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The information and schematic are from the web site Pintle Injector Blog.

A pintle injector is a variable area injector consists of a movable pintle, an annular nozzle and the central pintle nozzle. Propellants from the center and annular nozzles collide near the pintle tip.

Either fuel centered or oxidizer centered approaches can be used in design. In oxidizer centered approach, there is a moving pintle at the center that controls the oxidizer and an annular gap on the outside of the inner body that controls the fuel.

All the pintle injector diagrams I have found show that moving the pintle tip could close the central pintle nozzle, but wouldn’t affect annular gap on the outside of the inner body. The pintle tip does not cover the annular gap. By enlarging the pintle tip and extending the outer edge of the annular gap, moving the pintle could regulate and shut off the flow of both the fuel and oxidizer.

Is this what is done in the Merlin engine?

How many pintle injectors are in the Merlin combustion chamber?

Does the Merlin gas generator use pintle injectors?

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Here is an image on how this can be done.

enter image description here

It can be done using a sleeve that covers the central pintle. The sleeve also has a "bump" in it that controls the gap around the annular gap. So the sleeve covers the center flow along with the annular gap. The Apollo Lunar Module Decent Engine did something similar, but cant find a good cross-section view that shows this. It had a throttling capability of 10/1. Not sure if it was face shut-off.The problem you run into is the when using LOX you're dealing with cryogenics. Sealing is tough since you have a metal/metal surface used as a seal. Not sure if this is how they do it on the Merlin engine, but the image was taken from a Tom Mueller talk. I'm pretty sure that each engine has has just one injector since that is what past designs at TRW have done. Two injectors would change the mixing characteristics. Hope this helps.

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  • $\begingroup$ Here's the link the the injector cross-section I was referring to. google.com/…: $\endgroup$
    – Robert T
    Commented Aug 10, 2017 at 7:19
  • $\begingroup$ Stackexchange uses imgur to incorporate images into questions and answers. If you go back and edit your question, once you are in edit mode, look at the top toolbar like this i.sstatic.net/eY0w8.png If you click that, you'll get a box where you can drag-and-drop an image from your device, or choose the option to use a URL. $\endgroup$
    – uhoh
    Commented Aug 10, 2017 at 7:20
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    $\begingroup$ @uhoh: Users with less than 10 rep can't post inline images at all, IIRC. $\endgroup$
    – Nathan Tuggy
    Commented Aug 10, 2017 at 7:21
  • $\begingroup$ @NathanTuggy My up vote raised rep to 11, I am not sure if the "privilege upgrade" is instantaneous thought. $\endgroup$
    – uhoh
    Commented Aug 10, 2017 at 7:22

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