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Turbine technology

Guy Norris/LOS ANGELES

The newest US combat aircraft shattering the skies over Farnborough in 2010 will be powered by an engine which, compared to its 1990s' ancestor, will have double the thrust-to-weight ratio, yet will cost one-third less to make and to maintain.

Futuristic though these goals may seem, they are well on the way to being achieved by the US Integrated High Performance Turbine Engine Technology (IHPTET) programme. This joint government/industry initiative is now in its tenth year and the fruits of its labours are to be seen at this year's Farnborough show - like the fan technology for the General Electric F118 powering the Northrop Grumman B-2.

IHPTET is roughly 50% funded by Government and 50% by industry, and is tasked with "doubling propulsion capability by the turn of the century". As this milestone approaches, the IHPTET team is reviewing progress and planning new goals for 2010 and beyond. As the programme covers expendable engines - turboshafts and turboprops, as well as turbojets and turbofans - industry participants cover the full range of US propulsion specialists and include AlliedSignal, Allison, General Electric, Pratt & Whitney, Teledyne and Williams.

The focus for all IHPTET efforts is a series of engine demonstrators. These assess the characteristics and performance of advanced components in a realistic engine environment. In the large turbofan/turbojet class, these demonstrators are the Advanced Turbine Engine Gas Generator (ATEGG) and the Joint Technology Demonstrator Engine (JTDE). Components are tested first in an ATEGG, which is basically a core engine. A low pressure system is then wrapped around the core and tested as a JTDE.

CRUCIAL MILESTONE

The two main turbofan teams, GE-Allison and P&W, are reaching crucial milestones as they complete phase II and begin to move towards phase III (see box). Testing of the latest GE-Allison core, the XTC76/2, was due to begin within days of the start of the Farnborough show and could total up to 200h over the next three months. "At that point, we're going to take the core and directly build it into a JTDE engine," says GE manager of advanced military engine technology Harvey Maclin.

The XTC76/2 represents the last phase II turbojet/turbofan core work for the team before it finishes phase II with a full engine demonstrator, the XTE76/1, around July 1999. "That engine will have a fixed exhaust-nozzle architecture and will be capable of delivering the same SFC [specific fuel consumption] as a conventional engine with a standard exhaust," says Maclin. The "fluidic nozzle" fixed exhaust system is "-probably the key to our being selected as prime [for phase II]", he says. "Exhaust systems are either the number one or two maintenance actions, so, if we can eliminate that, we're on track to achieve that goal".

With fluidic area and thrust vectoring control, the fixed nozzle is expected to reduce weight by 60% and costs by 25%. The system, which diverts air from the compressor into the nozzle throat to alter the direction of the main jet, also reduces the infrared (IR) signature. "We have run component tests and we know we have vectoring capability up to 7¼ or 8¼," says Maclin.

GE's earlier work on the axisymmetric vectoring exhaust nozzle (AVEN) has not been forgotten. "The AVEN didn't have the complexity that most people imagine," adds Maclin. A low signature version of the AVEN demonstrated IR values comparable to current two-dimensional convergent/divergent nozzles, but at 50% less weight, 60% less cost and with 300 fewer parts.

"At the conclusion of the JTDE test, we'll bring the XTE76/1 back, refurbish it and send it to NASA to demonstrate its T3 [combustor inlet temperature] capability. The goal for phase II is a 200íF [93°C] improvement in T3, and we should be testing for that at the end of 1999," says Maclin. This is roughly "-one year behind where we wanted to be relative to the original schedule", he admits.

Other technologies in the new engine include forward-swept fan blades. "From earlier research work, we found that forward sweep gave between 1% and 2% added efficiency, and about a 50% improvement in operability," Maclin says. "It helps us achieve higher fan pressure ratios with fewer stages and we have a highly loaded high pressure-ratio compressor. The result is up to 2% improvement in compressor performance, plus a doubling of pressure ratio with only two additional stages."

Much of the fan research has come from teaming with Allison - originally "inspired" by the Government, which requested that the two companies combine for the IHPTET programme. "Everything is not bright and happy, but in general the team is working well. We are learning from each other and are moving along at a much faster rate as a result. Our goal is to allow both companies to achieve all technology levels at the same time," Maclin says.

Part of Allison's contribution was its much vaunted Lamilloy single-crystal turbine material, which has already achieved phase II turbine rotor inlet temperatures. "The jury is still out regarding GE's opinion of that," says Maclin, who thinks more data are needed. Lamilloy was "-a foot in the door [for Allison]. But we've learned since then that they have more capabilities than heat transfer. The one we are counting on is in combustion systems. They have a single annular combustor doing the same job as we did with a double configuration," says Maclin.

