Space Launch System
Artist's rendering of the SLS Block 1 crewed variant launching
|Country of origin||United States|
|Cost per launch ()||US$500 million (2012)|
|Diameter||8.4 m (330 in) (core stage)|
|70,000 to 130,000 kg (150,000 to 290,000 lb)|
|Family||Shuttle-Derived Launch Vehicles|
|Launch sites||LC-39, Kennedy Space Center|
|First flight||No later than November 2018|
|Notable payloads||Orion MPCV|
|Boosters (Block I)|
2 Space Shuttle Solid Rocket Boosters
|Thrust||16,000 kN (3,600,000 lbf)|
|Total thrust||32,000 kN (7,200,000 lbf)|
|Specific impulse||269 seconds (2.64 km/s)|
|Burn time||124 seconds|
|First Stage (Block I, IB, II) - Core Stage|
|Diameter||8.4 m (330 in)|
|Empty mass||85,270 kg (187,990 lb)|
|Gross mass||979,452 kg (2,159,322 lb)|
|Thrust||7,440 kN (1,670,000 lbf)|
|Specific impulse||363 seconds (3.56 km/s) (sea level), 452 seconds (4.43 km/s) (vacuum)|
|Second Stage (Block I) - ICPS|
|Length||13.7 m (540 in)|
|Diameter||5 m (200 in)|
|Empty mass||3,490 kg (7,690 lb)|
|Gross mass||30,710 kg (67,700 lb)|
|Thrust||110.1 kN (24,800 lbf)|
|Specific impulse||462 seconds (4.53 km/s)|
|Burn time||1125 seconds|
|Second Stage (Block IB, Block II) - Exploration Upper Stage|
|Thrust||440 kN (99,000 lbf)|
The Space Launch System (SLS) is a United States Space Shuttle-derived heavy launch vehicle being designed by NASA. It follows the cancellation of the Constellation program, and is to replace the retired Space Shuttle. The NASA Authorization Act of 2010 envisions the transformation of the Constellation program's 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 is to lift a payload of 70 metric tons to low Earth orbit (LEO), which will be increased with the debut of Block IB and the Exploration Upper Stage. Block II will replace the initial Shuttle-derived boosters with advanced boosters and is planned to have a LEO capability of more than 130 metric tons to meet the congressional requirement; this would make the SLS the most capable heavy lift vehicle ever built.
These upgrades will allow the SLS to lift astronauts and hardware to various beyond-LEO destinations: on a circumlunar trajectory as part of Exploration Mission 1 with Block I, to a near-Earth asteroid in Exploration Mission 2 with Block IB, and to Mars with Block II. The SLS will launch the Orion Crew and Service Module and may support trips to the International Space Station if necessary. SLS will use the ground operations and launch facilities at NASA's Kennedy Space Center, Florida.
Design and development 1
- Core stage 1.1
- Shuttle-derived solid rocket boosters 1.2.1
- Advanced boosters 1.2.2
Upper stage 1.3
- Confirmed upper stages 1.3.1
- Potential upper stages 1.3.2
- Interplanetary stage 1.3.3
- Assembled rocket 1.4
- Program costs 1.5
- Fabrication 1.6
- Alternatives 2
- Proposed missions and schedule 3
- See also 4
- References 5
- External links 6
Design and development
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. Four versions of the launch vehicle have been planned at various times – Blocks 0, I, IA, IB 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 and Block IA 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.
In 2011, NASA announced that development of the Orion spacecraft from the Constellation program will continue as the Multi-Purpose Crew Vehicle (MPCV) to be flown on SLS.
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 four RS-25 engines.
- Block 0 was an initial planning baseline version, from Shuttle components, using an 8.4 meter core stage and three RS-25D engines. However, NASA managers preferred designing the SLS core stage to use four RS-25 engines, skipping over the Block 0 configuration, as it would remove the need to substantially redesign the core stage to accommodate an extra engine.
- Block I and IB: 8.4 meter core with four RS-25D/E engines.
- Block II: Initially planned to use five RS-25D/E engines, Block II is now expected to use four engines like Block I and IB.
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.
Shuttle-derived solid rocket boosters
Blocks I and IB of the SLS will use modified Space Shuttle Solid Rocket Boosters (SRBs), extended from four segments to five segments. Unlike the Space Shuttle boosters, these will not be recovered and will sink into the Atlantic Ocean downrange. 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. 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.
