The United Launch Alliance (ULA) Delta 4 Heavy rocket uses an asymmetrical engine startup sequence because the three Common Booster Cores (CBCs) that power the rocket's first stage are not identical. Two of the CBCs are powered by Rocketdyne RS-68 engines, while the third CBC is powered by an Aerojet Rocketdyne AJ-10 engine. The RS-68 engines are more powerful than the AJ-10 engine, so starting them first creates an imbalance of thrust that could cause the rocket to tip over. To prevent this, the Delta 4 Heavy is equipped with a pintle thrust vector control (TVC) system that uses gimbaling engines to vector the thrust and keep the rocket stable during the engine startup sequence.
The RS-68 engines are started first, followed by the AJ-10 engine. Once all three engines are up and running, the TVC system is no longer needed and the gimbals are locked in place. This asymmetrical engine startup sequence is necessary to ensure a safe and successful launch of the Delta 4 Heavy rocket.