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Unlike the first stage, the second stage has just one gimballed engine, meaning it can counter pitch and yaw perturbations, but not roll perturbations.

Fins are effective for roll control, but at 50 miles up and above, the atmosphere is very thin, so after the first stage separates, the fins might not serve much purpose... or do they?

The dracos on the Dragon are in their element in those conditions, but do they fire with the second stage still attached?

Beside the fins and the dracos, there seems to be nothing else suited for roll control. The cold gas thrusters on the interstage are gone with the first stage, and there are none of these on the second stage or the Dragon module.

So my question is: how does the second stage of Falcon 9 control roll after the first stage and interstage separate and before the Dragon module separates? What means, specifically do they use? Thanks if you know and can share!

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    $\begingroup$ Note that most Falcon 9 launches do not even carry a Dragon to start with, so the second stage would be useless without its own attitude control system. You really can't rely on whatever payload satellite you're carrying to point the second stage as needed before each burn. Even if you the payload wanted to, it's going to be enclosed in a fairing half of the time. $\endgroup$ – TooTea Jul 10 at 20:37
  • $\begingroup$ Good point! Thanks! $\endgroup$ – Alex Jul 11 at 6:04
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Neither fins nor Dracos, a dedicated cold-gas system.

Source: https://space.stackexchange.com/a/23654

Confirmed by the Falcon 9 User's Guide (April 2020 version)

In addition, the second stage contains a cold nitrogen gas (GN2) attitude control system (ACS) for pointing and roll control.

enter image description here

(emphasis mine)

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  • $\begingroup$ You're right! Thanks! But where on the second stage are those cold gas thruster pods? They're visible near the grid fins on the interstage (which falls out with the first stage)... but I never noticed them on the second stage... Any idea how much thrust those cold gas thrusters produce---even a ballpark figure? $\endgroup$ – Alex Jul 11 at 6:07
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    $\begingroup$ Great followup questions! $\endgroup$ – Organic Marble Jul 11 at 13:37

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