Pintle injector
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The pintle injector is a type of propellant injector for a bipropellant rocket engine. Like any other injector, its purpose is to ensure appropriate flow rate and intermixing of the propellants as they are forcibly injected under high pressure into the combustion chamber, so that an efficient and controlled combustion process can happen.[1]
A pintle-based rocket engine can have a greater throttling range than one based on regular injectors, and will very rarely present acoustic combustion instabilities, because a pintle injector tends to create a self-stabilizing flow pattern.[2][3] Therefore, pintle-based engines are specially suitable for applications that require deep, fast, and safe throttling, such as landers.[4]
Pintle injectors began as early laboratory experimental apparatuses, used by Caltech's Jet Propulsion Laboratory in the mid-1950s, to study the mixing and combustion reaction times of hypergolic liquid propellants. The pintle injector was reduced to practice and developed by Space Technology Laboratories (STL), then a division of Ramo-Wooldridge Corp., later TRW, starting in 1960.[2]
There have been pintle-based engines built ranging from a few newtons of thrust up to several millions, and the pintle design has been tested with all the common and many exotic propellant combinations, including gelled propellants.[2] Pintle-based engines were first used on a crewed spacecraft during the Apollo Program in the Lunar Excursion Module's Descent Propulsion System,[4][2][5] however, it was not until October 1972 that the design was made public.[2][3] and U.S. patent 3,699,772 was granted to its inventor Gerard W. Elverum Jr.[6]
Working principle
A pintle injector is a type of coaxial injector. It consists of two concentric tubes and a central protrusion. Propellant A (usually the oxidizer, represented with blue in the image) flows through an outer tube, coming out as a cylindrical stream, while propellant B (usually the fuel, represented with red in the image) flows within an inner tube and impinges on a central pintle-shaped protrusion, (similar in shape to a poppet valve like those found on four-stroke engines), spraying out in a broad cone or a flat sheet that intersects the cylindrical stream of propellant A.[2][3]
In the typical pintle-based engine design, only a single central injector is used, differing from "showerhead" injector plates which use multiple parallel injector ports.[2]
Throttleability can be obtained either by placing valves before the injector, by moving the inner pintle or outer sleeve, or both.[2]
Many people have experienced throttleable pintle sprayers in the form of standard garden hose-end sprayers.[5]
Variants
In pintle engines that do not require throttling, the pintle is fixed in place, and propellant valves for startup and shutdown are placed somewhere else.[2]
A movable pintle allows for throttleability, and, if the moving part is the sleeve, the pintle itself can act as the propellant valve. This is called a Face Shutoff pintle. A fast-moving sleeve allows for the engine to be operated in pulses, and this is usually done in pintle-based RCS thrusters and missile divert thrusters.[2]
In a variant of the Face Shutoff pintle, the pintle itself is hydraulically actuated by the fuel via a pilot valve, and no extra valves are required between the engine and tanks. This is called an FSO (Face Shutoff Only) pintle.[2]
In some variants the pintle has grooves or orifices cut into it to produce radial jets in the flow of propellant B, this allows for extra unburned fuel to impinge on the walls of the combustion chamber, and provide fuel film cooling.[2][7] The pintle pictured here is of this type.
Advantages and disadvantages
Advantages
Compared to some injector designs, pintle injectors allow greater throttling of bipropellant flow rates, although throttling rocket engines in general is still very difficult. If only one central injector is used, the mass flow inside the combustion chamber will have two main recirculation zones which decrease acoustic instability without necessarily requiring acoustic cavities or baffles.[2][3]
The pintle injector design can deliver high combustion efficiency (typically 96–99%).[2][3]
If fuel is chosen for the inner flow (which is the case in most pintle-based engines), the injector can be tuned so that any excess fuel which is not reacted immediately as it passes through the oxidizer stream is projected onto the combustion chamber walls and cools them through evaporation, thus providing fuel film cooling to the combustion chamber walls, without incurring the mass penalty of a dedicated coolant subsystem.[2][7]
While pintle injectors have been developed for applications in rocket propulsion, due to their relative simplicity, they could easily be adapted for industrial fluid handling processes requiring high flowrate and thorough mixing.[8]
A given injector's performance can be easily optimized by varying the geometries of the outer propellant's annular gap and the central propellant slots (and/or continuous gap, if used). As this requires only two new pieces to be made, trying variations is usually cheaper and less time-consuming than with regular injectors.[2][3]
Disadvantages
Because combustion tends to occur in the surface of a frustum, peak thermal stresses are localized on the combustion chamber wall rather than a more evenly distributed combustion across the chamber section and more even heating. This has to be contemplated when designing the cooling system, or it might cause burn-through.[5][7][9]
The pintle injector is known to have caused throat-erosion problems in the early ablatively cooled Merlin engines due to uneven mixing causing hot streaks in the flow, however, as of 2021, it is not clear whether this is a problem that applies to all pintle-based engines, or this was a design problem of the Merlin.[7][10]
Pintle injectors work very well with liquid propellants and can be made to work with gelled propellants, but for gas–liquid or gas–gas applications, conventional injectors remain superior in performance.[9]
The pintle injector is desirable for engines that have to be throttled or restarted repeatedly, but it does not deliver optimal efficiency for fuel and oxidizer mixing at any given throttle rate.[9]
