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After test hits and flops, budget ups and downs, is US hypersonic propulsion development heading for oblivion or will a rallying call reinvigorate research?

When NASA’s diminutiveX-43A hypersonic demonstrator streaked across the skies over the Pacific at just under Mach 10 on 16 November 2004 it was hailed as a massive success for the agency and its Hyper-X partners.

Combined Cycle_big

Yet, as with so many other areas of US aeronautics research, the funding for the next stage of the effort – the larger X-43C – had already been withdrawn, and as the last demonstrator plunged to a watery grave in the ocean it seemed NASA’s entire hypersonic programme might soon follow it into oblivion.

The Hyper-X effort was aimed at demonstrating airframe-integrated “air-breathing” engine technologies that have the potential to dramatically increase payload capacity for reusable space launchers and hypersonic aircraft. Hyper-X was originally intended to include two additional X-43 variants beyond the -43A, of which there were three individual test vehicles.

Cancelled

The X-43B was to have demonstrated a combined-cycle engine capable of operating as a normal turbojet at low speeds and switching to scramjet (supersonic combustion ramjet) mode at high altitudes and speeds. The first X-43B flights were set for 2009 onwards after the completion of another Hyper-X test vehicle, the X-43C. This was intended to demonstrate the operation of a hydrocarbon-burning scramjet engine at speeds between Mach 5 and 7 around 2008.

However, along with a raft of other aeronautics projects, both vehicles were cancelled in March 2004 in the wake of a shift in NASA’s strategic goals following the announcement of President Bush’s vision for space exploration in January of that year. The failure of the first X-43A flight attempt in 2001 had badly affected the chances of further funding, and NASA knew that the success of the last two vehicles was critical to achieving any lasting credibility for its hypersonic research.

The 27 March 2004 flight, when the vehicle reached Mach 6.94 at 95,000ft (29,000m), and the Mach 9.68 flight the following November were therefore crucial achievements for the US hypersonic community. Thanks to these successes, the US Congress belatedly added $25 million to the NASA 2005 budget to continue low-level development work on the X-43C research vehicle.

But the money is little more than an attempt to sustain life with a drip feed, and a rallying call is being issued by industry, academia, NASA and elements of the Department of Defense to maintain the momentum in this critical area of propulsion research. Air-breathing hypersonic engines, they argue, are the true gateway to the future of space access, providing power for more affordable, reusable launch vehicles as well as a raft of long-range strike/reconnaissance and high-speed missile and intercept weapons. With time running out for the Space Shuttle, and the first steps just taken to prove the viability of air-breathing hypersonics, they argue why stop now?

The military significance of hypersonic technology is now seen as the key to keeping science and technology research in the area alive. To this end, attention is being increasingly focused again on the National Aerospace Initiative (NAI), a science and technology plan crafted in 2001 that would effectively integrate air and space technologies to serve the defence needs of the USA in three main areas: high-speed hypersonics, space access and space generations. Together, these three pillars are seen as supporting joint “transformational” work to enable the development of long-range time-critical strike, an on-demand space launch capability using a combination of rocket and air-breathing propulsion, and responsive spacecraft capability.

However, even though the NAI was rubber-stamped by the US National Research Council (NRC) in 2004 as “an effective instrument for assisting the DoD and NASA in their pursuit of technologies for our nation’s future military systems and its future space-launch needs”, the defence research and engineering community knows it needs to do more. The NAI has defined a stepping-stone approach towards the strike, intercept and space-access goals, but all the players are aware of the need to stay vigilant and to keep ahead of future threats to funding.

“We need some sort of national steering committee in hypersonics,” says Vincent Rausch, the deputy director of NASA Langley Research Center systems analysis and concepts directorate and former Hyper-X programme manager. “There are a lot of people who still don’t think this stuff works. Some of our biggest detractors now at least acknowledge it is possible.”

Speaking at the American Institute of Aeronautics and Astronautics 2005 Joint Propulsion Conference in Tucson, Arizona, Rausch said: “We need programmes that give more bang for the buck. There is still some hypersonic work going on at NASA, and maybe we need more.” Together with other research groups, industry and the DoD, the fledgling steering committee is putting together initial proposals. “One would be the spiral development of a two-stage launch system with a Mach 7 first-stage and an expendable upper stage, while the second would be further work on RATTLRS/RTA [Revolutionary Approach To Time Critical Long Range Strike Project/Revolutionary Turbine Accelerator], which provides lots of opportunity for collaboration and co-ordination,” says Rausch.

The US Navy-led RATTLRS is a demonstration programme aimed at developing technology for a high-speed cruise missile, and is a joint project involving Lockheed Martin, the USAF and NASA. At least three demonstration flights will be made in the 2008-9 timeframe, with the goal of demonstrating acceleration to Mach 3 under turbine power alone, followed by high-speed cruise at Mach 4 with a turbine-based combined-cycle (TBCC) engine.

