A black-coated vehicle the size and shape of a surfboard will carry the hopes for a rebirth of hypersonics technology when it flies later this year.
If successful, NASA's X-43 Hyper-X will claim the crown of the world's fastest aircraft, held for over 30 years by the X-15. Unlike the X-15, which reached Mach 6.7 on rocket power, the X-43 is designed to fly beyond M7 on air-breathing propulsion - demonstrating the supersonic-combustion ramjet (scramjet) engine which lies at the heart of most hypersonic vehicle designs.
The high and low points of past hypersonic research were just years apart. Over a decade from the mid-1980s, US hopes of breaking the last great barrier in aerospace soared, then crashed, when it became clear that the technology was far from ready. The aftermath has seen the emergence of a more realistic approach to maturing the technology, which could lead to hypersonic missiles by 2010 and spaceplanes by 2025 - at the earliest.
When the X-30 National Aerospace Plane (NASP) programme was launched in 1986, with a Kennedyesque call from President Reagan for a Mach 25 "Orient Express" to be flying by the end of the century, little of the required technology had been demonstrated. The X-30 was to be a full-scale demonstrator for a single-stage-to-orbit (SSTO) spaceplane and required hydrogen-fuelled scramjet and high-temperature materials technology that proved to be then out of reach.
When the NASP was cancelled in 1994, the projected cost of the programme had tripled to over $10 billion and the first flight of an X-30 had slipped more than a decade, to the early years of the new century. Although they tried to co-operate on scaled-down hypersonic flight experiments, NASA and the USAir Force soon went their separate ways, respectively pursuing technology demonstrations for hydrogen- and hydrocarbon-fuelled scramjets.
Two programmes emerged from the wreckage of the X-30: NASA's Hyper-X and the USAF's HyTECH (now called HySET). Together they are developing technology for hypersonic vehicles ranging from M8 missiles to M25 spaceplanes.
With delivery of the first X-43 air vehicle to NASA Dryden in November, the Hyper-X programme is entering a critical phase. The vehicle will be mated to its Pegasus air-launched booster this month and is set to fly at the end of May. The vehicle will be dropped from NASA's B-52, boosted to 100,000ft (30,500m) and released to demonstrate the ignition and operation of a hydrogen-fuelled scramjet at M7.
This first flight, and a second M7 test set for December, will allow direct comparison with windtunnel tests of the same engine at the same speed, says deputy programme manager Don Gatlin. The third flight, planned for September 2001, will be at M10 - "faster than we can go on the ground," he says. "An operational vehicle would go even higher than M10, depending on the mission".
If successful, the 3.7m (12ft)-long X-43 will be the first aircraft to fly under scramjet power - if only briefly, the engine running for just over 5s before the vehicle splashes down off the California coast. Supersonic combustion should start the instant fuel is introduced into the engine, but Gatlin says a hypergolic mixture of hydrogen and silane will also be injected "to make sure".
Scramjets in Australia
There is a race, of sorts, to flight test a scramjet. Under its HyShot programme, the University of Queensland, Australia, is to demonstrate supersonic combustion in two flights of a hydrogen-fuelled scramjet experiment atop Terrior Orion sounding rockets to be launched from Woomera in June. Each flight will provide 5s of scramjet operation at around M8 as the rocket re-enters the atmosphere, allowing correlation with tests in the university's hypersonic shock tunnel.
"The objectives of the HyShot programme can be equated to breaking the sound barrier in flight, but for HyShot it will be the combustion sound barrier that is broken," says vice-chancellor Professor John Hay. The $1 million programme is supported by Australian, German, UK and US research agencies.
European scramjet technology efforts have taken a typically low-key path over the last few years, but several French programmes, and particularly growing co-operation between France and Russia, look set to redress the balance.
In the first half of 2000, the Moscow Aviation Institute (MAI) will deliver the world's first variable-geometry scramjet to Aerospatiale Matra's Bourges test centre, near Paris. This will mark the culmination of a co-operative project to develop an engine that - given the lack of any other programme - will lead Europe's work on SSTO launch vehicles, and be aimed at future hypersonic missiles.
