Delta 4-Heavy rocket deploys spysat on final planned mission from ‘Slick Six’

EDITOR’S NOTE: Updated Sept. 25 with satellite sightings and radio signal detections.

ULA’s Delta 4-Heavy rocket climbs away from Space Launch Complex 6 at Vandenberg Space Force Base to begin the NROL-91 mission Saturday. Credit: Brian Sandoval / Spaceflight Now

United Launch Alliance sent a triple-core Delta 4-Heavy with a top secret U.S. government spy satellite into orbit Saturday from California’s Central Coast, closing out a chapter in the tangled history of a launch pad originally built to support military astronaut missions on Titan rockets and space shuttles.

The spy satellite on-board the Delta 4-Heavy is owned by the National Reconnaissance Office, which discloses few details about its spacecraft. The circumstances of the launch Saturday — its launch site, rocket configuration, and flight track after liftoff — led independent analysts to conclude the Delta 4 likely carried a high-resolution electro-optical surveillance satellite into orbit.

Two more Delta 4-Heavy rockets are left in ULA’s inventory for launches in 2023 and 2024 from Cape Canaveral as the company transitions to the next-generation Vulcan Centaur rocket, the replacement for the Atlas and Delta launcher families that date back to the dawn of the Space Age.

The Atlas 5 and Delta 4 rockets currently flown by ULA show little resemblance to their forebears, but the names are steeped in history. Saturday’s launch from Vandenberg Space Force Base was the 95th and final flight of a Delta rocket from the military spaceport about 140 miles (225 kilometers) northwest of Los Angeles, and the 387th launch overall of a rocket bearing the Delta name.

It was the 10th launch of a Delta 4 rocket — and the fifth in the Delta 4-Heavy configuration — from Space Launch Complex 6, a picturesque launch site nestled on a hillside bluff overlooking the Pacific Ocean. The sprawling launch pad, known as SLC-6 or “Slick Six,” faces an uncertain future after the last Delta 4 flight from the site Saturday. The Space Force, which owns the pad and leased it to ULA, doesn’t yet have another tenant lined up to take over the facility.

“For us, seeing the last Delta launch from Vandenberg is bittersweet, for sure. We’re sorry to see it go,” said Col. Bryan Titus, vice commander of Space Launch Delta 30, the unit in charge of Vandenberg Space Force Base and the Western Range.

Three Aerojet Rocketdyne RS-68A main engines roared to life in the final seconds of Saturday’s countdown at SLC-6, sending a plume of red-hot exhaust out each side of the flame trench under the pad. After a staggered start sequence, the hydrogen-fueled engines powered the 233-foot-tall (71-meter) Delta 4-Heavy off the pad at 3:25:30 p.m. PDT (6:25:30 p.m. EDT; 2225:30 GMT).

Ten seconds later, the rocket vectored thrust from its main engines to steer south-southeast from Vandenberg, taking an arcing trajectory over the Pacific Ocean riding 2.1 million pounds of thrust. The rocket broke the sound barrier in about 80 seconds as the RS-68A engines on the Delta 4-Heavy’s side boosters operated at full throttle, while the center engine throttled back to conserve fuel for the first few minutes of the flight.

The engines burned 5,000 pounds of liquid hydrogen and liquid oxygen propellants each second, quickly burning through the 450,000 gallons of cryogenic fluids loaded into the Delta 4-Heavy’s three orange common booster cores during the countdown.

The side boosters shut down their engines and separated from the core stage about 4 minutes into the flight, heading for uncontrolled splashdowns in the Pacific a few hundred miles downrange from Vandenberg. The core stage then throttled up its RS-68A engine to burn for another minute-and-a-half before separation from the Delta 4’s cryogenic upper stage, which did the rest of the work to place the top secret spy payload into orbit.

A Delta 4-Heavy rocket emerges from behind a hill a few seconds after liftoff Saturday from Vandenberg Space Force Base, California. Credit: United Launch Alliance

The upper stage ignited its RL10 engine and released its clamshell-like payload fairing a few seconds later, and ULA ended its live coverage of the mission. The remainder of the flight occurred in a government-ordered news blackout.

The NRO and ULA issued press releases a little more than two hours after liftoff, confirming a successful conclusion to the launch, officially designated NROL-91.

“The Delta 4-Heavy has proven to be an integral part of the NRO’s history, helping us build the architecture for the world’s best space-based intelligence, surveillance and reconnaissance,” said Chris Scolese, director of the NRO. “As our agency and the aerospace industry continue to innovate and evolve, we will explore new vehicles for launching payloads even more efficiently and effectively, with even greater capacity, agility, speed, and resilience. We are excited about the new technologies and partnerships that will define our next chapter.”

The NRO has flown payloads on 15 Delta 4 missions to date. The final two Delta 4-Heavy rockets set for launch from Cape Canaveral in 2023 and 2024 will also loft classified NRO satellites.

NRO satellites collect optical and radar imagery, intercept radio transmissions, and gather other intelligence data for U.S. government leaders, deployed military forces, and intelligence agencies.

