When these rockets crash, is it because their digital simulations are inaccurate? Why do they need data from the actual test is the question instead of just relying on a bullet proof simulation?
WRT Starship, I'm sure that certain aspects (like heat shield performance) can be simulated to acceptably high fidelity, but the entire system is beyond simulation. An accurate simulation would have to simulate the materials and mechanical behavior of every component, in an interconnected way, under a wide variety of stress states -- which is basically impossible with modern technology. Maybe in a few decades...
Ok chatgpt 21.2
Build me a rocket simulation
The failures Starship had were often to do with simpler engineering bugs that they’ve been ironing out, such as: leaks in piping caused by violent shaking, explosive gases accumulating in closed spaces, filters getting clogged by ice forming in the cryogenic tanks, and burn-through of an experimental heat shield design at moving joints.
Also combustion itself is not properly understood all the way down, so there is literally a big physics gap involved here.
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.
There is more to engineering than understanding the fundamental physics.
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.
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.