What are these big shiny metallic "lumps" on the bottom edge of each nozzle of each core of this Falcon Heavy image from the Teslarati article SpaceX reveals Falcon Heavy Block 5 in first official photo, timelapse. See also SpaceX tweet.

I'm thinking it could be related to damping vibrations or oscillations of the nozzle, preventing it from ringing like a bell?

I've never seen one in SpaceX videos of the single nozzle of the 2nd stage, but of course those are very different and much larger structures.

Based on the placement on the eight outer nozzles such that they fall directly between adjacent nozzles, I thought that they might be "bumpers", but that wouldn't explain the presence on the central engine's nozzle. Of course it might be there just so that the engines are identical, so that there aren't two kinds of first-stage nozzles.

Falcon Heavy (cropped, sharpened) from Teslarati

Falcon Heavy (cropped) from Teslarati

Falcon Heavy from Teslarati

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    $\begingroup$ I've heard they were bumpers for nozzle on nozzle contact, especially when they gimbal inwards during entry. I do not understand why this is preferred to bells touching though, and I haven't seen official info. Agreed on center just using the same bell, that's sensible. $\endgroup$
    – Saiboogu
    Apr 8, 2019 at 2:41
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    $\begingroup$ I was going to ask you why then the center one wouldn't need any bumpering, but then I realized that while six outer engines would result in a close-packed hexagonal, eight outer engines leaves the central engine some extra "personal space". $\endgroup$
    – uhoh
    Apr 8, 2019 at 2:46

2 Answers 2


They are bumpers, to prevent damage during gimbaling. These were added after an incident in testing where engine nozzles were damaged after they banged into each other.

The bumper functions as a crumple zone. Here's a cropped version of this photo, which shows the bumper on the 400th Merlin engine.

enter image description here

There's no primary source for this.

  • $\begingroup$ this seems like a hardware solution to a software problem. Why not just gimbal less? I guess that's for another question though. $\endgroup$
    – vero
    Oct 20, 2019 at 9:15

They are bumpers for when the engines move and gimbal, to avoiding ringing the bells.

You can see it on the 100th Merlin delivered image.

100th Merlin engine with bumper


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