How fast should the next US bomber fly? Could a high-speed missile do the same job? US research into high-Mach vehicles is seeking the answers
US hypersonics research is not dead, just headed in a different direction. The military need for long-range strike is adding impetus to research into high-speed aircraft and missiles, but there is debate over how fast the next US bomber needs to fly.
Boosted by the Mach 9.7 flight of the X-43A Hyper-X demonstrator in November 2004, hypersonic hopes plunged soon after when NASA terminated its pursuit of air-breathing reusable launch vehicles to focus instead on a low-risk rocket-and-capsule architecture for space exploration.
Taking the vanguard in the vacuum that ensued, several projects have kept US high-Mach research alive, but with a distinctly military intent. Where NASA was looking decades ahead at a successor to the Space Shuttle, today's research has "off ramps" that could produce practical near-term applications.
High-speed potential
Potential products of current research could include high-speed missiles, long-range strike aircraft and quick-reaction launch vehicles, but much more flight testing will be needed before defence planners will consider a hypersonic vehicle to be a viable option any time within the next two decades.
"The problem is technology maturity," says George Muellner, president, Boeing Advanced Systems. "The TRL [technology readiness level] is not appreciably different to when we shut down the X-30." An overambitious attempt to develop a Mach 25 single-stage-to-orbit spaceplane, the X-30 was cancelled in 1993 before being built - although the design was scaled down to produce the X-43A.
Research projects now under way are intended to bring high-speed technologies to a readiness level of 6 through flight testing. Achieving TRL 6 by demonstrating a prototype in a relevant environment is increasingly seen as the minimum level of maturity required for entry into development with acceptable risk.
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| X-51A will demonstrate scramjet acceleration |
The US Air Force's target of fielding a new long-range strike (LRS) capability by 2018 means only technologies already at or close to maturity will make the grade. Turbine- and ramjet-powered high-speed missiles could be candidates, but hypersonic vehicles with supersonic-combustion ramjets or turbine/scramjet combined-cycle engines are likely to be contenders only when looking further out, to 2025 or even 2035.
Two designs for high-speed strike weapons will be flight tested next year. Lockheed Martin is developing a high-Mach turbine-powered missile under the Revolutionary Approach to Time-Critical Long-Range Strike (RATTLRS) project led by the Office of Naval Research (ONR), while Boeing is developing the dual-combustion ramjet-powered HyFly hypersonic missile demonstrator for ONR and the Defense Advanced Research Projects Agency (DARPA).
RATTLRS aims to flight test a vehicle capable of accelerating from a subsonic launch to a supersonic cruise with only a turbine engine, then cruising at M3-plus for at least 5min - with the goal of exceeding 15min, for a range approaching 1,000km (500nm). "Five minutes is good enough to demonstrate relevant range. At 15min we get into the class of current cruise missiles," says Craig Johnston, RATTLRS programme manager at Lockheed's Skunk Works.
To demonstrate "joint tactical weapon system traceability", RATTLRS is representative of a 900kg (2,000lb)-class missile able to be launched from fighters, bombers, ships and submarines. "We want to make it a minor step to go from science and technology to development and acquisition," says Lawrence Ash, ONR programme manager. "Instead of testing something the size of a Mack truck, we have parameters around length, weight and diameter that keep us in the box."
Turbojet power
Key to RATTLRS - and certain other high-speed concepts - is a turbine engine able to operate to speeds of M3-4, beyond the normal limit of M2.5. Developed by Rolls-Royce's Liberty Works, the YJ102R turbojet has six times the specific power of the Lockheed SR-71's Pratt & Whitney J58. "That was a cruiser this is an accelerator. It does it a 6% of the weight, 25% of the diameter and 20% of the length - and it's dry: the SR-71 needed afterburner to accelerate and to cruise," says Ash.
In size and shape RATTLRS resembles an SR-71 nacelle, but the engine is much smaller, just 330mm (13in) diameter. "The J58 was four times the size, but only two times the dry power," says Johnston. Although similar, the flowpath is significantly simpler while maintaining performance that is hard to beat even today, according to Lockheed. "The SR-71 had a fairly complex flowpath and 1960s controls. We have very modern aerodynamics and control software," he says.
