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

Zoom in / United Launch Alliance's final Delta IV Heavy rocket is seen here in December as ground crews roll it to the launch pad at the Cape Canaveral Space Force Station in Florida.

This rocket ignites itself before going into space. It is the world's largest rocket fueled entirely by liquid hydrogen, a propellant that is challenging to handle but rewarding in its performance.

The Delta IV Heavy has been America's most powerful launch vehicle for nearly a decade and has been a cornerstone of the U.S. military's space program for more than 20 years. It is the most expensive commercially produced rocket in the world, driven not only by its external capability, but also by its complexity.

Now, United Launch Alliance's last Delta IV Heavy rocket is set to lift off from the Cape Canaveral Space Force Station in Florida on Thursday with a classified payload for the US government's spy satellite agency, the National Reconnaissance Office.

“It's an amazing piece of technology, 23 stories tall, a half million gallons of propellant and a quarter of a million pounds of thrust, and the most metal of all rockets, it will ignite itself before going into space,” he said. Tory Bruno, President and CEO of ULA. “Retirement (is important) of the future, moving to Vulcan, a low-cost high-performance rocket. But it's still sad.”

45th and final Delta IV

Weather permitting, the Delta IV Heavy will ignite its three hydrogen-fueled RS-68A engines at 1:40 pm EDT (17:40 UTC) on Thursday, opening a four-hour launch window. The three RS-68s fire in a staggered sequence, a permutation designed to minimize the burning hydrogen fireball around the base of the rocket when the engine starts.

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The Delta IV Heavy will certainly have a legacy of launching national security missions, flying through the Sun's outer atmosphere with NASA's Orion spacecraft on an orbital test flight in 2014 and NASA's Parker Solar Probe in 2018.

But the fireball will leave an indelible mark on the memories of anyone who saw a Delta IV Heavy missile. It all comes down to choosing super-cold liquid hydrogen as the fuel. Three RS-68 engines burn hydrogen with liquid oxygen as an oxidizer.

“We like those propellants because they have very, very high efficiency,” Bruno said. “In order to prepare the RS-68 engines, we start flowing very cold cryogenic propellant through them before they are ignited.

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

Delta IV rocket cores are wrapped in orange foam insulation. One of the reasons for this is to protect the rocket from the fireball, which would lead to the “very dramatic effect of a self-immolating booster” that looks like a “roasted marshmallow” on its way to space.

Seconds after the engines start, 12 hold-down bolts release the triple-core rocket from its restraints. More than 2 million pounds of thrust will propel the Delta IV Heavy from the launch pad east of Cape Canaveral. The RS-68 in the center core will be pushed down to protect the liquid hydrogen and liquid hydrogen propellant, while the rocket's two side boosters will burn through their propellants within four minutes.

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As Delta IV sheds its side boosters and falls into the Atlantic Ocean, the center core burns for another minute and a half. Minutes later, the first stage booster jettisons and the upper stage's RL10 engine ignites in the first of three burns required to propel the rocket's assorted cargo into orbit thousands of miles above Earth.

There is only a 30 percent chance of fair weather on Thursday. High winds and cumulus clouds are the primary concerns. The weather forecast improves for the chance of a backup release Friday afternoon.

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