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NASA's Space Launch System
by Ed Kyle, Updated October 16, 2016

sls-block1-1bs.jpg (21856 bytes)SLS Block 1 and Block 1B as of 2016

President Obama's February 1, 2010 cancellation of NASA's Constellation Program left the Agency with no post-Shuttle human launch capability.  The U.S. Senate rebelled against the President, passing, on August 5, 2010, the "NASA Authorization Act of 2010" which directed the Agency to build a new "Space Launch System" (SLS), a big new rocket for beyond low Earth orbit (LEO) exploration.  SLS was directed to be Shuttle-Derived, using existing Shuttle or Constellation/Ares contracts to the extent possible. 

The Senate's SLS was to be developed in two phases.   Phase One would not include an upper stage, but would be able to lift 70 to 100 tons (or, by some interpretations, metric tons or tonnes) to LEO.  Phase Two would add an upper stage, and probably other upgrades, to increase LEO payload to at least 130 tons (or tonnes).  SLS would serve as a back up for International Space Station commercial cargo and crew, carrying humans in a new "Multi Purpose Crew Vehicle" (MPCV) derived from the cancelled Orion spacecraft.   SLS/MPCV was to be operational by December 31, 2016.   


NASA had begun to study alternative heavy lift launch vehicles before Constellation was cancelled, in response to the 2009 Augustine Committee.  The "Heavy Lift Launch Vehicle" study published on May 20, 2010 compared Shuttle Derived designs against two kerosene-fueled alternatives, one of which looked like a modernized Saturn 5.  It contemplated dropping the Ares 5 RS-68 engines for more efficient SSME-derived RS-25 engines - a change that would allow use of a smaller ET-derived 8.4 meter diameter tank rather than the 10 meter tank planned for Ares 5. 

After the Constellation cancellation, NASA's Human Exploration Framework Team (HEFT) reconsidered launch vehicle plans as part of its broader program study.  By September 2010, HEFT recommended a baseline heavy lift design, identified as "5/5", that used two five-segment solid rocket boosters to lift an 8.4 meter diameter External Tank (ET)-derived core powered by five SSME-derived RS-25E engines.  Able to lift more than 100 tonnes to LEO without an upper stage, it would stand more than 100 meters, weigh more than 2,650 tonnes, and develop roughly 3,300 tonnes (about 7.5 million pounds) of thrust at liftoff.  This baseline SLS design was similar to the original "classic" Ares 5 design of 2005.

sls1s.jpg (8636 bytes)HEFT Alternatives

HEFT also contemplated, but did not recommend, an "interim" SLS named "4/3" that would have used four-segment boosters and a shorter core powered by only three RS-25 engines to lift 70 tonnes to LEO.  Its initial flights would actually use SSME engines pulled from retired Shuttle orbiters.  

Adding an upper stage later could increase "5/5" payloads to 130 tonnes LEO or more than 50 tonnes into deep space.  HEFT left open the question of upper stage engine type.    J-2X, an "RS-25E" based engine, or a cluster of RL-10 derived "Next Generation Engines" were contemplated.   

The Requirements Analysis Cycle

Following completion of the HEFT study, NASA initiated the "Requirements Analysis Cycle (RAC) studies, during which four teams of engineers analyzed alternative SLS designs.  Team 1 looked at Shuttle-Derived options.  Team 2 studied kerosene/LOX fueled alternatives.  Team 3 considered modular rocket designs using existing kerosene/LOX hardware.  Team 4 focused on cost reduction methods.  RAC results were provided in a March 2011 NASA briefing.

slsrac1s.jpg (23159 bytes)SLS RAC Team 1 Shuttle Derived Alternatives

Team 1 presented four growth "Blocks" in its results.  "Block 0" was essentially HEFT's "4/3", but with the standard Shuttle four segment solid rocket boosters replaced by stacking all but the middle segment of a five-segment booster.  The core was powered by three RS-25 engines, either existing "D" engines or new "E" engines.   "Block 0" could lift 70 tonnes to LEO, could orbit astronauts in an MPCV, and aimed for a 2016 operational date.                 

"Block 1" would use a pair of five-segment boosters and a longer core powered by five RS-25E engines.  This operational rocket, essentially the "5/5", would lift 100 tonnes to LEO by 2019.

