NASA's Space Launch System - Winners and Losers
by Ed Kyle, 06/17/2011

SLS Reference Design

In early July, 2011, NASA will announce its design choice for Space Launch System (SLS), the heavy lift launch vehicle specified by Congress in 2010’s NASA Authorization Act.   By most accounts, the design will differ little from NASA’s SLS “reference vehicle” design described earlier this year. 

That “Shuttle-Derived” SLS reference design used a pair of five segment PBAN boosters to lift an External Tank diameter core powered by five RS-25E (Space Shuttle Main Engine-derived) engines to lift 108.6 tonnes to a 30 x 130 nmi insertion orbit.  An additional upper stage, powered by one or two J-2X engines, could be added later in an “evolved” design to meet the 130 tonne goal.

Specific design details, such as the number of engines on the initial core stage or the number of solid booster segments, are not yet known, but clues were provided in a February 2011 NASA briefing to the Space Operations Committee of the NASA Advisory Council.  The briefing, by Christini Guidi, described studies performed by four Requirements Analysis Cycle (RAC) teams.  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.

Team 1:  Shuttle Derived Options

Figure 1:  SLS RAC Team 1 Shuttle Derived Alternatives

Figure 1 shows four Team 1 alternatives, or "Blocks", presented in the briefing. 

"Block 0" was powered by a pair of four segment solid rocket boosters, but these were not existing Shuttle four-segment SRBs.  Since no more Shuttle four-segment boosters remain in inventory, these “four-segment” boosters would be created by stacking all but the middle segment of a five-segment booster.  This configuration was named "RSRMV-1", for "Reusable Solid Rocket Motor, Five-Segment, Variant 1".  The 8.4 meter diameter ET-derived core would be powered by three RS-25 engines, either existing "D" engines or new "E" engines.   "Block 0", which would aim for a 2016 operational date, would lift 70 tonnes to LEO and would be able to orbit astronauts in NASA's Orion-based Multi-Purpose Crew Vehicle (MPCV).                 

"Block 1" would use a pair of five-segment boosters and a longer core powered by five RS-25E engines.  This operational rocket 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, replacing replace PBAN propellant, would provide the performance improvement.

Whether NASA will follow the four "Block" approach is yet to be revealed, but the reference five segment booster, five RS-25 engine design appears to remain the goal.  An initial SLS version that uses up existing Space Shuttle SSME and SRB steel casing inventory seems likely.  The solid motors may be expended, rather than recovered for reuse, to save money.  A follow-on SLS may involve competition for new boosters, with solid and liquid alternatives likely to be offered by ATK, NASA's current SRB contractor, and by ATK's competitors.  The Ares J-2X engine may still be in the running to serve as an upper stage engine for SLS, eliminating the need to develop an air-start RS-25. 

Although SLS will be “Shuttle-Derived” by outward appearance, it will be more “Ares-Derived” at the detail level, thanks to its use of Ares five-segment boosters, Ares J-2X upper stage engine, and Ares fabrication techniques for the core stage. 

Team 2:  Big Kerosene Rockets

RAC Team 2 looked at big kerosene/LOX rockets.  Figures 2-5 show some of the numerous alternative considered.

Figure 2:  RAC Team 2 Gas Generator Concepts

The first group of vehicles presented would use gas generator ("GG") main engines, each producing 2 million pounds of thrust.  These engines could be considered modern versions of the F-1 engines used to power Apollo's Saturn 5 first stage. 

The presentation identified a two-stage "Concept 1" (Concept 131.03.00), with a first stage powered by four "GG" engines and a second stage powered by a pair of J-2X engines.  This relatively compact, but very powerful rocket would be 10 meters in diameter and perhaps 82 meters tall, but would lift 101 tonnes to LEO.  

A growth version (Concept 131.00.00) would use a larger first stage powered by six "GG" engines.  This rocket, similar to a previous "Concept 103" described in an earlier article, would boost 142 tonnes to LEO.

