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The penultimate Delta IV Heavy was launched from Cape Canaveral SFS on June 22, 2023, for the NROL-68 mission, setting the stage for the program’s completion with the NROL-70 launch on the last Delta IV Heavy on April 9.
What began as the U.S. Air Force’s first operational ballistic missile ended more than six decades of service as one of the country’s oldest and most reliable space launch vehicles with a final flight for the National Reconnaissance Office.
The last Delta IV Heavy rocket lifted off from Cape Canaveral SFS on April 9 with the classified NROL-70 payload, marking the end of what was once the heaviest-lift expendable rocket in the U.S. as well as the Delta family.
- The Delta rocket program comes to a close with its final U.S. military mission
- Atlas boosters will be retired as well
“It is a bittersweet moment for us,” United Launch Alliance (ULA) CEO Tory Bruno told reporters in Cape Canaveral on March 27. “Delta has been around in one form or another for 60 years. There was the Thor [intermediate-range ballistic missile], then it was the Delta space launch vehicle. It went through Delta I, II, III into IV, and there were many models . . . and different configurations of Delta rockets. It just has a storied legacy, and it has done great things for our nation.”
ULA took ownership of the Delta rocket line as part of manufacturer Boeing’s 2006 joint venture with Lockheed Martin, which contributed its Atlas V booster to the partnership.
The Atlas’ days are numbered, too, with 17 missions remaining. ULA is replacing both the Atlas and the Delta with the less expensive and higher-performance Vulcan, a two-stage booster outfitted with a pair of Blue Origin BE-4 methane-fueled first-stage engines. While the rocket is fully expendable now, ULA plans to recover and reuse the BE-4s.
The Vulcan is designed to fly high-energy missions for the military and serve as a viable option for commercial satellite operators. Amazon has purchased 38 Vulcan launches to deploy part of its Kuiper broadband megaconstellation in low Earth orbit.
“Every rocket can do a range of missions, but there is one mission that it’s best at,” Bruno said before the April 9 launch. “[The Vulcan] is literally designed to be best at the mission we’re flying with this last Delta IV. Delta IV is three rockets bolted together. With a single-core Vulcan, we’re able to collapse that cost by 70%, making that mission a lot more practical. [The Vulcan] also is able to fly at a much higher tempo, which is going to be pretty important as the country faces the threat from Russia and especially China.”
In January, the Vulcan completed the first of two certification missions needed to fly National Space Security Launch payloads. The demonstration mission sent Astrobotics’ Peregrine lunar lander toward the Moon. Cert-2, which will carry Sierra Space’s Dream Chaser spaceplane to the International Space Station, is expected to launch this year.
ULA had hoped to fly the 16th and final Delta IV Heavy on March 28, but during an unplanned hold due to weather, a liquid pump on the gaseous nitrogen pipeline that provides pneumatic pressure to the launch vehicle systems failed. The pipeline, which is owned by NASA and managed by contractor Air Liquide, services launchpads at both Cape Canaveral SFS and Kennedy Space Center.
With its three hydrogen-fueled Aerojet Rocketdyne RS-68A main engines burning, the 235-ft.-tall Delta IV Heavy lifted off from Cape Canaveral’s Space Launch Complex 37 at 12:53 p.m. EDT on April 9 to begin the classified NROL-70 mission.
“This is such an amazing piece of technology—23 stories tall, a half-million gallons of propellant, a quarter--million pounds of thrust, and the most metal of all rockets, setting itself on fire before it goes to space,” Bruno said before the launch. “Retiring it [and] moving to Vulcan is obviously the future, but still sad.”
Flames shooting up the rocket’s three core stages during engine ignition stem from residual hydrogen propellant clinging to the sides of the booster following engine thermal conditioning.
“It is one of the reasons there’s SOFI [spray-on foam insulation]—that striking orange insulation on the boosters,” Bruno explained. “Part of that is for aero heating, but the thickness of it is to account for that effect.”
Over the years, the U.S Defense Department has been the primary customer for the Delta IV Heavy, purchasing 12 of its 16 flights. NASA bought two of the launches—one for the Parker Solar Probe, with an original contract valued at $389 million, and one for its 2014 Orion capsule flight test, at a cost of about $370 million.
NROL-70 marked the 389th launch of the Delta rocket, which debuted nearly 64 years ago on May 13, 1960. SpaceX’s Falcon 9 booster—the poster child of the new Space Age—is poised to surpass that record in 2024 after 11 years of flight.
“The legacy we hope for Delta is one of service. . . . That’s how I want Delta to be remembered,” Bruno said. “Even though Vulcan is the future, I’m personally sad to see it go.”
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