In its quest to propel manned space missions to the Moon, asteroids and onwards to Mars, NASA began its Space Launch System (SLS) programme in 2010.

While the initial Block I version of the SLS will be able to carry 70t to orbit, NASA envisages later missions requiring greater lift capacity of 130t.

As such, NASA intends to field later versions of the SLS using a pair of advanced boosters, more powerful than the ATK-built SLS Block I's five-segment solid rocket boosters, derived from technology used on the Space Shuttle.

To stimulate development in that arena, on 1 October NASA awarded three contracts totalling $137 million to demonstrate key booster technologies to mature competing designs.

ATK proposes using an advanced version of its expendable booster, which would use a new lightweight composite casing and a higher-energy hydroxyl-terminated polybutadiene fuel instead of the current polybutadiene acrylonitrile propellant.

At about 20,000kN (4.5 million lb) thrust, this advanced solid booster is expected to produce about 40% more thrust than the Block I variant. ATK will receive $51.3 million to construct and test elements of the oblique-nosed booster, including the design of the composite case, avionics and nozzles.

Dynetics, along with partner Pratt & Whitney Rocketdyne, proposes a booster design using two improved variants of the F-1A engine, a throttle version of the mighty F-1s that powered the Saturn V's lunar flights. The team will receive $73.8 million to build and test key elements of its 8,000kN thrust engine, including the gas generator and new simplified turbopump assembly.

The final $12.1 million contract was awarded to Northrop Grumman to demonstrate new techniques used in the production of composite fuel tanks, including out-of-autoclave curing and in-situ manufacturing.

A fourth contract is expected to be awarded to Aerojet to reduce development risks for a new, 4,500kN-thrust dual-combustion chamber liquid oxygen/kerosene-burning engine, called the AJ-1-E6, via demonstrations of combustion stability in the new design.

The Aerojet bid surprised some observers. While the AJ-1-E6 has a more efficient cycle than the Dynetics/Rocketdyne F-1A, it stands to produce less than 60% of its thrust, meaning four engines will be needed on each booster instead of two on F-1A derivatives.

Christopher Crumbly, manager of NASA's SLS advanced development office, is careful to appear unbiased in his comments with respect to the merits of the competing teams: "The F-1 has great advantages because it is a gas generator and has a very simple cycle," says Crumbly. "The ox[ygen]-rich staged combustion [of the AJ-1-E6] 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 teams using liquid fuel made play of their higher performance, which could allow the Block II SLS to lift 150t or more to low-Earth orbit, promising a great deal more flexibility for long-range missions.

In response, Donald Sauvageau, ATK's director of advanced programmes, notes that as long as the 130t criteria is met, overall development and life-cycle costs are more likely to be deciding factors.

"It is going to be a very tight competition between liquid and solid boosters," says Sauvageau.

Steve Cook, director of space technologies at Dynetics, notes that while liquids are preferable to solids on performance, super-high-efficiency powerplants are not necessary for booster engines.

"A gas generator is a much simpler engine," says Cook. "The point is not just to have performance for performance's sake, just performance to drive affordability."

Cook hails the reliability record of the F-1 and notes that many of the major elements of the redesigned engine have kept the same dimensions as previous iterations to avoid problems with combustion stability.

NASA will not launch a contest for the engine until 2015, and the winner may yet be a design not included in this round of development, with none of the winners of risk reduction contracts compelled to bid.

Equally, if one of the two current "liquid bids" does win, Aerojet, thanks to its GenCorp parent’s planned acquisition of Pratt & Whitney Rocketdyne, would be very much in pole position.

Source: Flight International