Delta IV

History

The Delta IV was the latest evolutionary development of the Delta rocket family, introduced to meet the requirements of the USAF's EELV program, later known as the National Security Space Launch (NSSL) program. Major changes from earlier Deltas included a switch from kerosene to liquid hydrogen fuel, requiring new tankage and a new engine. During development, a small variant was considered that would have used a Delta II second stage, an optional Thiokol Star 48B third stage, and the Delta II payload fairing atop a single Common Booster Core (CBC); this variant was dropped by 1999.

The first launch occurred in 2002, with the RS-68 becoming the first large liquid-propellant rocket engine designed in the United States since the Space Shuttle main engine in the 1970s. The guidance system, the L3 Technologies Redundant Inertial Flight Control Assembly (RIFCA), was common to that on the Delta II, though the software differed. Boeing initially intended to market Delta IV commercial launch services, but the vehicle entered a market with excess capacity and higher costs than comparable vehicles; in 2003 Boeing pulled it from the commercial market, citing low demand and high costs.

In 2005 Boeing stated it sought to return to commercial service, but ultimately all launches except the first (carrying the Eutelsat W5 commercial satellite) were paid for by the U.S. government. As of 2009, the USAF funded Delta IV EELV engineering, integration, and infrastructure work through contracts with Boeing Launch Services.

Delta IV Medium

The Delta IV Medium, also called single stick, was offered in four configurations: Medium, Medium+ (4,2), Medium+ (5,2), and Medium+ (5,4). The baseline Medium (Delta 9040) used a single Common Booster Core (CBC) and a 4 m Delta Cryogenic Second Stage (DCSS) derived from the Delta III but with enlarged tanks, and could place 4200 kg into geostationary transfer orbit (GTO).

The Medium+ (4,2) (Delta 9240) added two Orbital ATK-built GEM 60 solid rocket boosters (SRBs), increasing GTO capacity to 6150 kg. The Medium+ (5,2) (Delta 9250) used a larger 5 m DCSS and 5 m payload fairing, reducing GTO payload to 5072 kg due to added mass. The Medium+ (5,4) (Delta 9450) used four GEM 60 SRBs, achieving a GTO capacity of 6882 kg.

Vehicle description

Each Delta IV consisted of at least one Common Booster Core (CBC), each powered by one Aerojet Rocketdyne RS-68 engine burning liquid hydrogen and liquid oxygen. On Medium flights, the RS-68 ran at 102% rated thrust for the first few minutes, then throttled to 58% before cutoff. On the Heavy, the main CBC's engine throttled to 58% about 50 seconds after liftoff while the strap-on CBCs remained at 102%, conserving propellant; after booster separation, the main engine throttled back up to 102% then down to 58% before cutoff.

The RS-68 was mounted to the lower thrust structure by a four-legged thrust frame and enclosed in a composite conical thermal shield. Above the thrust structure were an aluminum isogrid liquid hydrogen tank, a composite centerbody, an aluminum isogrid liquid oxygen tank, and a forward skirt. A cable tunnel and a liquid oxygen feedline ran along the back of the CBC. The upper stage was the Delta Cryogenic Second Stage (DCSS), based on the Delta III upper stage but with increased propellant capacity.

Two versions existed: a 4 m diameter DCSS retired with the Delta IV Medium, and a 5 m diameter DCSS used on the Heavy. Each DCSS was powered by one RL10B-2 engine with an extendable carbon-carbon nozzle. A tapering composite interstage mated the 4 m DCSS to the CBC, while a cylindrical composite interstage mated the 5 m DCSS.

Launch history

The 45 Delta IV launches comprised 3 Delta IV Medium, 36 Medium+, and 16 Heavy. The first payload was the Eutelsat W5 communications satellite, launched on a Medium+ (4,2) from Cape Canaveral on November 20, 2002, into GTO. The first Delta IV Heavy launch, Heavy Demo, occurred in December 2004 after delays due to bad weather; due to cavitation in propellant lines, sensors on all three CBCs registered propellant depletion, causing premature shutdown of the strap-on and core engines, and the second stage burned until it ran out of propellant.

The payloads—DemoSat (a 6020 kg aluminum cylinder with brass rods) and NanoSat-2 (two small satellites, Sparky and Ralphie)—did not reach their planned orbits. NROL-22, the first Delta IV launched from SLC-6 at Vandenberg Space Force Base, flew on a Medium+ (4,2) in June 2006 carrying a classified NRO satellite. DSP-23, the first launch of a valuable payload on a Delta IV Heavy and the first Delta IV launch contracted by United Launch Alliance, lifted off from Cape Canaveral on November 10, 2007, carrying the 23rd and final Defense Support Program missile-warning satellite.

NROL-26, the first Delta IV Heavy EELV launch for the NRO, lifted off on January 18, 2009. NROL-32, a Delta IV Heavy launch carrying a satellite speculated to be the largest sent into space, lifted off on November 21, 2010, after a delay from October 19.

Did You Know?

• The Delta IV Medium was retired in 2019, with its final flight carrying a GPS III-2 satellite on August 22, 2019.
• The Delta IV Heavy's final flight, NROL-70, launched on April 9, 2024.