The Defense Advanced Research Projects Agency is exploring whether to pursue development of a hypersonic, reusable aircraft that could "skip" along the atmosphere and deliver military payloads anywhere in the world within about 2 hr.

LLNL's HyperSoar concept has attracted interest among USAF space officials.

The idea itself isn't new; aerospace engineers for years have harbored the notion that skipping along the atmosphere would be an efficient method to traverse great distances. But after years of mulling the concept, Darpa appears ready to take the concept off the drawing board toward a demonstrable capability, called HyperSoar.

Darpa Director Anthony J. Tether said the aircraft would travel at a speed of around 6,700 mph., or Mach 10, and operate primarily at altitudes of 115,000-200,000 ft. After climb-out, the aircraft would travel along a sine-wave flight path, with the propulsion system engaging when HyperSoar is at about 115,000 ft.--the skip altitude--to boost the aircraft back to 200,000 ft. before repeating the cycle. During a skip, the aircraft would be exposed to about 1.5g, Tether said, with a skip required about every 400 km.

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A notional aircraft design would be 213 ft. long and 79 ft. at its widest point. Moreover, Tether indicated the system would likely have a waverider design.

HyperSoar is supposed to be a high lift-over-drag aircraft that would use hydrogen-based propulsion, accelerate out of the atmosphere, and glide back into the atmosphere, according to Darpa officials. The hope is this approach will "improve fuel consumption and reduce average thermal loading over the mission." Those effects, officials said, "should result in greater mission radius and reduced thermal management requirements."

Among the technical problems Darpa engineers believe they must solve are occasional trajectory flight controls; a highly integrated airframe/ propulsion concept typical of hypersonic vehicles; combined-cycle accelerator propulsion, and cyclical thermal stress management.

Darpa officials have only begun to work with industry to determine what technologies might be available to realize the vision. At this point, the research organization wants contractors to outline what total system concepts are probable, as well as any technologies that could assist the effort. Of particular interest to the agency are potential efforts applicable to the aircraft/propulsion vehicle concept; rocket-based and turbine-based combined cycle or other propulsion technologies; lightweight, low-cost airframe technologies; and more mundane activities such as concept of employment development and modeling and simulation.

THE LAWRENCE LIVERMORE National Laboratory (LLNL) has for years been pushing the HyperSoar concept (AW&ST Sept. 7, 1998, p. 126). Its engineers conceived of an aircraft that would take off horizontally from a 10,000-ft. runway, accelerate to Mach 10 while climbing to 130,000 ft., and then shut down its rocket-based combined-cycle engines for a period in which the aircraft coasts. One of the primary engineers working on the concept at the time was Preston H. Carter, who now works in Darpa's Tactical Technology Office, which is managing the effort.

A bomber version would be able to carry almost 100,000 lb. of payload on a 6,200-mi. mission radius, LLNL estimates. The concept, at other times, is being envisioned as a rapid response reconnaissance system or a future airlifter. U.S. Air Force officials have had a long-standing interest in the project.