Flight demonstrations of the Defense Advanced Research Projects Agency’s Falcon hypersonic test vehicle could be closer to reality. Successful tests of a rocket-boosted scramjet in Australia and good results from thermal protection system work in the U.S. are bolstering prospects for the next steps in this ambitious hypersonic plan.

The Falcon project is aimed at developing and demonstrating technologies to support a global-reach hypersonic cruise vehicle (HCV) capable of delivering a 12,000-lb. payload at a distance of 9,000 naut. mi. from the continental U.S. in less than 2 hr. Flight tests of Falcon, particularly in its powered form in the shape of the planned HTV-3X version sometime beyond 2010, are heavily dependent on the success of a series of propulsion and structure technology demonstration tests.

Propulsion-related work for Falcon is being conducted under the Facet (Falcon combined-cycle engine test) series, and is examining the technology for a turbine-based combined cycle (TBCC) engine. This will integrate a dual-mode ramjet/scramjet and a Mach 4-plus turbojet to allow the reusable HTV-3X to take off horizontally, accelerate to Mach 6, and return to land on a conventional runway. Key to its operation will be an inward-turning inlet that forms the basis for the axisymmetric scramjet flowpath design just evaluated in the recent test in Australia.

Dubbed HyCause (Hypersonic Collaborative Australia-U.S. Experiment), the test is the first to attempt to measure the efficiency of this form of revised flowpath at sustained hypersonic speeds. Previous two-dimensional inlets, such as the rectangular design used on NASA’s X-43A, could only compress the airflow on the upper and lower surfaces, whereas the inward-turning design allows a larger area of a curved surface to be used for compression. In addition, the circular inlet provides improved stability for flow into the TBCC engine.

The HyCause launch—which was itself delayed in the wake of problems encountered during the past two launches under Australia’s HyShot series of hypersonic test flights—took place at the Woomera range on June 15 using a Talos Mk. 11/Castor 1 XM-33 sounding rocket. “We got some data,” says Steve Walker, Falcon manager and Darpa program manager for HyCause. Speaking at the American Institute of Aeronautics and Astronautics Joint Propulsion Conference in Cincinnati, he says, “We know the fuel came on, and the augmentor doors worked. We’re still going through it, and we are continuing to take a look at these flowpaths.”

In the test, conducted with Australia’s Defense Science and Technology Organization (DSTO), the vehicle reached an altitude of 285 naut. mi. (530 km.) and achieved Mach 10 during a near-vertical descent back through the atmosphere, during which the hydrogen-fueled scramjet engine was lit and fired for around 3 sec. The test provided data for comparison with wind tunnel trials of the inward-turning scramjet conducted at Buffalo, N.Y.-based Calspan.

The test also augurs well for the ongoing hypersonic joint venture between the U.S. and Australia, which in late 2006 was extended under a $54-million agreement to explore technologies for a future generation of air-breathing weapons (AW&ST Mar. 19/26, p. 40). The six-year Hypersonic International Flight Research Experimentation (HiFire) effort, led by the U.S. Air Force Research Laboratory and DSTO, is coordinating research performed with NASA, U.S. industry, the Australian Hypersonics Consortium and the University of Queensland’s hypersonics research group. Under HiFire, both AFRL and DSTO are designing experimental payloads and conducting ground testing while the Australian group integrates the payloads and performs launches. At least six of the HiFire flights will test technologies for next-generation waverider-configuration hypersonic vehicles powered by inward-turning ­scramjet designs.

Facet tests, meanwhile, are focused on the performance of a dual-mode ramjet combustor and a common nozzle. “We’ve done direct-connect combustor tests looking at round combustion and the ability to operate at lower Mach numbers, and we have good recent news on performance between Mach 2 and Mach 3,” says Walker. Static nozzle tests “are looking at how to get the flowpath to mix more,” he adds. Facet trials are due to culminate in the third quarter of 2008, “when we bring all these technologies together in free-jet tests.”

Mach 4-plus turbine engine technology is being addressed separately in the HiSted (high-speed turbine engine demonstration) program, under which Rolls-Royce’s Liberty Works’ XTE18 and Williams International’s XTE88 ramburner-configured turbojets are scheduled to run at Arnold Engineering Development Center in Tullahoma, Tenn., in 2008 and 2009, respectively. “HiSted engine tests are critical to show these turbine engines can go to Mach 4,” adds Walker.

Meanwhile, the first of two unpowered, expendable HTV-2s is on track to be launched by rocket in December 2008, Walker says. The tests—which will take place over hypersonic glide flights starting off at Mach 20-22 and covering up to 3,000 naut. mi. in around 30 min.—will demonstrate materials for long-duration hypersonic flight, in particular an advanced thermal protection system.

Manufacturing challenges involved in the development of the carbon-carbon aeroshell forced the original Falcon schedule to be changed and the HTV-1 step to be bypassed. “We’ve made lots of progress and are trying to get the lift/drag up on the vehicle,” Walker says. “We’ve also done more leading-edge work, and it looks like it’s really holding up and will be able to sustain a 40-min.-plus flight.” Progress by the Fort Worth-based aeroshell maker C-CAT (carbon-carbon advanced technologies) has been positive, he adds. “We’ve turned the corner on that, particularly in being able to produce it reliably and consistently.”

Given the recent progress, Walker appears upbeat on the prospects for HTV-3X, which, he underlines, “is not NASP”—a reference to the abandoned X-30 National Aerospace Plane project. “We’re only biting off a small piece, and we’re confident that if these things [tests] can happen, then we can do this demonstration. There are big challenges ahead, but I’m pretty optimistic,” he adds.