An upper stage would be added to "Block 1" by 2022 to create "Block 2".  This upper stage would be powered by a single air-start/restartable RS-25E variant.  This would be the ultimate 130 tonne to LEO SLS required to meet the Congressional requirements.

"Block 3", projected to fly in 2026, would use improved solid rocket motors to lift up to 150 tonnes to LEO.  HTPB propellant in composite cases, replacing replace PBAN propellant in steel cases, would provide the performance improvement.

SLS Final Choices

NASA Administrator Bolden decided to follow the HEFT recommendations, dropping the RAC 1 "Block 0" approach.  This avoided having to develop more than one core stage and more than one solid rocket motor design.  The 8.4 meter diameter stage would be used from the outset, for 1.5 stage Block 1, and would continue to be used for the 2.5 stage Block 2 once its upper stage was developed.   Congress forced another change when it required competition for SLS boosters right from the start, with ATK's five segment PBAN boosters, developed for Ares 1, only used for the first few Block 1 flights.  The competition would be open to both solid and liquid booster designs.  New boosters added to the Block 1 core would create a "Block 1A" SLS. 

sls-evolveds.jpg (14314 bytes)SLS Initial and Evolved Designs

NASA formally announced its SLS plans on September 14, 2011.  "Block 1" SLS was, at the time, expected to use "at least three" RS-25D/E core engines, but by February 2012 early design reviews had set the number of core engines to four.   

SLS Block 1 would launch MPCV spacecraft on developmental missions, using an "Interim Cryogenic Propulsion Stage" (ICPS) for in-space propulsion.  An initial "SLS-1" (later renamed "EM-1") unmanned test flight was projected for the end of 2017.  The second "SLS-2"("EM-2") launch would carry crew, but would not occur until the end of 2021.  The flights would use RS-25D engines from the existing Space Shuttle Main Engine (SSME) inventory and existing solid rocket booster casings.

ICPS, considered part of the SLS payload rather than the vehicle itself, would be inserted with a 24,224 kg MPCV into a 1,800 x -93 km suborbital trajectory by SLS.  The stage would then perform up to three burns to provide more than 3,050 m/s delta-V during its mission.

The ICPS description aligned closely with the capabilities of the existing Delta 4 Heavy upper stage.  Such a stage would be able to boost MPCV out of LEO on a circumlunar, or other deep space, mission.  The fact that Delta 4 Heavy was expected to be used for one or more MPCV boilerplate orbital test flights made the choice logical.

By February 2012, NASA had assigned "10000" as the "Vehicle Configuration Reference" (VCR) for SLS Block 1.

slsBlk1s.jpg (16390 bytes)Block 1 SLS on Launch Pad.  Actual SLS Vehicles will not be Painted White.

Block 1 would only fly twice, to be replaced by "SLS Block 1A", which would fly no earlier than 2023.  A free-for-all booster competition would lead to the development of Block 1A.  Potential boosters included Advanced Composite Booster (ACB) solids using either PBAN or HTPB propellants or kerosene/LOX Liquid Rocket Boosters (LRB) with new or existing engines.  Liquid boosters would be limited to about 5 meters diameter due to width restrictions of the VAB doors.  Either booster type would have to support the same load path as the five segment booster, which transferred most of its force through its upper section at the thrust beam that passed through the core intertank section.

During mid-2011, Aerojet and Teledyne Brown announced an alliance to develop a 227 tonne thrust-class engine for the booster competition.  The "AJ-500" engine would be based, in part, on the Russian NK-33/AJ-26 engine used by Antares.  Other competitors for the booster contract competition were likely to appear. 

Block 1A would lift at least 105 tonnes to LEO.  Its payloads could include ICPS or full-scale "CPS" stages.   A full-scale CPS might be able to boost Saturn 5-class payloads (about 45 tonnes) beyond LEO.  CPS was not defined, since its missions were not yet defined.   Proposed CPS designs included EELV-evolved stages powered by RL10 or RL10 follow-on "Affordable Upper Stage Engine" (AUSE) clusters and an Ares I Upper Stage derivative powered by a single J-2X.   Proposed propellant loadings ranged from 50 to 130 tonnes, with the heavier stages burning first to reach LEO and restarting to power payload beyond LEO. 

