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 oxidiser 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 oxidiser 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 type of injectors are
- Shower Head type
- Self Impinging doublet type
- Cross impinging triplet type
- Centrifugal or Swirling type
- Pintle injector type
The Pintle injector permits good mixture control of fuel and oxidizer over a wide range of flow rates. The pintle injector was used on the Apollo Lunar Module engines See Descent Propulsion System and the current Merlin and Kestrel engines designed by SpaceX and used on Falcon 9 and planned Falcon Heavy rockets.
The Space Shuttle Main Engine 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 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 SSME due to this design detail.
Valentin Glushko invented the centrifugal 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 fuel engine used a single impinging injector. German scientists in WWII experimented with impinging injectors on flat plates, used successfully in the Wasserfall missile.
Read more about this topic: Liquid-propellant Rocket, Injectors
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