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The Space Launch System (SLS) is planned to be a US super heavy-lift expendable launch vehicle, which has been under development since its announcement in 2011. It is the primary launch vehicle of NASA's deep space exploration plans, including the planned crewed lunar flights of the Artemis program and a possible follow-on human mission to Mars. SLS replaces the Constellation program's Ares V launch vehicle of 2005, which never left the development phase.

The initial variant of SLS, Block 1, was required by the US Congress to lift a payload of 70 t to low Earth orbit (LEO), but exceeded that requirement with a rated payload capacity of 95 t. this variant is planned to launch Artemis 1, 2, and 3. The later Block 1B is intended to debut the Exploration Upper Stage and launch the notional Artemis 4 through Artemis 7. Block 2 is planned to replace the initial Shuttle-derived boosters with advanced boosters and would have a LEO capability of more than 130 t, again as required by Congress. Block 2 is intended to enable crewed launches to Mars. The SLS will launch the Orion spacecraft and use the ground operations and launch facilities at NASA's Kennedy Space Center in Florida.

Vehicle description
The SLS is a Space Shuttle-derived launch vehicle and will have the ability to tolerate a minimum of 13 tanking cycles due to launch scrubs and other launch delays before launch. The assembled rocket is to be able to remain at the launch pad for at least 180 days and can remain in a stacked configuration for at least 200 days.

Core stage
The Space Launch System's core stage is 212 ft long and 27.6 ft in diameter and mount a Main Propulsion System (MPS) incorporating four RS-25 engines. The core stage is structurally similar to the Space Shuttle external tank, and initial flights will use modified RS-25D engines left over from the Space Shuttle program. Later flights will switch to a cheaper version of the engine not intended for reuse.

The core stage is fabricated at the NASA's Michoud Assembly Facility and is common across all currently planned evolutions of the SLS to avoid the need for substantial redesigns to meet various payload mandates.

Block 1 and 1B boosters
Blocks 1 and 1B of the SLS will use two five-segment Solid Rocket Boosters (SRBs) based on the four-segment Space Shuttle Solid Rocket Boosters. Modifications to the five-segment boosters included the addition of a center booster segment, new avionics, and lighter insulation. The five-segment SRBs provide approximately 25% more total impulse than the Shuttle SRB and will not be recovered after use.

Block 2 advanced boosters (late 2020s)
The advanced boosters for Block 2 were intended to be selected through the Advanced Booster Competition, which was to be held in 2015.

Several companies proposed boosters for this competition:
 * Aerojet, in partnership with Teledyne Brown, offered a booster powered by three new AJ1E6 LOX/RP-1 oxidizer-rich staged combustion engines, each producing 1100000 lbf thrust using a single turbopump to supply dual combustion chambers. On 14 February 2013, Aerojet was awarded a $23.3 million, 30-month contract to build a 550,000 lbf main injector and thrust chamber.
 * Alliant Techsystems (ATK) proposed an advanced SRB nicknamed "Dark Knight", which would switch to a lighter composite case, use a more energetic propellant, and reduce the number of segments from five to four.
 * Pratt & Whitney Rocketdyne and Dynetics proposed a liquid-fueled booster named Pyrios.

In 2013, the manager of NASA's SLS advanced development office indicated that all three approaches were viable.

However, the 2015 competition was planned in support of Block 1A. A later study found that the advanced booster would have resulted in unsuitably high acceleration, and NASA canceled Block 1A and the planned competition in 2014. In February 2015, it was reported that SLS is expected to fly with the five-segment SRB until at least the late 2020s, and modifications to Launch Pad 39B, its flame trench, and Mobile Launcher were being evaluated.

Booster Obsolescence and Life Extension program
The stock of SLS boosters is limited by the number of casings left over from the Shuttle program. There are enough to last through flight eight of the SLS, but a replacement will be required for further flights. On 2 March 2019, the Booster Obsolescence and Life Extension (BOLE) program proposed to use new solid rocket boosters built by Northrop Grumman Innovation Systems for further SLS flights. These boosters would be derived from the composite-casing SRBs in development for the OmegA launch vehicle, and are projected to increase Block 1B's payload to TLI by 3-4 metric tons, 1 mT below the payload capacity of Block II.

