3 ms·

Liquid fuel rockets complicate matters: think "inverted pendulum with a couple of stacked, full wine glasses that you absolutely must not spill balanced on top.

Liquid fuel rockets complicate matters: think "inverted pendulum with a couple of stacked, full wine glasses that you absolutely must not spill balanced on top. Oh, and there's a gremlin drinking from them so both CG and total mass are constantly moving."

thfuran 3y ago
Also, it's extremely windy.
goku12 3y ago
True. Liquids make control difficult. > full wine glasses that you absolutely must not spill balanced I'm not sure what you mean by spilling. The liquids are usually contained in fully closed tanks and cannot spill. The thrust usually keeps the liquid at the bottom around the engine intakes. However, liquid sloshing inside the tanks is a problem. Even with slosh dampers, sloshing liquid manages to create attitude disturbances that can cause control instability unless managed carefully.
- yetihehe 3y ago
  I think they also don't actually use wine, gremlins or any glass containers...
- TeMPOraL 3y ago
  My inner child of Doc Brown and Wernher von Kerman says, it's time to develop non-newtonian fuel sponges, which would release fuel if squeezed gently and steadily, but behave like solid if things start shaking too much, preventing sloshing.
  - yetihehe 3y ago
    Or maybe giant syringes? There can't be sloshing if there is no free space because top of cylinder just moves to adjust volume.
    - TeMPOraL 3y ago
      This actually makes perfect sense, so I'm hoping some rocket scientist will come and explain what are the "devils in the details" that prevent rockets from being built like this.
      - raducu 3y ago
        I'm sure it's easy: some liquids like oxygen and methane are stored under high pressure, you would need an incredibly well built seal and powerful piston. If it's a room temperature liquid probably there are less complicated solutions like baffles and bladders. And then you'd gave to over engineer the rocket iself to not break when tipping over, not just the fuel tanks.
    - littlestymaar 3y ago
      “Non-Newtonian fuel sponges” sounds way cooler though.
    - regularfry 3y ago
      It's a moving part to go wrong. But more than that, the fuel tank has very thin walls. I can imagine you don't want a solid object inside the tank that's able to get any momentum. You do get baffles in liquid fuel tanks to cut down the effect, and some people have tried flexible bladders inside the rigid tanks for a similar reason but they don't work well with cryogenic fuels.
    - raducu 3y ago
      When does a syringe become a piston?
      - yetihehe 3y ago
        When you remove outer wall?
        raducu 3y ago
        my bad, the piston is indeed just the inner moving part, my point was that we think of syringe as easy because it is not under pressure, but if you think in terms of car piston/cylinder you appreciate more why this can't work in a rocket fuel tank.
zardo 3y ago
In an inverted pendulum the support pushes up, in a rocket it pushes along the direction the rocket is pointed. You only need to steer the rocket, not balance it.
- regularfry 3y ago
  In a very real sense, you only need to "steer" the inverted pendulum, too.

thfuran 3y ago

Also, it's extremely windy.

goku12 3y ago

True. Liquids make control difficult. > full wine glasses that you absolutely must not spill balanced I'm not sure what you mean by spilling. The liquids are usually contained in fully closed tanks and cannot spill. The thrust usually keeps the liquid at the bottom around the engine intakes. However, liquid sloshing inside the tanks is a problem. Even with slosh dampers, sloshing liquid manages to create attitude disturbances that can cause control instability unless managed carefully.

yetihehe 3y ago
I think they also don't actually use wine, gremlins or any glass containers...
TeMPOraL 3y ago
My inner child of Doc Brown and Wernher von Kerman says, it's time to develop non-newtonian fuel sponges, which would release fuel if squeezed gently and steadily, but behave like solid if things start shaking too much, preventing sloshing.
- yetihehe 3y ago
  Or maybe giant syringes? There can't be sloshing if there is no free space because top of cylinder just moves to adjust volume.
  - TeMPOraL 3y ago
    This actually makes perfect sense, so I'm hoping some rocket scientist will come and explain what are the "devils in the details" that prevent rockets from being built like this.
    - raducu 3y ago
      I'm sure it's easy: some liquids like oxygen and methane are stored under high pressure, you would need an incredibly well built seal and powerful piston. If it's a room temperature liquid probably there are less complicated solutions like baffles and bladders. And then you'd gave to over engineer the rocket iself to not break when tipping over, not just the fuel tanks.
  - littlestymaar 3y ago
    “Non-Newtonian fuel sponges” sounds way cooler though.
  - regularfry 3y ago
    It's a moving part to go wrong. But more than that, the fuel tank has very thin walls. I can imagine you don't want a solid object inside the tank that's able to get any momentum. You do get baffles in liquid fuel tanks to cut down the effect, and some people have tried flexible bladders inside the rigid tanks for a similar reason but they don't work well with cryogenic fuels.
  - raducu 3y ago
    When does a syringe become a piston?
    - yetihehe 3y ago
      When you remove outer wall?
      - raducu 3y ago
        my bad, the piston is indeed just the inner moving part, my point was that we think of syringe as easy because it is not under pressure, but if you think in terms of car piston/cylinder you appreciate more why this can't work in a rocket fuel tank.

zardo 3y ago

In an inverted pendulum the support pushes up, in a rocket it pushes along the direction the rocket is pointed. You only need to steer the rocket, not balance it.

regularfry 3y ago
In a very real sense, you only need to "steer" the inverted pendulum, too.