Research Papers on pintle injector

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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.

INJECTORS IN ROCKET ENGINES

Injector as the name implies, injects the propellants into combustion chamber in the right proportions and right conditions to yield an efficient and stable combustion process. It also performs the structural task of closing of the top of the combustion chamber against the high pressure and temperature it contains. An injector has been compared to carburetor of an automobile engine. However, the injector, located directly over the high pressure combustion chamber performs many other functions related to the combustion and cooling process and is much more important to the function of the rocket engine than the carburetor is for automobile engine.

No other component of a rocket engine has as great impact upon engine performance as the injector. The measure of delivered performance (specific impulse) is the number of pounds of thrust provided per pound of propellant consumed per second. Each percentage point loss in combustion efficiency means a loss of same magnitude in overall I_s Propulsive efficiency.

High levels of combustion efficiency derive from uniform distribution of desired mixture ratio and fine atomization of liquid propellants. Local mixing within the injection-element spray pattern mixing within the injection-element spray pattern must take place at virtually a microscopic level to ensure combustion efficiencies approaching 100%.

Combustion stability is also very important requirement for satisfactory injector design. High performance can be secondary if the injector is easily triggered into destructive instability. At times, it may be seen that the design requirements for stability are counter to those for performance, since many of the high performance appear also to reduce the stability margin. Stable operation will depend in good part on the injector element selected and provision for damping any oscillatory phenomena. All systems that releases large amounts of energy have the potential for destructive oscillations, particularly if there is regenerative feedback (gain) between the combustion phenomena and the rate of energy release. This is true of the combustion process, because the temperature and pressure variations can directly impact the rates of vaporization and reaction. Stable operation can be achieved by either damping or detuning these processes.

Mass distribution is another important design parameter for successful injector/combustor interaction, can be difficult to achieve truly uniform fashion across the injector face. Good injector design includes a computation of the effective mass distribution and an assessment of design accuracy in this regard.

Mixture ratio distribution also plays an important part from the stand point of both performance and chamber compatibility. With combustion chambers made of metals (copper, nickel, steel) that are fuels, it is important to avoid the scrubbing of chamber walls by high temperature oxidizing streams. Most injector patters are designed to avoid this possibility and generally provide an excess of fuels to the above mentioned areas.

Pintle Injector

This is the magical rocket Injector used in rocket engines used to land 12 Apollo Astronauts on Moon.

A pintle injector is a variable area injector consists of a movable pintle, an annular nozzle and the central pintle nozzle. Propellants from the center and annular nozzles collide near the pintle tip. These collisions creates the mixing of fuel and oxidizer. The angle between the pintle base and the conical surface is called Pintle angle and it has significant impact on atomization of propellants.

Origin of pintle injector hails to some laboratory apparatus used to study the propellant mixing in NASA JPL in the mid 1950’s. A company named TRW (now part of Northrop Grumman) developed one injector in 1960, later in 1972 the design patent was publically released. Whenever a pintle injector related discussion takes place, TRW, is one of the first names mentioned. They deserve this recognition not only for the pioneer work started in the 60’s, that eventually led to a patent, but also because they have employed pintle injectors on hundreds of engines, ranging from a 10-Newton thruster up to the experimental 3-mega-Newton Low Cost Pintle Engine (LCPE). Till 2000 they have developed over 60 different pintle injectors and 130 engines were flown successfully.

Spray from Face Shut off type pintle injector.

Either fuel centered or oxidizer centered approaches can be used in design. In oxidizer centered approach, there is a moving pintle at the center that controls the oxidizer and an annular gap on the outside of the inner body that controls the fuel. 

Schematic of a Face Shutoff type Pintle injector.

Advantages	Disadvantages	Rockets using these injectors
Throttle able	Wall compatibility problems	LEM decent engine, SpaceX Merlin engine,BE-3 engine, Grasshopper engine,Kesterl Engine of Falcon 9 second stage.
Proven dependability	No correlations for level of mixing and spray size.
Simple structure and easy to manufacture
Large thrust per element
Good Combustion stability
Wider spray angles enables single injection element instead of multiple elements and subsequent weight reduction.

When developing a throttleable rocket engine, variable area injector is known as a suitable choice because it is difficult to control thrust using a fixed shape injector with high-efficiency. These ability to throttle the thrust is essential when trying to reuse the first stage of the rocket engine otherwise allowed to burn in the atmosphere after the desired use.

However, variable area injectors, especially a pintle injector, have recently attracted attention as next-generation injectors because of their associated benefits with respect to relatively newer engines. The pintle injector has a lot of attractive features such as the simple structure, throttling capability and combustion stability. Due to the throttling capability and combustion stability, the pintle injector has been used for a propellant injection system of a rocket engine which is required to operate under wide thrust range, and throttling capability is required.^.

Pintle injector has wide spray angles, only one injector can cover the overall combustion chamber. It means that a heavy injector plate with many injector elements could be alternated to one unit of the pintle injector. It saves weight, there by improving the thrust to weight ratio of the engine. 

Pintle Injector Cold flow Testing for SpaceX's Raptor engine.

Researchers demonstrate a pintle injector in a Grasshopper and Merlin of the Space X company and prove that the pintle injector has the potential to become the most preferred injector in the future. Numerous research groups have attempted to develop a pintle-injector engine, including Purdue University and the National Defense Scientific and Technical University in China.

Flame from a Slit type Pintle injector.

There were no combustion instability noted,even after scaling down thrust in scaling over a range of 50,000:1 in thrust and 250:1 in chamber pressure and 25 different propellant combinations. You can clearly see the two swirling zones on the left and right side of flame. This facilitates mixing and combustion stability.

There are two types of pintle injectors. They are Face shut-off type and Slit type. The top most picture shows the face shut off type. 

Pintle of a Slit type pintle injector with slits on the circumference of the pintle.
Another type of pintle injector is the face shut off type, it is the pintle configuration used in the Lunar Landing Module. This injector over the years made several modifications and slit type was choosen as the more preferred injector. Face shut off type was studied extensivly by Min Son and team from Korea Aerospace university. They have quantified the different parameters that afffect the performance of a face shut off type pintle injector.

Spray patternator comparison of face shut off type and slit type pintle injector.

A slit type pintle injector has several advantages over the face shut off type pintle injector. Slit type provides more even distribution of fuel and oxidizer inside the combustion chamber. Face shut off type has wider spray sometimes it is so wide that it will spray more propellants into the combustion chamber walls alone. The above graph clearly shows that face shut off type has very less propellant spray in the centre of the spray. However the Slit type has almost constant spray distribution through out the spray. Please be noted that the pintle angle of face shut off type pintle injector was 25 degree which might be a reason for more wide spray. However, thrust control by slit type pintle injector is expected to be poorer compared to the other injector.


Merlin 1D is famous pintle engine from SpaceX. This pintle engine uses face shut off type pintle injector. Pintle is spring loaded and tightly pressed againist the pintle post. High pressure from LOX side pushes the pintle to open. By doing this SpaceX avoided upstream LOX valves which are costly as well as technically difficult to operate. This can also avoid the sequencing computer. Face shut off on a bigger engine is difficult to acheive, but they have done it marvelosly.  But SpaceX owner elon musk wanted this design. So Space X started designing pintle engine using face shut off technique from 2011. They blew up several hardware while perfecting it. At last they made it perfect and enjoy its benefits now. This allowed Falcon 9 rocket to precisly control thrust as well as reduce the weight of pintle engine.Pintle engine always gives weight advantage over engines that use other type of injectors.