ICPS - Block 1
The Interim Cryogenic Propulsion Stage (ICPS) is planned to fly on Artemis 1. It is a minimally modified Delta IV 5 m Delta Cryogenic Second Stage (DCSS) powered by a single RL10B-2. Block 1 will be capable of lifting 95 t to LEO in this configuration if the ICPS is considered part of the payload. Artemis 1 will be launched into an initial 1,800 by suborbital trajectory to ensure safe disposal of the core stage. ICPS will then perform an orbital insertion burn at apogee and a subsequent translunar injection burn to send Orion towards the moon. The ICPS for Artemis 1 was delivered by ULA to NASA about July 2017, and was housed at Kennedy Space Centre until at least November 2018. ICPS (not EUS) is planned for Artemis-1, 2, and 3. ICPS will now be human-rated for the crewed Artemis-2 flight.

EUS - Block 1B and 2
The Exploration Upper Stage (EUS) is planned to fly on Artemis 3. Similar to the S-IVB, the EUS would have completed the SLS ascent phase and then re-ignited to send its payload to destinations beyond low-Earth orbit. It was expected to be used by Block 1B and Block 2, share the core stage diameter of 8.4 meters, and be powered by four RL10 engines.

Development history


SLS is intended to replace the retired Space Shuttle as NASA's flagship vehicle. Following the cancelation of the Constellation program, the NASA Authorization Act of 2010 envisioned a single launch vehicle usable for both crew and cargo. SLS is to have the world's highest ever total thrust at launch, but not the world's highest ever payload mass. In 2013, SLS was projected to possibly be the most capable super-heavy lift vehicle ever built.

Program history
During the joint Senate-NASA presentation in September 2011, it was stated that the SLS program had a projected development cost of $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 schedule were considered optimistic in an independent 2011 cost assessment report by Booz Allen Hamilton for NASA.

An unofficial 2011 NASA document estimated the cost of the program through 2025 to total at least $41 billion for four 95-t launches (1 uncrewed, 3 crewed), with the 130-t version ready no earlier than 2030.

The Human Exploration Framework Team (HEFT) 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 per launch is a reasonable target cost for SLS.

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.

By April 2014, it was decided to cancel Block 1A and the related advanced booster competition due in 2015.

In August 2014, as the SLS program passed its Key Decision Point C review and entered 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 period.

In October 2018, NASA's inspector general reported that the Boeing core stage contract had made up 40 percent of the $11.9 billion spent on SLS as of August 2018. By 2021, core stages were expected to have cost a total of $8.9 billion, which is twice the initial planned amount.

In December 2018, NASA estimated that yearly budgets for 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. This budget did not include any money for the Block 1B and Block 2 variants of SLS. It is uncertain whether these future variants of SLS will be developed. Several launches previously planned for the SLS Block 1B are now expected to fly on commercial launcher vehicles such as Falcon Heavy, New Glenn, Omega, and Vulcan. However, the request for a budget increase of $1.6 billion towards SLS, Orion, and crewed landers along with the launch manifest seem to indicate support of the development of Block 1B, debuting Artemis 3. The Block 1B will be used mainly for co-manifested crew transfers and logistical needs rather than constructing the Gateway. An uncrewed Block 1B is planned to launch the Lunar Surface Asset in 2028, the first lunar outpost of the Artemis program. Block 2 development will most likely start in the late 2020s, after NASA is regularly visiting the lunar surface and shifts focus towards Mars.

In May 2019, NASA's Office of Audits reported that the SLS Block 1's marginal cost per launch is to be at least $876 million. By comparison, a Saturn V launch cost roughly $1.23 billion in 2016 dollars. A letter from the White House to the Senate Appropriations Committee revealed that the SLS's cost per launch is estimated at "over $2 billion" after development. NASA did not deny this cost and an agency spokesperson stated it "is working to bring down the cost of a single SLS launch in a given year as the agency continues negotiations with Boeing on the long-term production contract and efforts to finalize contracts and costs for other elements of the rocket".

