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.

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.

Or maybe giant syringes? There can't be sloshing if there is no free space because top of cylinder just moves to adjust volume.

I believe https://en.wikipedia.org/wiki/MGM-52_Lance used roughly this configuration, with a gas generator pushing the back of the pistons. As I remember, the oxidizer and fuel pistons were physically connected (concentric, with concentric propellant tanks) to simplify getting the mixture correct.

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.

“Non-Newtonian fuel sponges” sounds way cooler though.

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.

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.

Some fighter planes go this. They use a bladder they compress, works the same.

When does a syringe become a piston?

When you remove outer wall?

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.

I think they also don't actually use wine, gremlins or any glass containers...

Also, it's extremely windy.

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.

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