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SLS Block II illustration
The boosters of the Block II version of the SLS, capable of placing up to 130 metric tons into low Earth orbit, could potentially be powered by derivatives of the F-1 used on the Saturn V. (credit: NASA)

SLS Block II drives hydrocarbon engine research


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At the 63rd International Astronautical Congress (IAC) held in Naples, Italy, last October, Steve Cook presented a paper titled “Enabling an Affordable, Advanced Liquid Booster for NASA’s Space Launch System.” Cook was the program manager of the Ares 1 crew launch vehicle under NASA’s Constellation program. NASA developed that launch vehicle using the Solid Rocket Motor technology developed for the Space Shuttle program.

The association of NASA and the US Air Force regarding launch vehicles and propulsion goes back to the space agency’s founding.

Today, Cook is director of space technologies at Dynetics, based in Huntsville, Alabama. The company is partnering with Pratt & Whitney Rocketdyne (PWR) to revisit the Apollo-era F-1 engine evaluate the feasibility of employing proven elements of that massive hydrocarbon engine design to enable NASA to achieve the payload goal of 130 metric tons for the Block II Space Launch System. However, additional impetus to pursue this development comes from the Air Force and its desire to switch from the Russian RD-180 core booster engine used on the Atlas V.

The US Air Force Research Laboratory Propulsion Directorate has established a new program called the Hydrocarbon Boost Technology Demonstrator. Two companies, Aerojet and PWR, are involved in this program. Dale Thomas, associate director-technical at MSFC, speaking at the Fifth Von Braun Memorial Symposium in Huntsville last October, said, “We have to have higher lift capabilities out of the Space Launch System, and it turns out one of the options we are looking at is RP-based engines, which potentially intersect with the core-stage engine for the Atlas V.”

The association of NASA and the US Air Force regarding launch vehicles and propulsion goes back to the space agency’s founding. In fact, it was the Air Force that issued the requirement for a rocket engine with a thrust of one million pounds-force (4.4 million newtons) that led to the research and development of the F-1 long before it was taken over by NASA for the Apollo program. The F-1 made it possible for the Saturn V to send Apollo to the Moon. With the demise of Project Constellation and reboot of the Ares 5 into the SLS, some saw the F-1 as a possible option to achieve the final 130-metric-ton goal for the SLS (see “A new hydrocarbon engine for America?”, The Space Review, June 14, 2010).

A renewed interest in a hydrocarbon engine for SLS boosters

At the IAC, Aerojet unveiled its AJ-1E6 engine design, with the targeted one million pounds of thrust. Julie Van Kleeck, vice president of Aerojet’s space and launch systems, announced at that time the company was pursuing a demonstration contract under the NASA SLS program. “We are negotiating for a contract involving technology risk reduction for an engine of one million pounds thrust,” she said at the time. This follows awards already given to Dynetics and ATK.

There is a wrench in the works of this effort, though. In July, GenCorp, the parent company of Aerojet, announced plans to purchase Pratt & Whitney Rocketdyne for $550 million. The sale is awaiting approval of the Federal Trade Commission, which is expected sometime during the first half of 2013. Some industry analysts believe GenCorp will be in a win-win situation if NASA should decide to go with either the Aerojet AJ-1E6 or a new variant of the F-1. Indeed, GenCorp may decide to permit parallel programs to continue for both engines.

The Aerojet AJ-1E6 is an oxygen-rich staged combustion design having two combustion chambers, much like the RD-180 already on the Atlas V, but this is a new engine entirely of Aerojet’s design and manufacture. Its one million pounds of thrust is considerably less than that of an F-1A. A total of four AJ-1E6 engines would be required for each of the two boosters on the Block II SLS.

Within NASA, the operative phrase in this new engine development is risk reduction. That translates into flight-proven hardware that can eliminate or drastically reduce engine systems development and thus overall cost.

Dynetics’ approach is to use proven legacy hardware from the F-1 and components developed for the F-1A (which did not see serial production) and incorporate a simplified F-1A turbopump and exhaust duct, a new hot-isostatic press-bond main combustion chamber, and a new optimized 12:1 channel-wall nozzle having simplified construction less expensive than the previous F-1 nozzle design. The engine would produce 1.8 million pounds (8 million newtons) of thrust at sea level. Only two such engines would be required for each of the two SLS boosters in Block II configuration.

“The high-cost, non-recurring engineering typical of engine development was accomplished during the Apollo-Saturn program,” Cook stated, “eliminating significant risk. This permits the current focus to be on affordability rather than technical feasibility.”

Within NASA, the operative phrase in this new engine development is risk reduction. That translates into flight-proven hardware that can eliminate or drastically reduce engine systems development and thus overall cost. “Those risk reductions are focused heavily around affordability,” Cook added, “because a big deal on the Space Launch System is affordability, while also giving NASA additional performance margin above their 130-metric-ton requirement, on the order of 20 metric tons.”

One thing is certain. There are far more potential engine sales for an Air Force booster with hydrocarbon engine than the SLS, which is projected to fly no more than twice per year, and more likely just one launch a year. Having an American-designed and -manufactured rocket engine on the Atlas V would also allay concerns among some members of Congress and the Air Force itself. So, this, too, is an impetus to develop a new hydrocarbon engine.

Solid Rocket Motors are still a possibility

ATK is very much in the running, proposing an improved performance solid rocket motor for the Block II SLS. The company is supplying the five-segment solid boosters for the Block I SLS and is working on even more powerful motors for the Block II SLS. ATK is exploring a new lightweight composite four-segment motor casing and higher-energy hydroxyl-terminated polybutadiene propellant instead of the current polybutadiene-acrylonitile propellant. ATK has received a $51.3 million study contract from NASA to pursue this research development. This engine has a targeted thrust of 4.5 million pounds of thrust (20 million newtons) at sea level.

There are far more potential engine sales for an Air Force booster with hydrocarbon engine than the SLS.

NASA has shown no propulsion preference on the Block II SLS. It is using much the same approach it has with respect to commercial crew capsule design and launch vehicle selection. Christopher Crumbly, manager of NASA’s SLS advanced development office, is noncommittal with respect to the PWR/Dynetic’s F-1 design, Aerojet's AJ-1E6, and ATK’s proposed design.

“The F-1 has great advantages because it is a gas generator and has a very simple cycle,” he told an industry online journal. “The oxygen-rich staged combustion [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 can work.”

The evolution of the Block II design is years in the future. The SLS core stage successfully passed a preliminary design review in December 2012, with the critical design review set for some time in 2014. First launch of the Block I SLS is currently planned for 2017, and flights of the Block II SLS that will take astronauts beyond low Earth orbit won’t occur until the 2020s.

Fortunately, the US will likely have a new hydrocarbon rocket engine that can both power the Block II SLS and help meet the country’s military payload needs for many years to come, if necessary. Only time, cost and reliability, will tell the ultimate decision.


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