Editing Liquid-propellant rocket (section)

===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.