Space Launch System
The Space Launch System is an American super heavy-lift expendable launch vehicle used by NASA. As the primary launch vehicle of the Artemis Moon landing program, SLS is designed to launch the crewed Orion spacecraft on a trans-lunar trajectory. SLS first launched on 16 November 2022 for the uncrewed Artemis I mission.
Development of SLS began in 2011 as a replacement for the retiring Space Shuttle and the canceled Ares I and Ares V launch vehicles. SLS was built using a combination of Shuttle components, including solid rocket boosters and RS-25 engines, and new technology such as the Core Stage. The project has seen mismanagement, budget overruns, and delays. The first launch, required by Congress to take place by 2016, occurred nearly six years later.
All Space Launch System flights are to be launched from Launch Complex 39B at the Kennedy Space Center in Florida. The first three SLS flights are expected to use the Block 1 configuration, comprising a core stage, extended Space Shuttle boosters developed for Ares I, and the Interim Cryogenic Propulsion Stage upper stage. The improved Block 1B configuration, with the purpose-built Exploration Upper Stage, is to be introduced on the fourth flight; a further improved Block 2 configuration with new solid rocket boosters is planned for the ninth flight.
Description
The SLS is a Space Shuttle-derived launch vehicle. The rocket's first stage is powered by one central core stage and two outboard solid rocket boosters. All SLS Blocks share a common core stage design but differ in their upper stages and boosters.Core stage
Together with the solid rocket boosters, the core stage is responsible for propelling the upper stage and payload out of the atmosphere to near orbital velocity. It contains the liquid hydrogen and liquid oxygen tanks for the ascent phase, the forward and aft solid rocket booster attach points, avionics, and the Main Propulsion System, an assembly of the four RS-25 engines, associated plumbing and hydraulic gimbal actuators, and equipment for autogenous pressurization of the vehicle's tanks. The core stage provides approximately 25% of the vehicle's thrust at liftoff, the rest coming from the solid rocket boosters.The stage measures long by in diameter and is visually similar to the Space Shuttle external tank. It is made mostly of 2219 aluminum alloy, and contains numerous improvements to manufacturing processes, including friction stir welding for the barrel sections, and integrated milling for the stringers. The first four flights will each use and expend four of the remaining sixteen RS-25D engines previously flown on Space Shuttle missions. Aerojet Rocketdyne refits these engines with modernized engine controllers, higher throttle limits, as well as insulation for the high temperatures the engine section will experience due to their position adjacent to the solid rocket boosters. Later flights will switch to an RS-25 variant optimized for expended use, the RS-25E, which will lower per-engine costs by over 30%. The thrust of each RS-25D engine has been increased from, as on the Space Shuttle, to on the sixteen modernized engines. The RS-25E will further increase per-engine thrust to. The first test firing of a RS-25E took place in June 2025 and was declared a success.
Solid Rocket Boosters
Shuttle derived
Blocks 1 and 1B of the SLS will use two five-segment solid rocket boosters. They use casing segments that were flown on Shuttle missions as parts of the four-segment Space Shuttle Solid Rocket Boosters. They possess an additional center segment, new avionics, and lighter insulation, but lack a parachute recovery system, as they will not be recovered after launch. The propellants for the solid rocket boosters are aluminum powder, which is very reactive, and ammonium perchlorate, a powerful oxidizer. They are held together by a binder, polybutadiene acrylonitrile. The mixture has the consistency of a rubber eraser and is packed into each segment. The five-segment solid rocket boosters provide approximately 25% more total impulse than the Shuttle Solid Rocket Boosters.BOLE
The stock of SLS Block 1 to 1B boosters is limited by the number of casings left over from the Shuttle program, which allows for eight flights of the SLS. On 2 March 2019, the Booster Obsolescence and Life Extension program was announced, with the goal of developing new solid rocket boosters for SLS Block 2. These boosters will be built by Northrop Grumman Space Systems, and will be derived from the composite-casing solid rocket boosters then in development for the canceled OmegA launch vehicle, and are projected to increase Block 2's payload to to low Earth orbit and at least to trans-lunar injection. the BOLE program is under development, with first firing of a prototype taking place in June of that year, which experienced a nozzle failure, exploding about two minutes into the burn.Upper stages
Interim Cryogenic Propulsion Stage
The Interim Cryogenic Propulsion Stage is a temporary upper stage for Block 1 versions of SLS, built by United Launch Alliance, a joint venture of Boeing and Lockheed Martin. The ICPS is essentially an "off-the-shelf" Delta Cryogenic Second Stage, with minimal modifications for SLS integration. The ICPS is intended as a temporary solution and slated to be replaced on the Block 1B version of the SLS by the next-generation Exploration Upper Stage, under design by Boeing.The ICPS used on the Artemis I mission was powered by a single RL10B-2 engine, while the ICPS for Artemis II and Artemis III will use the RL10C-2 variant. Block 1 is intended to be capable of lifting to low Earth orbit in this configuration, including the weight of the ICPS as part of the payload. At the time of SLS core stage separation, Artemis I was traveling on an initial transatmospheric orbital trajectory. This trajectory ensured safe disposal of the core stage. ICPS then performed orbital insertion and a subsequent trans-lunar injection burn to send Orion towards the Moon. The ICPS will be human-rated for the crewed Artemis II and III flights.
