Delta IV Heavy

The Delta IV Heavy (Delta 9250H) is an expendable heavy-lift launch vehicle, the largest type of the Delta IV family. It is the world's third highest-capacity launch vehicle in operation, behind NASA's Space Launch System and SpaceX's Falcon Heavy and closely followed by CASC's Long March 5. It is manufactured by United Launch Alliance (ULA) and was first launched in 2004. ULA will retire the Delta IV Heavy in 2024. Future ULA launches will use the new Vulcan Centaur rocket. As of June 2023, one flight remains.

The Delta IV Heavy consists of a central Common Booster Core (CBC), with two additional CBCs as liquid rocket boosters instead of the GEM-60 solid rocket motors used by the Delta IV Medium+ versions. At lift off, all three cores operate at full thrust, and 44 seconds later the center core throttles down to 55% to conserve fuel until booster separation. The two side boosters burn out at 242 seconds after launch and are separated as the core booster throttles back up to full thrust. The core burns out 86 seconds later, and the second stage completes the ascent to orbit.

The launch vehicle uses three RS-68 engines, one in the central core and one in each booster. On the last seconds of countdown, the hydrogen fuel flows through the engines and upwards along the booster body, and after the ignition that hydrogen inflames, making the characteristic fireball and charred look of the booster.