Blue Origin submitted a proposal to replace the Exploration Upper Stage with an alternative to be designed and fabricated by the company, but it was rejected by NASA in November 2019 on multiple grounds. These included lower performance compared to the existing EUS design, unsuitability of the proposal to current ground infrastructure, and unacceptable acceleration in regards to Orion components.

Funding history
For fiscal years 2011 through 2018, the SLS program had expended funding totaling $13.999 billion in nominal dollars. This is equivalent to $15.109 billion in 2018 dollars using the NASA New Start Inflation Indices.

Excluded from the prior SLS costs are:
 * Costs of the predecessor Ares V / Cargo Launch Vehicle (funded from 2008 to 2010)
 * Costs for the Ares I / Crew Launch Vehicle (funded from 2006 to 2010, a total of $4.8 billion in development that included the 5-segment Solid Rocket Boosters that will be used on the SLS)
 * Costs to assemble, integrate, prepare and launch the SLS and its payloads such as Orion (funded under the NASA Ground Operations Project, currently about $400M per year)
 * Costs of payloads for the SLS (such as Orion)

Included in the prior SLS costs are:
 * Costs of the interim Upper Stage for the SLS, the Interim Cryogenic Propulsion Stage (ICPS) for SLS, which includes a $412M contract


 * Costs of the final Upper Stage for the SLS, the Exploration Upper Stage (EUS) (funded at $85M in 2016, $300M in 2017 and $300M in 2018 )

There are no current NASA estimates for the average costs per flight of SLS, nor for the SLS program recurring yearly costs once operational. In 2016, the projected annual cost for Orion, SLS, and ground systems was $2 billion or less. NASA associate administrator William H. Gerstenmaier has said that per flight cost estimates will not be provided by NASA.

On May 1, 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.

Constellation
From 2009 to 2011, three full-duration static fire tests of five-segment SRBs were conducted under the Constellation Program, including tests at low and high core temperatures, to validate performance at extreme temperatures. The 5-segment SRB would be carried over to SLS.

Early SLS
During the early development of the SLS a number of configurations were considered, including a Block 0 variant with three main engines, a Block 1A variant with upgraded boosters instead of the improved second stage, and a Block 2 with five main engines and the Earth Departure Stage, with up to three J-2X engines. In February 2015, it was determined that these concepts would exceed the congressionally mandated Block 1 and Block 1B baseline payloads.

On 14 September 2011, NASA announced the new launch system, which is intended to take the agency's astronauts farther into space than ever before and provide the cornerstone for future US human space exploration efforts in combination with the Orion spacecraft

On 31 July 2013, the SLS passed the Preliminary Design Review (PDR). The review included not only the rocket and boosters but also ground support and logistical arrangements. On August 7, 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.

In 2013, NASA and Boeing analyzed the performance of several EUS engine options. The analysis was based on a second stage usable propellant load of 105 metric tons, and compared stages with four RL10 engines, two RL60 engines, or one J-2X engine.

In 2014, NASA also considered using the European Vinci instead of the RL10. The Vinci offers the same specific impulse but with 64% greater thrust, which would allow for the same performance at lower cost.

Northrop Grumman Innovation Systems has completed full-duration static fire tests of the five-segment SRBs. Qualification Motor 1 (QM-1) was tested on March 10, 2015. Qualification Motor 2 (QM-2) was successfully tested on June 28, 2016.

Current SLS


, three SLS versions are planned: Block 1, Block 1B, and Block 2. Each will use the same core stage with four main engines, but Block 1B will feature the Exploration Upper Stage (EUS), and Block 2 will combine the EUS with upgraded boosters.

In mid-November 2014, construction of the first core stage hardware began using a new welding system in the South Vertical Assembly Building at NASA's Michoud Assembly Facility. Between 2015 and 2017 NASA test fired RS-25 engines in preparation for use on SLS.

As of late 2015, the SLS program was stated to have a 70% confidence level for the first crewed Orion flight by 2023.