Combustion represents one of the most interesting advances in the XTC76/2, which sports a "trapped vortex" system. This eliminates the traditional "dome" approach to combustion, relying on a series of vortex generator-like devices built into the combustor liner instead of nozzles or swirl cups. The trapped vortex flame stabilisation concept, as it is formally known, is being tested in the second sector of the combustor. It is expected to produce a more compact, robust and affordable combustor design with reduced length, weight and pressure loss.

The XTC76/2 ATEGG will also demonstrate GE's "coupled turbine" concept, which integrates the low pressure and high pressure turbines, allowing them to counter-rotate. "We do this so we don't have to bend the inlet guide vanes. We call it coupled because they have to be designed together," says Maclin.

TRANSISTION POTENTIAL

Meanwhile, P&W wraps up IHPTET phase II with the XTE-66/1 JTDE, which is expected to begin tests in November, according to advanced military engines programme manager Jim Reid. The engine is scheduled for "-about three months of tests and, if it is as successful as XTC-66 and the data is good, then there could be a lot of potential for transition [of key technologies] to the mainline products", he adds.

"With XTE-66 we will demonstrate a thrust-to-weight improvement of 50% over the baseline. We're pretty confident, although it has got a lot of new technologies - all of them promising - built into it," he adds. The demonstrator also forms the initial prototype of P&W's next generation PW7000 fighter engine family. Like the XTE-66, the PW7000 is planned around the XTC-66 core which was also the forerunner of the core of P&W's commercial PW6000 turbofan and PW8000 geared fan engines.

In the meantime, the XTC-66 core is being inspected after completing tests. Component performance "-met or exceeded" expectations, says P&W, which was pleased with results from the advanced compressor flowpath design which went from design to tests without the normal rig test. The core ran at temperatures "-1,000°F [537°C] hotter than were achievable 10 years ago, and at full lifetime capability", Reid says. "This engine has the same pressure ratio in five stages that the current engine has with 10. We are doing more with less and have fewer parts to maintain, so there's an instant affordability benefit. That's why we're using the XTC-66 as the basis for the PW6000, which is a very low cost JT8D replacement. It needs five stages of compression instead of 10 or 14."

P&W is focused on spinning off applications of its IHPTET activities as quickly as possible. This began early in the programme, says Reid. "The Government funded GE and P&W for phase I, but only P&W achieved the targets. For phase II, GE and Allison teamed to demonstrate the actual goals, which allowed P&W to focus on more product-applicable goals," he says.

The result is that P&W is seeking a 50% thrust-to-weight improvement, while the GE-Allison team is still determined to reach the original IHPTET phase II goal of 60%. "From a purist perspective we are therefore not as high in performance terms, but just getting a 30% to 50% improvement is the equivalent of climbing Mount Everest," Reid says, adding: "GE-Allison is almost two years behind us."

The adoption of derivatives of P&W's F119 engine as the baseline for the Joint Strike Fighter (JSF) contenders is also linked to IHPTET. One version of the XTC-66 core, the -66/SC, was made into the Core and Engine Structural Assessment Research (Caesar) vehicle and used as a transition to the JSF119.

"It was a structural core and we ran it to the limit. Testing was finished on that last year, and now we have 2,000 cycles on the JTDE equivalent, the XTE-66/SE [structural engine]," says Reid. Development of a demonstrator core sized for the JSF119 is under way with the XTC-67/1. A subsequent XTE-67/1engine is planned which is sized for the JSF application.

The XTC-67/1 is due for test in early 2000, with the XTE-67/1 provisionally planned for testing from mid-2000 onwards. "Plans are in place to demonstrate [IHPTET] phase III goals in 2003," he adds.

GE-Allison is also under contract for a phase III core engine, the XTC-77/1 which will run in the fourth quarter of 2001, says Maclin. The team also has started negotiations on the phase III JTDE, the XTE-77/1, and "-expects to be under contract in October". Only one team will be contracted to demonstrate the actual phase III goals, but the other will be "-positioned to move into a slot if the prime fails," he adds.

There are plans to take IHPTET beyond phase III. "We've established a set of goals for phase IV, but there is no funding pencilled in for this beyond 2003," says Reid. "Our job is to put together a compelling argument for IHPTET. Maybe it is time to go for the affordable approach. In the past it may have looked like a hot rod, but these days it needs to be an affordable hot rod."

