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65-487: NSSL may refer to: National Security Space Launch , US government procurement contract regime for orbital space launches National Severe Storms Laboratory , one of seven regional laboratories of the US National Oceanic and Atmospheric Administration Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with

130-471: A Delta Cryogenic Second Stage (DCSS)) and a 4-meter payload fairing. The Delta IV Medium was capable of launching 4,200 kg to geostationary transfer orbit (GTO). From Cape Canaveral, GTO is 1804 m/s away from GEO. The mass of fairing and payload attach fittings have been subtracted from the gross performance. The Delta IV Medium+ (4,2) (Delta 9240) had the same CBC and DCSS as the Medium, but with

195-672: A 4 m (13 ft) diameter DCSS that was retired with the Delta IV Medium and a 5 m (16 ft) diameter DCSS that remains in service with the Delta IV Heavy. The 4 m diameter version lengthened both Delta III propellant tanks, while the 5-meter version has an extended diameter liquid hydrogen tank and a further lengthened liquid oxygen tank. Regardless of the diameter, each DCSS is powered by one RL10B-2 engine, with an extendable carbon-carbon nozzle to improve specific impulse. Two different interstages are used to mate

260-806: A 5 m payload fairing. Delta IV CBCs and DCSSs were integrated horizontally before being transported to the launchpad. In 2020, ULA announced retirement of the Delta IV after 5 more launches. The final flight occurred in April 2024. Each Atlas V launch vehicle is based on a Common Core Booster powered by one NPO Energomash RD-180 engine with two combustion chambers and a Centaur upper stage powered by one or two Pratt & Whitney Rocketdyne RL10A-4-2 engines. Up to five Aerojet Rocketdyne Graphite-Epoxy Motor solid rocket boosters can be added to increase vehicle performance, and two diameters of payload fairing are available. A three-digit naming convention

325-472: A LEO payload of 14,800 kg (32,600 lb), 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

390-425: A Payload Attach Fitting (240 kg to 1,221 kg depending on payload). Possible future upgrades for the Delta IV included adding extra strap-on solid motors, higher-thrust main engines, lighter materials, higher-thrust second stages, more (up to eight) strap-on CBCs, and a cryogenic propellant cross feed from strap on boosters to the common core. At one point, NASA planned to use Delta IV or Atlas V to launch

455-465: A competitive contract award to launch national security spacecraft was conducted between United Launch Alliance (ULA), Northrop Grumman Innovation Systems (NGIS), Blue Origin , and SpaceX. Two providers were to be selected to launch spacecraft to a number of reference orbits. In October 2018, the USAF awarded development funding to ULA, NGIS, and Blue Origin to further the design of their rockets before

520-562: A contract under the EELV program to launch a GPS Block III satellite payload to MEO . The USAF began the process of competitively selecting the next generation NSSL vehicles in 2018. Announced performance requirements include: Category A payloads fit within a 4 m diameter payload envelope , category B payloads fit within a 5 m diameter payload envelope, and category C payloads require an extended 5 m diameter envelope. The USAF and United States Space Force (USSF) plan to use

585-413: A draft RFP for "Phase 3" of NSSL, which covers the five fiscal years 2025-2029. Unlike the earlier NSSL contracts, Phase 3 is divided into two "lanes". Lane 1 covers less demanding missions and is structured to encourage new launch providers. Lane 2 is more similar to Phase 2 and requires each provider to be able to handle all NSSL requirements. However, Lane 2 allows for up to three providers instead of just

650-647: A first launch of New Glenn in 2021. In the event, no phase 2 funding from the US government was forthcoming after August 2020 when ULA Vulcan and SpaceX were selected by the Air Force. Blue Origin is considered likely to continue building and testing New Glenn, in part since they were already privately funding development prior to the Air Force NSSL competition. Northrop Grumman was awarded US$ 792 million of phase 1 funding in 2018 for development of OmegA. OmegA

715-425: 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 29 June 2012. ULA phased out the baseline RS-68 engine with

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780-459: A market in commercial launches, and Delta IV launch costs are 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. As of 2009, the USAF funded Delta IV EELV engineering, integration, and infrastructure work through contracts with Boeing Launch Services (BLS). On 8 August 2008,

845-522: A maximum of up to 9,200 kg (20,300 lb) 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. The Vulcan Centaur is planned to replace the Atlas V and Delta IV rockets. Vulcan Centaur had been projected to enter service by 2023, using the BE-4 methane -fueled rocket engine, but the first Vulcan launched on 8 January 2024. The Atlas V

