Space Launch System

Core stage

The SLS core stage is built by Boeing at NASA's Michoud Assembly Facility in New Orleans. It measures 65 meters (213 ft) in length and 8.4 meters (28 ft) in diameter, matching the diameter of the Space Shuttle external tank to allow NASA to leverage Shuttle-era experience. The stage is visually similar to the Shuttle tank due to its rust-colored spray-on insulation.^[21][22]

The stage contains liquid hydrogen and liquid oxygen propellant tanks, the attach points for the solid rocket boosters, avionics, equipment for autogenous pressurization of the tanks, and the Main Propulsion System (MPS), an assembly of four RS-25 engines with associated plumbing and hydraulic gimbal actuators. The first core stages reused MPS plumbing removed from the three remaining Space Shuttle orbiters following their decommissioning. The core stage, when combined with the solid rocket boosters, is capable of propelling the Orion spacecraft into a highly elliptical orbit without the upper stage firing, though the upper stage is required for trans-lunar injection.^[14][23][24]

The core stage structure is primarily made of 2219 aluminum alloy, and compared to the Space Shuttle external tank, several manufacturing improvements were incorporated.^[25][26][27] Production began in 2014, but delays in manufacturing, testing, and integration postponed the readiness of the first flight article by several years.

The first four SLS missions will each use four of the remaining fourteen RS-25D engines left over from the Space Shuttle program. The final two engines needed were assembled from existing RS-25D spare parts.^[28]Aerojet Rocketdyne refurbished these engines with modernized controllers, expanded throttle capability, and additional insulation to handle the higher thermal environment caused by proximity to the solid rocket boosters.^[29] Later flights will transition to the RS-25E, optimized for expendable use, being faster to build and costing 30% less.^[30][31] Thrust for the refurbished RS-25D engines has been increased from 492,000 to 513,000 lb_f (2,188 to 2,281 kN), while the RS-25E is rated at 522,000 lb_f (2,321 kN) per engine.^[32][33] The first test firing of an RS-25E occurred in June 2025 and was declared successful.^[34]

Solid rocket boosters

Shuttle-derived

The first eight SLS flights are planned to use two five-segment solid rocket boosters derived from the four-segment Space Shuttle Solid Rocket Booster manufactured by Northrop Grumman. The booster for the SLS incorporates casing segments previously flown as part of the Shuttle program, with the addition of a center segment to increase performance. The boosters feature updated avionics and lighter insulation but do not include a parachute recovery system, as they are expended after launch.^[35]

The boosters are filled with an ammonium perchlorate composite propellant, consisting of aluminum powder (fuel) and ammonium perchlorate (oxidizer), bound together with polybutadiene acrylonitrile. The mixture has a rubber eraser-like consistency and is cast into each segment.^[36]

At liftoff, the two boosters together produce more than 75 percent of the total thrust required to propel SLS. The five-segment configuration provides approximately 25 percent greater total impulse than the Shuttle-era boosters.^[37][38]

The available inventory of Shuttle-era booster casing segments limits use of this configuration to eight SLS flights.^[39]

BOLE

The Booster Obsolescence and Life Extension (BOLE) program was announced in 2019 to develop replacement solid rocket boosters for SLS once the supply of Shuttle-derived hardware is exhausted. In 2021, NASA awarded Northrop Grumman a $3.2 billion contract to produce Shuttle-derived boosters for five missions (Artemis IV–VIII) and to design, develop, and test the new BOLE booster.^[40]

The BOLE design replaces Shuttle-era steel motor cases with carbon-fiber composite cases, which are lighter and stronger. It also substitutes the hydraulic thrust vector control system with an electronic system and uses a different propellant formulation derived from Northrop Grumman's commercial solid rocket motors.^[40]

Each BOLE booster is designed to produce approximately 3.9 million lb_f (17 MN) of thrust,^[40] about 19 percent greater than the five-segment Shuttle-derived boosters, thereby increasing overall payload capability.^[41][42][43]

As of 2025,^[update] BOLE remains in development. A prototype was test-fired in June 2025 but experienced a nozzle failure approximately 15 seconds before the end of the motor firing. However, the long-term future of the program is uncertain: SLS was authorized for only five flights at that time, and no funding has been provided for production of operational BOLE boosters.^[40]

Upper stage

Interim Cryogenic Propulsion Stage

The Interim Cryogenic Propulsion Stage (ICPS) is the upper stage used on the initial Block 1 configuration of the SLS. Three ICPS units were built by United Launch Alliance (ULA), a joint venture of Boeing and Lockheed Martin. The ICPS is derived from the Delta Cryogenic Second Stage (DCSS), incorporating minimal modifications for integration with SLS. Following the discontinuation of the DCSS, further ICPS production is no longer available as an option.

