On September 14, 2011, NASA announced its design selection for the new launch system, declaring that it, in combination with the Orion spacecraft, would take the agency's astronauts farther into space than ever before and provide the cornerstone for future US human space exploration efforts.
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 that would have upgraded the vehicle's boosters instead of its second stage, and a Block 2 with five main engines and a different second stage, the Earth Departure Stage, with up to three J-2X engines. In February 2015, it was reported that NASA evaluations showed "over performance" versus the baseline payload for Block 1 and Block 1B configurations.
Three versions of the SLS launch vehicle 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 a more powerful second stage called the Exploration Upper Stage (EUS), and Block 2 will combine the EUS with upgraded boosters. Block 1 has a baseline LEO payload capacity of 95 metric tons (105 short tons) and Block 1B has a baseline of 105 metric tons (116 short tons). The proposed Block 2 will have lift capacity of 130 metric tons (140 short tons), which is similar to that of the Saturn V. Some sources state this would make the SLS the most capable heavy lift vehicle built; although the Saturn V lifted approximately 140 metric tons (150 short tons) to LEO in the Apollo 17 mission.
On July 31, 2013, the SLS passed the Preliminary Design Review (PDR). The review encompassed all aspects of the SLS's design, 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 April 2017, NASA announced that the schedule for the maiden flight would slip to 2019. In November 2017, the EM-1 maiden flight slipped further to June 2020.
Artist's rendering of a SLS Block 1
The Space Launch System's Core Stage will be 8.4 meters (28 ft) in diameter and use four RS-25 engines. Initial flights will use modified RS-25D engines left over from the Space Shuttle program; later flights are expected to switch to a cheaper version of the engine not intended for reuse. The stage's structure will consist of a modified Space Shuttle external tank with the aft section adapted to accept the rocket's Main Propulsion System (MPS) and the top converted to host an interstage structure. It will be fabricated at the Michoud Assembly Facility.
The core stage will be common across all currently planned evolutions of the SLS. Initial planning included studies of a smaller Block 0 configuration with three RS-25 engines, which was eliminated to avoid the need to substantially redesign the core stage for more powerful variants. Likewise, while early Block 2 plans included five RS-25 engines on the core, it was later baselined with four engines.
Blocks 1 and 1B of the SLS will use two five-segment Solid Rocket Boosters (SRBs), which are based on the four-segment Space Shuttle Solid Rocket Boosters. Modifications for the SLS included the addition of a center booster segment, new avionics, and new insulation which eliminates the Shuttle SRB's asbestos and is 860 kg (1,900 lb) lighter. The five-segment SRBs provide approximately 25% more total impulse than the Shuttle SRB and will not be recovered after use.
Orbital ATK (formerly Alliant Techsystems, part of Northrop Grumman since mid-2018) has completed full-duration static fire tests of five-segment SRBs. These include successful firings of three developmental motors (DM-1 to DM-3) from 2009 to 2011. The DM-2 motor was cooled to a core temperature of 40 °F (4 °C), and DM-3 was heated to above 90 °F (32 °C) to validate performance at extreme temperatures. Qualification Motor 1 (QM-1) was tested on March 10, 2015. Qualification Motor 2 was successfully tested on June 28, 2016. It was the final ground test before Exploration Mission 1 (EM-1).
For Block 2, NASA plans to switch from Shuttle-derived five-segment SRBs to advanced boosters. This will occur after development of the Exploration Upper Stage for Block 1B. Early plans would have developed advanced boosters before an updated second stage; this configuration was called Block 1A. By 2012 NASA planned to select these new boosters through an 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 AJ1E6 engines, which would be a newly developed LOX / RP-1oxidizer-rich staged combustion engine. Each AJ1E6 engine would produce 4,900 kN (1,100,000 lbf) thrust using a single turbopump to supply dual combustion chambers. On February 14, 2013, NASA awarded Aerojet a $23.3 million, 30-month contract to build a 2,400 kN (550,000 lbf) main injector and thrust chamber.
Alliant Techsystems (ATK) proposed an advanced SRB nicknamed "Dark Knight". This booster would switch from a steel case to one made of lighter composite material, use a more energetic propellant, and reduce the number of segments from five to four. It would deliver over 20,000 kN (4,500,000 lbf) maximum thrust and weigh 790,000 kg (1,750,000 lb) at ignition. According to ATK, the advanced booster would be 40% less expensive than the Shuttle-derived five-segment SRB. It is uncertain if the booster will allow SLS to deliver the mandated 130 t to LEO without the addition of a fifth engine to the core stage, as a 2013 analysis indicated a maximum capacity of 113 t with the baselined four-engine core.
