Pintle injector

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]