NASA assigned the following VCR numbers to the Block 1A concepts.  Only two of these four vehicles, at most, would be developed.

11000 Block 1 core with two ACB topped by a cargo payload fairing
12000 Block 1 core with two LRB topped by a cargo payload fairing
13000 Block 1 core with two ACB topped by CPS/Orion/MPCV
14000 Block 1 core with two LRB topped by CPS/Orion/MPCV

SLS Block 2, which would not fly until after 2030 at the earliest, would be a Block 1A, potentially with a fifth RS-25E engine added to the core, topped by a new full-scale Large Upper Stage (LUS).  LUS would be powered by one to three J-2X engines, but neither engine choice nor number of engines was fixed.  Also not fixed was the basic stage construction or its propellant loading.  Concepts called for LUS, which would only be used to reach LEO, to hold as much as 210 tonnes of propellant when three J-2X engines were used..

Block 2 would be able to lift more than 130 tonnes to LEO or about half as much to escape velocity.  It would be a "monster rocket", weighing more than 2,900 tonnes, standing 110-120 meters, and producing more than 4,100 tonnes of liftoff thrust. 

NASA's VCR numbers for Block 2 were 21000-24000, aligned with the Block 1A numbers.

slsBlk1As.jpg (3436 bytes)Graphic of SLS Block 1 Launch Shows RS-25 Core Engine Pattern

NASA would budget about $18 billion to develop and fly the SLS-1/EM-1 development mission.  This would include about $10 billion to develop "Block 1", $6 billion for Orion/MPCV, and $2 billion to create launch facilities and other infrastructure.  SLS would fly from Kennedy Space Center, where it would be stacked in the Vehicle Assembly Building on a new mobile launch platform and launched from Launch Complex 39B.   Program costs were expected to average $3 billion per year, but that budget would probably only support one SLS/Orion flight every two years. 

NASA's SLS mission was yet to be determined.  The Agency described a potential human mission to an asteroid by 2025, but no specific asteroid or mission plan had been selected.  The Agency mentioned that SLS could lay the groundwork for future human missions to Mars, but such missions appeared to be decades distant at best.

Block 2 SLS was similar to Vehicle 27.3, the recommended Cargo Launch Vehicle from NASA's 2005 Exploration Architecture Systems Study (ESAS).  27.3 was capable of lifting 146 tonnes to LEO or 60 tonnes to a trans-lunar trajectory when it used HTPB boosters and a twin-J-2S powered upper stage loaded with 207 tonnes of propellant.  Subsequent NASA studies postulated even heavier propellant loads.

j2xe10001s.jpg (17755 bytes)J-2X Testing

J-2X Engine E10001 at Stennis

SLS would benefit from Ares 1 program development efforts.  During 2011, for example, the first J-2X engine, E10001, was assembled and test fired at Stennis Space Center in Mississippi.  The engine was erected into the A-2 test stand during June.  It performed an initial "burp test" on July 14 and reached main stage during a 3.7 second test on July 26.   The engine fired for 7 seconds in early August before attempting a 50 second burn mid-month, but the latter test had to be aborted after 32 seconds.  Residual propellants created a "back-fire" shortly after shutdown that caused some damage to the engine.  The engine was removed, repaired, and by early October returned to the stand.  It performed a 40 second test and then, by the end of November a 500 second "mission duration" test. 

By year's end, a second J-2X Power Pack assembly had been installed in the A-1 test stand in preparation for more extensive development testing using only turbopumps and engine valves.  It performed firings lasting up to 22.5 minutes in an extensive series of tests during 2012.  Power Pack testing ended in December 2012.

In 2013, Engine 10002 was on test stands A2 and A1 for 13 firings during February-September of 2013.  One test lasted 885 seconds while total firing time for the engine passed 5,200 seconds.  Engine 10003 began testing that November, performing a dozen firings through April 2014 that totalled 3,760 seconds.  Meanwhile, Engine 10004 was in fabrication. 

It was at this point that the J-2X program halted as NASA's plans shifted and as preparations began for RS-25 testing on the same stands.. 

