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NASA's Space Launch System
by Ed Kyle, Updated 9/16/2010

sls1s.jpg (8636 bytes)Space Launch System Candidates

On August 5, 2010, the United States Senate passed its "NASA Authorization Act of 2010" which would direct the Agency to build a new "Space Launch System" (SLS) - a big new Shuttle-Derived rocket that could eventually be used to boost astronauts far beyond low Earth orbit (LEO).  By mid-September the bill remained just a bill because the U.S. House of Representatives had yet to take up the issue, though it was expected to do so soon.  The House originally appeared disposed to fight for continuing NASA's essentially-cancelled Constellation Program, with its Ares 1 and Ares 5 rockets.  Whether the conflicting approaches could be reconciled, and whether sufficient funding would be provided for either approach, remained open questions. 

For its part, the White House signaled support of the Senate bill, a significant shift from President Obama's February 1, 2010 NASA funding proposal.  That Groundhog Day proposal cancelled Constellation, Orion, and Ares outright.  It directed the Agency to commercially outsource its International Space Station crew carrying missions.  Human exploration beyond low earth orbit was essentially shelved, with decisions on non-International Space Station human missions put off for at least five years.  Lunar landings were off the table altogether, and the end of Shuttle technology and infrastructure appeared at hand. 

The Senate and House rebelled against the President.  Hearings were held.  Former astronauts denounced the commercial plan and decried abandonment of Shuttle technology.  Op-ed arguments raged.   Then, as summer waned, pen was put to paper and deals were struck.  U.S. Senator Nelson, from Florida, who once rode Shuttle Columbia to orbit, played a major role in the process.

When it came to Space Launch System, the Senate bill was quite specific.  It called for SLS to be a Shuttle follow-on project that would use existing contracts to the extent possible.  The bill called for continued ground testing of the Shuttle's solid rocket motors, essentially directing their use by SLS.   It seemed clear that Senators envisioned Space Shuttle Main Engines (SSMEs) powering an "in-line" core stage boosted by SRBs.  The bill also called for use of existing Shuttle launch infrastructure to the greatest extent possible, which clearly meant Kennedy Space Center's Launch Complex 39.  It even directed NASA to complete the A-3 test stand at Stennis Space Center, and told the Agency to complete the job by September 30, 2013.  Since the A-3 stand was designed to test the new J-2X upper stage engine, it seemed possible that even J-2X still had a heartbeat, at least in chambers of the U.S. Senate.

Dead, apparently, was the big kerosene fueled, all-liquid super heavy rocket discussed in recent months, along with its U.S.-built engine.  Dead also, by all appearances, were advanced "Phase 2" EELV concepts.  Quietly forgotten was any lunar return.  The new goal would be only "Beyond Earth Orbit".   

There was more.  SLS would be developed in two phases.   Phase One would not include an upper stage, but would be able to lift 63 to 90.7 metric tons (tonnes) to LEO.  Phase Two would add an upper stage, and probably other upgrades, to increase LEO payload to at least 118 tonnes.  SLS would be designed to serve as an emergency back up for International Space Station commercial cargo providers.  SLS would also be designed to carry humans in a new "Multi Purpose Crew Vehicle" (MPCV), which would be operational by December 31, 2016.   

Most dramatic was the proposed timeline.  The Senate would have NASA add one additional Shuttle mission, keeping Kennedy Space Center alive well into 2011.  Meanwhile, rather than wait five years before deciding whether or not to develop a heavy lifter, NASA would begin work on SLS immediately. 

"4/3" Versus ""5/5"

What would SLS look like?  It isn't difficult to speculate.   NASA has studied Shuttle-derived heavy lift concepts for more than three decades

An "inline" core built using Shuttle External Tank (ET) tooling, powered by three Space Shuttle Main Engines (SSMEs), boosted by a pair of four segment reusable solid rocket boosters (RSRBs), has been shown, repeatedly during studies, capable of lifting more than 75 tonnes directly to LEO, easily meeting the Senate's initial 70 tonne SLS goal.  Sometimes described as a "4/3" model for its use of four segment boosters and three SSMEs, such a rocket would stand 90 or more meters tall, weigh more than 2,060 tonnes, and produce more than 3,060 tonnes (6.7 million pounds) of thrust at liftoff - comparable to a Shuttle stack.   

The existing active SSME (RS-25D) 15-engine inventory could support three or four "4/3" launches beginning in 2017.  A follow-on, expendable RS-25E variant could subsequently enter service, but not until 2019 or so, after a six-year development program had run its course.  Development costs for a "4/3" with no upper stage were projected to total about $11-15 billion.  

During the summer, NASA's Human Exploration Framework Team (HEFT) studied SLS and recommended development of a bigger, more powerful "5/5" alternative.  The "5/5" would use two five-segment solid rocket boosters to lift a core powered by five RS-25 engines.  It would stand more than 100 meters, weigh more than 2,700 tonnes, and develop roughly 3,300 tonnes (about 7.5 million pounds) of thrust at liftoff.  "5/5" should be able to lift more than 100 tonnes to LEO.  The "5/5", a design similar to the original "classic" Ares 5 design of 2005,  would take longer (six months to a year) and cost more (perhaps $12-18 billion) to develop than "4/3". 

Several years after either "4/3" or "5/5" entered service, SLS could be augmented with a cryogenic upper stage.  The stage would serve both to perform the final burn to reach LEO, and to provide beyond-LEO propulsion.  The HEFT study left open the question of engine-type.  J-2X could do the job.  So could a "RS-25E" engine based on SSME or a cluster of RL-10 derived "Next Generation Engines".  A "4/3" upper stage could boost 85 tonnes to LEO or 36 tonnes to escape velocity.  Atop the larger "5/5" rocket, a cryogenic stage would be able to lift 118 tonnes or more to LEO and more than 50 tonnes into deep space.  The cryogenic stage would cost more than $3 billion to develop.

The Fixed Cost Problem

Fixed costs present a difficult challenge for SLS.  The HEFT work projected annual average budgets of nearly $3.6 billion to support a "5/5" SLS during its first two decades.  The costs included an average of $2.6 billion annually for the launch vehicle, $230 million per year for the upper stage, and $800 million per year for ground operations.  SLS would have to fly twice per year to meet its projected $1.8 billion per-flight cost, but HEFT did not foresee enough missions to support such a flight rate. 

Instead, HEFT evaluated a "Design Reference Mission" identified as "DRM-4" that would only require a total of nine "5/5" launches during a 21 year period.  The flights would initially be used to validate the rocket and the human spacecraft that it would launch.  Then, beginning in 2029, three of the "5/5" rockets would be used to launch a human mission to a "Near Earth Object" asteroid.  The total cost for the heavy-lift rockets, upper stages, and ground operations would be $75.6  billion, or nearly $8.4 billion per launch.

Clearly, a rocket that cost $8.4 billion to fly would be a non-starter politically.  The HEFT study was merely an evaluation of a single mission type, but it showed that NASA will either need to conjure more missions for SLS, to support a steady annual flight rate, or that it will have to slash SLS fixed costs.   The same HEFT study projected costs for "commercial" launches to be less than $500 million per flight.  NASA could buy seven 25 tonne to LEO "commercial" launches each year for the cost of an SLS program.  A "5/5" would have to fly at least twice per year, year after year, to offer monetary savings and performance benefits compared to the "commercial" EELV-Heavy-class option.

Whatever decision is made in Congress and at the White House in coming weeks will decide NASA's future for decades.


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SPACE LAUNCH REPORT
by
Ed Kyle