910-624: A similar facility for launches from SLC-6 pad at Vandenberg Space Force Base . The latest evolutionary development of the Delta rocket family , Delta IV was introduced to meet the requirements of the United States Air Force's (USAF) Evolved Expendable Launch Vehicle (EELV, now National Security Space Launch (NSSL)) program. While the Delta IV retains the name of the Delta family of rockets, major changes were incorporated. Perhaps

975-414: A slight performance loss for most medium configurations. The Delta IV Heavy required non-standard CBCs for the core and boosters. Payload capacities after RS-68A upgrade 407 km x 51.6° 1800 m/s residual Payload capacities with original RS-68 Version Fairing CBCs SRBs Payload to LEO 407 km x 51.6° Payload to GTO 1800 m/s residual Launches *Masses include

1040-453: A subsequent competition for award to build the launch vehicles. SpaceX received none of this funding to develop their designs, as both the Falcon 9 and Falcon Heavy had completed development and were already flying. Blue Origin was awarded $ 500 million of phase 1 funding for further development of New Glenn as a potential competitor in future contracts. As of 2019, Blue Origin expected

1105-595: Is a super heavy-lift rocket developed and produced by SpaceX. It has been certified for the NSSL program after the STP-2 launch completed on 25 June 2019, as confirmed by the commander of the Air Force Space and Missile Systems Center, Lt. Gen. Thompson. He clarified: "I certified them to compete last year" and "one of the requirements behind certification is to fly three missions." This requirement has been satisfied by

1170-483: Is expected to stay in service for a few years after Vulcan's inaugural launch, and the Delta IV Heavy was discontinued in April 2024. The Delta IV Medium (also referred to as 'single stick' ) was available in four configurations: Medium, Medium+ (4,2), Medium+ (5,2), and Medium+ (5,4). The Delta IV Medium (Delta 9040) was the most basic Delta IV. It featured a single CBC and a modified Delta III second stage, with 4-meter liquid hydrogen and liquid oxygen tanks (called

1235-750: Is used for the Atlas V configuration identification. The first digit represents the payload fairing diameter, either 4.2 meters (indicated by a 4) or 5.4 (indicated by a 5) meters. The second digit is for the number of solid rocket boosters (0 to 5) to be used, and the third digit the number of RL-10 engines on the Centaur upper stage (1 or 2). As an example, an Atlas V 551 has a 5.4-meter payload fairing, 5 SRBs, and 1 RL-10. In August 2021, ULA announced that Atlas V would be retired, and all 29 remaining launches had been sold. The last launch for NSSL happened on 30 July 2024. As of July 2024 , fifteen launches remain, all for non-DoD launches. The main features of

1300-632: The Delta IV based around Common Booster Cores and the Delta Cryogenic Second Stage , while Lockheed Martin developed the Atlas V based around Common Core Boosters and the Centaur upper stage. In 2003, Boeing was found to be in possession of proprietary documents from Lockheed Martin. The USAF moved 7 launches from Delta IV to Atlas V. To end litigation and competition for a limited market, both companies agreed to form

1365-568: The Evolved Expendable Launch Vehicle launch system program, the initial program goal was to make government space launches more affordable and reliable , leading to the development of the Boeing Delta IV and Lockheed Martin Atlas V EELV families. These remained the primary launch vehicles for U.S. military satellites , and were later joined by the Falcon 9 developed by SpaceX . On 1 March 2019,

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1430-399: The Evolved Expendable Launch Vehicle (EELV) program, the Delta IV became a United Launch Alliance (ULA) product in 2006. The Delta IV was primarily a launch vehicle for United States Air Force (USAF) military payloads, but was also used to launch a number of United States government non-military payloads and a single commercial satellite. The Delta IV had two main versions which allowed

1495-708: The Falcon Heavy test flight in February 2018, Arabsat-6A in April 2019, and the STP-2 launch in June 2019. Falcon Heavy has been certified for two Phase 1A reference orbits and as of 2019, "it's not certified for all of our most stressing national security space orbits," Thompson said. The USAF is working with SpaceX to mature their Falcon Heavy's design. As of December 2023, it has flown three classified national security flights: USSF-44, USSF-67, and USSF-52. In 2018,

1560-819: The United Launch Alliance (ULA) joint venture. Each company has a 50% stake in ULA. Prior to 2012, the USAF was advancing another program, the Reusable Booster System and other follow-on technologies, and up to early 2012 issued contract awards for various aspects of it, before the program was cancelled in October 2012. In December 2012, the DoD announced a re-opening of the EELV-class launch vehicle market to competition while authorizing

