20 years from now:

Ok chatgpt 21.2

Build me a rocket simulation

The simulatable stuff is almost perfect. It's the stuff that can't be simulated that fails.

Take the last flight as an example. The booster experienced what was (probably) a structural failure in the propellant fuel lines. Simulating stress in the structure under static conditions is quite straightforward. Simulating the stress as the rocket ascends vertically and the tanks empty is hard, but doable.

Simulating the dynamic loading as the rocket flips? The fuel sloshes around, the sloshing fuel changes the kenimatics of the rocket, the kenimatics of the rocket change how the fuel sloshes, the engines try to correct adding a new force, the thrust from the engines creates increased force on the fuel increasing the pressure to the pumps, the performance of the engines changes because of the new fuel flow, that alters the acceleration further causing fuel to slosh, gass bubbles are intrained in the fuel from all the sloshing thus altering its flow/sloshing behavior, valves open and close creating pressure waves in the fuel that travel up and down the fuel lines (the water-hammer effect alone being enough to burst the pipes if valve closing is not well-timed), and the rocket itself flexes as all this happens, testing every exact detail of the manufacturing which you have to go out to the factory and physically measure. No simulation software ever imagined can handle all that coupling of systems.

The usual solution is to make some conservative estimates (the center-of-mass of the fuel will move by at most some amount, bubbles will last at most some time, the engines will have so much control authority, etc). But that requires experience. And this is aerospace, so safety margins are tiny.

A perfect theory of physics that exactly predicts the behavior of atoms doesn't really help you much when you're trying to predict the behavior of a spaceship containing approximately 10^33 atoms. Any such prediction is going to involve an enormous amount of heuristic approximation.

Just as one example, a spacecraft moving through a fluid atmosphere and with fluid fuel/oxidizer burning in its combustion chamber is going to involve incredibly complex turbulent fluid flows. And turbulence is something that we famously don't have good high-level theories (approximations) for.

> We’re not learning new physics are we

We are - at least in terms of model fidelity. There's limitations to how much CFD/simulations you can do. That kind of data (+other sensors) is used to refine models - thermal, aerodynamics, structures. Especially with starship, they are able to stream out live-video and data so that they get it even if the vehicle breaks up. Controlled hypersonic flight of such structures has been done very few times. There's stuff that can be learned from previous vehicles like the Space Shuttle but there are a lot of things that are very different - different control surfaces, flight profiles, thermal management etc.

Because there's a million and one things that could go wrong? Yes, we can make simulations. But the simulations are based on assumptions about the physical world. When the physical world doesn't obey those assumptions, things start to deviate from the simulation. Plus, simulations aren't perfect. We can't simulate every atom of the rocket and every atom of the atmosphere, so we have to approximate things. Errors creep in.

There is more to engineering than understanding the fundamental physics.

There isn't enough computing power in the world to simulate something like Starship at the atomic level, even if you assume you know all possible starting conditions and your software is absolutely correct.

I believe it's because the space of physics interacting with specific designs of specific components in the context of a large system with other specific components is very, very big and thus it is not feasible to just "simulate everything" ahead of time.

Also combustion itself is not properly understood all the way down, so there is literally a big physics gap involved here.

The physics is the easy part. Starship consists of thousands (millions?) parts that are all manufactured to varying precision. These tolerances can add up and cause unexpected failures.