September 24 at 3:51 pm E.T. from SLC-6 at Vandenberg Space Force Base
The last flight out of Vandenberg for the Delta IV Heavy has happened today. Launching the NROL-91 mission, ULA (United Launch Alliance) has closed an era in space flight on the West coast. There are two more Heavies remaining, but both will lift off from Cape Canaveral, Florida and will launch in 2023, and 24. ULA has opted to retire their Atlas and Delta rockets in favor of their next generation Vulcan Centaur. The Delta IV Heavy is the most powerful rocket in the arsenal of ULA’s launching fleet. Designed with three Delta IV cores bolted together, each powered by an RS-68A engine from Aerojet Rocketdyne. These engines will produce around 2.1 million pounds of thrust at lift off, bringing the vehicle to the speed of sound in just eighty-one seconds as it soars over the Pacific Ocean.
NROL-91 will be the forty-third flight of a Delta IV since 2002, and the fourteenth flight of the Delta IV Heavy since 2004. It marks the ninety-fifth and final flight of a Delta rocket from Vandenberg and the tenth and final Delta IV from the California coast. Launching from SLC-6, a launch complex originally designed for military astronauts and NASA space shuttles. Space Shuttle Enterprise was stacked there previously for ground demonstrations and atmospheric test flights which included the external tank and SRBs (Solid Rocket Boosters) back in 1985. Things changed after the Challenger accident in 86, and the Air Force abandoned plans to launch the shuttle from Vandenberg. SLC-6 sat abandoned for years after. In the 1990’s Lockheed Martin briefly had plans for the complex and their Athena rockets but those missions did not fully utilize the shuttle era infrastructure. In 2006 Boeing took of the launch complex and modified the shuttle infrastructure for the Delta IV rocket, first launching there in June of 2006. Later in 2006 ULA was formed when Boeing and Lockheed Martin merged their Delta / Atlas programs and gained control of the pad. Northrop Grumman also planned to us SLC-6 for its OmegA rocket, which was proposed to the Pentagon in competition for military launch contracts but lost out when the military chose ULA’s Vulcan, and SpaceX’s Falcon 9 / Heavy launch vehicles to compete for contracts through 2027. At this time there are no current future plans for the launch complex after this mission.
Officially the NROL–91 mission is classified, but we know that the NRO (National Reconnaissance Office) is the eyes and ears of the United States government. They are responsible for developing, acquiring, launching and operating America’s reconnaissance satellites, along with the operations associated with data processing facilities in support of national security. The NRO uses a variety of satellites to meet their mission parameters. From small satellites to more traditional larger satellites the NRO has a wide range of intelligence requirements to carry out research and development efforts along with assisting in emergency and disaster relief efforts not just in the U.S. but across the globe.
Though we can’t say with any certainty, the secret payload onboard this last Delta IV heavy is likely a new sharp eyed electro-optical Keyhole type imagine platform for the NRO. The mission parameters closely match a previous Delta IV Heavy launch from January of 2019, on the NROL-71 mission. Using maritime exclusion zones, and other publicly available information it is estimated that the mission will aim to place the satellite in a low Earth orbit at a latitude of around 250 miles with an inclination of about 74 degrees to the equator as it launches South-Southeast from Vandenberg over the Pacific Ocean.
At seven seconds before launch the three RS-68A engines will ignite in sequence from the starboard booster to the port with the core igniting two seconds later. This staggered start helps mitigate the effects of hydrogen build up around the base of the vehicle. At this point the 233-foot-tall rocket and its 1.8 million pounds will start to move vertically as it builds to its full 2.1 million pounds of thrust. around 89 seconds into flight the vehicle will reach Max-Q and then 1.4 seconds later will push past the speed of sound at Mach 1. At five minutes and thirty-five seconds into flight the first stage boosters will shut down and separate from the second stage. Shortly after the Delta IV’s payload fairing will also separate and the NROL-91 spacecraft will be exposed to space for the first time as it continues on attached to the second stage of the vehicle to its desired orbit.