There are 3 main injector element classification: non impinging jets, impinging jets, and hybrid. The elements are all varied in the method of atomization and mixing. All element types have advantages and disadvantages and the selection of an element type is a function of propellants and desired performance/robustness trade-off. Let’s first go over the different types of injector elements.
Non impinging jets – These are very common and are usually broken into two varieties: the showerhead and the impinging jet. Showerheads were one of the first injectors used in the V-2 and the Aerobee sustainer engine, and they are still used near chamber walls for film cooling. It is basically an array of axial jets and, as such, mixing and atomization are slow for these injectors, but overall uniformity can be good. Showerheads are not often used as main elements because of the low performance but, as I already mentioned, they are used near chamber walls for cooling. I have always wanted to use 3D printing to make a very fine array of showerhead elements to see how that would perform.
Concentric elements are broken into 2 major groups: shear coaxial, which is a tube of propellant A surrounding a rod of propellant B mixing by shear forces, and swirl coaxial, where one or both of the propellants are swirled leading to an injected cone of propellant. These are both very common in gas-liquid injectors, such as the RL-10 and the J2 LOX-H2, but they are hard to make work for a liquid-liquid injection and surprisingly complex as small geometric changes can have signification performance and stability effect.
Impinging Jets – This is basically direct impingement of jets of propellants. Most of the mixing and atomization occurs at the impingement point of the jets and thus it is important to get the correct momentum ratios to achieve good mixing and atomization. This is done by either varying the pressure drop on the injectors or by adjusting the number of elements. The sizing of the correct momentum ratio is covered in various sources like NASA SP-809 pg 19. Usually these injectors are 1-on-1 (doublet element), 2-on-1 (triplet), 2-on-2 (quad), or 4-on-1 (Pentad).
Like-on-Like impinging is when jets of the same propellant combine and atomize, then mix downstream. This is a good and stable element, but doesn’t have quite as high efficiency as other options. Aligning elements for element to element mixing is key to performance. The elements are also used near chamber walls for reduced wall heating.
Unlike impinging is when jets of dissimilar propellants combine at the impingement points. These elements are an efficient and common choice for liquid to liquid injectors. They do have some stability and injector face heating issues, but are very quick mixing when made properly.
Hybrid Elements- Every other element type gets lumped in here, but it mostly consists of pintle elements and splash-plate injectors, amongst other weirder designs. Pintles are the most common of the other elements and have been used on many designs including the LEM descent engine and SpaceX Merlin engines. The design is a post with radial holes for propellant A at the end of the post and a tube of propellant forms at the base. The design has some injector face heating and streaking issues, but it also has good performance and chamber wall comparability. It is also uncommon for usually having only 1 injector element per chamber.
Friday we will cover the Earendel rocket injector elements.