The Wide Range Ramjet (WRR) combustor has run many times already at MAI, but its arrival in France will herald the beginning of a programme to evaluate the data and determine whether a variable-geometry scramjet can be made to work in a practical application.
Aerospatiale Matra has 30 years experience in ramjet design and is the European leader in the field. It received a boost with the mid-1990s Programme de la Recherche pour la Propulsion Hypersonique Avancée (PREPHA), funded mainly by the French Government and bringing together Aerospatiale Matra and Dassault, engine makers Snecma and SEP and research agency Onera.
Since 1997, Onera and Germany's DLR have been leading an internal programme, JAPHAR, aimed at defining an experimental vehicle able to fly autonomously at M4-8. This extends the PREPHA research and is based mainly on developing the necessary numerical codes for computational aerothermodynamics. It also focuses on the eventual development and ground testing of a hydrogen-fuelled, dual-mode ramjet featuring subsonic combustion up to M6 and supersonic combustion thereafter.
Prepha ended last year, having "given us initial knowhow in scramjet and dual-mode ramjet component design - including the inlet, combustor, injection struts and nozzle - and in hypersonic air-breathing vehicle systems". This knowledge will be applied to "space launchers, missiles and experimental flight vehicles".
The main product of the programme was the Chamois combustor, tested repeatedly at up to M6, leading to new techniques for measuring the extremely high temperatures, pressures and gas speeds associated with supersonic combustion. Different combustion chamber shapes were analysed and several types of fuel injector tested. A new combustor is under development that will be tested at M7.5.
The Chamois combustor is built of stainless steel and has an inlet area of around 0.05m² (0.54ft²). It is essentially of a box with up to three horizontal struts into which hydrogen fuel is injected. The position, angle and number of struts can be varied - but only between tests. The strut leading edges are water cooled and combustion lasts between 3s and 10s.
The WRR variable-geometry combustor also measures 0.05m² at the inlet. Initial combustion is subsonic, using kerosene. The combustor then moves to supersonic operation, using hydrogen fuel. Propulsive efficiency is optimised using computer-controlled moveable panels in the combustor. The device is designed to be capable of sustained running at M12.
Variable geometry
The test programme at Bourges will use the variable-geometry feature to set up optimum conditions for running the scramjet at up to M6.5. It is hoped this will lead to a comprehensive understanding of the transition between subsonic and supersonic combustion, "to help make decisions on dual-mode combustors", says Aerospatiale Matra.
The tests will also look at the use of two fuels in the same engine - kerosene for the atmospheric phase and hydrogen for the space portion of the mission. Finally, attention will focus on an active fuel-cooling system for the movable injection struts and combustor walls.
Studies will concentrate on the potential use of the engine for an air-breathing SSTO space launcher, comparing the results with those from PREPHA. Initial results indicate the variable-geometry combustor holds real promise for a future SSTO vehicle.
NASA also has an SSTO spaceplane in its sights, and the Hyper-X programme's X-43 is essentially a 10% scale model of the original X-30. As such, the vehicle is a "waverider" - shaped to use the shockwave flowfield to generate lift and compress air entering the engine. The airframe-integrated engine configuration favoured for hypersonic designs, where the forward fuselage acts as the compressor and the aft as the nozzle, will be flight tested for the first time with the X-43.
NASAis looking beyond Hyper-X along the road to a third-generation reusable launch vehicle that could enter service after 2025. The next step could be a 15m-long experimental vehicle that would be air launched, boosted to M4 then accelerated under ramjet/scramjet power to M6-7 to study the subsonic-supersonic combustion transition. The unmanned vehicle would be recovered to a low-speed landing, Gatlin says.
Beyond that there are "dreams" of a larger experimental vehicle, still unmanned, that would be able to take off horizontally, he says. This would require development of a propulsion system that would work from zero airspeed to hypersonic Mach numbers. The favoured candidate, Gatlin says, is the rocket-based combined cycle (RBCC) engine.