The "secret sauce", says Johnston, is Lamilloy, an R-R-developed material made from laminated layers of perforated metal through which compressor bleed air is blown to provide transpiration-like cooling of combustor and turbine components. "Lamilloy can operate at very high temperatures, so air goes to thrust not cooling," he says.
"RATTLRS takes the engine, puts it into an integrated system that is tactically relevant, with room for fuel, payload, guidance and navigation, and flight demonstrates it," says Ash. Engine ground tests are planned for early next year, with flights from a Lockheed NC-130F drone launch aircraft to begin late in 2007. "We get to TRL 6 with the first successful flight," he says.
With three times the speed of a cruise missile, an operational weapon could hit a target in 5-10min, cruising at 70,000ft and diving at M4 to penetrate 9-15m under ground, or alternatively dispensing submunitions subsonically or supersonically. "For the time-sensitive stand-off mission there is a wide range of targets to look at, and a range of payload types, not just kinetic energy," says Ash.
While a competitor to RATTLRS, HyFly is a propulsion demonstration, not missile development, programme, says John Fox, Boeing's HyFly programme manager. The goal is to achieve speeds up to M6 and ranges up to 740km using a hydrocarbon-fuelled hybrid ramjet/scramjet, to demonstrate technology for a penetrating missile that, like RATTLRS, could be launched from an aircraft, ship or submarine.
In flight tests planned for early next year, HyFly will be launched from a Boeing F-15, rocket-boosted to M3.5, then accelerated to M6 by its dual-combustion ramjet. Developed by Johns Hopkins University Applied Physics Laboratory and Aerojet, this comprises a ramjet "pre-burner", or subsonic gas generator, that feeds its fuel-rich exhaust into a scramjet, where supersonic air is added and combustion completed.
HyFly weighs 1,100kg including booster. "A tactical version would weigh less, and be in the same cost range as a cruise missile," says Fox. "The technology is becoming interesting to the services for next-generation cruise missiles, and we are getting it ready for development, but there is no current transition plan."
The same high-speed strike requirement that produced HyFly also led to the X-51A, a scramjet engine demonstrator (SED) that is planned to fly late in 2008, and reach M6.5. Led by the US Air Force Research Laboratory (AFRL), and also called WaveRider, the project follows on NASA's Hyper-X as it uses a version of the HyTECH hydrocarbon-fuelled scramjet once planned for the cancelled X-43C.
In a step towards the X-51A, Pratt & Whitney Rocketdyne (PWR) in April completed tunnel tests of its GDE-2 ground demonstrator engine at M5.
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| F-22 armed with RATTLRS high-Mach missile is a long-range strike option |
Fuel-cooled scramjet
Intended as the centre engine for the X-43C, GDE-2 is the first flight-weight, fuel-cooled scramjet to be tested, and has reduced risk for the SED engine that will power the X-51A, says Bob Mercier, deputy for technology at AFRL's aeropropulsion directorate.
In the HyTECH scramjet, JP-7 jet fuel is circulated through the structure to cool the engine and heat the fuel before it is injected into the combustor. GDE-2 was the first to use closed-loop fuel control, and flight-weight valves. It also demonstrated the transition from ethylene, which is used while the engine heats up, to JP-7 fuel. "We start with ethylene because it is more reactive," says Mercier.
"We will start the SED the same way," says Curtis Berger, PWR's X-51A programme manager. GDE-2 is a bit longer and wider than the 230mm-wide SED, which is also lighter because of its welded rather than bolted construction and fixed- rather than variable-geometry inlet. "But the combustor, cooling scheme and control system are the same," he says.
The first SED scramjet, SJX61/1, is to begin ground runs in October, and will be tested at low, mid and high Mach number as the X-51A is intended to demonstrate the ability to accelerate and operate over a range from M4.5-6.5. The SJX61/2 flight-clearance engine will run next year, and be followed by four SJY61 flight engines.
Launched from a Boeing B-52 and rocket-boosted to M4.5-5, the expendable X-51A will separate and accelerate to M6.5 under its own power. "It will be the first scramjet to accelerate over a range of Mach numbers. We expect it to transition from mixed subsonic/supersonic to pure supersonic combustion," says Mercier. Although the scramjet will power the vehicle for just 5min, "once it is thermally balanced, we could run it for as long as we like", says Berger.