Confidence article builds for the core stage began on January 5, 2016 and were expected to be completed in late January of that year. Once completed the test articles will be sent to ensure structural integrity at Marshall Spaceflight Center. A structural test article of the ICPS was delivered in 2015, with the core stage for Artemis 1 completing assembly in November 2019.

The first flight of SLS has slipped multiple times: first to 2019, then to June 2020, then to April 2021, and finally to November 2021.

The first core stage left Michoud for comprehensive testing at Stennis in January 2020. The static firing test program at Stennis, known as the Green Run, will operate all the core stage systems simultaneously for the first time.

Criticism
The SLS has been criticized on the basis of program cost, lack of commercial involvement, and the non-competitive nature of a vehicle legislated to use Space Shuttle components.

In 2009, the Augustine commission proposed a commercial 75 t launcher with lower operating costs, and noted that a 40-60 t launcher was the minimum required to support lunar exploration.

In 2011–2012, the Space Access Society, Space Frontier Foundation and The Planetary Society called for cancellation of the project, arguing that SLS will consume the funds for other projects from the NASA budget. U.S. Representative Dana Rohrabacher and others proposed that an orbital propellant depot should be developed and the Commercial Crew Development program accelerated instead. A NASA study that was not publicly released and another from the Georgia Institute of Technology showed this option to be possibly cheaper. In 2012, the United Launch Alliance also suggested using existing rockets with on-orbit assembly and propellant depots as needed. The lack of competition in the SLS design was highlighted. In the summer of 2019, a former ULA employee claimed that Boeing, NASA's prime contractor for SLS, viewed orbital refueling technology as a threat to SLS and blocked further investment in it.

In 2011, Mars Society/Mars Direct founder Robert Zubrin suggested that a heavy lift vehicle could be developed for $5 billion on fixed-price requests for proposal.

In 2010, SpaceX's CEO Elon Musk claimed that his company could build a launch vehicle in the 140–150 t payload range for $2.5 billion, or $300 million (in 2010 dollars) per launch, not including a potential upper-stage upgrade. In the early 2010s, SpaceX went on to start development of SpaceX Starship, a planned fully reusable super-heavy launch system. Reusability is claimed to allow the lowest cost super-heavy launcher ever made. If the price per launch and payload capabilities for the Starship are anywhere near Musk's claimed capabilities, the rocket will be substantially cheaper than the SLS.

In 2011, Rep. Tom McClintock and other groups called on the Government Accountability Office (GAO) to investigate possible violations of the Competition in Contracting Act (CICA), arguing that Congressional mandates forcing NASA to use Space Shuttle components for SLS are de facto non-competitive, single source requirements assuring contracts to existing shuttle suppliers. Opponents of the heavy launch vehicle have critically used the name "Senate launch system". The Competitive Space Task Force, in September 2011, said that the new government launcher directly violates NASA's charter, the Space Act, and the 1998 Commercial Space Act requirements for NASA to pursue the "fullest possible engagement of commercial providers" and to "seek and encourage, to the maximum extent possible, the fullest commercial use of space".

In 2013, Chris Kraft, the NASA mission control leader from the Apollo era, expressed his criticism of the system as well. Lori Garver, former NASA Deputy Administrator, has called for canceling the launch vehicle alongside Mars 2020 rover. Phil Plait has voiced his criticism of SLS in light of ongoing budget tradeoffs between Commercial Crew Development and SLS budget, also referring to earlier critique by Garver.

In 2019, the Government Accountability Office found that NASA had awarded Boeing over $200 million for service with ratings of good to excellent despite cost overruns and delays. , the maiden launch of SLS is expected in 2021.

On May 1, 2020, after NASA awarded a contract extension to manufacture 18 additional RS-25 engines for $1.79 billion, Ars Technica highlighted that within the entire RS-25 contract, each engine worked out at a cost of $146 million each, and that the four engines for each SLS will cost more than $580 million. They critically commented that for the cost of just one engine, six more powerful RD-180 engines could be purchased, or nearly an entire Falcon Heavy launch with two-thirds of the SLS lift capacity.