Lessons learned

The programme will leverage lessons learned from the now abandoned NASA RTA, as well as other ongoing research efforts, including the US Defense Advanced Research Projects Agency (DARPA) and USAF Falcon hypersonic technology vehicles programme. Lockheed is teamed with Allison Advanced Development to develop technologies for the TBCC based on the YJ102R developmental engine which, says the engine maker, provides more than six times the specific thrust of the engines in the SR-71 reconnaissance aircraft.

RATTLRS fits the NAI roadmap perfectly because it not only produces a relatively short-term technology, but it tackles the thorny issue of transition, which is rapidly becoming the main focus for future hypersonic studies. Michael Richman, associate director of aerospace technology for the Office of the Deputy Undersecretary of Defense, says: “We need an increased emphasis on near-term technologies – either those on the verge of a breakthrough in capability that can be transitioned immediately, or those that can be taken ’off the shelf’. What we need, however, is work in mode transition from low speed to high speed, and back again. We need a turbine that will get us towards transition to space access or a scramjet.”

With most of the hypersonic roadmap plotted for the rest of this decade, Richman says a few holes remain. “We feel we have a lot of data that will help us transition to future expendable systems, but there is a gap towards reusable systems and we need to identify the next step. We feel ‘mode transition’ is probably that, and over the next few months we’ll be working with industry and NASA to develop a technology-demonstration programme that will see that happen.”

So what does the roadmap contain and, other than RATTLRS, what are the main programmes under the NAI umbrella?

US NAVY/DARPA HYFLY A joint effort involving DARPA, the US Navy, Boeing and Aerojet, the HyFly programme is on the verge of a series of flight tests that are to run from early 2006 to around mid-2007. Also aimed at demonstrating key enabling technologies for a high-speed tactical strike weapon capable of sustained efficient cruise at M6, the concept is based around a rocket-boosted ramjet-scramjet. Also known as a dual combustion ramjet, the ramjet element of the engine provides a “pilot”, or combustion source, for the scamjet combustor.

This is believed to be a lower-risk solution as it has a lower Mach number (M3.3) at which the engine can be started than a pure scramjet. Ground tests have shown operability over the M3.5-6.5 speed range. Eight flight tests are scheduled, two to be half-scale, short-duration flying engines at altitudes simulating full-scale engine operating conditions.

The other six will be full-scale engines used to test high-temperature treatments in the airframe (titanium) and engine (ceramics). For potential space access, the HyFly concept would be adapted to couple the scramjet with a turbine engine in a combined-cycle system. Tests conducted in 2003 in the Arnold Engineering Development Center’s aerodynamic propulsion test unit marked the first time a fully integrated hypersonic cruise missile engine using conventional liquid-hydrocarbon fuel was tested at critical flight takeover conditions.

Invention of the HyFly dual-combustion ramjet engine is credited to the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

USAF/DARPA SED The scramjet engine demonstration (SED) programme involves Pratt & Whitney and Boeing, and is a descendent of several previous efforts including the Affordable Rapid Response Missile Demonstration and the liquid hydrocarbon-fuelled scramjet engine developed under the USAF‘s HyTech programme. “The success of these two led us to press on the SED,” says P&W Space Propulsion hypersonic and combined- cycle programmes manager Joaquin Castro. “It’s not a missile, but a propulsion demonstrator intended to validate the operation and performance of a scramjet engine in an operating environment as close as possible to reality. We’re looking for data on acceleration and cruise performance, as well as evaluating its potential application for a strike weapon, global reach and even access to space.”

Ground tests of SED are due to begin by mid-2006, with flight tests running from 2008 into 2009. “We will fly for minutes, not seconds,” says Castro. “When you come to put a real system into tests you have to run it over hours. Running something for milliseconds or seconds doesn’t cut it. We’re going to have to go out and flight test – there’s simply no substitute.”

Flight tests are set to culminate in a series of sorties in which a vehicle, powered by the actively fuel-cooled SED research engine, will be boosted from around M4.5 to speeds in excess of M6.5.

DARPA/USAF FALCON The DARPA/ Lockheed Martin Falcon programme (Force Application and Launch from Continental United States), announced in 2003, is aimed at developing a range of enabling technologies to support the development and test of a range of fast-reaction, global-reach, hypersonic vehicles. High-speed propulsion and integration of the propulsion system forms only two parts of the entire effort, which is scheduled to test and produce a range of small launch vehicles as well as a series of hypersonic test vehicles (HTV).

The goal is to demonstrate technology for a 2025-timeframe hypersonic cruise vehicle (HCV) capable of striking targets up to 17,000km (9,000nm) from the USA in under 2h. Lockheed is designing the HCV, with Aerojet working on the engine, which features an “inward-turning” flowpath. This means the inlet focuses the flow towards the centre of the engine, which has a circular cross-section. The flowpath is therefore three-dimensional, instead of the rectangular two-dimensional cross-section of all previous hypersonic designs. The round engine has reduced internal surface area with better pressure recovery and less frictional heating. Ground tests of the flowpath are being undertaken at Calspan-University of Buffalo Research Center.

