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Future thrust

The next two months will see the debut of key military engine technologies on both sides of the Atlantic. Eurojet's EJ200 should be flown for the first time, while Pratt & Whitney was due to begin testing its latest combat core, the XTC-66.

Europe's latest collaborative military engine programme should become airborne with the flight of Eurofighter 2000 Development Aircraft 3 (DA3), the first to be fitted with the EJ200 turbofan.

The XTC-66 is P&W's first attack on Phase II of the US Department of Defence's (DoD) three-phase, Integrated High Performance Turbine Engine Technology (IHPTET) initiative.

The thrust of this critical DoD programme is to secure near-revolutionary performance increments for the next generation of combat-aircraft power plants.

The technology used in the XTC-66 could boost fighter-engine thrust-to-weight ratio by up to 60%, compared to the baseline YF119 developed for the Advanced Tactical Fighter (ATF) competition.

By 2003, under the IHPTET programme, Allison Engines, P&W, and now General Electric, aim to demonstrate engine thrust-to-weight ratio up by 100% and cut fuel consumption by up to 38%.

Progress has been rapid since IHPTET began in the 1980s - Allison and P&W have achieved their performance goals ahead of schedule; confidence is high that IHPTET targets, once believed impossible, will be met; and early planning for "beyond IHPTET" is under way.

Allison, which teamed with GE in 1994, to pursue further IHPTET contracts, has successfully run what many in the industry classify as, one of the most advanced cores in the world.

In 1994, the company's XTC-16/1B variable-cycle core achieved stable operation at turbine-rotor temperatures, which had not been considered attainable until the end of 1997.

In 1995 it plans to run a new version, dubbed the XTC-76/2, featuring single-crystal Lamilloy turbine blades. Lamilloy is an Allison-developed technology whose "transpirational" cooling uses pathways within laminated alloy to produce "pores" for heat loss.

The XTC-76/2 will also use metal-matrix composite (MMC) materials in the compressor. The MMC likely to be used in the core is a gamma titanium aluminide, which has half the density of the nickel-based alloys normally used. The reduced mass of the rotating unit means the compressor ring and containment case can also be made lighter.

The Allison/GE team has been selected by the DoD as the major-contractor team to demonstrate the Phase II goals. Harvey Maclin, manager of GE's advanced engine operations, says: "The team uses integrated component teams to perform the design, analysis and hardware procurement. The work is not split by component, but by the people who have the best skills, thereby achieving the highest level of integration and confidence that the goals will be achieved."

To ensure the recent acquisition of Allison by Rolls-Royce does not compromise any "top secret," or "above top secret" work, the Indianapolis-based organisation has created a wholly owned subsidiary, named Allison Advanced Development, to handle restricted and classified programmes such as IHPTET. Larry Burns, chief engineer of advanced projects, is expected to become manager. Maclin says the "...GE/Allison approach to Phase II is to demonstrate a balanced system featuring advanced materials, cooling technology, advanced aerodynamics and cycle architecture."

Phase II goals include running the core engine at turbine-inlet temperatures (TIT) higher than levels achieved during the first phase. With the XTC-65 core developed for P&W's XTC-65/2 Joint Technology Demonstrator Engine (JTDE), inlet temperatures exceeded those recorded in current advanced-technology engines by more than 200°C, beating the Phase I TIT target by more than 35°C.

Towards the end of the third quarter of 1995, the joint Allison/GE team will design the hardware for the Phase II variable-cycle Advanced Technology Engine Gas Generator (ATEGG) and JTDE.

GE's future in advanced fighter engines is considered to be tied to variable-cycle technology (VCE), particularly since its development of the YF120, an operational VCE which was competed against P&W's YF119 for the ATF competition.

As part of its drive for VCE supremacy, GE is concentrating on core-driven fan engines. In these power plants a secondary fan, driven from the core as an extended high-pressure (HP) compressor first stage, helps supercharge the air travelling down the bypass duct.

The secondary fan improves the fuel efficiency of a VCE. Maclin says that the Allison/GE team hopes to complete "the component demonstration of its variable- cycle-core-drive fan stage" in the fourth quarter of 1995.

Earlier work to test a GE-developed core-driven fan and compressor was recently completed at the US Air Force's Wright Laboratories at Wright Patterson AFB, Ohio. The components under test incorporated new materials, which helped prevent twisting of the extended fan blades, a common problem of earlier core-driven-fan concepts.

During the third quarter of 1995, Allison and GE will demonstrate an advanced high-pressure ratio swept-fan design, just as the first demonstrations are due to begin of the core engine technology in an ATEGG test.

The Allison/GE team and P&W each won new ATEGG contracts from the US government under recently announced IHPTET awards. Maclin stresses that these tests will be of the Allison/GE core in its fixed-cycle mode. Full VCE tests are not scheduled until "late 1996, or early 1997." Tests of a full core-driven fan JTDE could follow in late 1997.

