The Delta IV Heavy rocket, whose time has come and gone, will fly again

This is a rocket that sets itself on fire before heading into space. It is the world's largest rocket powered entirely by liquid hydrogen, a propellant that is difficult to handle but rewarding in its efficiency.

Delta 4 Heavy has been America's most powerful launch vehicle for nearly a decade, and has been a cornerstone of the US military space program for more than 20 years. It is also the most expensive commercially produced missile in the world, a fact that is due not only to its huge capacity, but also to its complexity.

Now, United Launch Alliance's last Delta IV Heavy rocket is scheduled to lift off Thursday from Cape Canaveral Space Force Station in Florida, carrying a secret payload for the National Reconnaissance Office, the U.S. government's space spy agency.

“It's an amazing piece of technology, 23 stories high, half a million gallons of propellant and a quarter of a million pounds of thrust, and it's the most metallic rocket ever, as it sets itself on fire before it goes into space,” he said. Tori Bruno, President and CEO of ULA. “His retirement is (the key to) the future, moving to Vulcan, which is a lower-cost, higher-performance rocket. But it's still sad.”

Delta 45 and the last IV

Weather permitting, Delta IV Heavy will light up its three RS-68A hydrogen-fueled engines at 1:40 PM EDT (17:40 UTC) on Thursday, opening a four-hour launch window. The three RS-68 rockets will be fired in a staggered sequence, an alternation designed to reduce the hydrogen fireball that ignites around the base of the rocket during engine startup.

Delta 4 Heavy is sure to have a legacy in 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.

But the fireball will leave an indelible mark in the memories of anyone who witnessed the launch of the Delta 4 Heavy. It all comes down to choosing ultra-cold liquid hydrogen as the fuel. The three RS-68 engines burn hydrogen with liquid oxygen as the oxidizer.

“We like those propellants because they are very high-performance,” Bruno said. “In order to prepare the RS-68 engines to have super-cooled propellant flow through them, before they are ignited, we start the flow of that propellant.

“Hydrogen is lighter than air, so after it flows through the engine into the flame trench, it rises. When the engines are finally full and ready to go and we start running the pumps, we actually drop the main load (of propellant), we ignite it, and that flame carries the hydrogen column Which clings to the side of the booster and rises up.

The Delta IV rocket cores are covered in orange foam insulation. One reason for this is to protect the rocket from the fireball, which results in a “very dramatic effect of a self-burning booster” that looks like a “toasted marshmallow” as it heads into space.

A few seconds after the engines start, 12 securing bolts will explode, freeing the three-core rocket from its restraints. More than 2 million pounds of thrust will power Delta IV Heavy from the launch pad eastward from Cape Canaveral. The RS-68 in the central core will be throttled to preserve liquid hydrogen and liquid hydrogen fuel, while the rocket's side boosters will burn in less than four minutes.

Once the Delta IV fires its side boosters and falls into the Atlantic Ocean, the central core suffocates and burns for another minute and a half. A few moments later, the first-stage booster is jettisoned, and the upper stage's RL10 engine ignites in the first of three burns necessary to propel the rocket's secret cargo into orbit thousands of miles above Earth.

There is only a 30 percent chance of suitable weather for takeoff on Thursday. High winds and cumulonimbus clouds are the main concerns. The weather forecast is improving for a backup launch opportunity Friday afternoon.