The blast deflectors on conventional carriers are used to protect personnel and equipment (such as another aircraft waiting to use the same catapult) from the blast of the aircraft taking off. Catapults on an American carrier are at the front of the ship with most of the flight deck behind them or half way back with still a large area behind with operations on going. On the Queen Elizabeth ships the F-35B will start it's run from the rear of the ship and travel the full length before using the ski jump to fly away. Nothing behind the jet when it starts its run.