1625-474: The Delta IV rocket to China and was sentenced to 15 years. In March 2015, ULA announced plans to phase out the Delta IV Medium by 2018. With the exception of the first launch, which carried the Eutelsat W5 commercial communications satellite, all Delta IV launches have been paid for by the US government. In 2015, ULA stated that a Delta IV Heavy is sold for nearly US$ 400 million. The possibility of

1690-645: 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

1755-452: The EELV program in 1994, following many years of government-funded studies into improved systems and architecture. The intent was to replace legacy vehicles, including Delta II , Atlas II , and Titan IV . EELVs were to reduce costs by being based on standardized fairings, liquid core vehicles , upper stages, and solid rocket boosters . A Standard Payload Interface bus was also proposed as a way to save money and improve efficiency. Reducing

1820-627: The East coast of the United States used Space Launch Complex 37 (SLC-37) at the Cape Canaveral Air Force Station . On the West coast, polar-orbit and high-inclination launches used Vandenberg Space Force Base 's Space Launch Complex 6 (SLC-6). Launch facilities at both sites are similar. A Horizontal Integration Facility (HIF) is situated some distance from the pad. Delta IV CBCs and second stages to be mated and tested in

1885-472: The Falcon 9 in its Block 5 version include two stages , both powered by LOX and RP-1 , with nine Merlin 1D engines on the first stage and one Merlin 1D Vacuum engine on the second stage. This launcher features a reusable first stage and fairings, which lowers the cost per mission. GPS-IIIA USA-289 was the first NSSL-type B5 Falcon 9 launch. The launch occurred on December 23, 2018. The Falcon Heavy

1950-635: The HIF before they are moved to the pad. The partial horizontal rocket assembly of the Delta IV is somewhat similar to the Soyuz launch vehicle , which is completely assembled horizontally. The Space Shuttles , the past Saturn launch vehicles , and the Space Launch System are assembled and rolled out to the launch pad entirely vertically. Movement of the Delta IVs among the various facilities at

2015-506: The Heavy, the main CBC's engine throttles down to 58% rated thrust around 50 seconds after liftoff, while the strap-on CBCs remain at 102%. This conserves propellant and allows the main CBC to burn after booster separation. After the strap-on CBCs separate, the main CBC's engine again throttles up to 102% before throttling back down to 58% prior to main engine cutoff. The RS-68 engine is mounted to

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2080-515: The Medium+ (5,2) could launch 5,072 kg to GTO. The Delta IV Medium+ (5,4) (Delta 9450) was similar to the Medium+ (5,2), but used four GEM 60s instead of two, enabling it to lift 6,882 kg to GTO. To encapsulate the satellite payload, a variety of different payload fairings were available. A stretched Delta III 4-meter diameter composite payload fairing was used on 4-meter Medium versions, while an enlarged, 5-meter diameter composite fairing

2145-615: 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 30 September 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, was the first person convicted under the Economic Espionage Act of 1996 . Chung passed on classified information on designs including

2210-431: The USAF to proceed with a block buy of "up to" 36 boosters from ULA. At the same time, another 14 boosters were to be procured competitively beginning in 2015, with the initial launches to be performed in 2017. The USAF signed a contract at that time with SpaceX for two launches in 2014 and 2015 to serve as proving flights to support the certification process for the Falcon 9 v1.1 and Falcon Heavy . In April 2014, after

2275-595: The USSF selected SpaceX (F9 and FH) and ULA's yet-to-be-certified Vulcan Centaur to supply US military launch requirements in 2022–2027. New Glenn was considered for the first time in NSSL Phase 3 Lane 1 bidding. Delta IV Medium flew with two or four SRBs on a single Common Booster Core . The DCSS had 4 m diameter and 5 m diameter versions, with matching diameter payload fairings. Delta IV CBCs and DCSSs were integrated horizontally before being transported to

2340-415: The addition of two Orbital ATK -built 1.5-m (60-in) diameter solid rocket booster Graphite-Epoxy Motors (GEM 60s) strap-on boosters to increase payload capacity to 6,150 kg to GTO. The Delta IV Medium+ (5,2) (Delta 9250) was similar to the Medium+ (4,2), but had a 5-m–diameter DCSS and payload fairing for larger payloads. Because of the extra weight of the larger payload fairing and second stage,

2405-402: The award of two development agreements, the total amount was more than $ 3 billion. Boeing was awarded a contract for 19 out of the 28 launches; Lockheed Martin was awarded a contract for the other 9. Boeing received $ 1.38 billion, and Lockheed Martin received $ 650 million for the launches. Boeing and Lockheed Martin were each awarded US$ 500 million for the final phase of the bid. Boeing developed

