Pintle injector

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A pintle injector is a type of propellant injector for a bipropellant, liquid fueled rocket engine.

It was first used on a flight vehicle during the Apollo Program in the Lunar Excursion Module's Descent Propulsion System. Pintle injectors are currently used in SpaceX's Merlin engines.


Pintle injector image
Fuel in red, oxidizer in blue.

A pintle injector is a type of coaxial injector. It consists of two concentric tubes and a central protrusion. Propellant A (blue in the image) flows through an outer tube, coming out as a cylindrical stream, while propellant stream B (red in the image) flows in an inner tube and impinges on a pintle-shaped protrusion, similar to a poppet valve as seen on automotive engines, which causes B to spray outward in a broad cone or a flat sheet that intersects the cylindrical stream of propellant A. In some variants the pintle has grooves cut into it to produce radial jets in the flow of propellant B. In the typical design, only a single central injector is used, differing from "showerhead" injector plates which use multiple parallel injector ports.[1]

Many people have experienced throttleable pintle sprayers in the form of standard garden hose-end sprayers.[2]


The origins of the pintle injector were early laboratory experimental apparatus, used by Caltech and JPL in the mid-1950s, to study propellant 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; however, it was not until October 1972 that U.S. Patent 3,699,772 for invention of the pintle injector was granted to Gerry Elverum, of TRW and made public.[1]

Advantages and disadvantages[edit]

Compared to some injector design archetypes pintle injectors allow greater throttling of bipropellant flow rates, though it is worth noting that throttling rocket engines in general is still very difficult. If only one central injector is used, the mass flow inside the combustion chamber will include two main recirculation zones which decrease acoustic instability without necessarily requiring acoustic cavities or baffles.[1]

The pintle injector design can deliver high combustion efficiency (typically 96–99%)[1]

Having either oxidizer or fuel of a bipropellant in the center tap have trade offs, but fuel is often chosen for the center tap. One advantage of this is that the injector can be tuned so that the propellant which is not reacted immediately as it passes through the oxidizer stream is projected onto the combustion chamber walls cooling the walls through evaporation.[3]

Another view of pintle injector.
Another view, showing more clearly how fuel and oxidizer flow.

Pintle injectors have been developed for applications in rocket propulsion, but could potentially have high flowrate in other applications due to their relative simplicity.[4]

Engines known to use pintle injectors[edit]

Engine name Manufacturer Fuel Oxidizer
Descent Propulsion System TRW Inc. Aerozine 50 N2O4
Executor ARCA Kerosene LOX
Fastrac NASA RP1 LOX
Kestrel SpaceX RP1 LOX
Merlin SpaceX RP1 LOX
TR-106 TRW Inc. LH2 LOX


  1. ^ a b c d Dressler, Gordan A.; Bauer, J. Martin (2000). "TRW Pintle Engine Heritage and Performance Characteristics" (PDF). AIAA. AIAA-2000-3871. Retrieved 14 May 2017.
  2. ^ Fischer, Dave. "Pintle Injector Rocket Engines". National Space Society Blog. National Space Society. Retrieved 2013-08-15.
  3. ^ Dressler, Gordan A.; Bauer, J. Martin (2000). "TRW Pintle Engine Heritage and Performance Characteristics" (PDF). AIAA: 2, 3. AIAA-2000-3871. Retrieved 17 Oct 2018.
  4. ^ Heister, S.D. (25 February 2011). "Chapter 28: Pintle Injectors". In Ashgriz, Nasser. Handbook of Atomization and Sprays: Theory and Applications (2011 ed.). New York: Springer. pp. 647–655. ISBN 978-1-4419-7263-7. Retrieved 14 May 2017.

External links[edit]