The RBCC concept essentially integrates a rocket into a ramjet/scramjet to provide thrust at low speed and once the vehicle is outside the atmosphere. Aerojet has ground-tested a scale model of its Strutjet RBCC from zero speed at sea level to M8 at 140,000ft (42,700m), and believes the concept is ready for flight testing.
The Strutjet acts as an air-augmented, ducted rocket on take-off, a ramjet from M2.4 to M6, a scramjet from M6 to beyond M8, and a pure rocket thereafter - all within the same flowpath. The key to the engine is wedge-shaped vertical struts in the inlet which compress the incoming air, inject the fuel and house the rocket thrusters. The Strutjet will run on hydrogen, preferred for space launchers, or hydrocarbon fuel, preferred for atmospheric vehicles.
The US Air Force, meanwhile, has continued its efforts to develop technology for hydrocarbon-fuelled scramjets. The first use is likely to be in a hypersonic strike missile, but the USAF is looking in the long term at applications including a global-range, quick-reaction reconnaissance/strike aircraft - manned or unmanned - and a military spaceplane.
A scramjet developed by Pratt & Whitney for the HyTECH programme has been selected by Boeing Phantom Works to power the weapon it is designing under the US Defense Advanced Research Projects Agency's Affordable Rapid-Response Missile Demonstrator (ARRMD) programme. This could lead to testing in late 2002 of a M6.5 weapon.
Boeing has selected a waverider configuration for the ARRMD, "like the X-30, but a narrow, skinny version", says programme manager Joe Hoerter. The missile will be air-launched subsonically, at 30,000-40,000ft, and boosted by strap-on solid rocket motors to M4.5, where the scramjet will take over and accelerate the weapon to M6.5 and 100,000ft. The specification calls for a missile able to cover 750km (400nm) in 7min and strike with an impact velocity of 20m/s (4,000ft/min).
ARRMD is also required to achieve a $200,000 unit cost over a 3,000-missile production run - dramatically less than today's cruise missiles. "The operational utility will depend on its affordability," says Hoerter. "The simplicity of the scramjet approach helps. The big issue is the affordability of the booster. If we can start the scramjet at a lower Mach number, we can use a smaller booster."
There is no urgent requirement for a hypersonic missile, he says. "We are not being driven by a need date, but by a technology push." If development began around 2005, a weapon could be in service around 2010.
Last year, the French arms procurement agency decided to extend the PREPHA research and fund a new research programme, Promethee. Its goal is to explore the feasibility of a dual-mode hydrocarbon-fuelled ramjet/ scramjet to power an air-launched missile.
A generic missile has already been designed. Capable of M8, it would be 6m long and weigh around 1,700kg (including solid propellant boosters). Three concepts were evaluated: pure scramjet operating from M5 to M8; fixed-geometry dual-mode ramjet/scramjet operating from M2.3 to M8; and variable-geometry ramjet/scramjet operating from M1.8 to M8.
Perhaps not surprisingly, in view of the work under way with the Russian variable-geometry scramjet, the latter concept was chosen.
A full-scale model of the Promethee concept is in design and should enter tests in early 2001. While it has not yet been admitted, it is likely that the Russian work will be incorporated into the programme, meeting the aim of a "single future flight experimentation programme aimed at demonstrating the positive aeropropulsive balance of a very high-speed engine".
In this wording can be seen the main - and still unknown - challenge of scramjets. Can such engines be made to produce positive thrust? If the answer is yes, the future of missile and aircraft technology is about to undergo its most dramatic advance yet.
If Hyper-X and HyShot prove that scramjets can work in flight, there is no shortage of concepts waiting in the wings, including heirs to Reagan's original vision of an Orient Express. One is the Lawrence Livermore National Laboratory's HyperSoar, an M10 aircraft that would avoid the heat build-up of hypersonic flight by "skipping" along the atmosphere's edge.
The HyperSoar would fly to 130,000ft, shut off its engines and coast back into the atmosphere, restarting the air-breathing scramjets and skipping back into space. The laboratory says a 90min flight from the USA to Japan would require 25 skips. It estimates demonstration of the HyperSoar would cost less than $500 million.
While such a roller-coaster ride is still decades away, hypersonics look set for a revival in the new century.
Source: Flight International