“We like to say we’re the nation’s eyes and ears in space,” said Space Force Col. Chad Davis, director of the NRO’s Office of Space Launch. “So we provide that capability for our warfighters, allied and U.S., for our national decision makers to put them in the best spot they possibly can to either make decisions or execute what they need to on the battlefield.”

Satellite trackers using publicly-available information on the Delta 4 rocket’s flight track after liftoff predicted the mission would place its payload into an orbit less than 250 miles (400 kilometers) above Earth, and at an orbital inclination of 73.6 degrees to the equator. It turned out the prediction was close to the mark.

Sky watchers spotted the satellite from the NROL-91 mission in the skies over the Netherlands early Sunday. Scott Tilley and Cees Bassa, both radio astronomers, also detected encrypted radio transmissions from the satellite, suggesting the spacecraft was healthy and in the expected orbit after launch.

The NROL-91 mission likely deployed a sister satellite to a payload launched on a Delta 4-Heavy rocket on the NRO’s NROL-71 mission in January 2019, according to Marco Langbroek, a Dutch archaeologist and an expert in the orbits of military spacecraft.

Both satellites launched into similar orbits. Seasoned trackers of spy satellites believe the NROL-71 and NROL-91 missions launched the first members of a new generation of Keyhole, or KH-11, optical reconnaissance satellites.

But previous KH-11 satellites launched into sun-synchronous polar orbits, at a different inclination than the 73.6-degree orbits achieved on the NROL-71 and NROL-91 missions. A Delta 4-Heavy launch from Vandenberg last year — between NROL-71 and NROL-91 — deployed its NRO payload into a more conventional sun-synchronous orbit, which is tailored for regular, repeatable observations of strategic sites, military installations and other targets of interest to U.S. intelligence agencies.

The KH-11 satellites are essentially bus-sized telescopes peering down on Earth, with primary mirrors measuring 7.9 feet (2.4 meters) across, the same size as the mirror on the Hubble Space Telescope. The ultra-sharp, very-high-resolution imagery produced by such satellites is believed to be unparalleled, and the spy craft relay their observations to the ground via the NRO’s dedicated network of communications satellites.

The NRO’s patch for the NROL-91 mission. Credit: National Reconnaissance Office

Delta 4-Heavy missions from California in 2011 and 2013 deployed gap-filler KH-11-type satellites to continue supplying the government with reconnaissance imagery after the NRO canceled a contract with Boeing for a replacement line of optical imaging craft as part of the Future Imagery Architecture program. Keyhole satellites before 2011 launched on Titan rockets from Vandenberg.

The NRO eventually selected Lockheed Martin — the same company that built the past generation of KH-11 satellites — to construct at least two new-generation spacecraft, introducing new technology and other upgrades into the spy satellite constellation.

The new electro-optical surveillance satellites, sometimes called KH-11 Block 5, have the same 2.4-meter mirror diameter as the earlier Keyhole-type Earth-imaging platforms, according to past public statements by government officials.

The launches of NROL-71 and NROL-91 into an orbit at a different inclination than the KH-11 satellites customarily fly caused some spy satellite observers to consider whether those missions carried a different kind of NRO payload, such as a radar imager. But without clear evidence otherwise, the consensus remains that NROL-71 and NROL-91 most likely delivered KH-11-type optical telescopes to orbit.

There are no other known hefty satellites in the NRO’s pipeline that require the Delta 4-Heavy’s lift capability. The Delta 4-Heavy can deliver payloads up to 62,540 pounds (28.3 metric tons) to low Earth orbit, making the Delta 4-Heavy the most powerful rocket in ULA’s fleet, and one of the most powerful operational launchers in the world.

Saturday’s launch was the the 43rd Delta 4 flight overall, and the 14th in the Delta 4-Heavy configuration, created by combining three expendable hydrogen-fueled booster cores together to haul the military and NRO’s most massive payloads into orbit.

“The National Reconnaissance Office puts absolutely exquisite capabilities on orbit,” Davis said. “And the natural extension to that is the Delta 4-Heavy is used for some of those even most exquisite and most sensitive kinds of capabilities that I wish we could share with the broader community. But that’s part of the idea, is that people don’t know that we can do the kinds of things that we can do in space, and that vehicle is just part and parcel to those absolutely exquisite capabilities on orbit delivering for this nation.”

After retirement of the Delta 4-Heavy, the NRO’s heaviest payloads will launch on SpaceX’s Falcon Heavy rocket or ULA’s Vulcan. Both are less expensive than the Delta 4-Heavy, which is priced at more than $400 million per mission.


Built among hills that hide the pad from public view, the SLC-6 launch site was originally constructed to support flights of military astronauts and NASA space shuttles.

The Air Force developed the SLC-6 launch site in the 1960s for the Manned Orbiting Laboratory program. Pentagon officials envisioned launching military astronauts on top of Titan rockets for orbital spy missions, but the program was canceled in 1969 before any launches from the SLC-6 pad.

NASA and the Air Force agreed in the 1970s to launch space shuttles from SLC-6, allowing astronauts to conduct military missions and deploy spy satellites in polar orbit. The 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.

The space shuttle Enterprise at Space Launch Complex 6 for fit checks in 1985. Credit: U.S. Air Force

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. SLC-6 hosted its first launch of the triple-core Delta 4-Heavy rocket configuration in January 2011.

