I'm not familiar with the specifics of Falcon9 redundancy. But in theory, losing many engines can be completely recoverable, because all 9 engines share a singl

I'm not familiar with the specifics of Falcon9 redundancy. But in theory, losing many engines can be completely recoverable, because all 9 engines share a single fuel tank. Outside of atmosphere, a rocket's final speed and orbit depend on three factors: the mass of the payload, the mass of the fuel, and the inherent engine design efficiency. Notably, it does not depend on thrust. Burning one engine for 9 seconds results in a speed boost equal to burning 9 engines for 1 second. Even if an engine fails and another one is set to reduced thrust, the fuel is still there, and can be burned by remaining engines. Of course, this is a huge simplification, and loss of thrust while still in atmosphere or near the ground might be a problem.