5 ms·

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

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??

pmcjones 5y ago
Gimbals -- https://en.wikipedia.org/wiki/Gimbal#Rocket_engines https://en.wikipedia.org/wiki/Gimbal#Rocket_engines
sephamorr 5y ago
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_thrust https://en.wikipedia.org/wiki/Thrust_vectoring#Gimbaled_thru...
- walleeee 5y ago
  If you watch a video of a booster or starship coming down you can see the engines twitching around, pretty cool
zlsa 5y ago
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 https://www.youtube.com/watch?v=3pD5CZZfnYk Merlin TVC test: https://www.youtube.com/watch?v=Pigsq5rt-mY https://www.youtube.com/watch?v=Pigsq5rt-mY
- bigyikes 5y ago
  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?
  - perl4ever 5y ago
    Probably just as important to save on complexity.
    - zlsa 5y ago
      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?
      - bolangi 5y ago
        IOW, the best part is no part.
  - sgtnoodle 5y ago
    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.
- hcrisp 5y ago
  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 https://youtu.be/Y_9FZDnCaoU
zardo 5y ago
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.
polishdude20 5y ago
Here's a little montage of a project I did involving thrust vectoring for a model rocket! https://youtu.be/pobaR6kSajg https://youtu.be/pobaR6kSajg
sgtnoodle 5y ago
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.
chedabob 5y ago
You can see them adjusting in the last 30 seconds or so of this SN15 recap: https://www.youtube.com/watch?v=7CZTLogln34 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/starship_raptor_gimbal_visualized/ 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.

pmcjones 5y ago

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

sephamorr 5y ago

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

walleeee 5y ago
If you watch a video of a booster or starship coming down you can see the engines twitching around, pretty cool

zlsa 5y ago

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 https://www.youtube.com/watch?v=3pD5CZZfnYk Merlin TVC test: https://www.youtube.com/watch?v=Pigsq5rt-mY https://www.youtube.com/watch?v=Pigsq5rt-mY

bigyikes 5y ago
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?
- perl4ever 5y ago
  Probably just as important to save on complexity.
  - zlsa 5y ago
    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?
    - bolangi 5y ago
      IOW, the best part is no part.
- sgtnoodle 5y ago
  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.
hcrisp 5y ago
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 https://youtu.be/Y_9FZDnCaoU

zardo 5y ago

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.

polishdude20 5y ago

Here's a little montage of a project I did involving thrust vectoring for a model rocket! https://youtu.be/pobaR6kSajg https://youtu.be/pobaR6kSajg

sgtnoodle 5y ago

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.

chedabob 5y ago

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