NASA will eventually switch from Shuttle-derived five-segment SRBs to upgraded boosters These may be of either the solid rocket or liquid rocket booster type. NASA originally planned to incorporate these advanced boosters in the Block IA configuration of SLS, but this was superseded by Block IB, which will continue to use five-segment SRBs combined with a new upper stage, after it was determined that the Block IA configuration would result in high acceleration which would be unsuitable for Orion and could result in a costly redesign of the Block I core. Prior to the selection of Block IB, NASA intended to begin the Advanced Booster Competition, which would have selected an advanced booster in 2015. Though NASA is no longer planning on selecting new boosters for the first flights of SLS, competitors for the SLS Block II advanced booster include:
- Aerojet, in partnership with Teledyne Brown, with a domestic version of an uprated Soviet NK-33 LOX/RP-1 engine, an engine derived from the NK-15 initially 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 would be powered by eight AJ-26-500 engines, or four AJ-1E6 engines 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, 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 proposed 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 would achieve no more than 113 t to low earth orbit with the current core stage with four RS-25 engines.
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 upper stage for the interim SLS Block I is designated the Interim Cryogenic Propulsion Stage and uses a single RL10 engine. The SLS Block IB's 2nd stage is designated the Exploration Upper Stage (EUS) and uses four RL10 engines. Prior to the selection of the EUS, NASA considered the Earth Departure Stage, a second stage powered by two or three J-2X engines, which has been dropped in favor of the RL10 powered EUS.
Confirmed upper stages
- Block I, scheduled to fly only Exploration Mission 1 (EM-1) by November 2018, will use a modified Delta IV 5 meter Delta Cryogenic Second Stage (DCSS), referred to as the Interim Cryogenic Propulsion Stage (ICPS). This stage will be powered by a single RL10B-2. SLS will be capable of lifting 70 metric tons 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 along with the Orion crew capsule, where the stage will perform an orbital insertion burn and then a translunar injection burn to send the uncrewed Orion on a circumlunar excursion.
- Block IB's second stage, scheduled to debut on Exploration Mission 2 (EM-2), will use the 8.4 meter Exploration Upper Stage (EUS), previously named the Dual Use Upper Stage (DUUS), powered by four RL10 engines. The EUS is to 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. An analysis by NASA and Boeing prior to the selection of the EUS indicated an upper stage with four RL10 engines would be capable of lifting about 93 metric tons to orbit with an upper stage propellant load of 231,000 lb (105,000 kg). The EUS design calls for a propellant load of up to 285,000 lb (129,000 kg).
- Block II, not expected until the 2030s, would combine the Block IB EUS with advanced boosters and be capable of placing more than 130 metric tons into LEO or up to 155 metric tons into LEO with the liquid booster designs.
Potential upper stages
Prior to the selection of the EUS for Block IB, 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 I and ICPS, which will carry 27.1 metric tons. These options are the following:
- Block I SLS without an upper stage would be capable of delivering 70 t to low earth orbit (LEO), and, using the ICPS, 20.2 t to Trans-Mars injection (TMI) and 2.9 t to Europa.
- A 4 engine RL10 upper stage could deliver 93.1 t to LEO, 31.7 t to TMI and 8.1 t to Europa.
- A 2 engine MB60 (an engine comparable to the RL60) upper stage could deliver 97 t to LEO, 32.6 t to TMI and 8.5 t to Europa.
- A single engine J-2X upper stage, with higher thrust than other options, could deliver 105.2 t to LEO, but the lower specific impulse of the J-2X would decrease its beyond-LEO capability to 31.6 t to TMI and 7.1 to Europa.
Robotic exploration missions to Jupiter's water ice moon - Europa, are increasingly seen as well suited to the lift capabilities of the Block IB SLS.
An additional beyond LEO engine for interplanetary travel from Earth orbit to Mars orbit, and back, is being studied as of 2013 at Marshall Space Flight Center with a focus on nuclear thermal rocket (NTR) engines. In historical ground testing, NTRs proved to be at least twice as efficient as the most advanced chemical engines, allowing quicker transfer time and increased cargo capacity. The shorter flight duration, estimated at 3-4 months with NTR engines, compared to 8-9 months using chemical engines, would reduce crew exposure to potentially harmful and difficult to shield cosmic rays. NTR engines, such as the Pewee of Project Rover, were selected in the Mars Design Reference Architecture (DRA).
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.
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, 3 manned), 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, the cost for a Saturn V launch was US$185 million in 1969 dollars, which is roughly US$1.2 billion in 2014 dollars.
On July 24, 2014, Government Accountability Office audit predicted that SLS will not launch by the end of 2017 as originally planned since NASA is not receiving sufficient funding.
In mid-November 2014, construction began on the first SLS using the new welding system at NASA's Michoud Assembly Facility, where major rocket parts will be assembled.
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- Ares V - cargo vehicle design for the Constellation Program of the 2000s.
- Comparison of orbital launchers families
- Comparison of orbital launch systems
- DIRECT - Ares V competitor, but with smaller payload capacity, based on Jupiter (rocket family).
