COMMONLY USED INJECTION ELEMENTS IN ROCKET ENGINES

                                          1.3     LIKE IMPINGING

Like impinging (or self-impinging elements) impinge the injected streams directly on other streams of the same propellant. The most common of these, a doublet type, has two like-fluid streams angles together to an impact point, producing a fan shaped spray of droplets similar to that of an unlike doublet. These fans can also travels in a direction determined by the resultant of the momentum vectors of the incoming streams before the impingement. However there is no mixing within this fan, since only one reactant is present in each. Energy dissipated by the impingement atomizes the liquids. Orientation of the initial fans for the secondary impingement and overlapping of these sprays mixes the two propellants. Again this efficient mixing is related to the arrangement of fuel doublet adjacent to the oxidizer doublet.

1.     Studies have shown that mass and mixture ratio distributions are the functions of element size, spacing between fuel and oxidizer fans, fan inclination or cant angle.

2.     Spray drop size is the function of orifice size, injection velocity, impingement angle and impingement distance.

3.     These injector elements were used in large Lox/RP1 injectors for F1 rocket engine used in Saturn v rocket, Atlas 1^st stage booster and sustainer and first stage of Titan 1 engines.

Table 1.

Advantages	Disadvantages	Rockets Using these elements
Easy to manifold	Requires increased axial distance to mix fuel and oxidizer	Gemini LV first stage,
Well understood	Sensitive to design tolerances	Titan 1 and 2- first stages
Proven dependability		Redstone, Jupiter, Thor.
Good mixing		Atlas boosters
Very stable element		H1, F1 engines
Not subjected to blow apart		Uppers Stage VEGA

1.4         UNLIKE IMPINGING

Unlike impingement doublet is the commonly used element for storable propellant engines. Consist of single fuel and oxidizer streams separated at an angle impinging at a prescribed distance. Commonly used angle is 60° and 45°. They accomplish atomization and mixing by direct impingement of fuel and oxidizer jets. This impingement provides direct mechanical mixing by dissipative exchange of momentum. Virtually all mixing and atomization happens near the vicinity of the impingement point. Since all mixing happens near the impingement point, ignition and chemical reactions occur near the injector face and there by results in a high heat flux near injector face.

Table 2.

Advantages	Disadvantages	Rockets using these elements
Proven dependability	Subject to blow apart with hypergolic propellants	Used in reaction control engines of Apollo LEM ascent engines
Good overall mixing	Wall compatibility problems due to
Simple to manifold	Sensitive’s to design tolerances
Extensively studied	Performance sensitive to continuous throttling

1.5         TRIPLET

Different injector elements

The mismatch in stream size and momentum between the oxidizer and fuel in unlike elements will force the spray away from the desired axial direction and distort the fan, resulting in poor mixing. This problem may be avoided unlike triplet elements. They consists of two outer jets impinging on a centrally located axial jet. A typical spray pattern will be narrower than an equivalent doublet, and the mass more concentrated as a result. Unlike –triplet injectors have demonstrated high levels of mixing and resultant combustion efficiency, but also tend to be sensitive to stability problems.

1.6         NON IMPINGING

In these types of injector elements mixing and atomization are controlled by shearing of liquid by gas. The most common type is the coaxial configuration, characterizes the SSME injector and other oxygen/hydrogen engines. The coaxial, or concentric, injection element usually has a slow moving central stream of liquid oxidizer surrounded by a high velocity concentric sheet of gaseous fuel. It has a well-earned reputation as high-performance, stable injection element for gaseous fuel and liquid oxidizer. The liquid oxidizer is deliberately injected at lower velocities, with the usual injection pressure drop accomplished by an upstream metering orifice in each element, and diffused to a reduced velocity in the tubular LOX post. On the other hand, the fuel injection pressure is turned into high injection velocity in the annular gap around the LOX post. 

Mixing, atomization of the liquid, and mass distributions are provided by the shearing action of the high velocity gaseous fuel on the surface of the liquid. The coaxial element is less well suited to liquid fuels, or even very dense gaseous fuels, since the velocity relationships required to make it work well are difficulty to obtain.

One of the other notable element is shower head type. It is one of the first injector used on a production rocket. It was used in German V2 rocket and X-15 engine.