Read our 2011 story detailing the history of the SLC-6 launch pad, written just before the first Delta 4-Heavy flight from Vandenberg.

SLC-6 is iconic here,” Titus said. “It’s legendary. It’s kind of what people think about when they think about Vandenberg. It’s had many lives. It started in the mid 1960s. They were going to support something called the Manned Orbiting Laboratory. In the ’70s and ’80s, it was built up for a shuttle program, and they ended up not going down that path. And then in the ’90s and 2000s, we started going down this path of launching Deltas from there, and we’ve had mission success after mission success out of that place for the last almost 20 years.

Northrop Grumman planned to use the SLC-6 launch pad for its now-canceled OmegA rocket, which the company proposed to the Pentagon in a competition for military launch contracts. But the Defense Department selected ULA’s Vulcan rocket 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.

And now ULA will leave SLC-6 after the final Delta 4-Heavy launch from the West Coast spaceport.

“SLC-6 has done us well,” Titus said. “I think that everyone here at Vandenberg has a warm spot in their heart for that place, and we’re going to make sure that it’s continued to be utilized. What we don’t know is exactly what that’s going to look like. But I can say that, first of all, we’re working closely with ULA to make sure that we have a smooth transition. And secondly, there are many other launch service providers out there that could find utility in that location. There’s a lot of infrastructure there. So I’m fairly confident that it will be utilized. We just don’t know exactly how yet.”

ULA’s other launch pad at Vandenberg, Space Launch Complex 3-East, will be converted to support flights of the new Vulcan Centaur rocket in the next few days. SLC-3E is currently used to launch Atlas 5 rockets, and the final Atlas 5 flight from Vandenberg is scheduled for Nov. 1.

Like the Delta rocket family, the Atlas 5 will be retired in the coming years for replacement by the Vulcan. The final Atlas 5 missions will be based out of Cape Canaveral.

Gary Wentz, ULA’s vice president of commercial and government programs, said the company chose the SLC-3E launch pad for the Vulcan rocket over SLC-6 for a few reasons. One was the similarity between the Atlas 5 rocket and the Vulcan, which both use Centaur upper stages, the same payload shroud design, and the same type of solid rocket boosters.

Another reason was that SLC-6 is more expensive to maintain, and ULA is seeking to reduce costs to compete with SpaceX for military launch contracts. SLC-6 has the largest footprint of any launch site at Vandenberg, and is on a similar scale to NASA’s two Apollo-era launch pads at Kennedy Space Center — pads 39A and 39B.

This wide view of SLC-6 shows the pad’s Mobile Service Tower at right, surrounding and protecting the Delta 4-Heavy rocket before launch on the NROL-91 mission. The Mobile Assembly Shelter, originally built for the space shuttle, is at center with the U.S. flag. Credit: Stephen Clark / Spaceflight Now

SLC-6 has a 325-foot-tall Mobile Service Tower used to help stack and protect rockets on the pad, a structure originally built for the Manned Orbiting Laboratory program. There’s also a 270-foot-tall Mobile Assembly Shelter that dates back to the 1980s for space shuttle missions. Both buildings move on rail tracks.

Wentz said ULA will “safe and secure” the launch pad after the final Delta 4-Heavy launch Saturday, ensuring that all hazardous systems to sit in an “idle condition” until officials decide what to do next with SLC-6.

There are no immediate plans to demolish any of the structures at SLC-6, and ULA will turn over responsibility for the pad back to the Space Force, which will try to find a new tenant for the pad. Until then, officials don’t expect any significant changes to the site.

ULA will not lay off any of its workforce at Vandenberg, Wentz said. Instead, the workers will transition to ready for the final Atlas 5 launch at the West Coast spaceport, then prep for Vulcan missions.

“From a workforce perspective, the team is upbeat,” Wentz said. “There’s no concern about their future. We’re not in a position where we’re stepping down and going to have to lay off folks. So there are folks that are sticking around to launch these vehicles.”

Startup launch companies like Firefly Aerospace and Relativity Space have signed agreements with the Space Force to fly their small satellite boosters from Vandenberg. SpaceX currently leases a launch pad at Vandenberg for Falcon 9 rocket missions, and ULA will continue using SLC-3 for the Atlas 5 and Vulcan rockets. Another small launch pad, SLC-8, has at least one more space launch on its schedule — a Northrop Grumman Minotaur 4 rocket set to take off in 2023.

“The way that I think we envision this working is that once we identify who’s interested in taking over SLC-6, then we’ll work with them to understand what they want to do in order to facilitate launching whatever type of booster they have,” Titus said.

“Once we identify who it’s going to be, we’ll probably look at the current state of the infrastructure, identify some things that I think would be value-added, maybe some things that don’t need to be there anymore, and then that company will probably go through the same process that ULA went through 15 or 20 years ago when they took a pad that was designed for the space shuttle, and converted it to a pad that supports a Delta launch vehicle,” Titus said.

“So it’s really going to be up to the commercial provider to decide what changes they want to make, but we’ll be there to support and enable that in any way possible.”

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