- Energia - comparable vehicle to SLS Block I for Low Earth Orbit excursions.
- Exploration of Mars
- Manned mission to Mars
- Morpheus Lander - rocket powered lander designed to use propellants manufactured on Mars.
- Nautilus-X - proposed deep space habitat module.
- Saturn MLV - modified super heavy lift Saturn V of 1960s, designed for Mars missions by 1980s.
- Saturn V ELV - enlarged Saturn V design concept of 1960s, with strapon Titan IV solid rocket boosters.
- Saturn V-3 - upgraded Saturn V design concept of 1960s, using F-1A 1st stage engines & HG-3 2nd stage engines.
- Shuttle-Derived Heavy Lift Launch Vehicle - concept vehicle with a lower lift capability than Saturn V.
- Magnum (rocket) - a 1990s concept with a similar lift capability to Block I and Block IB but lower than the Saturn V.
- Space Exploration Vehicle
- Space policy of the Barack Obama administration
- Space Shuttle successors
|SLS-1/EM-1||By November 2018||Block I||Send uncrewed Orion/MPCV on trip around the Moon.|
|SLS-2/EM-2||2021||Block IB||Send the Orion (spacecraft) with four crew members to an asteroid that had been robotically captured and placed in lunar orbit two years in advance.|
Other proposed missions
- 2024+ Single Shot MSR on SLS, a crewed flight with a telerobotic Mars Sample Return (MSR) mission proposed by NASA's Mars Program Planning Group. The time frame suggests SLS-5, a 105 t Block 1A rocket to deliver an Orion capsule, SEP robotic vehicle, and Mars Ascent Vehicle (MAV). “Sample canister could be captured, inspected, encased and retrieved tele-robotically. Robot brings sample back and rendezvous with a crew vehicle." The mission may also include a “Possible Mars SEP (Solar Electric Power/Propulsion) Orbiter”.
- Potential sample return missions to Europa and Enceladus have also been noted.
- Deep Space Habitat (DSH), NASA's planned usage of spare ISS hardware, experience, and modules for future missions to asteroids, Earth-Moon Lagrangian point and Mars.
- Skylab II, proposal by Brand Griffin, an engineer with Gray Research Inc working with NASA Marshall, to use the upper stage hydrogen tank from SLS to build a 21st-century version of Skylab for future NASA missions to asteroids, Earth-Moon Lagrangian point-2 (EML2) and Mars.
- SLS DoD Missions, the HLV will be made available for Department of Defense and other US Government agencies to launch military or classified missions.
- Commercial payloads, such as the Bigelow Commercial Space Stations have also been referenced.
- Additionally “Secondary Payloads” mounted on SLS via an Encapsulated Secondary Payload Adapter (ESPA) ring could also be launched in conjunction with a "primary passenger" to maximize payloads.
- Monolithic telescope mission, SLS has been proposed by Boeing as a launch vehicle for the ATLAST Space Telescope. This could be an 8 m monolithic telescope or a 16 m deployable telescope at Earth-Sun L2.
- Solar probe mission, SLS has been proposed by Boeing as a launch vehicle for Solar Probe 2. This probe would be placed in a low perihelion orbit to investigate corona heating and solar wind acceleration to provide forecasting of solar radiation events.
- Uranus mission, SLS has been proposed by Boeing as a launch vehicle for a Uranian probe. The rocket would “Deliver a small payload into orbit around Uranus and a shallow probe into the planet’s atmosphere.” The mission would study the Uranian atmosphere, magnetic and thermal characteristics, gravitational harmonics as well as do flybys of Uranian moons.
Strategic Timeframe DRMs
- GEO mission – a dual launch mission separated by 180 days to Geostationary Orbit. The first launch would comprise an SLS with a CPS and cargo hauler, the second an SLS with a CPS and Orion MPCV. Both launches would have a mass of about 110 t.
A set of lunar missions enabled in the early 2020s ranging from Earth Moon Lagrangian point-1 (EML-1) and low lunar orbit (LLO) to a lunar surface mission. These missions would lead to a lunar base combining commercial and international aspects.
- The first two missions would be single launches of SLS with a CPS and Orion MPCV to EML-1 or LLO and would have a mass of 90 t and 97.5 t respectively. The LLO mission is a crewed twelve day mission with three in Lunar orbit. Its current description is “Low Lunar Orbit (LLO): Crewed mission to LLO. Expected drivers include: SLS and CPS performance, MPCV re-entry speed, and LLO environment for MPCV”.
