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On the SuperHeavy booster, the 33 engines break down into two sets, 20 outer engines that cannot relight (That equipment is actually on the pad itself) and cannot gimble.

The inner 13 consist of a ring of 10 and then 3 more in the middle that on B8 and before, used a hydraulic system to manage the gimble movements. These used big HPU's (hidden under the aerocovers, forming the chines on the side of the booster) to provide hydraulic power/fluid.

With Booster 9 and later, the engines have been changed to use an electric motor system for gimble and TVC support.

Elon Musk did a Twitter Space (captured in this Youtube) on what they know so far and one of the comments that came out of it could be written as:

As things continued to deteriorate through T+62 seconds, more aft heat shield damage, and at T+85 seconds, thrust vector control (TVC) is lost. The loss of the hydraulics is why TVC was lost, and without the hydraulics there could be no separation of Starship. Stage separation uses the hydraulics to operate.' (Taken from a commenter, transcribing the audio, on Behind the Black)

So, on B8, hydraulics for TVC and separation of stages.

On B9, no hydraulics for TVC needed, will separation differ? Or will a smaller HPU be installed to support it?

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Starship does not use a hydraulic / pneumatic stage separation system.

Starship's stage separation is inspired by the satellite deployment system used on Falcon 9 launches of Starlink v1 and v1.5 satellites: the Falcon 9 upper stage gets put into an end-over-end rotation and simply releases the tension rod that fixes the satellites to the payload adapter. This sudden loss of the centripetal force then "flings" the satellites out into space due to the centrifugal force.

The Starship stack will use a similar maneuver to separate the Starship upper stage from the Super Heavy booster. The Super Heavy booster will actually start its 180° flip for the boost back burn shortly before booster engine cutoff. This will "kick" the Starship away from the booster.

Elon Musk explained this in one of the "Starbase Tours" with Tim Dodd, the Everyday Astronaut on August 4, 2021 after 40:45:

The attachment system is also extremely simple and mostly relies on the fact that the Super Heavy booster is pushing the stack from below. There are only a couple of completely passive alignment pins, and a few hooks. The hooks are the only moving part of the system and need to be retracted, but they could easily be electrically actuated (and maybe already are).

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    $\begingroup$ Thanks for the explanation. I had only heard garbled versions of it and couldn't believe it. Still seems like a terrible idea but we'll see. $\endgroup$
    – Organic Marble
    Commented May 2, 2023 at 0:08
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    $\begingroup$ It follows Musk's 5 steps of engineering: 1) Your requirements are dumb. Make them less dumb. 2) Delete the part / process step – if you are not realizing that deleting it was a mistake and have to add it back in at least 10% of the time, you haven't been deleting aggressively enough. (cough flame diverter cough water deluge cough) 3) Third, and only third: optimize. The biggest trap for a smart engineer is to optimize a thing that should not exist. 4) Accelerate the cycle time. 5) Automate. (4 and 5 refer to mass production.) Starship stage sep is currently at step #2. $\endgroup$
    – Jörg W Mittag
    Commented May 2, 2023 at 7:43
  • $\begingroup$ If you read my question, I quote Elon saying that Separation failed because the hydraulic system failed. So there is some component of the process that is hydraulic, even if not all of it. $\endgroup$
    – geoffc
    Commented May 2, 2023 at 20:28
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    $\begingroup$ @geoffc: Except Elon didn't say what's in your quote. In fact, he said the opposite: he said that stage sep didn't fail because it was never attempted, as the criteria for stage sep were never hit. When he talks about stage sep, he says it wasn't attempted, and when he talks about hydraulics, he talks about TVC, not stage sep. He talks about hydraulics in the context of fault isolation and that it is hard to have fault isolation if you have a common manifold feeding hydraulic pressure to multiple actuators. He then continues that the problem is fixed in B9+ with independent electric motors. $\endgroup$
    – Jörg W Mittag
    Commented May 3, 2023 at 7:57
  • $\begingroup$ If it already works well between 2nd stage and payload for F9, isn't starship separation from superheavy supposed to happen in atmospheric drag ? $\endgroup$
    – user19132
    Commented May 9, 2023 at 19:13

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