IHPTET TIMETABLE
Phase 1 (1991)
AllisonXTC-15
GEXTC-45
GEXTE-45
P&WXTC-65
P&WXTE-65/1 (interim phase I demo)
P&WXTE-65/2 (phase I goal demo)
Phase II (1997-99)
AllisonXTC-16 (initial phase II demo)
P&WXTC-66/1B (rebuild of -66/1A, phase II 90% goal demo)
P&WXTC-66/SC (Caesar core test/JSF119 transition)
P&WXTE-66/1 (planned 90% phase II goal demo)Phase II (continued)
P&WXTE-66/SE (Caesar engine test/JSF119 transition)
GE-AE*XTC-76/2 (ATEGG test, core fan phase II demo) - see note 1
GE-AE*XTC-76/3 (phase II core demo - T3)
GE-AE*XTE-76/1A,B (phase II engine demo)
Phase III (2003)
P&WXTC-67/1 (initial phase III core)
P&WXTE-67/1 (initial phase III engine demo)
GE/AE*XTC-77/1 (initial phase III core)
GE/AE*XTE-77/1 (initial phase III JTDE)
*Allison advanced development. Note 1 XTC 76/2 was a variable cycle engine ATEGG test

 

THRUST VECTOR FACTOR

Howard Gethin/LONDON

Current developments in military propulsion are aimed firmly towards reducing life-cycle costs rather than increasing aircraft performance. "With the lack of a clear superior threat, the emphasis is on cost reduction,"says GE.

That may be so but, despite talk of dogfighting belonging to the past, thrust vectoring continues to attract attention. If the engine manufacturers have their way, most of the next generation of fighters will at least have the option for thrust vectoring.

Russian manufacturers have no doubts. Klimov has developed a vectoring nozzle for the MAPO MiG-29's RD-33 engine. Designated the RD-133, and with a projected +/-15ívectoring capability, this engine has completed ground testing. It is uncertain whether the RD-133 will be flight tested, given the emergence of the upgraded MiG-29SMT (with fixed nozzles) over the projected advanced MiG-35 and apparent MAPO reluctance to fund the programme. A further modification, the VKS-10 nozzle, is thought to be for China's new Chengdu F-10 fighter.

Lyulka Saturn, meanwhile, is designing the AL-55 thrust vectoring turbofan, intended for future advanced trainers and light attack aircraft. One intended application is the MiG-AT, now powered by the Snecma/Turboméca Larzac.

Lyulka Saturn has the distinction of having the most advanced operational thrust vectoring system, on the Sukhoi Su-30MK-6 and to be retrofitted to India's Su-30MKIs. So far, the Su-30MKI nozzles have only single-axis movement, but they have been realigned to operate 32¼ outwards from the vertical plane, providing a considerable lateral force when the nozzles are moved differentially.

Lyulka Saturn continues development of the AL-41F engine for a future Russian MFI (multifunktsionalni logkiy istrebitel) fighter project, despite the lack of an airframe programme. Delays with the AL-41F, details of which remain scant, resulted in Perm Aviadvigatel providing modified D-30F6 turbofans to power Sukhoi's S-37 forward swept wing demonstrator. It is not known whether the AL-41 has thrust vectoring as standard.

Western Europe has shown a belated interest in thrust vectoring. Perhaps the keenest proponent has been Germany, with Daimler-Benz Aerospace (Dasa) continuing work on the X-31 enhanced fighter manoeuvrability demonstrator. The three-nation (Germany, Sweden and the USA) vector programme will get under way with the signing in September of a memorandum of understanding, leading to a further series of X-31 test flights with an axisymmetric vectoring nozzle developed by Dasa and ITP of Spain. These may lead to tailless flights.

Germany's MTU and ITP are continuing development of a three-dimensional thrust vectoring nozzle for the Eurojet EJ200 powering the Eurofighter EF2000, but despite a successful first engine test with the nozzle, there is as yet no firm customer. Eurofighter maintains that there is no current requirement for thrust vectoring. Flight testing of the nozzle on the EF2000 is planned for early 2001.

MTU is responsible for integration of the nozzle with the engine and software changes, while ITP will build the nozzle mechanism. This gives 20° of thrust deflection, using a three-ring system.

Eurojet is offering an uprated EJ200, the EJ230, for the Saab JAS39 Gripen, aimed principally at export customers. The engine manufacturer plans 700h of flight tests for the 23,000lb-thrust (102kN) engine, which will be fitted with an ITP-designed axisymmetric thrust vectoring nozzle.

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