- The lunar surface mission set for the late 2020s would be a dual launch separated by 120 days. This would be a nineteen-day mission with seven days on the Moon's surface. The first launch would comprise an SLS with a CPS and lunar lander, the second an SLS with a CPS and Orion MPCV. Both would enter LLO for lunar orbit rendezvous prior to landing at equatorial or polar sites on the Moon. Launches would have masses of about 130 t and 108 t, respectively. Its current description is “Lunar Surface Sortie (LSS): Lands four crew members on the surface of the Moon in the equatorial or Polar Regions and returns them to Earth,” “Expected drivers include: MPCV operations in LLO environment, MPCV uncrewed ops phase, MPCV delta V requirements, RPOD (Rendezvous, Proximity Operations and Docking), MPCV number of habitable days.”
- Five Near Earth Asteroid (NEA) missions ranging from “Minimum” to “Full” capability are being studied. Among these are two NASA Near Earth Object (NEO) missions in 2026. A 155-day mission to NEO 1999 AO10, a 304-day mission to NEO 2001 GP2, a 490-day mission to a Potentially Hazardous Asteroid such as 2000 SG344, utilizing two Block IA/B SLS vehicles, and a Boeing proposed NEO mission to NEA 2008 EV5 in 2024. The latter would start from the proposed Earth-Moon L2 based Exploration Gateway Platform. Utilising a SLS third stage the trip would take about 100 days to arrive at the asteroid, 30 days for exploration, and a 235-day return trip to Earth.
- Forward Work Martian Moon Phobos/Deimos, a crewed Flexible Path mission to one of the Martian moons. It would include 40 days in the vicinity of Mars and a return Venus flyby.
- Forward Work Mars Landing, a crewed mission, with four to six astronauts, to a semi-permanent habitat for at least 540 days on the surface of the red planet in 2033 or 2045. The mission would include in-orbit assembly, with the launch of seven SLS block II heavy lift vehicles (HLVs) with a requirement of each being able to deliver 140 metric tons to low earth orbit (LEO). The seven HLV payloads, three of which would contain nuclear propulsion modules, would be assembled in LEO into three separate vehicles for the journey to Mars; one cargo In-Situ Resource Utilization Mars Lander Vehicle (MLV) created from two HLV payloads, one Habitat MLV created from two HLV payloads and a crewed Mars Transfer Vehicle (MTV), known as "Copernicus", assembled from three HLV payloads launched a number of months later. Nuclear Thermal Rocket engines such as the Pewee of Project Rover were selected in the Mars Design Reference Architecture (DRA) study as they met mission requirements being the preferred propulsion option because it is proven technology, has higher performance, lower launch mass, creates a versatile vehicle design, offers simple assembly, and has growth potential.
- ISS Back-Up Crew Delivery – a single launch mission of up to four astronauts via a Block 1 SLS/Orion-MPCV without an Interim Cryogenic Propulsion Stage (ICPS) to the International Space Station (ISS) if the Commercial Crew Development program does not come to fruition. This potential mission mandated by the NASA Authorization Act of 2010 is deemed undesirable since the 70 t SLS and BEO Orion would be overpriced and overpowered for the mission requirements. Its current description is “delivers crew members and cargo to ISS if other vehicles are unable to perform that function. Mission length 216 mission days. 6 crewed days. Up to 210 days at the ISS.”
Tactical Timeframe DRMs
- BEO Uncrewed Lunar Fly-by – Exploration Mission 1 (EM-1), a reclassification of SLS-1, is a single launch mission of a Block I SLS with ICPS and a Block 1 Orion MPCV (Multi-Purpose Crew Vehicle), with a destination of 70,000 km past the lunar surface. Its current description is “Uncrewed Lunar Flyby: Uncrewed mission Beyond Earth Orbit (BEO) to test critical mission events and demonstrate performance in relevant environments. Expected drivers include: SLS and ICPS performance, MPCV environments, MPCV re-entry speed, and BEO operations.”
- BEO Crewed Lunar Orbit – Exploration Mission 2 (EM-2), a reclassification of SLS-2, is a single launch mission of a Block I SLS with ICPS and lunar Block 1 Orion MPCV with a liftoff mass around 68.8 t with SLS’ Payload Insertion of 50.7 t, which would be a ten to fourteen day mission with a crew of four astronauts who would spend four days in lunar orbit. Its current description is “Crewed mission to enter lunar orbit, test critical mission events, and perform operations in relevant environments”. The destination for EM-2, as of 2013, is regarded to be a captured asteroid in lunar orbit, to be conducted by no later than 2021.
Some of the currently proposed NASA Design Reference Missions (DRM) and others include:
Proposed missions and schedule
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".
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. Based upon extrapolations of increased payload lift capabilities from past experience with SpaceX's Falcon launch vehicles, SpaceX CEO Elon Musk guaranteed that his company could build the conceptual Falcon XX, a 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-funded MCT launch vehicle, powered by nine Raptor engines, has also been proposed for lofting very large payloads from Earth in the 2020s.
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.