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Comment by 2bitencryption | Hacker News Reader

from this article, it seems like the thrusters must be doing lots of little micro-adjustments, similar to balancing a pencil upright on your hand, or riding on a Segway, correct?

I'm super interested in understanding what the actual mechanism is like for "aiming" the thrusters in different directions. I imagine they must need freedom of movement in a full 360-degree circle to adjust in all possible directions. How the hell is that achieved??

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Here's a little montage of a project I did involving thrust vectoring for a model rocket!

https://youtu.be/pobaR6kSajg

There's a fundamental difference between balancing a rocket on its thrust, and an object on a surface. The surface pushes up, the rocket thrusts along its axis. When the pencil strays from the balance point, the normal force of the support produces a net torque pushing it further out of balance. The rocket doesn't have that problem.

What you're looking for is Trust Vector Control(ler). Most modern rocket engines physically gimbal the engine around with ~two linear actuators around a fixed point at the top of the engine. All of the plumbing and connections to the rest of the rocket must then be flexible.

https://en.wikipedia.org/wiki/Thrust_vectoring#Gimbaled_thru...

If you watch a video of a booster or starship coming down you can see the engines twitching around, pretty cool

A "fun" problem with thrust vectoring and multiple engines is that the engines can most likely physically interfere with each other when actuated to their opposite extremes. There may or may not be some software somewhere to explicitly prevent nozzle collisions, but it's kind of a no-win situation if that software needs to kick in. If the controllers are commanding thrust angles that the engines can't physically achieve, then the rocket is probably going to blow up anyway. In some ways it's better to just let the engines smash into each other and hope for the best, if it gives the controller slightly more control authority in exceptional circumstances.

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

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

You can see them adjusting in the last 30 seconds or so of this SN15 recap: https://www.youtube.com/watch?v=7CZTLogln34

This animation shows some of the movements the engines can pull off https://www.reddit.com/r/SpaceXLounge/comments/kf25kr/starsh...

In the recap video they are only using two engines though. I think on SN15 they relit all 3, and then the guidance computer picked either the two most stable (in the case one of them didn't relight properly), or the two with the best lever arm for the landing trajectory, then shut the remaining one down. Earlier landings tried just one (I think there's enough thrust and maneuverability to do it) but there was no redundancy if it failed.

Gimbals -- https://en.wikipedia.org/wiki/Gimbal#Rocket_engines

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