LUS1.jpg (17508 bytes)Block 1B

Boeing Large Upper Stage Proposal, 2013

During 2013, Boeing provided details of its proposed "Large Upper Stage" alternatives in an American Institute of Aeronautics and Astronautics (AIAA) paper authored by Benjamin Donahue and Sheldon Sigmon of Boeing's Huntsville Exploration Launch Systems group.  The paper showed that a stage weighing just under 120 tonnes, powered by four RL10-C2 engines would be able to boost more payload on trans-lunar and trans-Mars missions (39.1 tonnes and 31.7 tonnes) than a similar stage powered by one J-2X engine.  The stage would use an 8.4 meter diameter LH2 tank and a 5.5 meter diameter LOX tank, with the tanks joined by a composite x-frame.

The 4xRL10 stage design seemed to be gaining favor as time passed, potentially consigning J-2X to the shelf.  Its adoption could also mothball plans for the Advanced Booster competition, saving money that would have been needed for that development effort.

NASA's choice, apparently made during April 2014, created a new version named "SLS Block 1B" that would keep the Block 1 solid boosters and core, but replace the ICPS upper stage with a new, larger Exploration Upper Stage (EUS).  EUS would be powered by four RL10-C engines and would weigh about 140 tonnes at liftoff.  SLS Block 1B would be able to lift at least 105 tonnes to low earth orbit or more than 31 tonnes on a trans-Mars trajectory.  The latter payload number increased to as much as 33 tonnes as early development proceeded.

An eventual "Block 2B" would use EUS, but upgrade the boosters.  As time passed, NASA dropped the "B" in favor of SLS "Block 2", even though this "Block 2" differed from original plans.

e0525b.jpg (13174 bytes)Development Progress

E0525 January 9, 2015 Test

By early 2014, physical signs of SLS development progress were beginning to appear at Michoud and Kennedy Space Center, as reported in the Space Launch Report story "Progress on NASA's Space Launch System and Orion". 

On January 9, 2015, RS-25 engine E0525 peformed its first successful 500 second test on the A-1 test stand at NASA's Stennis Space Center near Bay St. Louis, Mississippi.  The test provided engineers with data on the engine controller and on inlet pressure conditions relative to its new application on the SLS core stage.  It was the first RS-25 (SSME) hot fire test since the end of Space Shuttle testing in 2009.

The test was the culmination of a long build up.  After months of test stand modifications, E0525 was originally installed on the A-1 test stand during July, 2014.  It was subsequently removed and reinstalled on October 24, 2014.  A chill test followed a month later.  Testing was expected to continue in April, 2015 following upgrades to the high pressure cooling water system.   Eight tests, totaling 3,500 seconds, were planned for the E0252 development engine. Another development engine would follow to perform 10 tests totaling 4,500 seconds. The latter engine would use a new-design flight-type controller.

5sbdm2s.jpg (13011 bytes)Five Segment Booster DM-2 Test

By the end of 2011, ATK had performed three successful five segment booster tests at its Utah test site.  The tests took place on September 10, 2009 (DM-1), August 31, 2010 (DM-2), and September 8, 2011 (DM-3).   A precursor test of a five segment motor, assembled using an extra segment from a Shuttle four-segment motor, occurred on October 23, 2003 (ETM-3).  ATK had also performed recovery system tests for five segment motor for Ares I, but no plans were being made to recover the SLS boosters. 

In early May, 2014, NASA and ATK completed a series of structural loads tests on the SLS booster forward skirt at ATK's facility in Promontory, Utah.  The forward skirt was tested under simulated lift-off and ascent conditions before finally being tested to failure.  The final test demonstrated the structure's maximum load.

qm1.jpg (16654 bytes)QM-1

Orbital ATK performed the first SLS qualification motor (QM-1) test firing on March 11, 2015. The five-segment motor produced about 1,633 tonnes of thrust during a more than two-minute test firing at the company's Promontory, Utah test site. It was the first of two planned qualification motor tests. The test, performed with propellant heated to its maximum specified range,  included new avionics that controlled motor ignition and nozzle vectoring.

