Delta IV Heavy. The Delta IV Heavy (Delta 9250H) was an expendable heavy-lift launch vehicle, the largest type of the Delta IV family. It had the highest capacity of any operational launch vehicle in the world after the retirement of the Space Shuttle in 2011 until the Falcon Heavy debuted in 2018, and it was the world's third highest-capacity launch vehicle in operation at the time of its retirement in 2024. It was manufactured by United Launch Alliance (ULA) and was first launched in 2004. Delta IV Heavy was the last operating member of the Delta IV family, and its final flight was on 9 April 2024. It is succeeded by the Vulcan Centaur rocket.

The Delta IV Heavy first stage consisted of a central Common Booster Core (CBC), with two additional CBCs as liquid rocket boosters instead of the GEM-60 solid rocket motors used by the Delta IV Medium+ versions. At lift-off, all three rocket engines would operate at full thrust, and 44 seconds later the central engine would throttle down to 55% to conserve fuel until the other two engines separate. The latter engines burn out at 242 seconds after launch and are separated as the central engine throttles back up to full thrust. The central engine burns out 86 seconds later, and the second stage completed the ascent to orbit.

The launch vehicle used three RS-68 engines, one in the central core and one in each booster. On the last seconds of countdown, the liquid hydrogen fuel would flow through the engines and upwards along the booster body, and after the ignition that hydrogen inflamed, creating the characteristic fireball and charred look of the booster.

History 

The Delta IV line of rockets were developed by Boeing. The program was later transferred to United Launch Alliance. The Delta IV Heavy was the most powerful member of the line, which also includes the smaller Delta IV Medium. The Delta IV Heavy can lift  to low Earth orbit and  to geostationary transfer orbit (GTO). It was an all liquid-fueled launch vehicle, consisting of an upper stage, one main booster and two strap-on boosters.

The first launch of the Delta IV Heavy on 21 December 2004 carried a boilerplate payload and was a partial failure. Cavitation in the liquid-oxygen propellant lines caused shutdown of both boosters eight seconds early, and the core engine nine seconds early; this resulted in a lower staging velocity for which the second stage was unable to compensate. The payload was left in a lower than intended orbit.

Its first operational payload was the DSP-23 satellite, successfully launched in 2007; it was then used to launch a further five visual and electronic reconnaissance satellites for the National Reconnaissance Office (NRO) through 2013. In December 2014, the Delta IV Heavy was used to launch an uncrewed test flight of the Orion spacecraft, designated Exploration Flight Test-1 (EFT-1). After several delays, the mission was successfully launched at 12:05 UTC on 5 December 2014.

On 12 August 2018, the Delta IV Heavy with an additional Star 48BV third stage was used to launch the Parker Solar Probe into an elliptical heliocentric orbit.
In May 2023, the final Delta IV Heavy core and boosters finished construction, officially ending Delta IV production and making way for the Vulcan launch vehicle. The final Delta IV heavy would launch on April 9, 2024.

Delta IV's final flight was initially scheduled for March 29, 2024 for the National Reconnaissance Office; its actual launch date was April 9, 2024 after a postponement.

Capabilities 
Capacity of the Delta IV Heavy:
 Low Earth orbit (LEO), 200 km × 28.7°: 
 Low Earth orbit (ISS), 407 km × 51.6°: 
 Geosynchronous transfer orbit (GTO): 
 Geosynchronous orbit (GEO): 
 Lunar transfer orbit (LTO): 
 Mars transfer orbit: 

The Delta IV Heavy's total mass at launch was approximately  and produce around  of thrust to power the rocket skyward at liftoff.

Launch history

Comparable vehicles 

Current:
 Long March 5 (geostationary transfer orbit)
 Long March 5B (low Earth orbit)
 Long March 7A (geostationary transfer orbit)
 Falcon Heavy
 Proton-M
 Vulcan Centaur
 Angara A5
 Ariane 6
 New Glenn

Retired or cancelled:
 Ariane 5 (retired)
 Atlas V Heavy (proposed, never developed)
 Saturn IB (retired)
 Titan III (retired)
 Titan IV (retired)

See also 

 Comparison of orbital launch systems
 Comparison of orbital rocket engines
 National Launch System, (1991–1993) study

References

External links 

Delta (rocket family)
Vehicles introduced in 2004