The SLS Block 1 has a conical frustum-shaped interstage called the Launch Vehicle Stage Adapter between the core stage and the ICPS. It consists of sixteen aluminum-lithium panels made of 2195 aluminum alloy. Teledyne Brown Engineering is its builder. The first one cost $60 million, and the next two cost $85 million together.
Exploration Upper Stage
The Exploration Upper Stage is planned to first fly on Artemis IV. The EUS will complete the SLS ascent phase and then re-ignite to send its payload to destinations beyond LEO. It is expected to be used by Block 1B and Block 2. The EUS shares the core stage diameter of 8.4 meters, and will be powered by four RL10C-3 engines. It will eventually be upgraded to use four improved RL10C-X engines., Boeing is developing a new composite-based fuel tank for the EUS that would increase Block 1B's overall payload mass capacity to trans-lunar injection by 40 percent. The improved upper stage was originally named the Dual Use Upper Stage, but was later renamed the Exploration Upper Stage.Block variants
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, 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.
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. In December 2018, NASA estimated that yearly budgets for the SLS will range from $2.1 to $2.3 billion between 2019 and 2023.
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. One Gateway component that had been previously planned for the SLS Block 1B is expected to fly on the SpaceX Falcon Heavy rocket.
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.
Budget
NASA has spent $29.0 billion on SLS development from 2011 through 2024, in nominal dollars. This is equivalent to $35.4 billion in 2025 dollars using the NASA New Start Inflation Indices.In 2025, the Enacted NASA Budget for Exploration, which includes SLS, is approximately the same again as 2024.
In January 2024 NASA announced plans for a first crewed flight of the Orion spacecraft on the SLS, the Artemis II mission, no earlier than March 2026.
Included in the above SLS costs above are the Interim Cryogenic Propulsion Stage, a $412 million contract and the costs of developing the Exploration Upper Stage.
Excluded from the SLS cost above are the costs to assemble, integrate, prepare and launch the SLS and its payloads, funded separately in the NASA Exploration Ground Systems, currently at about $600 million per year, and anticipated to stay there through at least the first four launches of SLS. Also excluded are payloads that launch on the SLS, such as the Orion crew capsule, the predecessor programs that contributed to the development of the SLS, such as the Ares V Cargo Launch Vehicle project, funded from 2008 to 2010 for a total of $70 million, and the Ares I Crew Launch Vehicle, funded from 2006 to 2010 for a total of $4.8 billion in development, including the 5-segment Solid Rocket Boosters used on the SLS.
Despite calls from the Trump administration to terminate the SLS program after Artemis III, the 2025 One Big Beautiful Bill Act included $4.1 billion to fund SLS rockets for the Artemis IV and Artemis V missions, with mandated minimum spending of $1.025 billion per year from FY 2026 through 29. However, as a compromise, lawmakers have suggested eliminating the EUS, and directed NASA to evaluate alternatives such as the Centaur V or the GS2 upper stage used on the New Glenn rocket.