The Space Launch System (SLS) is a United States Space Shuttle-derived heavy launch vehicle being designed by NASA. It follows the cancellation of theConstellation Program, and is to replace the retired Space Shuttle. The NASA Authorization Act of 2010 envisions the transformation of the Ares I and Ares V vehicle designs into a single launch vehicle usable for both crew and cargo.
The SLS launch vehicle is to be upgraded over time with more powerful versions. Its initial Block I version, without an upper stage, is to lift a payload of 70 metric tons to orbit. The final Block II version with an integrated upper Earth Departure Stage is to have, depending on the configuration, a payload lift capability of at least 130 metric tons to low earth orbit, 12 metric tons above that of Saturn V, which would make the SLS the most capable heavy lift vehicle ever built.
SLS is to be capable of lifting astronauts and hardware to near-Earth destinations such as asteroids, the Moon, Mars, and most of the Earth's Lagrangian points. SLS may also support trips to the International Space Station, if necessary. The SLS program is integrated with NASA's Orion Crew and Service Module, with astronauts returning to earth in a capsule-shaped, four-person crew module. SLS will use the ground operations and launch facilities at NASA's Kennedy Space Center, Florida. The first flight-test of the Block I variant of the vehicle, Exploration Mission 1, is scheduled to fly in 2017.
On September 14, 2011, NASA announced its design selection for the new launch system, declaring that it would take the agency's astronauts farther into space than ever before and provide the cornerstone for future US human space exploration efforts. Since the announcement, four versions of the launch vehicle have been revealed – Blocks 0, I, IA and II. Each configuration utilizes different core stages, boosters and upper stages, with some components deriving directly from Space Shuttle hardware and others being developed specifically for the SLS. Block II of the SLS, the most capable variant, was initially depicted as having five RS-25E engines, upgraded boosters and an 8.4-meter diameter upper stage with three J-2X engines. Along with its baseline 8.4 meter diameter payload fairing a longer but thinner 5-meter class payload fairing with a length of 10 m or greater is also considered for propelling heavier payloads to deep space. Since then a number of changes have been made, with Block 0 no longer in design and the final Block II design being dependent on an ongoing booster competition and further analysis. The initial Block I two-stage variant will have a lift capability of between 70,000 and 77,000 kg, while the proposed Block II final variant will have similar lift capacity and height to the original Saturn V.By November 2011, NASA had selected five rocket configurations for wind tunnel testing, described in three Low Earth Orbit classes; 70 metric tons (t), 95 t, and 140 t.
On May 24, 2011, NASA announced that development of the Orion spacecraft from the Constellation program will continue as the Multi-Purpose Crew Vehicle (MPCV).
On July 31, 2013 the SLS passed the Preliminary Design Review (PDR). The review encompassed all aspects of the SLS' design, not only the rocket and boosters but also ground support and logistical arrangements. Successful completion of the PDR paves the way for Gate-C approval by NASA senior administration, enabling the project to move from design to implementation.
The core stage of the SLS is common to all vehicle configurations, essentially consisting 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 stage will utilize varying numbers and versions of the RS-25 engine depending on the configuration to be used:
In addition to the thrust produced by the engines on the core stage, the first two minutes of flight will be aided by two rocket boosters mounted to either side of the core stage. Early configurations (Blocks 0 and I) of the SLS are set to use modified Space Shuttle Solid Rocket Boosters (SRBs), with either four or five segments depending on configuration These boosters will not be recovered and will sink into the Atlantic Ocean downrange. The boosters for Block IA and Block II configurations will use upgraded boosters from the selection of improved booster bids. These may be of either the solid rocket or liquid rocket booster type.
Alliant Techsystems (ATK), the builder of the Space Shuttle SRBs, has completed three full-scale, full-duration static tests of the five-segment rocket booster that will be used in Blocks 0 and I. Development motor (DM-1) was successfully tested on September 10, 2009; DM-2 on August 31, 2010 and DM-3 on September 8, 2011. For DM-2 the motor was cooled to a core temperature of 40 degrees Fahrenheit (4 degrees Celsius), and for DM-3 it was heated to above 90 °F (32 °C). In addition to other objectives, these tests validated motor performance at extreme temperatures. Each five-segment SRB has a thrust of 3,600,000 lbf (16 MN) at sea level.
