what is a pintle injector...???

          The pintle injector is one of the most unique injectors that have been used in liquid rocket engines. Originally developed as a laboratory experimental apparatus to study propellant mixing and combustion reaction times of hypergolic liquid propellants. In a bipropellant engine, one of the propellants flows down the inside of the pintle and is ejected radially through a series of holes or slots near the tip of the pintle while the other propellant leaves the manifold through an annular sheet around the base of the pintle and as a result vigorous mixing and atomization occurs from the collision of the radial jets with the thin liquid sheet. The resultant flowfield yields a curved combustion zone that is substantially different from those formed by "Flat Face type" injectors.

          The pintle injectors enjoy several advantages over other types of liquid bipropellant injectors. The design is inherently simpler than the face type injectors in the sense that there is only one injector element, but the single element can have multiple holes. In any case the pintle injectors have lesser number of injection sites than the face type injectors. The second advantage is it's inherent combustion stability. The pintle engines have never reported any cases of combustion instability which reduces risk and the need for stability aids such as baffles. The third attractive feature of the pintle injector is its throttleability. Throttling ratios of 10-20 : 1 have been demonstrated with hypergolic propellants.

          The injector flows and combustion have been much less studied than those of flat face type injectors and all the designs and analysis are within the industry and not available for general public. The major issues of concern that tend to complicate the development of these injectors are as follows. Manufacturing issues related to maintaining the required gap between injector holes. As the pintle tip lies in the re-circulation zone, it is subjected to high heat flux. In engines using hypergolic propellants, because of local combustion, the pintle tips can get damaged. To alleviate this, size of re-circulation zones should be minimized.

          Very little research work has been carried out on these injectors and much information related to them is not available in open literature. By integrating the design of the pintle injector with that of the combustion chamber, better results can be achieved and it even helps reduce the size of the combustion chamber.

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