Fifty years after the U.S. Air Force first began to recognize the true challenges of air-breathing hypersonic flight for weapons and aircraft, a cohesive plan is emerging that finally may enable those long-held goals to be achieved.
Unlike many earlier road maps, however, the new plan is measured in decadal, rather than annual, targets and appears to accommodate both the technological difficulties of the tasks and the realities of defense science and technology (S&T) spending in a time of austerity.
Despite the painfully slow progress from the days of the ramjet-powered Martin Marietta Advanced Strategic Air Launched Missile (Asalm) of the late 1970s to the most recent flights of theX-51A scramjet demonstrator, the plan recognizes that speed remains an Air Force priority for its warfighting capabilities.
The Air Force defense S&T vision now calls for efforts to support development of a hypersonic strike weapon by 2020, and a penetrating, regional intelligence, surveillance and reconnaissance (ISR) aircraft—probably piloted—by 2030. The service intends to achieve for strike weapons a technology readiness level (TRL) of 6, the jumping off point for full-scale development, by the start of fiscal year 2018. The target for a hypersonic aircraft is the far lower TRL 4 maturity level by 2020.
“Right now, the need is not necessarily there, but we can accelerate it if need be,” says Christopher Clay, deputy chief of the Science and Technology Div. of the office of the deputy assistant secretary of the Air Force for science, technology and engineering. The bottom line, he says, is that “one day, hypersonic capability will have to be an option for the U.S.”
The plan essentially funnels together the outcomes from a broad range of Air Force Research Laboratory (AFRL),(Darpa), and other national and international efforts that have either ended, been canceled, are in process or in flux with uncertain funding. They range from the X-51A, with one more flight to go, to the canceled Darpa Blackswift, as well as related Facet combined-cycle and HiSted high-speed turbine engine technology demonstrations. Others include the US-Australian HIFiRE (hypersonic international flight research experimentation) fundamental research effort and AFRL robust scramjet.
“There were a lot of things going on at AFRL and none had critical mass. So we said, 'Let's pick two areas and see what progress we can make,'” says Clay. “Time-critical strike is on a pretty good pace, whereas the TBCC [turbine-based combined cycle] aircraft side is on a slower pace.” The outline plan “provides the basis for us to talk to other agencies,” he adds.
For the first time in high-speed strike weapons development, the plan also embraces “some level of international collaboration,” notes Clay. Although declining to offer additional details, the area of joint work is believed to involve development of compact boosters—one of several key technologies on the high-speed weapon road map. Others include multimode seekers, high-speed guidance systems in a GPS-denied environment, aerodynamic configurations, structures, materials and thermal-protection systems, as well as compatibility with current and future fighters and bombers. The hypersonic weapon, like Darpa's now-ended ArcLight high-speed strike project, should also be compatible with the U.S. Navy's Mk. 41 vertical launch system.
Many of these technology targets have fallen out of mission analyses that have guided planners as they set out the road map. For high-speed weapons, the chief study is known as Technologies for Responsive Precision Air-Land-Sea Strike (Trespals2). “Basically, the question we asked was, 'How fast is fast enough?'” says Clay. Trespals2, modeling and simulation, and technology development work will support the start of a High-Speed Strike Weapon (HSSW) demonstration.
The demo program, which will get underway around March 2013, will culminate with live-fire tests toward the end of the decade. “We are looking at flights starting in 2017 and, if they are successful, these will run through 2018 and 2019,” says Clay, adding that the plan calls for “six to seven flights.” The key focus for the flights is to increase confidence in various technology aspects of the weapon system ranging from guidance to propulsion. Demonstration goals include successful strikes on targets “up to hundreds of miles” away, says the Air Force.
“We need to show precision strike, as well as demonstrate it will work with existing aircraft systems and can be fitted internally for bombers and externally for fighters. It will also be net-enabled and include variable ordnance effects,” Clay says. “So we are working advanced guidance systems, selectable ordnance packages, as well as efficient, high-speed expendable propulsion concepts. It has also got to be lightweight and low-cost, not even double the cost for a subsonic weapon. The mission cost, from an affordability standpoint, should be there. The idea is to get the unit cost down to where it makes the mission viable. Without that, hypersonics is not an option.”