The QM-1 test was delayed by months when debonded propellant/liner/insulation was found in the aft segment after propellant was cast in early 2013. A replacement segment exhibited the same problems, which led to a long engineering investigaiton. Eventually, ATK determined that the problem was caused by use of a new type of insulation, an issue solved by revised casting processes.


sls_blk1-cdr.jpg (5890 bytes)SLS Passes Critical Design Review

SLS Block 1 Rendering Released by NASA at CDR

On October 22, 2015, NASA announced that SLS Block 1 had completed its critical design review (CDR) during July at Marshall Space Flight Center in Huntsville, Alabama. The CDR included reviews of the core stage, the solid rocket boosters, and the liquid fuel engines. By the time of the announcment, all the major components for the first SLS were entering production,

The next program steps for SLS were design certification and flight readiness review. Design certification would occur in 2017 after the launch vehicle is built, integrated, and tested. Flight readiness review would occur just before the planned 2018 flight.

By the end of October, 2015, NASA had completed the first developmental test firings of an RS-25 engine. The Orion spacecraft CDR was approaching and preparations were being made for the second SLS booster qualification test in Utah. At Michoud, structural test articles for the core and upper stages were either completed or in production.

sls-evo.jpg (15456 bytes)SLS Block 1, Block 1B Crew, Block 1B Cargo, and Block 2 Cargo Design Progression as of October 22, 2015.

The CDR press release noted that the SLS core design would not change as the launch vehicle progressed through Block 1B and Block 2 design phases in the future.  An Exploration Upper Stage (EUS), likely similar to Boeing's proposed Large Upper Stage with four RL10-C2 engines, would replace the ICPS to create Block 1B.  Advanced boosters would then replace the existing SRBs to create Block 2. 

qm2-4.jpg (17405 bytes)QM-2 

QM-2 Test Firing on June 28, 2016

Orbital-ATK performed the QM-2 five-segment motor test firing at Promontory, Utah on June 28, 2016.  This was the final booster test firing before the first SLS flight.  The booster was cooled to   40 degrees Fahrenheit before the test to qualify it to the cold end of its temperature range.  The motor produced a maximum of about 3.6 million pounds of thrust during its 126 second burn.



Space Launch System Details (Subject to Change)