The ICPS flown on Artemis I was powered by a single RL10B-2 engine. The two ICPS units built for Artemis II and Artemis III use a single RL10C-2.^[44][45][46]

The SLS uses a conical frustum-shaped interstage known as the Launch Vehicle Stage Adapter (LVSA) between the core stage and the narrower diameter ICPS. The LVSA consists of sixteen aluminum-lithium panels made of 2195 aluminum alloy and is built by Teledyne Brown Engineering.^[47] The first unit cost approximately $60 million, with the next two costing $85 million combined.^[48]

Block variants

Funding

During the joint Senate-NASA presentation in September 2011, it was stated that the SLS program had a projected development cost of US$18 billion through 2017, with $10 billion for the SLS rocket, $6 billion for the Orion spacecraft, and $2 billion for upgrades to the launch pad and other facilities at Kennedy Space Center.^[58][59] These costs and schedules were considered optimistic in an independent 2011 cost assessment report by Booz Allen Hamilton for NASA.^[60] An internal 2011 NASA document estimated the cost of the program through 2025 to total at least $41 billion for four 209,000 lb (95 t) launches (1 uncrewed, 3 crewed),^[61][62] with the 290,000 lb (130 t) version ready no earlier than 2030.^[63] The Human Exploration Framework Team estimated unit costs for 'Block 0' at $1.6 billion and Block 1 at $1.86 billion in 2010.^[64] However, since these estimates were made, the Block 0 SLS vehicle was dropped in late 2011, and the design was not completed.^[14]

In September 2012, an SLS deputy project manager stated that $500 million is a reasonable target average cost per flight for the SLS program.^[65] In 2013, the Space Review estimated the cost per launch at $5 billion, depending on the rate of launches.^[66][67] NASA announced in 2013 that the European Space Agency will build the Orion service module.^[68] In August 2014, as the SLS program passed its Key Decision Point C review and was deemed ready to enter full development, costs from February 2014 until its planned launch in September 2018 were estimated at $7.021 billion.^[69] Ground systems modifications and construction would require an additional $1.8 billion over the same time.^[70]

In October 2018, NASA's Inspector General reported that the Boeing core stage contract had made up 40% of the $11.9 billion spent on the SLS as of August 2018. By 2021, development of the core stage was expected to have cost $8.9 billion, twice the initially planned amount.^[71] In December 2018, NASA estimated that yearly budgets for the SLS will range from $2.1 to $2.3 billion between 2019 and 2023.^[72]

In March 2019, the Trump administration released its fiscal year 2020 budget request for NASA, which notably proposed dropped funding for the Block 1B and Block 2 variants of SLS. Congressional action ultimately included the funding in the passed budget.^[73] One Gateway component that had been previously planned for the SLS Block 1B is expected to fly on the SpaceX Falcon Heavy rocket.^{[74][needs update]}

On 1 May 2020, NASA awarded a contract extension to Aerojet Rocketdyne to manufacture 18 additional RS-25 engines with associated services for $1.79 billion, bringing the total RS-25 contract value to almost $3.5 billion.^[75][31]

Early plans

The SLS was created by an act of the U.S. Congress in the NASA Authorization Act of 2010, Public Law 111–267, in which NASA was directed to create a system for launching payloads and crew into space that would replace the capabilities lost with the retirement of the Space Shuttle.^[108] The act set out certain goals, such as being able to lift 70–100 tons into low earth orbit with evolvability to 130 tons, a target date of 31 December 2016 for the system to be fully operational, and a directive to use "to the extent practicable" existing components, hardware, and workforce from the Space Shuttle and from Ares I.^[108]: 12

On 14 September 2011, NASA announced their plan to meet these requirements: the design for the SLS, with the Orion spacecraft as payload.^{[109][110][111][112]}

The SLS has considered several future development routes of potential launch configurations, with the planned evolution of the blocks of the rocket having been modified many times.^[98] Many options, all of which just needed to meet the congressionally mandated payload minimums,^[98] were considered, including a Block 0 variant with three main engines,^[14] a variant with five main engines,^[98] a Block 1A variant with upgraded boosters instead of the improved second stage,^[14] and a Block 2 with five main engines plus the Earth Departure Stage, with up to three J-2X engines.^[17]