Pratt & Whitney Rocketdyne and Dynetics proposed a liquid-fueled booster named "Pyrios". Each booster would use two F-1B engines which together would deliver a maximum thrust of 16,000 kN (3,600,000 lbf) total, and be able to continuously throttle down to a minimum of 12,000 kN (2,600,000 lbf). The F-1B would be derived from the F-1 engine, which powered the first stage of the Saturn V. It would have been easier to assemble, with fewer parts and a simplified design, while providing improved efficiency and a thrust increase of 110 kN (25,000 lbf). Estimates in 2012 indicated that the Pyrios booster could increase Block 2 low-Earth orbit payload to 150 t, 20 t more than the baseline.
Christopher Crumbly, manager of NASA's SLS advanced development office in January 2013 commented on the booster competition, "The F-1 has great advantages because it is a gas generator and has a very simple cycle. The oxygen-rich staged combustion cycle [Aerojet's engine] has great advantages because it has a higher specific impulse. The Russians have been flying ox[ygen]-rich for a long time. Either one can work. The solids [of ATK] can work."
Later analysis showed the Block 1A configuration would result in high acceleration which would be unsuitable for Orion and could require a costly redesign of the Block 1 core. In 2014, NASA confirmed the development of Block 1B instead of Block 1A and called off the 2015 booster competition. 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 SLS's Mobile Launcher Platform were evaluated based on SLS launching with solid-fuel boosters.
An RL10 engine, like the one pictured above, will be used as the second stage engine in both the ICPS and EUS upper stages.
Interim Cryogenic Propulsion Stage
Block 1, scheduled to fly Exploration Mission 1 (EM-1) in 2020, will use the Interim Cryogenic Propulsion Stage (ICPS). This stage will be a modified Delta IV 5–meter Delta Cryogenic Second Stage (DCSS), and will be powered by a single RL10B-2. Block 1 will be capable of lifting 95 t in this configuration, however the ICPS will be considered part of the payload and be placed into an initial 1,800 km by -93 km suborbital trajectory to ensure safe disposal of the core stage. ICPS will perform an orbital insertion burn at apogee, and then a translunar injection burn to send the uncrewed Orion on a circumlunar excursion. In May 2018, NASA updated the payload capability of the SLS Block 1 from 70 to 95 metric tons to low Earth orbit.
Exploration Upper Stage
The Exploration Upper Stage (EUS) was scheduled to fly on Exploration Mission 2 (EM-2). It was expected to be used by Block 1B and Block 2 and, like the core stage, have a diameter of 8.4 meters. The EUS is to be powered by four RL10 engines, complete the SLS ascent phase and then re-ignite to send its payload to destinations beyond low-Earth orbit, similar to the role performed by the Saturn V's 3rd stage, the J-2 powered S-IVB. Because of delays in building the mobile launch platform needed to hold the more powerful EUS, the EM-2 flight might launch earlier than planned but it will not use the EUS, it will not carry a module for the Lunar Gateway and it will not orbit the Moon.
Other upper stages
Artist's impression of the Bimodal Nuclear Thermal Rocket engines on the Mars Transfer Vehicle (MTV). Cold launched, it would be assembled in-orbit by a number of Block 2 SLS payload lifts. The Orion spacecraft is docked on the left.
In 2013, NASA and Boeing analyzed the performance of several second stage options. The analysis was based on a second stage usable propellant load of 105 metric tons, except for the Block 1 and ICPS, which will carry 27.1 metric tons. The ICPS upper stage and upper stages using four RL10 engines and two MB60/RL60 engines and one J-2X engine were studied. 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 a reduction of one or two of the four second stage engines for the same performance for a lower cost. Robotic exploration missions to Jupiter's water-ice moon Europa are increasingly seen as well suited to the lift capabilities of the Block 1B SLS.
Rendering of the SLS Block 1 with its older black-and-white paint scheme, showing core stage, two 5-segment SRBs, and the smaller upper stage.
In mid-November 2014, construction of the first SLS rocket began using the new welding system at NASA's Michoud Assembly Facility, where the Core Stage will be assembled.
The SLS 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 a minimum of 180 days and can remain in stacked configuration for at least 200 days without destacking.
In January 2015, NASA began test firing RS-25 engines in preparation for use on SLS. Tests continued throughout Spring of 2015. Further testing was conducted in 2016 and 2017.
Multiple facilities throughout the country have started full scale fabrication of different segments of the launch vehicle. Orbital ATK began casting propellant for the solid rocket boosters and manufacturing parts for the boosters in 2016. The company test fired a solid rocket booster in early 2015, and a second booster in June 2016.
