The Falcon 9 fairing uses four pneumatic pushers to impart a positive separation force between the fairing halves. The pushers all mount on half of the fairing and push against the other half.

But there is an odd feature on these pushers that I can't understand. The pusher rods are hollow, and on the other half of the fairing are needle pins that clearly fit inside the rod bore.

The picture below has the pusher cylinders on the left (green circles) and the needle pins on the right (orange circles). You can see the tips of the hollow cylinder rods in red. I'll try to post a better picture of them, but trust that they are hollow.

enter image description here

Why would they do this? The legs have pushers and there the rods have solid ends to push against a plate on the legs. The interstage has pushers and these have solid ends to push against brackets on the backside of the second stage.

So the leg and interstage pushers didn't need hollow rods. Why did the fairing? What is the purpose of fitting needle pins through the rods of pneumatic pushers?

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    $\begingroup$ I can't think of the word I'm looking for so I've asked Technical term for loose drawers jamming more readily than those with tighter clearance? in Engineering SE. $\endgroup$ – uhoh Jun 12 at 10:50
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    $\begingroup$ If they are pneumatic, I'm guessing filling the rods with pressurized gas will give piston-like action, longer travel can impart more speed. In both legs and interstage there are other mechanisms to provide further motion (leg actuator, second stage main engine) so any small separation is sufficient while here all the speed is imparted by the pushers and required to make the fairing clear the sides of the rocket. $\endgroup$ – SF. Jun 12 at 10:56
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    $\begingroup$ Thanks @uhoh. So it seems the needle pins would introduce lash (the term given in a comment to your post) if they are not properly designed? But couldn't they avoid that risk altogether by using standard pushers (without needle pins nested inside hollow pusher rods)? $\endgroup$ – Alex Jun 12 at 21:06

This is a guess, but it's what I'd do (and it's too long for a comment). I think that designing the pushers like this gives you three big advantages:

  • firstly they help locate the two halves of the fairing: once the rods are over the pins then that defines quite well where the two halves will sit and it does so well before the two halves are together, so they serve the additional purpose of being essentially a jig when you're assembling the thing;
  • secondly as you're putting the fairing together you get to make sure all the pushers are mated well before the fairing is together, rather than having to do some awkward (and probably very difficult) inspection once it's together;
  • finally, if the separation of the fairing happens as it hinges open from the base during the process of separation, then because the pushers can't skate sideways, then you can save a bunch of mass that would otherwise need to add to make the pushers rigid.
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    $\begingroup$ Oh, there's an interesting thought. The needle pins could serve to constrain the motion of the pushers---like a train on rails, until the needle pins exit the pushers, at which point you already have a good couple of feet of clearance and proper separation velocity if all goes well. $\endgroup$ – Alex Jun 12 at 21:02

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