In the initial announcement of the design of the SLS, NASA also announced an "Advanced Booster Competition", to select which boosters would be used on Block 2 of the SLS.^{[109][113][24][114]} Several companies proposed boosters for this competition, all of which were indicated as viable:^[115] Aerojet and Teledyne Brown proposed three booster engines each with dual combustion chambers,^[116] Alliant Techsystems proposed a modified solid rocket booster with lighter casing, more energetic propellant, and four segments instead of five,^[117] and Pratt & Whitney Rocketdyne and Dynetics proposed a liquid-fueled booster named Pyrios.^[118] However, this competition was planned for a development plan in which Block 1A would be followed by Block 2A, with upgraded boosters. NASA canceled Block 1A and the planned competition in April 2014, in favor of simply remaining with the Ares I's five-segment solid rocket boosters, themselves modified from the Space Shuttle's solid rocket boosters, until at least the late 2020s.^[98][119] The overly powerful advanced booster would have resulted in unsuitably high acceleration, and would need modifications to Launch Complex 39B, its flame trench, and Mobile Launcher.^[120][98]

On 31 July 2013, the SLS passed Preliminary Design Review. The review included not only the rocket and boosters but also ground support and logistical arrangements.^[121]

On 7 August 2014, the SLS Block 1 passed a milestone known as Key Decision Point C and entered full-scale development, with an estimated launch date of November 2018.^[69][122]

Proposed cancellation

On 7 February 2025, Boeing, the primary contractor for the SLS, informed its employees working on the rocket program that they may face layoffs when the company's contract expires in March. The announcement coincided with the anticipated release of the presidential budget, suggesting the Trump administration might propose canceling the SLS program.^[133]

On 2 May 2025, the Trump administration released its fiscal year 2026 budget proposal for NASA, which calls for terminating the SLS and Orion spacecraft programs after Artemis III.^[134][135] The budget proposal described the SLS as "grossly expensive", noting that it costs $4 billion per launch and has exceeded its budget by 140 percent. The budget allocates funding for a program to transition to "more cost-effective commercial systems", a move projected to save NASA $879 million.^[136]

The 2025 One Big Beautiful Bill Act included funding for SLS rockets for the Artemis IV and Artemis V missions, but a clause directed NASA to evaluate alternatives to the EUS.^[99][100]

Construction

As of 2020^[update], three SLS versions are planned: Block 1, Block 1B, and Block 2. Each will use the same Core stage with its four main engines, but Block 1B will feature the Exploration Upper Stage (EUS), and Block 2 will combine the EUS with upgraded boosters.^{[142][57][143]}

The ICPS for Artemis 1 was delivered by ULA to NASA about July 2017^[144] and was housed at Kennedy Space Center as of November 2018.^[145]

Launches

Originally planned for late 2016, the uncrewed first flight of SLS slipped more than twenty-six times and almost six years.^{[note 3]} As of earlier that month, the first launch was originally scheduled for 8:30 am EDT, 29 August 2022.^[206] It was postponed to 2:17 pm EDT (18:17 UTC), 3 September 2022, after the launch director called a scrub due to a temperature sensor falsely indicating that an RS-25 engine's hydrogen bleed intake was too warm.^[195][196] The 3 September attempt was then scrubbed due to a hydrogen leak in the tail service mast quick disconnect arm, which was fixed; the next launch option was at first a period in late^[201][202] October and then a launch in mid-November, due to unfavorable weather during Hurricane Ian.^{[200][207][198]} It launched on 16 November.^[208][209]

NASA originally limited the amount of time the solid rocket boosters can remain stacked to "about a year" from the time two segments are joined.^[210] The first and second segments of the Artemis I boosters were joined on 7 January 2021.^[211] NASA could choose to extend the time limit based on an engineering review.^[212] On 29 September 2021, Northrop Grumman indicated that the limit could be extended to eighteen months for Artemis I, based on an analysis of the data collected when the boosters were being stacked;^[156] an analysis weeks before the actual launch date later extended that to December 2022 for the boosters of Artemis I, almost two years after stacking.^[213]

In late 2015, the SLS program was stated to have a 70% confidence level for the first Orion flight that carries crew, the second SLS flight overall, to happen by 2023;^{[214][215][216]} as of November 2021^[update], NASA delayed Artemis II from 2023^[217] to May 2024.^[218] In March 2023, NASA announced they had delayed Artemis II to November 2024,^[219] in January 2024 the mission was further delayed to September 2025,^[220] and in December 2024 it was announced that the launch was pushed back to April 2026.^[221]

Funding

As the Space Shuttle program drew to a close in 2009, the Obama administration convened the Augustine Commission to assess NASA's future human spaceflight endeavors. The commission's findings were stark: NASA's proposed Ares V rocket, intended for lunar and Martian missions, was unsustainable and should be canceled. The administration further advocated for a public-private partnership, where private companies would develop and operate spacecraft, and NASA would purchase launch services on a fixed-cost basis.^[245]