2470-514: The back of the CBC is a cable tunnel to hold electrical and signal lines, and a feedline to carry the liquid oxygen to the RS-68 from the tank. The CBC is of a constant, 5 m (16 ft) diameter. The upper stage of the Delta IV was the Delta Cryogenic Second Stage (DCSS). The DCSS was based on the Delta III upper stage but has increased propellant capacity. Two versions have been produced:

2535-463: The cancellation of OmegA. ULA was awarded $ 967 million of phase 1 funding for further development of Vulcan Centaur as a potential competitor in future contracts. On 12 August 2019, ULA submitted Vulcan Centaur for phase 2 of the USAF's launch services competition. As of July 2024, Vulcan Centaur has completed one certification flight launch and another is left be fully certified for national security launches. On 12 August 2019, at least three of

2600-512: The case of Delta IV, the vehicle was completed on the launch pad inside a building. This Mobile Service Tower (MST) provides service access to the rocket and protection from the weather and is rolled away from the rocket on launch day. A crane at the top of the MST lifts the encapsulated payload to the vehicle and also attached the GEM 60 solid motors for Delta IV Medium launches. The MST is rolled away from

2665-402: The conclusion of the Delta rocket family. Each Delta IV consists of at least one Common Booster Core (CBC). Each CBC is powered by one Aerojet Rocketdyne RS-68 engine, which burns liquid hydrogen and liquid oxygen . On flights of the Medium, the RS-68 ran at 102% rated thrust for the first few minutes of flight, and then throttled down to 58% rated thrust before main engine cutoff. On

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2730-658: The cost of launches and ensuring national access to space were the two main goals of the USAF space launch/EELV program. Some of the reasons why assured access to space is a priority for the United States are stated in National Presidential Directive Number 40, which reads: Access to space through U.S. space transportation capabilities is essential to: The United States, therefore, must maintain robust, responsive, and resilient U.S. space transportation capabilities to assure access to space. Procurement of EELV boosters for military space launch

2795-474: The differences between the Delta II and Delta IV. The RIFCA featured six ring laser gyroscopes and accelerometers each, 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

2860-487: The family to cover a range of payload sizes and masses: the Medium (which had four configurations) and Heavy . The final flight of Medium occurred in 2019. The final flight of Heavy was in April 2024. Delta IV vehicles were built in the ULA facility in Decatur, Alabama . Final assembly was completed at the launch site by ULA: at the horizontal integration facility for launches from SLC-37B pad at Cape Canaveral and in

2925-441: The first stage and DCSS. A tapering interstage that narrowed down from 5 m to 4 m diameter was used to mate the 4 m DCSS to the CBC, while a cylindrical interstage is used to mate the 5 m DCSS. Both interstages were built from composites and enclosed the liquid oxygen tank, with the larger liquid hydrogen tank making up part of the vehicle's outer mold line. Delta IV launched from either of two rocket launch complexes . Launches on

2990-418: The four companies submitted their final bids for the launch services competition. SpaceX bid the existing Falcon 9 and Falcon Heavy , while Blue Origin was expected to bid New Glenn, ULA bid Vulcan Centaur, and NGIS's bid status was not reported. Blue Origin also filed a pre-award protest of the request for proposal arguing that the requirements were ambiguous. The US Department of the Air Force announced

3055-453: The launch of Delta flight 371 on 25 March 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

3120-465: The launches over the five-year period. The contract type for the phase 2 contracts is new for NSSL launches as it will be a "firm-fixed-price, indefinite-delivery" type of launch contract. The awards in August 2020 are a major part of "the transition of the national security launch program to take advantage of commercial innovation and private investments in launch vehicles." In 2023, USSF released

3185-729: The launches were contracted, SpaceX sued the United States Air Force, arguing that the RD-180 engines, produced in Russia by the government owned NPO Energomash and used by the Atlas V, violated sanctions against the Russian government. The USAF and SpaceX settled the lawsuit in Jan 2015 by opening up more launches to competitive bidding. The USAF certified the Falcon 9 in May 2015, and in 2016 SpaceX won

3250-451: The launchpad. The Delta IV Medium retired after the 22 August 2019 launch of a GPS-III satellite . The mission used a Delta IV M+(4,2) two SRBs, and a 4 m diameter DCSS and payload fairing, the final use of the 4 m fairing. The Delta IV Heavy launch vehicle used three Common Booster Cores (CBCs) each powered by a Pratt and Whitney Rocketdyne RS-68A engine, a Delta Cryogenic Second Stage 5 m (DCSS) powered by an RL10 , and