  Oct 2005
LV 27.3
Oct 2005
LV 27.3 w/ EDS
SLS Block 1
2011 Baseline
SLS Block 1
with ICPS (2012 spec)
SLS Block 1B
with EUS (2014)
SLS Block 2
with EUS (2014)
Boosters (Each) 5 Segment 5 Segment 5 Segment 5 Segment 5 Segment New Boosters Liquid or Solid
ATK Composite Shown
GLOW (tonnes) 751.084 t 751.22 t 729.8 t 731.885 t 731.885 t ~793 t
Propellant Mass (tonnes) 650.751 t 650.87 t 626.10 t 631.495 t 631.495 t ~709 t
Burnout Mass (tonnes) 100.333 t 100.33 t 103.7 t 100.390 t 100.390 t ~84 t
Diameter (meters) 3.71 m 3.71 m 3.71 m 3.71 m 3.71 m 3.71 m
Height (meters) (to top of frustum) 53.87 53.87 m 53.87 m 53.87 m 53.87 m 53.87 m
Liftoff Thrust (vac. tonnes) 1,578.29 t 1,578.6 t 1,592.47 t 1,428.83 t 1,428.83 t 2,041 t
Specific Impulse (sea level/vacuum, seconds) 242 s/265.4 s 237s/265.5 s 237s/267.4 s 237s/267.4 s 237s/267.4 s ~259/286 s
Burn Time (sec) 132.5 s 132.5 s 126.6 s 128.4 s 128.4 s 110 s
Propellant HTPB HTPB PBAN PBAN PBAN HTPB (Example Case)
Core Stage 5xSSME 5xSSME 4xRS25D 4xRS25D 4xRS25D 4xRS25D
GLOW (tonnes) 1,102.33 t 1,092.0 t 1,068.3 t 1,091.4516 t 1,091.4516 t 1,091.4516 t
Usable Propellant Mass (tonnes) 1,004.71 t 1,002.5 t 978.9 t 979.4516 t 979.4516 t 979.4516 t
Burnout Mass (tonnes) 91.155 t 89.38 t 89.38 t ~112 t ~112 t ~112 t
Dry Mass (tonnes) 81.964 t 76.12 t 76.12 t ~102 t ~102 t ~102 t
Diameter (meters) 8.384 m 8.384 m 8.384 m 8.384 m 8.384 m 8.384 m
Height (meters) 64.27 m 64.27 m 62.54 m 62.54 m 62.54 m 62.54 m
Thrust (sea level/vacuum, tonnes) 948 t/1,162 t 948 t/1162 t 758.4 t/929.6 t 758.4 t/929.6 t 758.4 t/929.6 t 758.4 t/929.6 t
Specific Impulse (sea level/vacuum., seconds) 361.3 s/452.1 s 361.3 s/452.1 s 366 s/452.1 s 366 s/452.1 s 366 s/452.1 s 366 s/452.1 s
Burn Time (sec) 411.5 s 408.2 s 476 s 476 s 476 s 476 s
Interstage       4.9896 t 4.9896 t 4.9896 t
Second Stage   2xJ2S   Interim Cryogenic Propulsion Stage (Delta 4 Heavy Upper Stage Assumed) Exploration Upper Stage (EUS), 4xRL10-C3 Exploration Upper Stage (EUS), 4xRL10-C3
GLOW (tonnes)   229.78 t   31.2075 t ~143.6 t ~143.6 t
Usable Propellant Mass (tonnes)   207.69 t   26.8529 t ~129.3 t ~129.3 t
Burnout Mass (tonnes)   22.063 t   4.3546 t ~15.6 t ~15.6 t
Dry Mass (tonnes)   19.344 t   3.7649 t ~14.3 t ~14.3 t
Diameter (meters)   8.384 m   5.1 m 8.384 m 8.384 m
Height (meters) (including interstage)   22.74 m   13.7 m ~18 m ~18 m
Thrust (vac., tonnes)   249.02 t   11.2492 t 44.9 t 44.9 t
Specific Impulse (vac., seconds)   451.5 s   461.5 s 462.5 s 462.5 s
Burn Time, seconds   377 sec   1,118 s 1,090 s 1,090 s
Propellants   LOX/LH2   LOX/LH2 LOX/LH2 LOX/LH2
Payload Fairing 40.09 x 8.38 m 22.16 x 8.38 m   12.8 x 5.1 m 19.12 x 8.38 m 25 x 8.38 m
Dry Mass (tonnes) 10.6 t 5.836 t ~10.6 t 8.1647 t ~10 t 12.11 t
SLS Totals            
GLOW (tonnes)(incl payload) 2734.27 t 2900.30 t ~2,650 t ~2,500 t ~2,700 t ~2,900 t
Height (meters)(including payload) 104.36 m 109.02 m 92.3 m 97.56 m 100 m cargo
111 m crew
~111 m cargo
Height (meters) (not including payload) 64.27 m 87.01 m 64.7 m 64.7 m ~81.4 m ~81.4 m
Payload (tonnes) to 48 x 296 km x 28.5 deg 125.1 t 146.6 t >70t (~95 t likely) N/A ~105 t ~130 t
Payload (tonnes) Trans-Lunar N/A 60.6 t N/A 24.5 t ~39 t ~45 t
Payload (tonnes) Trans-Mars N/A ~49.0 t N/A 19.5 t ~32 t ~50 t


NASA's Exploration Systems Architecture Study (ESAS), Final Report, NASA-TM-2005-214062, November 2005.

"Heavy Lift Launch Vehicles with Existing Propulsion Systems", Donahue,, Boeing, AIAA 2010-2310, April 2010.

Heavy Lift Launch Vehicle Study Briefing, NASA, May 2010.

Ares Development Motor 2 Ground Test Fact Sheet, ATK, August 2010.

Human Exploration Framework Team, Briefing, September 2010.

Space Launch System Status, Briefing, September, 2010.

Cryogenic Propulsion Stage, NASA, February 2011.

ATK Advanced Booster for SLS, ATK, October 2012.

The Space Launch System Capabilities for Enabling Crewed Lunar and Mars Exploration, Boeing, October 2012.


by: Ed Kyle

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Ed Kyle