The recommendations faced fierce opposition from senators representing states with significant aerospace industries. In response, in 2011, Congress mandated the development of the SLS. The program was characterized by a complex web of political compromises, ensuring that various regions and interests benefited, maintaining jobs and contracts for existing space shuttle contractors.^[246][247] Utah Senator Orrin Hatch ensured the new rocket used the Shuttle's solid boosters, which were manufactured in his state. Alabama Senator Richard Shelby insisted that the Marshall Space Flight Center design and test the rocket. Florida Senator Bill Nelson brought home billions of dollars to Kennedy Space Center to modernize its launch facilities.^[248][249]

Almost immediately, Representative Tom McClintock called on the Government Accountability Office to investigate possible violations of the Competition in Contracting Act, arguing that the requirement that Shuttle components be used on SLS were non-competitive and assured contracts to existing suppliers.^{[250][251][252]}

The Obama administration's 2014 budget called for canceling SLS and turning over space transportation to commercial companies. The White House sent Lori Garver, the NASA deputy administrator, along with astronaut Sally Ride and other experts to defend the proposal, saying the SLS program was too slow and wasteful. However, Senators Shelby and Nelson quickly moved to fight efforts to cut the program and were ultimately victorious.^[253][245] After retirement from NASA, Garver would go on to recommend cancellation of the SLS.^[254]

During the First Trump administration, NASA administrator Jim Bridenstine suggested to a Senate committee that the agency was considering using the Falcon Heavy or Delta IV Heavy rocket to launch Orion instead of SLS. Afterward, the administrator was reportedly called into a meeting with Senator Shelby, who told Bridenstine he should resign for making the suggestion in a public meeting.^[245]

In 2023, Cristina Chaplain, former assistant director of the GAO, expressed doubts about reducing the rocket's cost to a competitive threshold, "just given the history and how challenging it is to build."^[141]

Management

In 2019, the Government Accountability Office (GAO) noted that NASA had assessed the performance of contractor Boeing positively, though the project had experienced cost growth and delay.^[255][256] A March 2020 report by Office of Inspector General found NASA moved out $889 million of costs relating to SLS boosters, but did not update the SLS budget to match. This kept the budget overrun to 15% in FY 2019;^[244]: 22 an overrun of 30% would have required NASA to request additional funding from the U.S. Congress^{[244]: 21–23} The Inspector General report found that were it not for this "masking" of cost, the overrun would have been 33% by FY 2019.^{[244]: iv, 23} The GAO stated "NASA's current approach for reporting cost growth misrepresents the cost performance of the program".^{[257]: 19–20}

Proposed alternatives

In 2009, the Augustine commission proposed a commercial 165,000 lb (75 t) launcher for lunar exploration.^[258] In 2011–2012, the Space Access Society, Space Frontier Foundation, and The Planetary Society called for the cancellation of the project, arguing that the SLS would consume the funds for other projects from the NASA budget.^{[259][250][260]} U.S. Representative Dana Rohrabacher and others^[who?] proposed the development of an orbital propellant depot and the acceleration of the Commercial Crew Development program as an alternative to the SLS program.^{[259][261][262][263][264]}

An unpublished NASA study^[265][266] and another from the Georgia Institute of Technology found these approaches could have lower costs.^[267][268] In 2012, United Launch Alliance also suggested using existing rockets with on-orbit assembly and propellant depots as needed.^[269][270] In 2019, a former ULA employee alleged that Boeing viewed orbital refueling technology as a threat to the SLS and blocked investment in the technology.^[271] In 2010, SpaceX's CEO Elon Musk claimed that his company could build a launch vehicle in the 310,000–330,000 lb (140–150 t) payload range for $2.5 billion, or $300 million (in 2010 dollars) per launch, not including a potential upper-stage upgrade.^[272][273]

Former NASA Administrator Charlie Bolden, expressed that the SLS could be replaced in the future in an interview with Politico in September 2020. Bolden said that the "SLS will go away ... because at some point commercial entities are going to catch up." Bolden further stated, "They are really going to build a heavy-lift launch vehicle sort of like SLS that they will be able to fly for a much cheaper price than NASA can do SLS. That's just the way it works."^[274]

Shortly after the New Glenn 9×4 rocket variant was announced in November 2025, journalist Eric Berger noted that it would offer lift capacity approaching that of the SLS Block 1 while retaining a reusable first stage and a larger payload fairing, and could cost less than one-tenth as much per launch.^[275]