops thrust in the opposite direction to the exhaust gas flow. In these reactions, the reactants may be a single solid, a single liquid, or multiple liquid. As opposed to a gun, there is gas generation for seconds or even minutes.

Performance parameters

Specific impulse

Specific impulse is the total energy delivered while the propulsion system is active; if TotalImpulse is treated as the integral of the thrust delivered over the active time, changing mass-ratio drops out of the equation. Since only the engine is being considered here, that simplification is reasonable, but in real systems, mass-ratio becomes even more complex, because complex missiles and space launch vehicles are built up from stages of rockets. When a stage completes its contribution to the flight energy, it is jettisoned, so practical mass-ratio does not only change as a function of propellant release, but of the release of entire rocket engines.

I_sp=TotalImpulse/TotalMass

Solid fuel systems

Solid propellants allow much simpler construction, as they do not need pumps to move them into a combustion chamber. Indeed, black gunpowder, as the first solid propellant, was introduced in the 13th century.

Solids are stable in storage for an appreciable period of time. Their greatest disadvantage is that, other than some experimental devices, they burn until there is no more propellant, and the burning rate cannot be adjusted in real time. The highest-energy solid propellants do not produce as much energy as the highest-energy liquid propellants, but this is not a major restriction; all modern military rockets used for propulsion, as opposed to directional adjustment, are solid-propellant.

Liquid fuel systems

First demonstrated in 1926, liquid engines offer much more control over thrust, and generally higher energy than solid propellants. usually mix two liquids, a fuel and an oxidizer, although there are monopropellants. Every fuel-oxidizer system has advantages and disadvantages. The highest-energy mixes, such as hydrogen and oxygen, use cryogenic, intensely cold liquids, which are difficult to store and pump. Cryogenic systems also need an ignition system.

Storable liquid propellants, such as a hydrazine-based compound and red fuming nitric acid or a relative, can be held at normal temperatures, but tend to be very toxic and corrosive. The combinations are often hypergolic, igniting on contact. The hypergolic property removes the need for an ignition system, but can cause catastrophes if a leak brings them into unintended contact.

At a minimum, a liquid fuel system has propellant storage, a means of transferring the propellant(s) to the combustion chamber, one or more combustion chambers, and one or more subsystems to control the release of the products of combustion. There have been exotic variants, such as pumping fluid. to be used as reaction mass, but energized not by a chemical reaction but by heat generated from controlled nuclear fission.

Transfer

While some transfer systems rely on pressurized tanks, perhaps with a variable-flow valve, most high-performance transfer requires pumps. Pumps need a source of power, which is usually separate from the main reaction. For example, the V-2 pumps used concentrated hydrogen peroxide breakdown to produce a gas stream that ran pumps for the liquid oxygen oxidizer and ethanol fuel.

Transfer can be arranged to serve multiple purposes. Especially with cryogenic propellants, liquids can be piped around the outside of combustion and exhaust components, both to cool the components and preheat the propellants.