For SLS Block II, NASA has begun the Advanced Booster Competition that is expected to end in 2015. On June 17, 2011, Aerojet announced a strategic partnership with Teledyne Brown to develop and produce a domestic version of an uprated Soviet NK-33 LOX/RP-1 engine, an engine derived from the NK-15initially designed to lift the unsuccessful N-1 Soviet moonshot vehicle, with each engine's thrust increased from 394,000 lbf (1.75 MN) to at least 500,000 lbf (2.2 MN) at sea level. This booster, with eight AJ-26-500, or four AJ-1E6 engines is to compete against the Shuttle-derived solid rocket booster for the later Blocks of the SLS launch vehicle. On February 14, 2013, NASA awarded a $23.3 million 30-month contract Aerojet to build a full-scale 550,000-pound thrust class main injector and thrust chamber to be used in the advanced booster. Two standard Aerojet AJ-26 engines, together producing a combined 735,000 lbf (3.27 MN) of sea level thrust, successfully lifted the Antares rocket in 2013.
Pratt & Whitney Rocketdyne, and Dynetics have entered the competition with a booster design known as "Pyrios", which would use two F-1B engines derived from the F-1 LOX/RP-1 engine that powered the first stage of the Saturn V vehicle in the Apollo program. In 2012 it was determined that if the dual-engined Pyrios booster was selected for the SLS Block II, the payload could be 150 metric tons (t) to Low Earth Orbit, 20 t more than the baseline 130 t to LEO for SLS Block II.In 2013, it was reported that in comparison to the F-1 engine that it is derived from, the F-1B engine is to have improved efficiency, be more cost effective and have fewer engine parts.Each F-1B is to produce 1,800,000 lbf (8.0 MN) of thrust at sea level, an increase over the 1,550,000 lbf (6.9 MN) of thrust of the initial F-1 engine.
ATK is proposing an advanced SRB named "Dark Knight" with more energetic propellant, a lighter composite case, and other design improvements to reduce costs and improve performance. ATK states it provides "capability for the SLS to achieve 130 t payload with significant margin" when combined with a Block II core stage containing five RS-25 engines. However, the advanced SRB is to achieve no more than 113 t to low earth orbit with the current core stage with four RS-25 engines for Block I and Block II vehicles.
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."
The SLS will make use of several upper stages in its various configurations:
Studies through 2012 indicated that a heavy-lift rocket capable of delivering approximately 140 t to LEO is needed for NASA's manned mission to Mars. The third stage, beyond LEO engine, for the interplanetary leg of this mission, tasked with transporting cargo and crew from earth orbit to Mars orbit, and back, is being studied at Marshall Space Flight Center with project simulations on nuclear thermal rocket (NTR) engines and the goal of developing a Nuclear Cryogenic Propulsion Stage. The project will see rocket engines at least twice as efficient as their most efficient chemical counterparts, a level of thrust and efficiency required for propelling the necessary mass of cargo to support exploration during crewed missions to Mars and beyond. NTR engines, such as the Pewee of Project Rover, were selected in the Mars Design Reference Architecture(DRA). An NTR equipped Mars transfer vehicle would cut down on trip times and therefore reduce the amount of time the crew would be exposed to the most penetrating cosmic rays, it would also save money as over $1.5 billion of investment on their development and successful ground testing was spent during Project Rover and related projects.
Before launch, the SLS will have the ability to tolerate a minimum of 13 tanking cycles due to launch scrubs and other launch delays. 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.
During the joint Senate-NASA presentation in September 2011, it was stated that the SLS program has 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 are 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 70 t launches (1 unmanned in 2017, 3 manned starting in 2021), with the 130 t version ready no earlier than 2030. 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 was dropped in late 2011 and is no longer being designed, and 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 in 1969, the cost of a Saturn V including launch was US$185 million (inflation adjusted US$1.19 billion in 2014).