Editing Liquid-propellant rocket (section)

==Injectors==
The injector implementation in liquid rockets determines the percentage of the theoretical performance of the [[Rocket engine nozzle|nozzle]] that can be achieved. A poor injector performance causes unburnt propellant to leave the engine, giving poor efficiency.

Additionally, injectors are also usually key in reducing thermal loads on the nozzle; by increasing the proportion of fuel around the edge of the chamber, this gives much lower temperatures on the walls of the nozzle.

===Types of injectors===
Injectors can be as simple as a number of small diameter holes arranged in carefully constructed patterns through which the fuel and oxidizer travel. The speed of the flow is determined by the square root of the pressure drop across the injectors, the shape of the hole and other details such as the density of the propellant.

The first injectors used on the V-2 created parallel jets of fuel and oxidizer which then combusted in the chamber. This gave quite poor efficiency.

Injectors today classically consist of a number of small holes which aim jets of fuel and oxidizer so that they collide at a point in space a short distance away from the injector plate. This helps to break the flow up into small droplets that burn more easily.

The main types of injectors are
* Shower head
* Self-impinging doublet
* Cross-impinging triplet
* Centripetal or swirling
* [[pintle injector|Pintle]]

The pintle injector permits good mixture control of fuel and oxidizer over a wide range of flow rates. The pintle injector was used in the [[Apollo Lunar Module]] engines ([[Descent Propulsion System]]) and the [[Kestrel (rocket engine)|Kestrel]] engine, it is currently used in the [[Merlin (rocket engine)|Merlin]] engine on [[Falcon 9]] and [[Falcon Heavy]] rockets.

The [[RS-25]] engine designed for the [[Space Shuttle]] uses a system of fluted posts, which use heated hydrogen from the preburner to vaporize the liquid oxygen flowing through the center of the posts<ref>Sutton, George P. and Biblarz, Oscar, ''Rocket Propulsion Elements'', 7th ed., John Wiley & Sons, Inc., New York, 2001.</ref> and this improves the rate and stability of the combustion process; previous engines such as the F-1 used for the [[Apollo program]] had significant issues with oscillations that led to destruction of the engines, but this was not a problem in the RS-25 due to this design detail.

[[Valentin Glushko]] invented the centripetal injector in the early 1930s, and it has been almost universally used in Russian engines. Rotational motion is applied to the liquid (and sometimes the two propellants are mixed), then it is expelled through a small hole, where it forms a cone-shaped sheet that rapidly atomizes.  Goddard's first liquid engine used a single impinging injector.  German scientists in WWII experimented with impinging injectors on flat plates, used successfully in the [[Wasserfall]] missile.

===Combustion stability===

To avoid instabilities such as ''chugging,'' which is a relatively low speed oscillation, the engine must be designed with enough pressure drop across the injectors to render the flow largely independent of the chamber pressure. This pressure drop is normally achieved by using at least 20% of the chamber pressure across the injectors.

Nevertheless, particularly in larger engines, a high speed combustion oscillation is  easily triggered, and these are not well understood. These high speed oscillations tend to disrupt the gas side boundary layer of the engine, and this can cause the cooling system to rapidly fail, destroying the engine. These kinds of oscillations are much more common on large engines, and plagued the development of the [[Saturn V]], but were finally overcome.

Some combustion chambers, such as those of the [[RS-25]] engine, use [[Helmholtz resonator]]s as damping mechanisms to stop particular resonant frequencies from growing.

To prevent these issues the RS-25 injector design instead went to a lot of effort to vaporize the propellant prior to injection into the combustion chamber. Although many other features were used to ensure that instabilities could not occur, later research showed that these other features were unnecessary, and the gas phase combustion worked reliably.

Testing for stability often involves the use of small explosives. These are detonated within the chamber during operation, and causes an impulsive excitation. By examining the pressure trace of the chamber to determine how quickly the effects of the disturbance die away, it is possible to estimate the stability and redesign features of the chamber if required.