X-43A team claims $500-million air-breathing first stage could handle 80% of payloads
Last Mile
A reusable air-breathing space-launch first stage growing out of the successful X-43A Hyper-X test flight in March could be ready to send 20,000-lb. payloads to low-Earth orbit (LEO) by about 2015, say the engineers who developed the Mach 7 testbed.
Holding an operational first stage to that speed instead of trying for stage separation in the Mach 12-15 range would make the development challenges manageable and perhaps affordable, the team says in a paper delivered to the International Astronautical Congress (IAC) in Vancouver. The Mar. 27 X-43A flight test showed that existing wind tunnels could provide the "relevant environment" for testing the remaining technology needed, eliminating the requirement for an expensive large-scale flight demonstrator or new ground facilities.
"The technology has matured a lot," says Charles R. McClinton of Langley Research Center, technology manager on NASA's Hyper-X program. "In the U.S. over the last 40-plus years we've spent over $4 billion. NASA's successful X-43 flight tests validate the design tools, the design concepts and its future applications."
NASA's new exploration systems organization inherited the Hyper-X program when the U.S. agency was reorganized to carry out President Bush's call for human exploration beyond Earth orbit, with the Moon as a way station. It quickly canceled the follow-on X-43C project, deeming it unable to meet the heavy-lift requirements of a return to the Moon (AW&ST Mar. 22, p. 23).
NOW NASA'S aeronautics organization is trying to find a way to sustain the expertise built up in earlier hypersonics programs like the National Aero Space Plane. They have been aided on Capitol Hill by senators from Missouri and Tennessee, where the X-43A was developed and fabricated by Boeing Phantom Works and ATK GASL (formerly Micro Craft of Tennessee and GASL of Ronkonkoma, N.Y.). The Pentagon also is interested in NASA's hypersonic propulsion expertise for missiles and space access, and has worked on the technology with the civilian agency through the National Aerospace Initiative.
The X-43A launch scheduled for November is the last one and marks the end of current NASA hypersonic funding. Langley and NASA headquarters aeronautics officials are proposing ground research starting in Fiscal 2006 or 2007 to keep hypersonics alive, but it is not clear yet whether it will be part of the President's proposed budget.
An application like an air-breathing first stage that could be available just as the exploration initiative begins to need a lot of rides to LEO could revive the hypersonics effort. While McClinton concedes "we're never going to get heavy lift; we're not a threat to the rocket guys," a vehicle able to take 20,000 lb. to LEO would cover about 80% of the launch market identified by NASA's old Next-Generation Launch Technology program.
Although operational costs would be somewhat lower than for expendable rockets--$1,700 per lb. to LEO versus $2,500 for a baseline expendable--the main benefit would be in safety and reliability. Citing a study published by the American Institute of Aeronautics and Astronautics in July 2003, the X-43 team claims a Mach 7 hypersonic first stage could cut the risk of vehicle/payload loss to one in 4,000 from one in 50, chiefly through its abort capabilities.
The Mach 7 vehicle proposed by the authors of the IAC paper, including Hyper-X Program Manager Vincent L. Rausch, would use a Turbine-Based Combined Cycle (TBCC) engine burning hydrocarbon fuel for the turbojet and liquid hydrogen for the ramjet/scramjet phase of flight. The turbojets--a variant of the F135 or F110 engine--would push the first stage, an expendable upper stage and the payload from horizontal takeoff to about Mach 2.5. At that speed the transition to ramjet/scramjet operations is less challenging than at the higher Mach 3.5-4 transition speeds needed for a Mach 12-15 first stage.
The technology required to build such a stage is more in hand than what would be necessary for the higher-speed version. The stage structure would be aluminum or titanium, while the cryogenic tanks could be aluminum-lithium instead of lighter but more challenging composites. Thermal protection would be based on space shuttle technology for the most part, although "leading edges may require cooling for durability." McClinton and the other authors concede "significant technical challenges" would remain in building a reusable Mach 7 stage.
CHIEF AMONG THEM is integrating turbojet and ramjet/scramjet elements in a single TBCC engine. But at a maximum speed of Mach 7, the problem would be less severe because existing ground test facilities likely can do the job, based on the experience of March's X-43A flight.
"The Mach 7 flight of X-43 demonstrated that wind tunnels are a relevant environment for scramjet demonstration to TRL [technology readiness level] 6," the paper states. "Therefore the authors conclude TBCC engine tests in wind tunnels meet the TRL 6 requirement."
The maximum level of 9 means proven under operational conditions, and level 1 is the earliest stage of concept and research. Level 6 means a representative model has been tested in a relevant environment, such as a wind tunnel.
In the Mar. 27 test the 12-ft.-long X-43A produced thrust in scramjet mode after a boost from an air-launched Pegasus rocket first stage, setting a new record for jet-powered flight (AW&ST Apr. 5, p. 28). A second flight to Mach 10 with engine modifications and improved carbon-carbon leading-edge thermal protection is in the final stages of preparation and expected by mid-November, McClinton said.
"Technology levels are nearing completion for early production vehicles, and we think these could be put into place by 2015," McClinton told a well-attended IAC technical session. "Less than half a billion dollars are needed to complete the technology development for the first stage."
I don't know--I hate to be skeptical especially about something that does have promise, but haven't they been saying this since about 1985 when the Reagan administration approved some of the first budgets for developing NASP (National Aerospace Plane--America's space version of Concorde?)
Using a hypersonic air breathing craft as a booster stage could work--but I doubt very much it is going to save much money. The complexities involved in spaceflight make just about any endeavour a real technological reach.
I am hopeful but somewhat skeptical of this project.