The HSSW program is being managed by the AFRL at Wright-Patterson AFB, Ohio, and Eglin AFB, Fla., where the demonstration effort will be based. Following an initial industry day, the Air Force is encouraged so far. “We think from our studies we can have a rapid response,” says Clay. A second industry meeting day next week will pave the way for a request for proposals in January 2013. “The HSSW bridges a gap between a technology demonstrator and a capability demonstrator,” he says.
Despite one partial success and two failures on three flights of the AFRL's X-51A scramjet engine demonstrator, plans are proceeding to fly the fourth and final vehicle in mid-2013. “Air Force leadership remains committed to scramjet research,” says Charlie Brink, AFRL X-51A program manager. “The idea of a weapon that can fly 600 nautical miles in 10 minutes is starting to gain significant traction. The warfighter community is starting to realize the potential utility of something like this,” says Brink, who is slated to become HSSW deputy program manager, overseeing the airframe and propulsion.
In the May 2010 first flight of the X-51A, the hydrocarbon-fueled scramjet burned for 140 of a planned 300 sec. Leaking seals between the engine and nozzle ended the flight prematurely, but the scramjet accelerated the vehicle. The June 2011 second flight ended when the inlet unstarted during scramjet ignition, and the August third flight ended before the scramjet could ignite when a malfunctioning control fin caused the vehicle to tumble and break up. Fixes have been identified for the fourth flight.
That funding has been found within a tight budget for the fourth X-51A shows the level of support for the program. “There is a lot of positive support for this program, but they want us to do better,” says Brink. And whatever happens on the fourth flight, he believes that “continued research in this area will occur.”
For high-speed ISR/strike, the Air Force has fairly well-defined capabilities already sketched out. The aircraft must survive a “day without space”—communication and navigation satellites—and be able to penetrate denied areas. With a TBCC propulsion system, it will be capable of a Mach 4-plus cruise speed and operation from a conventional runway.
The development of a hypersonic aircraft “is probably a significant step up in the technology required and risk over weapons,” says Clay. “So it is a slower undertaking and much more expensive. We've laid out a reasonably paced program, which can be accelerated if the need arises.” The initial speed of Mach 4-plus was defined at the first meeting in December 2010. “Since then, we've migrated up to a higher speed of Mach 5,” he says.
A high-speed mission analysis research process was used to define the technology challenges and, along with studies of new concepts and vision vehicles, drove the desired speeds higher. “All of the studies have been leading us in the direction of Mach 5 and above. We've started to lay out what are the technologies required to make that real,” Clay says.
The outline plan calls for demonstrations of a turbine-to-dual-mode-ramjet/scramjet transition system around 2020. In tests of the Air Force/Darpa Blackswift ramjet/scramjet engine model, “we ran into problems in the TBCC and high-temperature turbine engine. There was a lot of significant ramjet/scramjet work done and since then there's been a lot of progress in high-temperature turbines that hold out promise that a program like this can be done,” Clay notes.
Testing will be focused on a subscale airframe, though likely with a half- or full-scale flowpath. “It becomes a technology testbed for other systems we need to develop,” says Clay, referring to a wide array of secondary objectives, including ceramic-matrix composite structures, advanced power and thermal management, sensors and affordability trades and initiatives.
The Air Force science and technology research plan is focused on tackling the major challenges associated with the aircraft's propulsion system and its integration into the airframe. Other focus areas cover power and thermal management, guidance and control, structures and materials, configuration and aerodynamics, as well as sensors.
“Getting through mode transition is very important,” Clay says. “One thing to be looked at is what we can do with commercial off-the-shelf turbines. Can I extend the speed of those up a bit and perhaps extend the [dual-mode ramjet] down a bit?” The tests will evaluate scramjets scaled eight to 16 times larger than those flown to date. A recently added technology test target includes nozzle sealing. “We've seen on X-51A how simple things, like how we do seals, can lead to problems,” he says.
The X-51A experience also reminded the Air Force of another key lesson. “Flight testing is hard, but it has to be done, and you have got to have enough flight-test vehicles to get you where you need to go,” says Clay.