3315-418: The lower thrust structure of the CBC by a four-legged ( quadrapod ) thrust frame and enclosed in a protective composite conical thermal shield. Above the thrust structure is an aluminum isogrid (a grid pattern machined out of the inside of the tank to reduce weight) liquid hydrogen tank, followed by a composite cylinder called the centerbody, an aluminum isogrid liquid oxygen tank, and a forward skirt. Along

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3380-432: The most significant change was 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,

3445-521: The next generation NSSL launch vehicles until at least 2030. See EELV_Secondary_Payload_Adapter (ESPA rings with radial ports) Since 2019 there have been four vehicles certified by the Department of Defense to conduct NSSL launches: Atlas V, Delta IV Heavy, Falcon 9 and Falcon Heavy. Delta IV Medium was retired in August 2019 and Delta IV Heavy retired in April 2024. After a two-year solicitation and competition process in 2018–2020, in August 2020

3510-423: The pad was facilitated by rubber-tired Elevating Platform Transporters (EPTs) and various transport jigs. Diesel engine EPTs are used for moving the vehicles from the HIF to the pad, while electric EPTs are used in the HIF, where precision of movement is important. The basic launchpad structure includes a flame trench to direct the engine plume away from the rocket, lightning protection, and propellant storage. In

3575-525: The program name was changed from EELV to National Security Space Launch (NSSL) to better reflect the growing commercial launch market and the changing nature of launch contracts, including the potential for reusable launch vehicles. The NSSL program launches the nation's most valuable military satellites; contracts to launch lower value payloads, such as those of the Space Test Program , are awarded using different methodologies. The USAF began

3640-702: 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

3705-440: The results of the approximately US$ 3.5 billion National Security Space Launch Phase 2 Launch Service Procurement on 7 August 2020. SpaceX and ULA were the two vendors selected via the competition to supply launches to the US military in the 2022–2026 timeframe. The US Space Force (USSF) plans 30–34 launches during these five fiscal years. ULA is expected to handle 60 percent of the launches while SpaceX will handle 40 percent of

3770-449: The rocket several hours before launch. At Vandenberg, the launch pad also has a Mobile Assembly Shelter (MAS), which completely encloses the vehicle; at CCAFS, the vehicle is partly exposed near its bottom. Beside the vehicle is a Fixed Umbilical Tower (FUT), which has two (VSFB) or three (CCAFS) swing arms. These arms carry telemetry signals, electrical power, hydraulic fluid, environmental control air flow, and other support functions to

3835-421: 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 7,500 kg (16,500 lb),

3900-476: The title NSSL . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=NSSL&oldid=1023031360 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages National Security Space Launch Started in 1994 as

3965-484: The two providers of Phase 2. Proposals were due December 15, 2023 and awards are expected in mid to late 2024. Phase 3 Lane 1 awardees were SpaceX, Blue Origin and ULA. The vehicles being: Falcon 9 , New Glenn and Vulcan Centaur , respectively. Delta IV Delta IV was a group of five expendable launch systems in the Delta rocket family . It flew 45 missions from 2002 to 2024. Originally designed by Boeing 's Defense, Space and Security division for

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4030-402: Was a rocket design with two main solid stages, a cryogenic upper stage, and the possibility of additional solid-rocket strap-on boosters. As of 2019, the first flight has been stated by NGIS to be expected in 2021. In the event, no phase 2 funding from the US government was forthcoming after August 2020 when ULA Vulcan and SpaceX were selected by the Air Force. In 2020, Northrop Grumman announced

4095-521: Was standard on the Delta IV Heavy, with an aluminum isogrid fairing also available. The aluminum trisector (three-part) fairing was built by Boeing and derived from a Titan IV fairing. The trisector fairing was first used on the DSP-23 flight. The Delta IV with the extended fairing is over 62 m (203 ft) tall. The last launch took place on 9 April 2024, carrying the NROL-70 satellite, marking

4160-482: Was to evolve to more closely match commercial practice. The initial bids came from four major defense contractors: Lockheed Martin , Boeing , McDonnell Douglas , and Alliant Techsystems . Each of the bids included a variety of concepts. Boeing initially proposed using the RS-25 Space Shuttle main engine. In October 1998 two initial launch services contracts (known as Buy 1) were awarded. Along with

4225-428: Was used on 5-meter Medium versions. The Medium (4,2) version last flew on 22 August 2019, marking the retirement of the Delta IV Medium variants. The Delta IV Heavy (Delta 9250H) combines a 5 m (16 ft) diameter DCSS and payload fairing with two additional CBCs. These are strap-on boosters which are separated earlier in the flight than the center CBC. As of 2007, a longer 5 meter diameter composite fairing

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