4 ms·
An unstable rocket (almost all of the big ones) is an inverted pendulum of sorts. The combined center of thrust and aerodynamic forces is usually below the C.G,
by goku12 3y ago
An unstable rocket (almost all of the big ones) is an inverted pendulum of sorts. The combined center of thrust and aerodynamic forces is usually below the C.G, and the rocket tends to tip over if not actively stabilized. To add a little, a slight margin of tipping is allowed, but the rocket tends to break up if the angle of attack crosses certain limit at high speeds.
- regularfry 3y agoLiquid 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 agoAlso, it's extremely windy.
- goku12 3y agoTrue. 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 agoI think they also don't actually use wine, gremlins or any glass containers...
- TeMPOraL 3y agoMy 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 agoOr 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 agoThis 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 agoI'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 agoIt'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 agoWhen does a syringe become a piston?
- yetihehe 3y ago
- zardo 3y agoIn 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 agoIn a very real sense, you only need to "steer" the inverted pendulum, too.