United Launch Alliance’s ground team recently completed a countdown rehearsal dress and installed a classified spy satellite payload on top of a Delta 4-Heavy rocket at Vandenberg Space Force Base in California, readying the triple-body launcher for liftoff Saturday.
The top secret spy satellite on top of the Delta 4-Heavy rocket is likely a new sharp-eyed electro-optical Keyhole-type imaging platform for the NRO, according to independent analysts. But the NRO has not disclosed details of the satellite, keeping with the spy agency’s policy of not commenting on details of its missions.
The Delta 4-Heavy rocket is set to blast off from Space Launch Complex-6, a sprawling launch pad on the southwestern edge of Vandenberg overlooking the Pacific Ocean, at 2:53 pm PDT (5:53 pm EDT; 2153 GMT) Saturday , according to ULA and the NRO. Officials previously said the launch window extends from 1:50 pm to 4:12 pm PDT.
The mission Saturday is codenamed NROL-91, and will mark the final Delta 4 launch from Vandenberg. It will be the third-to-last flight overall for the Delta rocket family, which ULA is retiring in favor of the next-generation Vulcan Centaur rocket. The Vulcan rocket will replace ULA’s Atlas and Delta rocket fleets.
The final Delta 4 launches are scheduled for 2023 and 2024 from Cape Canaveral. Those missions will also haul top secret spy satellites into orbit for the NRO.
Maritime exclusion zones published as part of public navigation warnings suggest the Delta 4-Heavy on the NROL-91 mission will fly south-southeast from Vandenberg over the Pacific Ocean. Based on publicly available data, analysts estimate the mission will aim to place its classified cargo into low Earth orbit at an latitude of around 250 miles (400 kilometers) and an inclination of approximately 74 degrees to the equator.
Those parameters closely match a previous Delta 4-Heavy rocket launch for the NRO in January 2019, designated NROL-71.
The Delta 4-Heavy is the most powerful missile in ULA’s fleet of operational rockets. Engineers designed the rocket with three Delta 4 rocket core bolted together, each powered by an RS-68A engine produced by Aerojet Rocketdyne. The Delta 4-Heavy’s main engines will produce about 2.1 million pounds of thrust at liftoff, powering the launcher faster than the speed of sound in 81 seconds as it soars downrange over the Pacific Ocean.
The core stage engine will throttle down its RS-68A engine to a partial thrust setting, conserving its supply of liquid hydrogen and liquid oxygen propellants while the side boosters fire at full throttle. The side boosters will shut down their RS-68A engines and jettison about four minutes into the mission, falling to predetermined drop zones in the Pacific. The core stage will burn until around about five-and-a-half minutes into the flight, then jettison to yield to the Delta 4’s hydrogen-fueled upper stage.
The upper stage will fire its RL10 engine moments later, and the rocket will release its nose fairing into a clamshell-like motion, revealing the top secret NRO satellite to space for the first time. At that point, the NROL-91 mission will enter a government-imposed news blackout, with ULA ending live coverage of the launch.
The rest of the mission through payload deployment will occur in secret. ULA and the NRO are expected to release a statement once the NROL-91 payload separates from the Delta 4’s upper stage.
ULA technicians assembled the Delta 4-Heavy rocket in a Horizontal Integration Facility near the SLC-6 launch pad after the first stage boosters and upper stage were delivered to California last year from the company’s factor in Decatur, Alabama. Ground teams rolled the Delta 4-Heavy to the SLC-6 launch pad earlier this year, then raised it vertical for a countdown dress rehearsal that involved loading cryogenic propellants into the rocket.
With that milestone complete, ULA recently installed the top secret NROL-91 payload atop the Delta 4-Heavy to cap off stacking of the 233-foot-tall (71-meter) rocket inside the launch pad’s mobile service gantry, which will retract away from the vehicle during Saturday’s countdown.
The NROL-91 launch will be the 43rd flight of a Delta 4 coat since 2002, and the 14th launch in the Delta 4-Heavy configuration since 2004. It’s also the 95th and final launch by the Delta rocket family from Vandenberg, and the 10th and final Delta 4 launch from the California spaceport.
There are no future users currently lined up for the SLC-6 launch pad after Saturday’s launch. Nestled among hills that hide the pad from public view, SLC-6 was originally constructed to support flights of military astronauts and NASA space shuttles.
NASA’s space shuttle Enterprise, used for ground demonstrations and atmospheric test flights, was stacked on the SLC-6 launch with external tank and solid rocket booster test articles for fit checks in early 1985.
But the Air Force abandoned plans to launch military space shuttle flights from Vandenberg in the wake of the Challenger accident in 1986, and the shuttle-era buildings at the SLC-6 site sat mothballed for years. Lockheed Martin used the SLC-6 pad for four launches of its light-class Athena rockets in the 1990s, but those missions didn’t utilize much of the shuttle-era infrastructure at the site.
Boeing took over the SLC-6 pad and modified the shuttle infrastructure for the Delta 4 rocket, which first launched there in June 2006. ULA was formed later in 2006 by the merger of Boeing’s Delta and Lockheed Martin’s Atlas rocket programs.
Northrop Grumman planned to use the SLC-6 launch pad for its planned OmegA rocket, which the company proposed to the Pentagon in a competition for military launch contracts. But the Defense Department selected ULA’s Vulcan missile and SpaceX’s Falcon 9 and Falcon Heavy launchers in 2020 for eligibility to compete for military launches scheduled through 2027.
Northrop Grumman shut down development of the OmegA rocket after losing the military launch procurement.
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