The latest evolutionary development of the
Delta rocket family, the Delta IV was introduced to meet the requirements of the USAF's EELV program, now known as
National Security Space Launch (NSSL) program. While the Delta IV retains the name of the Delta family of rockets, major changes were incorporated, the most significant being the switch from
kerosene to
liquid hydrogen fuel with new tankage and a new engine required. During the Delta IV's development, a small variant was considered. This would have featured the
Delta II second stage, an optional
Thiokol Star 48B third stage, and the Delta II payload fairing, all atop a single
Common Booster Core (CBC). The Small variant was dropped by 1999. In 2002, the Delta IV was first launched, with the
RS-68 becoming the first large
liquid-propellant rocket engine designed in the United States since the
Space Shuttle main engine (SSME) in the 1970s. The
L3 Technologies Redundant Inertial Flight Control Assembly (RIFCA) guidance system originally used on the Delta IV was common to that carried on the
Delta II, although the software was different because of the differences between the Delta II and Delta IV. The RIFCA featured six
ring laser gyroscopes and six
accelerometers, to provide a higher degree of reliability.
Boeing initially intended to market Delta IV commercial launch services. However, the Delta IV entered the space launch market when global capacity was already much higher than demand. Furthermore, as an unproven design it had difficulty finding a market in commercial launches, and Delta IV launch costs were higher than comparable vehicles of the same era. In 2003, Boeing pulled the Delta IV from the commercial market, citing low demand and high costs. In 2005, Boeing stated that it sought to return the Delta IV to commercial service. Ultimately, with the exception of the first launch, which carried the
Eutelsat W5 commercial communications satellite, all Delta IV launches were paid for by the US government. As of 2009, the USAF funded Delta IV EELV engineering, integration, and infrastructure work through contracts with Boeing Launch Services (BLS). On August 8, 2008, the USAF
Space and Missile Systems Center increased the "cost plus award fee" contract with BLS for US$1.656 billion to extend the period of performance through the September 30, 2008 (
FY09). In addition, a US$557.1 million option was added to cover FY10. In February 2010, naturalized citizen Dongfan Chung, an engineer working with Boeing, became the first person convicted under the
Economic Espionage Act of 1996. Chung passed on classified information on designs including the Delta IV rocket to China and was sentenced to 15 years.
RS-68A booster engine upgrade The possibility of a higher performance Delta IV was first proposed in a 2006
RAND Corporation study of national security launch requirements out to 2020. A single
National Reconnaissance Office (NRO) payload required an increase in the lift capability of the Delta IV Heavy. Lift capacity was increased by developing the higher-performance
RS-68A engine, which first flew on June 29, 2012. ULA phased out the baseline RS-68 engine with the launch of Delta flight 371 on March 25, 2015. All following launches used the RS-68A, and the engine's higher thrust allowed the use of a single standardized CBC design for all Delta IV Medium and M+ versions. This upgrade reduced cost and increased flexibility, since any standardized CBC could be configured for zero, two, or four
solid-propellant rocket boosters. However, the new CBC led to a slight performance loss for most medium configurations. The Delta IV Heavy required non-standard CBCs for the core and boosters. Masses include a Payload Attach Fitting (240 kg to 1,221 kg depending on payload). At one point, NASA planned to use Delta IV or Atlas V to launch the proposed
Orbital Space Plane, which eventually became the
Crew Exploration Vehicle and then the
Orion. Orion was intended to fly on the
Ares I launch vehicle, then the
Space Launch System after Ares I was cancelled. In 2009,
The Aerospace Corporation reported on
NASA results of a study to determine the feasibility of modifying Delta IV to be
crew-rated for use in NASA
human spaceflight missions. According to
Aviation Week & Space Technology the study, "found that a Delta IV heavy [...] could meet NASA's requirements for getting humans to low Earth orbit". A proposed upgrade to the Delta IV family was the addition of extra solid motors. The Medium+ (4,4) would have used existing mount points to pair the four GEM 60s of the M+ (5,4) with the upper stage and fairing of the (4,2). An M+ (4,4) would have had a GTO payload of , a LEO payload of , and could have been available within 36 months of the first order. It was also considered to add extra GEM 60s to the M+ (5,4), which would have required adding extra attachment points, structural changes to cope with the different flight loads, and launch pad and infrastructure changes. The Medium+ (5,6) and (5,8) would have flown with six and eight SRBs respectively, for a maximum of up to to GTO with the M+ (5,8). The Medium+ (5,6) and (5,8) could have been available within 48 months of the first order.
Retirement and replacement ULA was formed in December 2006 as a joint venture between Boeing and
Lockheed Martin Space, inheriting the
Atlas rocket family from Lockheed Martin and the Delta rocket family from Boeing. The
Atlas V offered better performance than the Delta IV Medium at a lower cost, and in March 2015 ULA announced plans to retire the Delta IV Medium by 2018. In 2014, ULA also began development of the
Vulcan Centaur to replace both the Atlas and Delta families. The Vulcan’s first stage shares design heritage with the Delta IV’s Common Booster Core and is manufactured in the same Decatur, Alabama, facility using much of the same equipment, but with a diameter about larger. It is powered by two
BE-4 methane-fueled engines developed by
Blue Origin. Compared with the liquid hydrogen used on the Delta IV, methane is denser and has a higher boiling point, allowing for smaller and lighter fuel tanks. surpassing the Atlas V's maximum of and approaching the Delta IV Heavy's capacity. The Vulcan Centaur was originally projected to enter service in 2023, but its first launch took place on January 8, 2024. The final Delta IV Medium launch (in a M+ 4,2 configuration) occurred on August 22, 2019, carrying a
GPS III-2 satellite,
USA-293, and the final Delta IV Heavy launch was on April 9, 2024, with the
NROL-70 mission. == Delta IV Medium ==