When flight tests are complete in 2009, the scramjet is expected to be at TRL 6, but the X-51 is viewed as a propulsion testbed, and not a potential weapon. There is now talk of an X-51B, which would test a combined-cycle engine being studied by AFRL and DARPA. This would combine the HyTECH scramjet with a high-Mach turbojet - a follow-on to the YJ102R - being pursued under the High-Speed Turbine Engine Demonstration (HiSTED) programme.
Under HiSTED, both Rolls-Royce and Williams International plan to demonstrate M4-plus turbojets by adding ramburners to small supersonic engines previously tested under the long-running Integrated High-Performance Turbine Engine Technology programme. Liberty Works' XTE18 engine, based on its XTL17 demonstrator run in 2005, is scheduled for testing in the first quarter of 2008, followed about a year later by Williams' XTE88.
Carbon-carbon ramburner
"XTL17 tested most of the technologies for RATTLRS," says Lance Chrisinger, AFRL's HiSTED programme manager, adding: "Williams ran a supersonic engine [XTL87] in 2004, and developed the basic design of the XTE88 under RATTLRS before the downselect." While aimed at expendable engines, the technologies developed apply to reusable engines. These include the carbon-carbon ramburner, which burns a mix of core air and bypass air ducted round the engine.
A propulsion system combining a high-Mach turbojet with a hydrocarbon-fuelled scramjet is in the plans for another US hypersonic demonstration - DARPA's Falcon project. While the other programmes focused on propulsion, Falcon set out to conquer the aerodynamics and materials challenges to sustained hypersonic flight through a series of unpowered test vehicles. Under plans now taking shape, the programme could culminate in a powered unmanned demonstrator capable of taking off from a runway, cruising at M6 and landing.
The demonstrator would be a subscale version of a hypersonic cruise vehicle (HCV) that could be operational by 2035 - a 38,500kg aircraft with 5,500kg payload capable of reaching anywhere in the world from the USA within 2h, at speeds up to M10. "It's the next level of hypersonics, and picks up where X-30 left off," says Frank Capuccio, Lockheed vice-president and general manager, advanced development programmes.
Lockheed plans to build two expendable HTV-2 test vehicles for flights in late 2008 and mid-2009. The vehicle will be boosted to over 300,000ft by rocket then glide at a "high teens" Mach number for 30min, says Steven Walker, DARPA Falcon programme manager. Plans then call for a reusable HTV-3 to be flown a couple of times at around M10 to demonstrate higher aerodynamic efficiency and reusable thermal protection.
DARPA also plans to ground-test a turbine-based combined-cycle (TBCC) propulsion system for a global-range HCV, says Bob Baumgartner, Skunk Works' Falcon programme manager. This features a Lockheed-designed "inward turning" inlet that provides air to either a Liberty Works high-Mach turbine engine or a Pratt & Whitney Rocketdyne dual-mode ramjet. Windtunnel tests of the inlet, ramjet and integrated nozzle are planned this year and could lead to subscale TBCC engine tests in 2007.
Whereas the X-43 and X-51 use the vehicle forebody and a planar inlet to capture and compress engine airflow, the Falcon has a three-dimensional "streamline-traced" inlet that turns the flow towards the centreline of the propulsion system. Advantages are lower weight, less area, higher pressure recovery and the ability to integrate the engine into the vehicle more efficiently for higher lift/drag ratio, says Walker.
Next generation
"The X-43 engine had no utility," says Capuccio, adding that mounting the Falcon's next-generation engines outboard "allows us to use the bottom of the aircraft for weapon release and the top of the aircraft for satellite release". Whereas an operational M10 HCV would be "B-52-sized", the M6, TBCC-powered HTV-3X flight demonstrator now being discussed would be about 15.2m long.
A hypersonic aircraft may be decades away, but Capuccio believes the next US bomber could be supersonic - M2.5-3.5 - and unmanned. Boeing's Muellner is doubtful: "You don't gain survivability until you get very fast. At M2.5 you don't gain much time compression. You have to get very much higher if you are to rely only on speed to survive." Whether hypersonics will play a role in US long-range strike plans will depend on the X-51A, he believes. "If it succeeds, there is potential for something in the 2025 timeframe." ■
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| Boeing conducted captive carry tests of HyFly |
Source: Flight International
