One of the most prolific families of space launch vehicles in U.S. history is preparing for its swan song.
United Launch Alliance (ULA) on Friday will attempt the 16th and final launch of its Delta IV Heavy rocket, one of the world’s most powerful—and expensive—commercially produced launch vehicles. The launch was initially scheduled for Thursday afternoon but was scrubbed a few minutes before takeoff.
The mission represents ULA’s 160th overall and the 45th and final flight for the Delta family of rockets as the manufacturer transitions to its Vulcan Centaur. Vulcan made its maiden voyage in January, carrying a Peregrine lunar lander for commercial customer Astrobotic.
“The Delta legacy will live on through Vulcan,” said Gary Wentz, vice president of government and commercial programs for ULA. “We also take this moment to celebrate the thousands of men and women who made the Delta program such a success over the decades. We carry their lessons and wisdom with us into the future.”
ULA is a joint venture between Lockheed Martin and Boeing. It produces the Delta and Atlas families of rockets, primarily for U.S. government use. Delta IV Heavy is the third-highest capacity launch vehicle in operation, behind NASA’s Space Launch System (SLS) and SpaceX’s Falcon Heavy.
The Mission
Friday’s mission, NROL-70, is on behalf of the U.S. National Reconnaissance Office (NRO), which develops and operates spy satellites to collect intelligence and support disaster relief and humanitarian efforts. NROL-70 is ULA’s 35th mission for the NRO and 99th for U.S. national security.
The mission’s payload is classified. But it is possibly intended to give the U.S. more eyes and ears in the stars, which could be used to listen into communications or radio transmissions, for example. Delta IV Heavy is the only rocket in the world that meets all of the requirements to perform the mission, according to ULA.
“The NROL-70 mission will strengthen the NRO’s ability to provide a wide range of timely intelligence information to national decision makers, warfighters, and intelligence analysts to protect the nation’s vital interests and support humanitarian efforts worldwide,” ULA said on its website.
The 235-foot-tall spacecraft will lift off from Space Launch Complex-37 at Cape Canaveral as early as 1:37 p.m. EDT Friday. On ascent, the rocket looks as if it is catching fire, but this is by design, as hydrogen gas used to cool it down before takeoff ignites and burns off. The process is mitigated by a staggered engine ignition, which reduces the amount of hydrogen burned.
First stage separation is expected to occur about five minutes into the mission, followed by the ignition of the main engine and jettisoning of the payload fairing. The spacecraft’s route and final destination are classified.
The Machine
Over six decades, Delta rockets have launched 388 times. About two-thirds of those launched from Cape Canaveral Space Force Station in Florida, the base for Friday’s mission. Delta IV rockets have successfully launched 44 times, carrying payloads on behalf of the NRO, NASA, Air Force, and Space Force.
Delta IV comes in three configurations: Medium+, with either two or four solid rocket motors, and Heavy. Each vehicle consists of a common booster core, upper stage, and payload fairing.
Delta IV Heavy features three common booster core tanks, which power a RS-68A engine system built by Aerojet Rocketdyne. RS-68A is the largest hydrogen-burning engine in existence, according to ULA. The engines burn cryogenic liquid hydrogen and liquid oxygen, each delivering about 700,000 pounds of thrust at sea level.
Atop the booster is a Delta Cryogenic Second Stage (DCSS), or upper stage, which is also fueled by cryogenic liquid hydrogen and liquid oxygen. It is powered by a single RL10C-2-1 engine, also produced by Aerojet Rocketdyne, that produces nearly 25,000 pounds of thrust. The DCSS avionics system provides guidance and flight control for the booster.
Encapsulating the spacecraft is a payload fairing: a three-piece shell designed to shield cargo from the launch and ascent. The payload fairing can be installed off pad, improving safety and minimizing the use of launch facilities.
The History
Incredibly, the Delta family of systems has been in use since 1960. Initiated by NASA in the late 1950s, the program is derived from the Thor intermediate-range ballistic missile, which was later modified into a space launch vehicle.
The inaugural Delta launch in 1960 was unsuccessful. But it paved the way for Delta rockets to launch the world’s first Telstar and Intelsat communications satellites, birthing the phrase, “Live, via satellite!” The launch vehicles also carried NASA’s Pioneer and Explorer scientific spacecraft and delivered the first weather observatory, the Tiros and Geostationary Operational Environmental Satellites (GOES), to space, revolutionizing weather forecasting.
Over the years, ULA updated Delta rockets to make them larger, more advanced, and more durable. The company installed larger first stage tanks, strap-on solid rocket boosters, and advanced electronics and guidance systems, increased the rocket’s propellant capacity, upgraded the main engine, and developed upper stage and satellite payload systems.
The earliest Delta models stood about 90 feet tall, with a mass of 112,000 pounds. Today, Delta IV Heavy towers 235 feet high and weighs 1.6 million pounds at launch. Liftoff thrust, meanwhile, has skyrocketed from 150,000 pounds in 1960 to 2.1 million pounds.
Later Delta models would help usher in the GPS era by sending constellations of navigation satellites into orbit. Delta II launched four dozen satellites over two decades, and Delta IV launched seven.
Delta II—which made its final flight in 2018—completed eight NASA missions to Mars, including the delivery of the Spirit and Opportunity rovers, over the course of 155 flights. It also flew missions to Mercury and visited asteroids, moons, and comets within the solar system.
Delta II has launched probes that “touched the sun,” uncovered exoplanets deep in the Milky Way, and scanned large swaths of the universe using infrared vision. In 2014, it launched the first orbital test flight of NASA’s Orion capsule, which will ferry astronauts around the moon and back during the Artemis II mission in 2025.
By 2002, Boeing had developed Delta IV for the Space Force’s Evolved Expendable Launch Vehicle (EELV) program. That year, the rocket made its debut flight carrying a Eutelsat 33B, its only commercial payload to date. It delivered its first Air Force payload the following year. In 2007, ULA launched the first operational Delta IV Heavy, sending a Space Force Defense Support Program (DSP) satellite into orbit.
The Legacy
Fifteen flights later, Delta IV Heavy is set to become the final Delta rocket to be retired. In addition, ULA has 17 remaining launches for Atlas V, the country’s longest-serving active rocket. Atlas V is cheaper to launch than its counterpart, but it uses Russian-made rather than American-made engines.
Once Delta IV and Atlas V are off the manifest, ULA will transition all launches to Vulcan, which is less expensive than both predecessors. Like previous ULA launch systems, Vulcan is expendable. It was designed primarily for the National Security Space Launch program, as well as for commercial launches such as January’s mission. Customers include Amazon’s Project Kuiper, which placed an order for 38 launches.
ULA will need to compete with the likes of SpaceX, which in 2023 launched more satellites than any other company. SpaceX in 2010 debuted its reusable Falcon 9 launch vehicle, which undercut Delta IV’s price tag. Delta IV, Falcon 9, and SpaceX’s Falcon Heavy, introduced in 2018, are all under contract with the Pentagon to launch expensive military satellites in the coming years.
In addition, SpaceX has an agreement with the Space Force to take over the vacant Space Launch Complex 6 at Vandenberg Space Force Base in California, which hosted Delta IV launches until 2022. The company may further look to acquire room at Space Launch Complex-37 at Cape Canaveral, where ULA will launch Friday barring any hiccups.
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