Complex sentence: The rocket's first and second stages performed nominally, and controllers reported that third stage ignition had occurred, however shortly afterwards the rocket began to underperform, tumble out of control, and deviate from its planned trajectory.
Split into simpler sentences: The rocket's first and second stages performed nominally, and controllers reported that third stage ignition had occurred. However, shortly afterward the rocket began to under-perform, tumbleing out of control, and deviating from its planned trajectory.