Delta IV Heavy

For most of the 21st century’s first two decades, when the United States needed to launch its heaviest, most valuable satellites, one rocket did the job: the Delta IV Heavy. It is also famous for setting itself briefly on fire at every liftoff.

Quick facts

• Type: Heavy-lift expendable launch vehicle (a rocket used once, not reused).
• Built and flown by: Boeing, then United Launch Alliance (ULA).
• First flight: December 21, 2004. Final flight: April 9, 2024 (a classified mission called NROL-70).
• Flights: 16 total, with 15 successes and one partial failure (the 2004 maiden flight).
• Height: about 72 m (235 ft), varying slightly with the size of the nose cone.
• Liftoff mass: about 733,000 kg (1,616,000 lb).
• Liftoff thrust: about 9,420 kN (2,120,000 lbf).
• Payload: about 28,370 kg (62,550 lb) to low Earth orbit; about 14,210 kg (31,330 lb) to a higher transfer orbit.
• Launch sites: SLC-37B in Florida and SLC-6 in California.
• Successor: the Vulcan Centaur.

What it is and how it works

The Delta IV Heavy was the biggest member of the Delta IV rocket family. Its trick was simple but powerful: take three identical rocket bodies, called Common Booster Cores (the standard fuel-and-engine modules that formed the base of every Delta IV), and bolt them together side by side. The result was a center core with two strap-on boosters, each powered by one Aerojet Rocketdyne RS-68A engine.

All three engines lit on the ground. The two outer boosters ran at full power, while the center core throttled down (eased off the throttle) to save fuel. After about four minutes, the two outer boosters ran dry and were jettisoned, and the center core throttled back up to keep climbing alone. Once it finished, the center core fell away and a second stage took over. That upper stage used a single RL10 engine, which could shut down and restart several times to place a payload precisely into a high or unusual orbit.

Every stage burned the same propellant: liquid hydrogen and liquid oxygen, two gases chilled until they turn to liquid. This pairing is very efficient, but hydrogen is also leaky and likes to pool. At ignition, leftover gas around the base lit up and produced the rocket’s signature fireball, briefly charring its orange foam insulation. This was completely normal and expected, like the puff of flame when a gas stove first clicks on, only enormous.

Why it matters

For roughly twenty years, the Delta IV Heavy was the trusted workhorse for America’s most demanding payloads. It carried the nation’s largest classified reconnaissance satellites for the National Reconnaissance Office (the U.S. agency that runs spy satellites), and at several points in its career it was the most powerful operational rocket in the U.S. fleet. It also launched landmark science and exploration missions.

Its all-hydrogen design was efficient but expensive, because hydrogen is hard to store and handle. That high cost is the main reason ULA retired it in favor of the cheaper, single-core Vulcan Centaur, which delivers comparable or greater heavy-lift performance. Even at retirement, the Delta IV Heavy ranked among the highest-capacity rockets still flying.

Notable examples

• RS-68A engine: the most powerful hydrogen-fueled rocket engine ever flown. Three of them powered every Delta IV Heavy.
• Orion EFT-1 (December 5, 2014): the first uncrewed orbital test of NASA’s Orion crew capsule, sent to a high altitude so it could reenter at about 20,000 mph to test its heat shield.
• Parker Solar Probe (August 12, 2018): a mission toward the Sun that needed so much energy it required a rare third stage added to the Heavy.
• NROL-70 (April 9, 2024): the final flight, a classified payload launched from Florida that closed out the rocket’s era.