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With each day, another milestone passes like clockwork for engineers and technicians preparing a United Launch Alliance Delta 2 rocket for liftoff next month from California’s Central Coast. With 154 missions in the books — that’s a lot for a space launcher — you might be right to say getting a rocket ready to fly is just another day at the office for the Delta 2 team.

But this summer’s launch campaign at Vandenberg Air Force Base, overlooking the Pacific Ocean between Los Angeles and San Francisco, is the last for the Delta 2 program after a run of launches spanning more than 29-and-a-half years.

The payload for the last Delta 2 flight is NASA’s ICESat 2 satellite, an orbiting platform that will use a laser to chart the melting of ice brought on by climate change.

“In a lot of ways, the Delta 2 launch of ICESat 2 is truly going to be the end of an era,” said Tim Dunn, NASA’s launch director for the ICESat 2 mission, in a recent interview. “It’s a little melancholy, but at the same time what a great history to celebrate.”

On Wednesday, controllers loaded super-cold liquid oxygen propellant into the Delta 2’s first stage on its launch pad at Vandenberg during a practice countdown — known as a wet dress rehearsal — that exercised ground teams and provided a checkout of the launch vehicle.

The successful wet dress rehearsal came three days after ULA and NASA relocated critical members of the Delta launch team from Cape Canaveral to Vandenberg after Sunday’s liftoff of a Delta 4-Heavy rocket with the Parker Solar Probe from Florida’s Space Coast.

With the practice countdown accomplished, crews at Space Launch Complex 2-West at Vandenberg will ready the Delta 2 rocket for attachment of the ICESat 2 spacecraft next week.

ICESat 2 — short for Ice, Cloud and land Elevation Satellite — arrived at the Vandenberg launch base in June from a Northrop Grumman Innovation Systems factory in Gilbert, Arizona. Since then, the satellite has been fueled with its own supply of maneuvering propellant, and technicians have verified ICESat 2 is ready for liftoff.

Liftoff is scheduled for a launch window opening at 5:46 a.m. PDT (8:46 a.m. EDT; 1246 GMT) on Sept. 15.

On its final flight, the Delta 2 will be propelled by a kerosene-burning first stage Aerojet Rocketdyne RS-27A main engine, four strap-on solid rocket boosters, and a restartable second stage AJ10-118K engine. The target for the Sept. 15 mission: A roughly circular polar orbit around 300 miles (500 kilometers) above Earth.

During its planned three-year mission, ICESat 2 will fire a downward-facing on-board laser with 10,000 pulses per second, precisely measuring the time it takes for the laser signals to bounce off Earth’s surface and back to the satellite. The measurements will yield information about the height of ice caps, vegetation, land and water surfaces, and clouds.

Scientists will use data from ICESat 2 — a replacement for NASA’s ICESat satellite which operated from 2003 until 2009 — to see where ice is melting and flowing, helping quantify and identify the impacts of warming global temperatures.

Once bolted to the adapter cone that will connect it to the Delta 2 second stage, ICESat 2 will be placed in a transport container for the short road trip from the Astrotech payload processing facility to the SLC-2W launch pad. Cranes will hoist the spacecraft — weighing around one-and-a-half tons — into the launch pad’s mobile gantry.

The moving service structure will roll back around the Delta 2, allowing workers to install the ICESat 2 spacecraft on the rocket and encapsulate it inside the launcher’s composite nose shroud, capping assembly of the 128-foot-tall (39-meter) vehicle. Then comes a sequence of testing to ensure the satellite is properly connected to the Delta 2, fueling of the rocket’s second stage with storable Aerozine 50 and nitrogen tetroxide propellants, and multiple readiness reviews to clear the mission for takeoff.

Workers at Vandenberg lifted the Delta 2’s first stage on its launch mount June 8, then added an interstage connector June 12. Ground crews mated four solid-fueled strap-on boosters in mid-June, and hoisted the Delta 2’s second stage into position June 21.

ULA’s Delta team powered up the Delta 2 in late June for several weeks of avionics and propulsion checkouts, then suspended the launch campaign to focus on the Delta 4-Heavy launch from Florida with NASA’s Parker Solar Probe.

The Parker Solar Probe launch was delayed several times before it lifted off early Sunday, prompting NASA and ULA to push back ICESat 2’s launch by three days, giving teams time to transition from the East Coast to the West Coast.

The upcoming mission will be the 155th flight of a Delta 2 rocket since the launcher family debuted with the deployment of a GPS navigation satellite on Valentine’s Day 1989.