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. The ICPS for EM-1 was slated for assembly in late January 2016, and a structural test article was delivered to NASA in 2015 for confidence testing.
Program costs and funding
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 Multi-Purpose Crew Vehicle 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 $41bn 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 $1.6bn and Block 1 at $1.86bn in 2010. However, since these estimates were made the Block 0 SLS vehicle was dropped in late 2011, and the design was not completed. 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.
NASA SLS deputy project manager Jody Singer at Marshall Space Flight Center, Huntsville, Alabama stated in September 2012 that $500 million per launch is a reasonable target cost for SLS, with a relatively minor dependence of costs on launch capability. By comparison, a Saturn V launch cost US$185 to US$189 million in 1969-1971 dollars or roughly $1.23 billion in 2016 dollars adjusted for inflation.
On July 24, 2014, Government Accountability Office audit predicted that SLS would not launch by the end of 2017 as originally planned since NASA had not been receiving sufficient funding.
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. As of February 2015 the Orion spacecraft was expected to enter its Key Decision Point C review in the first half of 2015.
For Fiscal Year 2015, NASA received an appropriation of US$1.7 billion from Congress for SLS, an amount that was approximately US$320 million greater than the amount requested by the Obama administration.
In 2018, NASA's inspector general stated in a report that the SLS will cost at least US$8.9 billion by 2021. That is double the amount that was stated initially.
Funding history and planning
For fiscal years 2011 through 2015, the SLS program had expended funding totaling $7.7 billion in nominal dollars. This is equivalent to $8.3 billion adjusting to 2016 dollars using the NASA New Start Inflation Indices.
Line Item Name
Actuals, 2011, Space Launch System (Formal SLS Program reporting excludes the Fiscal 2011 budget as being before "formulation start" in November 2011, Fiscal Year 2012)
For 2016, the SLS program funding, excluding the Exploration Upper Stage (EUS), was enacted at $1,915M with an additional $7,180M planned for 2017 through 2021. The SLS program has a 70% confidence level for initial program completion by 2023 according to the Associate Administrator for NASA, Robert Lightfoot.
The sum of the prior SLS program funding from 2011 to 2015, funding enacted for 2016.
Exclude costs for the Ares 1 / 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)
Exclude costs of the Upper Stage for the SLS, the EUS
Exclude 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)
Exclude costs of payloads for the SLS (such as Orion)
There are no NASA estimates for the SLS program recurring yearly costs once operational, for a certain flight rate per year, or for the resulting average costs per flight.
Others suggest it will cost less to use an existing lower payload capacity rocket (Atlas V, Delta IV, Falcon 9, or the derivative Falcon Heavy), with on-orbit assembly and propellant depots as needed, rather than develop a new launch vehicle for space exploration without competition for the whole design. The Augustine commission proposed an option for a commercial 75 metric ton launcher with lower operating costs, and noted that a 40 to 60 t launcher can support lunar exploration.
Mars Society founder Robert Zubrin, who co-authored the Mars Direct concept, suggested that a heavy lift vehicle should be developed for $5 billion on fixed-price requests for proposal. Zubrin also disagrees with those that say the U.S. does not need a heavy-lift vehicle.SpaceX's CEO Elon Musk stated in 2010 that he would "personally guarantee" that his company could build a launch vehicle in the 140–150 t payload range, for $2.5 billion, or $300 million per launch, but cautioned that this price tag did not include a potential upper-stage upgrade. SpaceX's privately fundedITS launch vehicle, powered by multiple Raptor engines, has also been proposed for lifting very large payloads from Earth in the 2020s.
Rep. Tom McClintock and other groups argue that the Congressional mandates forcing NASA to use Space Shuttle components for SLS amounts to a de facto non-competitive, single source requirement assuring contracts to existing shuttle suppliers, and calling the Government Accountability Office (GAO) to investigate possible violations of the Competition in Contracting Act (CICA). 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 cancelling the program.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.
Doubts have also been expressed about the utility and cost of depots. "Patrick R. Chai and Alan W. Wilhite of Georgia Tech presented a study early in 2011 estimating that depot tanks would lose about $12 million worth of propellant per month in low Earth orbit if protected only with passive insulation."
^Brian Fishbine, Robert Hanrahan, Steven Howe, Richard Malenfant, Carolynn Scherer, Haskell Sheinberg, and Octavio Ramos Jr. (December 2016). "Nuclear Rockets: To Mars and Beyond". National Security Science. Los Alamos National Laboratory.
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