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Comment by zlsa | Hacker News Reader

The thrust vector control method in this case is gimbaling, and it basically means using linear actuators (I think these are commonly hydraulic, and Falcon 9's Merlin engines actually use the kerosene fuel in the TVC hydraulic system) to point the engine in a different direction:

Raptor TVC test: https://www.youtube.com/watch?v=3pD5CZZfnYk

Merlin TVC test: https://www.youtube.com/watch?v=Pigsq5rt-mY

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Are you saying that they use kerosene fuel in place of hydraulic fluid? If so, wow, you just blew my mind. Would that be to save on weight?

Probably just as important to save on complexity.

Yeah, critically the rocket engine turbopump already produces very high pressure kerosene and liquid oxygen; so why not tap off some kerosene there instead of building a separate hydraulic pump system?

IOW, the best part is no part.

As others have said, there's already a turbopump providing kerosene at a high pressure. Also, the hydraulics are open loop; the dumped low pressure kerosene is fed into the engine. That's a lot simpler than a closed loop system, or an open loop system with a dedicated supply of fluid. There's no risk of running out of kerosene for the actuators because as soon as you do, you can't thrust anymore either and so the actuators don't matter.

They also use the kerosene to cool the engines' nozzles before it gets combusted. This keeps the nozzles from melting, and preheats the fuel so it combusts more efficiently.

Besides gimballed rocket bells, it looks like they also use side venting of fuel, oxidizer, or inert gas. It's not steady, it starts and stops apparently to give an extra torque moment when rotating the body or stabilizing the upper end just prior to and after landing.

https://youtu.be/Y_9FZDnCaoU

The big vents about midway up the vehicle are gaseous depressurization vents for the methane and liquid oxygen tanks; the RCS (reaction control system) that's designed to provide torque has thrusters at the nose and tail (to maximize the length of the lever arm.)

https://youtu.be/6iBnXy1ag8A?t=109

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