Lockheed’s dual-scramjet, waverider HCV is an unmanned, reusable vehicle designed to take off from a runway and cruise at M10 and 130,000ft, carrying a 5,400kg (12,000lb) weapons payload. Three unpowered, rocket-launched HTVs are planned: HTV-1, to fly around 2007, is an expendable vehicle with an 800s flight time at M10 and two expendable HTV-2s with 45min flight time are set to be followed in 2009 by three flights of two recoverable, reusable HTV-3s.

USAF HYTECH Forming the fun­d­a­­­­­­­­­mental platform for all air force hypersonic efforts, the Hypersonic Tech­nology (HyTech) programme has been consistently focused on the development of endothermic hydrocarbon fuel-cooled ground demonstrator scramjets with P&W.

Tests of the Ground Demonstration Engine (GDE-I) in 2003 helped develop technologies that supported NASA’s Hyper-X programme and are incorporated into the follow-on SED effort. HyTech continues in support of SED, as well as tests of a variable-geometry GDE-II that could power a revived X-43C. Tests of the P&W-developed GDE-II are to take place in a NASA Langley windtunnel in 2006. “This engine will be a workhorse, and should be able to run a number of times because we’ve put enough coatings on it to make it reusable. We’re confined to about 1min of run time because of the limitations of the test facility,” says Castro.

USAF/DARPA HISTED The USAF/DARPA high-speed turbine engine demonstration (HiSTED) is aimed at development of technologies for an M4-plus expendable turbine engine for tactical weapons and for a reusable TBCC engine. The combined-cycle application would be used for accelerating a flight vehicle to M4 plus, and would be integrated with a M4-7 hydrocarbon scramjet engine for hypersonic cruise vehicles as well as for access to space. Phase 2 ground tests are planned around late 2007 and late 2009, with Phase II focusing on development of the TBCC version. HiSTED, with SED, forms one of two funded propulsion concepts being pursued with the Falcon programme.

DARPA HYCAUSE The US-Australian Hypersonic Collaborative Australia/United States Experiment (HyCAUSE) is a joint effort by university-based scramjet propulsion specialists in the two countries and is a follow-on to the University of Queensland’s HyShot scramjet flight in July 2002, which achieved supersonic combustion in flight for the first time. Flight tests at M10 of an “inward-turning” scramjet on a sounding rocket are planned for November from Woomera in South Australia. Lessons learned from the HyCAUSE tests are expected to feed into the Falcon conceptual vehicle design.

US ARMY HYPERSONIC ENGINE TECHNOLOGY DEMONSTRATION PROGRAMME The US Army is study­ing scramjet engine concepts for a M10-12 hydrogen-fuelled vehicle. Advanced features under evaluation in­clude enhanced mixing of the fuel and air streams, cooling of the engine flowpath, drag reduction in the engine itself and boundary layer control in the engine flowpath. Component selection is due in 2007. A flowpath configuration will be decided in 2008 and the development of a flight weight engine in 2009.

Inward-turning inlet configurations, multiple flowpaths and multi-cycle ramjet/scramjet configurations are also being studied by Alliant Techsystems’ GASL unit, the hypersonic propulsion specialist that built the successful engine used by the X-43A. Through funds awarded to this company, NASA has effectively kept the X-43C effort alive and both groups are now working with Boeing on potentially reviving the effort in the near term.

“We are working on trade studies of a revised X-43C design that would demonstrate low-speed transition and use hydrocarbon fuel,” says Boeing Phantom Works hypersonics programme manager George Orton. “We’d also like to try and bring it back and reuse it,” says Orton, who adds that the proposed vehicle will be defined in the third quarter.

“There are indications that Congress would like to fund it in 2006, and we’re looking at having a preliminary design review in the new year and a critical design review in early 2007.” First flight could be as early as 2010, he adds. Orton sees a three-phase plan towards everyday applications of hypersonic propulsion beginning in 2010-15 with missiles. Phase 2, around the 2015 period, could see demonstrations of a reusable, recoverable vehicle, while Phase 3, from 2020, could see the deployment of global-reach hypersonic vehicles and air-breathing vehicles for space access.

P&W’s Castro adds: “We’re about to reach critical mass. But we have not yet developed a full appreciation of what this technology can do. I was a chartered member of the rocket mafia, but I’m a believer in air breathers now.”

Richman adds: “Within the next four years we’re going to see three additional flight tests in the Mach 3-4.5, 6, 7 and 8 areas. By 2009 we will have a lot of information and lots of things streaking across the sky. But to help feed it into the next step we’re going to need funding to develop it, and now’s the time to act.”

GUY NORRIS/TUCSON

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