Delta IV

Delta IV was a group of five expendable launch systems in the Delta rocket family introduced in the early 2000s. Originally designed by Boeing's Defense, Space and Security division for the Evolved Expendable Launch Vehicle (EELV) program, the Delta IV became a United Launch Alliance (ULA) product in 2006. The Delta IV was primarily a launch vehicle for United States Air Force (USAF) military payloads, but was also used to launch a number of United States government non-military payloads and a single commercial satellite. The Delta IV originally had two main versions which allowed the family to cover a range of payload sizes and masses: the retired Medium (which had four configurations) and Heavy. As of 2023, the Heavy has flown its penultimate mission, NROL-68. Payloads that would have previously fly on Medium and Heavy are moving to either the existing Atlas V or the forthcoming Vulcan. Delta IV vehicles were built in the ULA facility in Decatur, Alabama. Final assembly was completed at the launch site by ULA: at the horizontal integration facility for launches from SLC-37B pad at Cape Canaveral and in a similar facility for launches from SLC-6 pad at Vandenberg Air Force Base. The latest evolutionary development of the Delta rocket family, Delta IV was introduced to meet the requirements of the United States Air Force's (USAF) Evolved Expendable Launch Vehicle (EELV, now National Security Space Launch (NSSL)) program. While the Delta IV retains the name of the Delta family of rockets, major changes were incorporated. Perhaps the most significant change was the switch from kerosene to liquid hydrogen fuel, with new tankage and a new engine required. During the Delta IV's development, a small variant was considered. This would have featured the Delta II second stage, an optional Thiokol Star 48B third stage, and the Delta II payload fairing, all atop a single Common Booster Core (CBC). The Small variant was dropped by 1999.