Figure 3:  RAC Team 2 Staged Combustion Concepts

The next group would use 1.25 million pound thrust "oxygen rich staged combustion" ("ORSC") kerosene/LOX engines.  This engine might have replaced the Russian RD-180 engines currently used to power Atlas 5.  A joint NASA/U.S. Air Force development program was contemplated for "ORSC", but the Air Force has to date not expressed an interest to fund such a project.

A "Concept 2" (Concept 119.18.00) powered by six "ORSC" first stage engines and two J-2X upper stage engines would lift 112 tonnes to LEO.  This rocket would stand only about 70 meters tall.   

A growth version would add a pair of Atlas 5-like strap on boosters to increase payload into the 120-150 tonne range.

Figure 4:  RAC Team 2 Staged Combustion - All Kerosene Concepts

A third group would use kerosene fuel and "ORSC" engines for all of its stages. 

"Concept 3" (Concept 119.20.01) contemplated a first stage powered by seven "ORSC" engines, a second stage powered by one "ORSC", and an optional Centaur third stage.  This rocket would lift 91 tonnes to LEO with two stages or 96 tonnes with Centaur.

Although these designs would eliminate the need to develop J-2X, they suffered in performance without a high energy second stage, preventing any design from easily meeting the 130 tonne goal.

Figure 5:  RAC Team 2 Evolutionary Path Options

"Concepts 4 and 5" represented an interesting, but in the end probably difficult to implement, thought experiment. 

The ultimate goal of this group was to get to Concept 143.00.03, a three-stage rocket similar in many ways to Saturn 5.  It would use five 2-million pound thrust "GG" engines on Stage 1, six J-2X engines on Stage 2, and one J-2X on Stage 3.  This rocket would lift 142 tonnes to LEO.

Rather than waiting for new "GG" engines to be developed, an interim version named Concept 142.00.00 would fly.  It would use a pair of five-segment solid boosters to replace the kerosene first stage.  The second stage would sit between the boosters, but its J-2X engines would not ignite until the boosters had nearly completed their burn.  This interim rocket would have lifted 86 tonnes to LEO.

Although the kerosene/LOX alternatives offered lower operating costs than the "Shuttle-Derived" options, the cost to develop the new engines, either "GG" or "ORSC", were deemed prohibitive.  RAC 2 team members argued that the extra development would be worth the cost. 

Team 3:  Modularity

Figure 6 provides an overview of some RAC Team 3 modular concepts. 

Figure 6:  RAC Team 3 Modular Concepts

RAC Team 3 examined modular concepts that used 4 meter, 5.4 meter, and 8.4 meter diameter "common-cores". 

Four meters is close to the 3.81 meter diameter of the Atlas 5 Common Core Booster.  Several of the concepts used clusters of three to five of these cores to create a first stage.  Each core was powered by a single engine with a single thrust chamber.  An 8.4 meter diameter LH2/LOX second stage topped this vehicle, and all other RAC 3 vehicles.  In every 4 meter module case, strap on solid motors were added, indicating that the four meter diameter core concepts fell short of desired payload goals.

Alternatives that used clusters of three to five 5.4 meter cores appeared more capable, because solid motors were not added. Each core in this case was powered by either a pair of engines or by a single engine with two thrust chambers.   The 5.4 meter diameter value did not match with any known existing tank tooling.   Delta 4's Common Booster Core is 5.1 meters in diameter.

A final group used an External-Tank-like 8.4 meter diameter core, powered by four kerosene/LOX engines.  Small strap-on solid motors were used to increase payload. 

While the modular concepts provided flexibility and the possibility of shared elements with other rockets, their complexity was an issue.   Complexity, provided by the increased number of modules and strap-on boosters with their associated separation events, led to higher failure probabilities than other RAC team alternatives.

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SPACE LAUNCH REPORT
by Ed Kyle