Enlarge / United Launch Alliance's final Delta IV Heavy rocket, seen here in December when ground crews rolled it to the launch pad at Cape Canaveral Space Force Station, Florida. United Launch Alliance reader comments 2 This is the rocket that literally lights itself on
fire before it heads to space. It's the world's largest rocket entirely fueled by liquid hydrogen, a propellant that is vexing to handle but rewarding in its efficiency. The Delta IV Heavy was America's most powerful launch vehicle for nearly a decade and has been a cornerstone for the US military's space program for more than 20 years. It is also the world's most expensive commercially produced rocket, a fact driven not just by its outsized capability but also its complexity. Now, United Launch Alliance's last Delta IV Heavy rocket is set to lift off Thursday from Cape Canaveral Space Force Station,
Florida, with a classified payload for the National Reconnaissance Office, the US government's spy satellite agency. "This is such an amazing piece of technology, 23 stories tall, a half-million gallons of propellant and a quarter-million pounds of thrust, and the most metal of all rockets, setting itself on fire before it goes to space," said Tory Bruno, ULA's president and CEO. "Retiring it is (key to) the future, moving to Vulcan, a less expensive higher-performance rocket. But it’s still sad.” 45th and final Delta IV Weather permitting, the Delta IV Heavy will light up its three hydrogen-fueled RS-68A engines at 1:40 pm EDT (17:40 UTC) Thursday, the opening of a four-hour launch window. The three RS-68s will fire up in a staggered sequence, a permutation designed to minimize the hydrogen fireball that ignites around the base of the rocket during engine startup. The Delta IV Heavy will certainly have a legacy of launching national security missions, along with NASA's Orion
spacecraft on an orbital test flight in 2014 and NASA's Parker Solar Probe in 2018 on a mission to fly through the Sun's outer atmosphere. Advertisement But the fireball will leave an indelible mark in the memories of anyone who saw a Delta IV Heavy launch. It all comes down to the choice of super-cold liquid hydrogen as the fuel. The three RS-68 engines burn hydrogen along with liquid oxygen as the oxidizer. "We like those propellants because they’re very, very high performance," Bruno said. "In order to prepare the RS-68 engines to get that very cold cryogenic propellant flowing through them, before they’re ignited, we start flowing that propellant. "Hydrogen is lighter than air, so after it flows through the engine and into the flame trench, it then rises. When the engines are finally full and ready to go and we start spinning up the pumps, then we actually drop the main load (of propellant), we ignite it, and that flame carries on up that ... plume of hydrogen, which is clinging to the side of the booster and rising up.” The Delta IV rocket cores are covered in orange foam insulation. One of the reasons for this is to protect the rocket from the fireball, leading to a "very dramatic effect of a self-immolating booster" that has the appearance of a "toasted marshmallow" as it heads to space. A few seconds after the engines start, 12 hold-down bolts will blow to release the triple-core rocket from its restraints. More than 2 million pounds of thrust will power the Delta IV Heavy off the launch pad toward the east from Cape Canaveral. The RS-68 on the center core will throttle down to conserve liquid hydrogen and liquid hydrogen propellant, while the rocket's two side boosters will burn through their propellants in less than four minutes. Once the Delta IV lets go of its side boosters and falls into the Atlantic Ocean, the center core throttles up and burns for another minute and a half. A few moments later, the first stage booster jettisons, and the upper stage's RL10 engine ignites for the first of three burns needed to propel the rocket's classified cargo into an orbit thousands of miles above Earth. There's just a 30 percent chance of favorable weather for liftoff Thursday. High winds and cumulus clouds are the primary concerns. The weather forecast improves for a backup launch opportunity Friday afternoon.