Delta 2s, which sometimes lifted off two or three times per month in the 1990s and 2000s, have been replaced by a fleet of bigger launchers. ULA moved on to focus on the Atlas 5 and Delta 4 boosters, and SpaceX’s Falcon 9 rocket now carries up many of the same types of payloads that were once tailored for the Delta 2.

Many commercial and military missions once served by satellites sized to fly on the medium-lift Delta 2 launcher are now accomplished by smaller spacecraft, taking advantage of the miniaturization trend in technology. At the opposite end of the mass spectrum, many commercial communications satellites outgrew the Delta 2’s capabilities in the 1990s and 2000s.

Delta 2 rockets successfully launched 48 satellites for the U.S. Air Force’s Global Positioning System from 1989 through 2009, a two-decade span during which the satellite network became a part of the everyday lives of billions of people worldwide.

Other Delta 2s dispatched NASA’s first three Mars rovers — Sojourner, Spirit and Opportunity — toward the red planet, along with the MESSENGER mission to orbit Mercury, the Dawn mission to the asteroid belt, the Spitzer Space Telescope, the planet-hunting Kepler observatory, weather satellites, and dozens of commercial and military communications spacecraft.

From Vandenberg, Delta 2 rockets hauled the bulk of Iridium’s first-generation fleet of voice and data relay satellites into low Earth orbit on 12 launches from 1997 through 2002. Those satellites are now being replaced by an upgraded Iridium fleet launching on SpaceX Falcon 9 rockets.

The Globalstar satellite network, also designed for mobile communications, was deployed by a series of Delta 2 rockets launched from Cape Canaveral.

Ensuring a graceful end to the Delta 2 program

The last Delta 2 launch with an Air Force GPS satellite occurred in 2009, ending the military’s support of the program as the service contracted further launches with Atlas 5s and Delta 4s. The Delta 2’s run of launches at Cape Canaveral concluded in September 2011 with the liftoff of a pair of NASA science probes to study the moon’s gravity field.

Much of the Delta 2’s design dates to the dawn of the Space Age. The Delta rocket family was derived from the 1950s-era Thor intermediate range ballistic missile, and engineers have lengthened the Thor’s original 8-foot-diameter (2.4-meter) first stage several times, expanding the Delta’s propellant capacity.

The Delta rocket line has been on the brink of retirement several times, perhaps most notably in the 1980s, when the U.S. government sought to transition all of its satellite launches to the space shuttle. That policy changed in the aftermath of the Challenger accident in 1986, resulting in the creation of the Delta 2 and the restart of the Delta production line.

The Delta 2’s RS-27A main engine has design roots in engines developed for the Saturn 1 and 1B rockets of the 1960s.

No other Delta 2 rockets were under contract when NOAA’s Suomi NPP weather satellite launched on a Delta 2 flight in October 2011, leaving the program with an uncertain future.

But NASA gave the Delta 2 rocket another lease on life in 2012, with the purchase of three more launches from Vandenberg carrying two NASA Earth science satellites and NOAA’s JPSS 1 polar-orbiting weather observatory, which lifted off on on the most recent Delta 2 mission last November.

The space agency booked the ICESat 2 launch on a Delta 2 rocket in 2013, adding a fourth flight to the Delta 2 program’s final chapter. ULA has components for one additional Delta 2 launcher, but that rocket will not fly, and is likely bound for a museum display.

As with any long-lived flight program nearing retirement, NASA and ULA had to meticulously ensure they had enough spare parts to replace components that might be damaged during the final Delta 2 launch campaigns, according to Dunn.

In the final years of the program, officials kept an inventory spare parts for the launch vehicles and ground support equipment, and they faced concerns that Delta 2 personnel might want to leave the program because of its impending retirement, Dunn said.

“It’s been incredibly successful for us in managing spare parts,” Dunn said. “There were certain parts where the vendor went out of business, or quit making the parts many years ago. So while the vendor was still in business, ULA ensured that at least one spare was made for every critical component on the rocket, and then an evaluation of the entire rocket was made.

“To pick a term out of fairy tails, the ‘golden egg’ process was put in place,” Dunn said. “Certain components on the rocket, or its one-of-a-kind spare, were deemed ‘golden eggs’ and handled as such since the 2011 or 2012 timeframe. You would walk through the factory … and you would see certain items with a big ‘golden egg’ placard on them.”

“Here we are at the end of that process, we are still in very good shape in having at least one spare for all critical components,” Dunn said.

Over the last six decades, the builders of the Delta rocket family changed names through multiple corporate mergers and acquisitions, beginning with the Douglas Aircraft Company, then succeeded by McDonnell Douglas, Boeing and United Launch Alliance, formed in 2006 by the marriage of Boeing and Lockheed Martin rocket programs.

