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You can see them on the rim in a hexagonal pattern in the picture below. Also present in the latest version of the Dragon trunk, but less clearly than here. They alternate in color from nickel to silver to nickel to silver... The silver cartridges have wider holes but seem otherwise identical.

What are they? Why are there two types? How do they work? If they are separation devices, are they just locks, or are they also pushers? Are they pneumatic, pyrotechnic, spring types... ?

enter image description here

Edit 1:

And below is a picture from the matching top of the second stage. Again, this is an earlier Falcon/Dragon model, but the cartridges are more clearly visible here.

I circled the silver and nickel cartridges in matching colors. You can see they all have pins each with a hole here. These would presumably go inside the cartridges on the trunk.

Note that the silver rods are longer and thicker than the nickel green rods. They seem about twice as thick. Why? And could it be that least one of the two types of rods serves strictly to separate the trunk from the second stage, with the pin holes there to house a linear charge that would apply a positive force between the trunk and the second stage at separation?

enter image description here

Edit 2:

You can see similar but beefier cartridges at the bottom of the second stage and at the top of the interstage, where they must house the pneumatically actuated mechanical collets mentioned in the Falcon 9 manual. (The collets unlock the stages before pneumatic pushers apply a positive separation force to push them apart).

Below is the top of the interstage during assembly. This is an earlier version of the Falcon 9. The newer version seems to have fewer collets and these seem to be housed together with the pneumatic pushers, but that shouldn't matter here).

enter image description here

...And below is the bottom of the second stage during assembly. There are two types of cartridges, or brackets, here, but I know what they are. The wider ones match the collet cartridges on the interstage. The thinner ones match the cutouts for the peripheral pneumatic pushers on the interstage.

enter image description here

Mentioning the interstage separation collets just because they are similar in some ways. Not suggesting they are the same thing (and clearly I have doubts, because I went to lengths to ask this question :D)

Edit 3:

So (taking off from the very useful comments below) maybe the cartridges house explosive nut and bolt assemblies similar to the pyro devices used on the Apollo spacecraft? I've been resisting this option because pyro devices aren't testable on the ground (to use them or to test them is to destroy them)... And that would make them less attractive to SpaceX, even if neither the second stage nor the cargo trunk are recoverable. You'd still want to test.

But there is no trace of pneumatic cylinders, and the holes in the pins would seem natural only if you assume pyros... And... pyros have a proven history. Not one of them failed in the Apollo missions, if what I read is correct. And even if you can't test an individual pyro before flight, you can still test a batch of pyros from the same assembly line and know with high degree of confidence that the untested pyros on the rocket will work fine...

If SpaceX used pyros on the Falcon 9, and if the pyros were in just one place, this is no doubt where they'd be. I know they used pyros for the payload on Falcon 1 (it's in the manual for that rocket). But the payload fairing and Dragon trunk are different things. The payload fairing doesn't have those cartridges at the back, and the connection is to a conical payload adapter on top of the second stage. That payload adapter isn't used for the trunk...

Edit 4:

A view of the mating cartridges on the second stage and the Dragon trunk. Newer version of the Dragon. Must be the separation mechanism, but what type? Pneumatic, pyrotechnic? Locking collets, pushers, both? Pyro bolts and nuts?

enter image description here

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    $\begingroup$ Can you put in a picture where you circle the items you are describing? I am not sure what you are talking about. $\endgroup$
    – geoffc
    Jun 11, 2020 at 14:48

1 Answer 1

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What are they?

Interstage Umbilical Guillotines

Real Guillotine

Guillotine Schematic

I believe they are housings for pyrotechnic separation connections between the falcon 9 second stage and the dragon trunk. I found this NASA document describing the explosive devices on Apollo and the connections you describe look very close to one of the guillotines used on the Lunar Module.

Why are there two types?

They appear to be male and female ports of the same system, but it could also be that they are for separating two different diameter lines. It could also be that one set is purely structural and the other is for cutting lines.

How do they work?

An explosive charge is used on either side to propel a set of blades to cut the line.

I wish I could find more detailed information to better inform this answer, but SpaceX doesn't make it easy to find technical data regarding their spacecraft.

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  • $\begingroup$ The guillotines you reference sever electrical and/or fluid connections, not structural supports. Are you claiming the "cartridges" being asked about in the question involve electrical and/or fluid connections? $\endgroup$
    – Organic Marble
    Jun 11, 2020 at 18:52
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    $\begingroup$ @Organic Marble Yes, that is my guess. Especially considering the other posts mentioning that Dragon doesn't use pyrotechnics for structural separation. My main reasoning is the similarity in appearance. I'll add some additional pictures. $\endgroup$
    – A McKelvy
    Jun 11, 2020 at 19:49
  • $\begingroup$ @A-McKelvy Oh my god that is a match! I searched and searched and searched and couldn't figure out what they were. I stared at so many images. Those schematics are very close to the cartridges. Thank you!!!! $\endgroup$
    – user36480
    Jun 11, 2020 at 20:01
  • $\begingroup$ Interesting interpretation! $\endgroup$
    – Organic Marble
    Jun 11, 2020 at 20:06
  • $\begingroup$ But this begs a question: How then does the Dragon and trunk assembly separate from the second stage? These cartridges are the only possible connection I've ever seen between them. So they must do double duty of severing electrical and possibly pneumatic lines while also shearing off the structural components (bolts?) that hold the trunk on the second stage? But to shearing those connections isn't all that happens---video of Dragon tests shows a positive force is applied at separation... Could that positive force be derived from explosives? $\endgroup$
    – user36480
    Jun 11, 2020 at 20:10

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