WASHINGTON — The U.S. Air Force should hold off on major investments in a reusable launch vehicle concept until the economics are better understood but should continue and even expand smaller-scale efforts to reduce the technical risk of developing such a system, the National Research Council says in a new report.

Among other things, the Air Force Research Laboratory should build and fly more than one subscale prototype reusable booster system (RBS) under an existing program known as Pathfinder, the report said. “In addition, competition amongst RBS concepts should be maintained as long as possible to obtain the best system for the next generation of space launch,” the report said.

Lockheed Martin Space Systems of Denver, Boeing Phantom Works of Huntington Beach, Calif., and Andrews Space of Seattle are designing RBS prototypes under contracts awarded in December by the Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio. The indefinite-quantity, indefinite-delivery contracts have a total potential value of $250 million combined through 2019, with the initial task orders valued at about $2 million apiece, according to announcements by the Air Force and the companies.

As envisioned, the RBS features a reusable first stage powered by a liquid-kerosene-fueled engine and an expendable second stage fueled by liquid hydrogen. The vehicle would take off vertically and the first stage would, after releasing the second stage, return to the launch site and land horizontally.

The Air Force has notional plans for a fleet of eight such vehicles at Vandenberg Air Force Base, Calif., and Cape Canaveral Air Force Station, Fla., according to the report, “Reusable Booster System: Review and Assessment.” These plans were devised in response to a June 2011 Air Force Space Command vision document outlining strategies for maintaining U.S. dominance of space and cyberspace, the report said.

According to the Aerospace Corp., the Los Angeles-based not-for-profit company that performs engineering and other analyses in support of U.S. military space programs, developing and operating an eight-vehicle RBS fleet would provide significant long-term cost savings over the Evolved Expendable Launch Vehicle (EELV) program, which today launches the vast majority of U.S. military and intelligence satellites. But the National Research Council, tasked with conducting an independent analysis of the concept, said the business case for pursuing a large-scale RBS program is unclear.

The council’s expert panel questioned the RBS cost estimates, citing numerous uncertainties associated with reusable launch systems. While the estimates were based on industry-standard methodologies, they rely on historical data that do not draw on experience operating reusable launch systems, the report said.

The cost projections also assume the RBS’s first stage would utilize a U.S. variant of Russian hydrocarbon — or kerosene-fueled — engine technology, “but the cost risks associated with development of an operable engine are difficult to capture,” the report said. The first stage of the Atlas 5 rocket, one of two workhorses of the U.S. EELV fleet, is powered by the Russian-built RD-180 main engine, for which a U.S.-based production line was planned but never implemented.

Moreover, the report said, it is not clear what infrastructure would be needed to operate and maintain a fleet of RBSs, and therefore the costs associated with that requirement are uncertain.

The report also noted that the savings attributed to the RBS are based on current EELV program costs and do not account for the possibility of lower-priced competitors entering the market.

Denver-based United Launch Alliance, a Boeing-Lockheed Martin joint venture, has a virtual monopoly in the U.S. military market as prime contractor on the EELV program, whose costs have soared well beyond expectations for a variety of reasons. But Space Exploration Technologies Corp. of Hawthorne, Calif., is angling for that business with what it says will be lower-cost vehicles, and has several successful launches under its belt. Another company, Stratolaunch Systems of Huntsville, Ala., is developing a system that would launch rockets from an aircraft that incorporates components from a Boeing 747 jetliner.

“Given the significant number of commercial entities pursuing novel approaches to achieve launch capabilities, the future of space lift may look very different from those employed today,” the report said.

The report said reusable vehicles nonetheless hold promise for bringing down launch costs and that the Air Force, independent of whether it pursues the RBS program, should continue developing key technologies including oxygen-rich staged-combustion hydrocarbon engines; operations of vehicles that return to the launch site; vehicle-health monitoring systems; and adaptive guidance and control.

The return-to-launch-site, or rocketback, maneuver is a focus of the Pathfinder program. “The use of a rocketback maneuver for [return-to-launch-site] operations of an RBS has not yet been demonstrated, so this approach to reusability carries significant risk,” the report said. “Given these risks and the resulting parameter space for innovative solutions, the Pathfinder program should be executed in a manner wherein several vehicle designs are developed and flown.”

Warren Ferster is the Editor-in-Chief of SpaceNews and is responsible for all the news and editorial coverage in the weekly newspaper, the spacenews.com Web site and variety of specialty publications such as show dailies. He manages a staff of seven reporters...