The more powerful Delta 4 rocket built by ULA is a new design, incorporating different hydrogen-fueled engines, and wider propellant tanks than the Delta 2. Counting the Delta 4 family, the launch of ICESat 2 next month will be the 381st flight of a rocket carrying the Delta name since 1960.

NASA launch director shares his Delta 2 memories

“Delta has such a rich history, starting back in 1960,” Dunn told Spaceflight Now. “If you talk to real gray-beards on the program, it was on death’s door step probably at least three, maybe four times, when everyone thought for sure that was the last launch ever, and they were all out of jobs, and somehow it kept getting new life.

“For this version of Delta, we are certain that this one will not fly after ICESat 2,” he said.

After stints as an Air Force as a GPS satellite analysis officer and a Titan 4 launch controller, Dunn joined the Delta 2 program at Boeing in 1996, supporting dozens of missions as a launch-day guidance engineer. One of his first missions on the Delta 2 rocket was the launch of Mars Pathfinder in December 1996, which delivered NASA’s first rover to the red planet.

“One our first missions was the Mars Pathfinder mission,” Dunn said. “That was the very small rover that we sent to Mars back in the fall of ’96. That one was really satisfying to me because it was super-cool deep space mission with NASA, our customer at the time … We were sending something to crawl around on the surface of Mars.

Hot Wheels produced a toy version of Mars Pathfinder’s Sojourner rover before the mission’s launch.

“My son was just a couple of years old, so it something I could relate to him with toys,” Dunn said. “So Pathfinder sticks out.”

Dunn joined NASA’s Launch Services Program in 2000 as a Delta 2 avionics and flight controls engineer, then became NASA’s Delta 2 vehicle systems engineer in 2003, leading the agency’s technical team ensuring Delta 2 rockets were ready to loft science probes into the solar system.

“From a constellation build-up point of view, the Globalstar sequence of launches that we did back in the summer of ’99 timeframe,” Dunn said. “Those were really fun because of the repetition. I want to say we were launching every couple of weeks there, at least for a few of them, fire to fire.

“Supporting the Air Force through all those years, that’s a special memory with GPS,” Dunn said. “From 1989 when GPS 2-1 first launched, as you progressed through the ’90s, that was the time when GPS went from this military system that only the military used, to essentially a common everyday person’s utility. That was really fun to be supporting and sustaining the constellation that everyone was beginning to use. Fast forward to 2018, and (when) you talk to younger kids today about GPS, they can’t imagine life without GPS, on your phone, on your watch, in your car.”

Other Delta 2 launches high on Dunn’s favorites list include the back-to-back liftoffs in June and July 2003 with NASA’s Spirit and Opportunity Mars rovers. Opportunity, which took off second in 2003’s interplanetary launch window, had to use a version of the rocket known as the Delta 2-Heavy, the first Delta 2 to fly with uprated, more powerful solid rocket boosters.

“We launched them both in the summer, so we were able to hit the Mars window with two different launches from Complex 17 (at Cape Canaveral),” Dunn said. “One was a standard Delta 2-7900 series, and the other with the Heavy. That was a huge development effort, bringing the Heavy online, a real challenge because of the planetary window we had to hit.”

Dunn’s first mission as launch director was the last Delta 2 flight from Cape Canaveral in 2011.

“That one was very special,” he told Spaceflight Now in an interview.

Dunn said his move from the Air Force’s Titan 4 program, which primarily launched one-of-a-kind, costly U.S. military satellites, to the workhorse Delta 2 was a major change. The Titan 4 team vastly outnumbered the Delta 2 workforce, despite the Delta 2’s higher launch rate.

“I really enjoyed those Titan 4 launches, but it was hard to identify with the whole team, and the rocket was kind of a fairly complex rocket, especially with the Centaur second stage, just big and fun,” Dunn said. “Then I went to the Delta 2, and it was a little smaller. You could get your hands around the systems, and you could walk the tower a lot more easily, and the team that you were working with was kind of an order of magnitude smaller, so that made it feel more like family.

“That’s one of the things that I’ll miss,” Dunn said. “That may be replicated on some of the newer providers, especially the smaller providers that have smaller teams … Just my knowledge of the hardware systems on Delta 2, when you grow up and you invest so many years in a particular rocket, you really feel like you know it inside and out.”

The Delta 2’s iconic color will also become a part of space history with the rocket’s retirement.

“And of course that baby blue paint scheme,” Dunn said. “I’m going to miss that because I don’t see another rocket out there painted in baby blue just yet, but maybe there will be one coming.”

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