For the spaceship example, I’m fairly certain that we do know the theory behind how all those materials and processes are created though. It isn’t that we lost

For the spaceship example, I’m fairly certain that we do know the theory behind how all those materials and processes are created though. It isn’t that we lost the knowledge, but the infrastructure. Recreating the infrastructure to create the materials to create the rockets is possible, it’s just that recreating entire processes from scratch when they had already been phased out of our modern structures would be outrageously expensive, especially just to construct rockets.