The Space Launch System (SLS) is an American two-stage super heavy-lift expendable launch vehicle used by NASA. The primary launch vehicle for the Artemis program, SLS is designed to launch the four-person Orion spacecraft. The rocket first launched in November 2022, carrying the uncrewed Artemis I mission. Its only crewed launch has been for the Artemis II lunar flyby in April 2026, becoming the second launch vehicle to carry humans beyond low Earth orbit (LEO), after NASA's Saturn V of the Apollo program.
Development of SLS began in 2011 as a congressionally mandated replacement for the retiring Space Shuttle program and the cancelled Ares I and Ares V launch vehicles of the Constellation program, incorporating some hardware from both programs. Costing US$31.6 billion as of 2025, the project has been criticized for mismanagement, budget overruns, and delays, but ultimately succeeded in human spaceflight in 2026.
All SLS launches take place from Launch Complex 39B at the Kennedy Space Center in Florida. The new Boeing-built core stage is powered by four ex-Shuttle RS-25 engines. Attached to the core are two Northrop Grumman five-segment Solid Rocket Boosters, built for the Ares vehicles, tested on Ares I-X, developed from the Shuttle's four-segment SRB. SLS currently uses the Interim Cryogenic Propulsion Stage (ICPS) as its second stage, for the insertions to low Earth orbit and trans-lunar injection. ICPS uses the RL10 engine, and was derived from the Delta Cryogenic Second Stage designed by Japan's space agency. Aerojet Rocketdyne builds the RS-25 and RL10 engines.
Starting from Artemis V, no earlier than late 2028, SLS will use the Centaur V upper stage, developed for the Vulcan Centaur, instead of ICPS. NASA previously planned to upgrade SLS from its current Block 1 configuration to a Block 1B and Block 2, but cancelled these plans in February 2026, aiming to standardize on Block 1, to "reduce risk and maintain schedule stability". Block 1B was to use the Exploration Upper Stage, and Block 2 would have used new solid rocket boosters.
While SLS has the highest liftoff thrust of any rocket to ever carry humans, , SLS's payload capacity to trans-lunar injection of is only around half that of the Saturn V's payload. Thus for Artemis lunar landings, beginning with Artemis IV targeted early 2028, Orion is planned to dock with the Human Landing System (HLS) in lunar orbit or low Earth orbit, separately launched on a non-SLS rocket; SpaceX's Starship HLS and Blue Origin's Blue Moon are under development as HLS vehicles.
Its next scheduled flight is Artemis III, a 2027 low Earth orbit docking test between Orion and HLS vehicles. Beginning with Artemis V, NASA will transfer SLS operations to the Deep Space Transport LLC, a commercial launch provider consortium of Boeing and Northrop Grumman.
Components
The SLS is a Space Shuttle-derived launch vehicle. Its first stage consists of a central core stage powered by four engines, flanked by two space shuttle-derived solid rocket boosters.
NASA had planned to introduce upgraded variants of the rocket. The Block 1B configuration was to incorporate a larger, purpose-built upper stage, while the Block 2 configuration was to feature newly developed solid rocket boosters. On February 26, 2026, NASA announced that it would standardize the Block 1 configuration and pursue alternative upper-stage options.
Core stage
The SLS core stage is built by Boeing at NASA's Michoud Assembly Facility in New Orleans. It measures in length and 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.
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.
The core stage structure is primarily made of 2219 aluminum alloy, and compared to the Space Shuttle external tank, several manufacturing improvements were incorporated. Production began in 2014, but delays in manufacturing, testing, and integration postponed the readiness of the first flight article by several years. It uses ten barrel-shaped main sections, four domes, and seven rings.
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. 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. Later flights will transition to the RS-25E, optimized for expendable use, being faster to build and costing 30% less. Thrust for the refurbished RS-25D engines has been increased from , while the RS-25E is rated at per engine. The first test firing of an RS-25E occurred in June 2025 and was declared successful.
The SLS uses a conical frustum-shaped interstage known as the Launch Vehicle Stage Adapter (LVSA) between the core stage and the narrower diameter upper stage. The LVSA consists of sixteen aluminum-lithium panels made of 2195 aluminum alloy and is built by Teledyne Brown Engineering. The first unit cost approximately $60 million, with the next two costing $85 million combined.
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.
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.
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.
The available inventory of Shuttle-era booster casing segments limits use of this configuration to eight SLS flights.
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.
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.
Each BOLE booster is designed to produce approximately of thrust, about 19 percent greater than the five-segment Shuttle-derived boosters, thereby increasing overall payload capability.
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.
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, also built by Aerojet Rocketdyne. The ICPS units flown on Artemis II and planned for use on Artemis IV use a single RL10C-2.Rosenberg, Zach. "Delta second stage chosen as SLS interim" . Flight International, May 8, 2012.
Artemis III will have no ICPS, as it is only a mission to low Earth orbit (LEO), and instead its ICPS will be used for Artemis IV.
Exploration Upper Stage (cancelled) The Exploration Upper Stage (EUS) was originally planned to debut on Artemis IV as the upper stage for the Block 1B configuration. It would have shared the diameter of the SLS core stage and be powered by four RL10C-3 engines, with a later upgrade to improved RL10C-X engines. Although the EUS progressed through design review, it did not enter full-scale hardware production and was ultimately canceled in February 2026, prior to flight.
Centaur V (future) In February 2026, NASA announced plans to standardize the SLS on the Block 1 configuration and discontinue development of the EUS. With production of the DCSS ended, NASA selected an existing commercially developed upper stage for missions following depletion of the ICPS stock: ULA's Centaur V, which uses a pair of RL10 engines and is expected to provide modestly improved performance over the ICPS.
Block variants BlockBoostersCore enginesUpper stageLiftoff thrustPayload mass to...LEOTLI15-segment Shuttle-derived boosters ICPS with >1BEUS with RL10C-3RS-25E2>
Development 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. These costs and schedules were considered optimistic in an independent 2011 cost assessment report by Booz Allen Hamilton for NASA. An internal 2011 NASA document estimated the cost of the program through 2025 to total at least $41 billion for four launches (1 uncrewed, 3 crewed), with the version ready no earlier than 2030. The Human Exploration Framework Team estimated unit costs for 'Block 0' at $1.6 billion and Block 1 at $1.86 billion in 2010. However, since these estimates were made, the Block 0 SLS vehicle was dropped in late 2011, and the design was not completed.
In September 2012, an SLS deputy project manager stated that $500 million is a reasonable target average cost per flight for the SLS program. In 2013, the Space Review estimated the cost per launch at $5 billion, depending on the rate of launches. NASA announced in 2013 that the European Space Agency will build the Orion service module. 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. Ground systems modifications and construction would require an additional $1.8 billion over the same time.