Injectors
General description of injectors, types of injectors, and injector manufacturing.
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Overview
Injectors are needed to spray the bipropellants (i.e. fuel and oxidizer) into the combustion chamber in a way that controls the atomization, combustion rate, and combustion efficiency of a liquid engine. Injectors are a vital component of a liquid rocket engine that will affect how efficiently the energy of fuel is converted into the needed thrust for a rocket. There are a variety of injectors to choose from. When designing an injector, some factors to consider are the bipropellants used, engine application, viability, etc.
Types of Injectors
Pintle
A pintle injector consists of two concentric tubes and a pintle. The cylindrical tubes are responsible for carrying the propellants to the combustion chamber. Generally fuel will go through the inner tube while oxidizer goes through the outer tube. The pintle is a protrusion at the end, which allows the fuel carried on the inner tube to deflect at a certain angle. The fuels will meet and mix at the impinging point and proceed to combust. By varying the size of the annular and center gaps that the fuel passes through, this allows for throttling of the engine and controlling of the flow into the combustion chamber.
Advantages
A properly implemented pintle injector can achieve combustion efficiency adequate for liquid engines (96-99%). The design is relatively simple and has proven dependability. Performance can be easily optimized by varying the gap sizes. It works in engines that have to be restarted. Overall, this injector is a simple, adjustable, and high performance option.
Disadvantages
Pintle injectors only work well for liquid and gelled propellants. Thermal stress is more concentrated in the certain parts of the combustion chamber which can lead to burn through. Another disadvantage is that there are no correlations for level of mixing and spray size.
Showerhead
Similar to an actual showerhead, the propellants are fed in a straight path into the combustion chamber where they will then atomize and combust. The propellants are sprayed through holes that would maximize atomization.
Advantages
This is the simplest option being relatively easy to make and implement such as by repurposing a commercial showerhead and integrating it into the engine plumbing.
Disadvantages
Mixing is dependent on the turbulence of the propellants entering the combustion chamber. Otherwise, the propellants will go straight in and have poor mixing. In general, the combustion efficiency of a showerhead injector will be low compared to other options making it viable for experimental, non flying engines.
Impinging
Propellants are fed into the combustion chamber at certain angles. To achieve this, many holes are drilled into the face of the combustion chamber. The fuel and oxidizer manifolds can be spaced in different orientations to vary where and how much mixing occurs. Some stream patterns include doublet, triplet, and self-impinging stream patterns.
Advantages
If done correctly, this can achieve strong combustion efficiency and is scalable depending on the size of the combustion chamber.
Disadvantages
This design can be quite complicated to drill sets of holes correctly accounting for entry angles, fluid velocity, and mass flow rate. Atomization efficiency decreases at high entry velocities because droplets will scatter in different directions. The degree of precision and equipment needed for this to be viable is most likely beyond the budget of the club unless a cheaper solution is found.
Coaxial Swirl
As suggested by the name, coaxial swirl injectors consist of coaxial tubes that will feed the bipropellant into a mixing chamber through tangential inlet ports. The oxidizer flows into a swirling chamber at an angle such that it will swirl and then spray out into the combustion chamber to thoroughly atomize. The fuel is fed directly into the combustion chamber where it will atomize with the oxidizer.
Advantages
In theory, can achieve the highest combustion efficiency and thus the highest performance. The spray pattern is similar to a pintle injector but without the need for a pintle to deflect the fuel due to its angular momentum.
Disadvantages
The variables involved in swirling such as the speed and the angle at which the swirling oxidizer is injected can be more difficult to optimize for this injector.
Injector Manufacturing and Assembly
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