Falcon 9 2nd stage pusher; how far does it continue to extend as it accelerates separation? (currently unanswered) includes (and further explains) the GIF below.


  1. Is there a better term for this device than my colloquial "2nd stage pusher"?
  2. How common are they now? Are they limited to the F9 or fairly standard or somewhere in-between?

Falcon 9 2nd stage separation pusher

  • $\begingroup$ companion question: When were “2nd stage pushers” first used? $\endgroup$ – uhoh Dec 19 '19 at 0:55
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    $\begingroup$ Definitely not standard; explosive bolt separation plus “fire in the hole” or ullage motors on the upper stage to get clearance are far more common. F9 is the only large rocket I know of that relies on mechanical separators. $\endgroup$ – Russell Borogove Dec 19 '19 at 1:08
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    $\begingroup$ SpaceX is averse to pyrotechnic solutions and everything single-use in general. Others operate under assumption of trust a device built to the same specs as the test specimen will perform the same as the test specimen, In SpaceX all parts are tested to operate to specs, not just have the construction match them, so even errors in construction specs (like too loose tolerances) won't cause an in-flight failure. That's why Dragon's "Trunk" is coupled with electromagnetic clamps, not pyrotechnics - decoupling can be tested on the ground during integration. $\endgroup$ – SF. Jun 12 at 11:12
  1. Pneumatic pusher is the term normally used. The Falcon 9 manual itself refers to them as such. It's not just one pusher. Falcon 9 V1.1 has four: three on the rim of the interstage and one on a quadrapod mount inside the interstage.

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  1. I can't tell how common pneumatic pushers are in stage separation, but pyrotechnics seem by far the most common, and that suggests pneumatic pushers must be rare. Falcon 1 also used pneumatic pushers, and Falcon Heavy probably uses them too.

New Glenn, Blue Origin's rocket, uses pneumatic pushers for stage separation as well. From the New Glenn payload users' guide:

enter image description here

Not to go on too much of a tangent, but stage separation isn't the only place on the Falcons with pneumatic pushers.

The landing legs use them to kick the legs out the first few degrees so that the much much larger telescopic cylinders can extend them all the way down. (When stowed the cylinders are positioned so that they cannot extend even if pressure is applied---they must must be rotated a few degrees out first).

Pneumatic pushers are also used to separate the Dragon capsule from the trunk---e.g., during mission abort---and to separate the two halves of the bullet-shaped payload fairing (which the Falcon 9 manual says uses four pneumatic pushers).

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