As a result of the Allison/GE tie-up, some existing arrangements for GE's JTDE contract needed to be changed. "We have been informed by the USAF that we should move ahead with our JTDE design effort under the current contractual arrangement, while negotiations proceed to assure that the Phase II goals will be demonstrated before the end of 1997," says Maclin.

P&W meanwhile, with the broad experience of P&W XTE-65 under its belt, the pressing ahead with preparations for testing its successor P&W XTE-66.

The first Phase II core tests began in April, says Jimmy Reed, manager of P&W Government and Space Propulsion business Advanced Engineering Propulsion Center.

"Testing will continue for at least three to four months at Wilgoos Test Center near East Hartford [Connecticut]. Being a core engine (without a low-pressure system), it needs an altitude chamber to duplicate the correct conditions," says Reed.

Work on the XTE-66 ATEGG will eventually lead to "...running that core in 1998 with full Phase II capability. Before that we will have initiated Phase III work", Reed says. P&W is concentrating on inserting new technology in the Phase II engine.

"The primary new technology associated with XTE-66 is the super-cooling turbine." The advanced casting process used to produce in-built cooling will be tested on "blades, vanes and outer air-seals. We will be testing the next generation of super-cooling," says Reed.

MMCs also make a re-appearance in the core, having first appeared in quantity in the XTC-65 when it used a first-stage compressor made from a composite of titanium C alloy matrix and silicon-carbide fibres. "In that engine we used it at the front of the core, around the first stage of the HP compressor. In this core it will be used in the second and third stages are MMC," Reed adds.


Caesar (component and engine structural- assessment research) " a collective effort between P&W, Wright Laboratories and the F-22 System Program Office," says Reed. "It was put together to do high-endurance testing on advanced technology which usually runs for only 30 to 200h. In this case, we'll run it for several thousand hours." The high technology can be exposed to Caesar to see if it stands up to the typical rigors of operational life. If it does, the technology could be incorporated in new-production engines, or retrofitted to older ones.

Advanced features will be inserted into a basic F119 "shell" made up of a HP compressor, HP turbine and combustor. "Probably one of the most interesting aspects of the Caesar project is the compressor which will be fitted with blades made by Allison, GE, P&W and Rolls-Royce," says Reed.

Each company is supplying lightweight blades made from varying mixes of gamma-titanium-aluminide alloy. "They all have high strength, but very different blends. Using Caesar we will make an evaluation of their strength," says Reed.

Some earlier development problems with the F119 HP turbine caused P&W to "stand-down" the turbine section of Caesar until the F119 was solidified. The problems were resolved and "...that program is on track and going full-speed ahead," he says.

By early April seven F119 test engines had accumulated 4,300h, some 1,830h of which were made during endurance testing and a further hours were amassed during afterburner trials.


Development of the Eurojet's EJ200 has so far gone relatively smoothly, at least in comparison to some other elements of the Eurofighter EF2000 programme.

Daimler-Benz Aerospace (DASA) subsidary MTU experienced problems in compressor design, and integration of the full-authority digital engine-control (FADEC) system was not trouble-free.

The engine's performance envelope for the first year of test flights in the DA3, which will ensure the FADEC power plant interaction is as predicted has now been cleared.

For all four Eurojet partners - Italy's FiatAvio, Spain's ITP, MTU and the UK's Rolls-Royce - the success of the programme is critical. So far, the signs are relatively auspicious.

The two-spool turbofan has a three-stage low-pressure compressor and a five-stage high-pressure compressor, which is driven by a HP turbine, fitted with single crystal blades.

Total running hours for the engine, are around the 5,000h mark, including one engine some 600h of simulated service operation on one engine. While technically the EJ200 is progressing relatively smoothly, there are potential problems concerning work share.

Because of the continuing changes in the numbers of aircraft which some of the Eurofighter nations intend to take, engine and airframe work share will have to be re-allocated. UK production work will almost certainly have to be taken from Germany and Spain, a move, which could cause political problems.

Europe's competing power plant is France's Snecma M88. With delivery of the first production M88 for the Rafale fighter due at the end of 1996, Snecma is focussing on what must follow if it is to retain its long-established military engine capability.

Snecma is looking to the Saab JAS39 Gripen as the potential launch airframe for a more powerful derivative of the M88, a competition, which Eurojet is also addressing, with the EJ2000.

The M88-3, as it is called, is initially aimed at Sweden's need for a more powerful engine for the JAS39, now equipped with the General Electric/Volvo Flygmotor RM-12, a derivative of the GE F404.

One option is that this engine may be retained and given a "throttle boost", increasing thrust by 5%. Sweden may, however, demand a more potent solution to future Gripen needs and will be able to choose between a more advanced F404, the GE F414, Eurojet 200 or the M88-3.

Either of the latter three would confer a 20% thrust increase over the RM-12, for virtually no increase in weight. Massot considers the Snecma solution the best: "The M88 fits very well into the same engine bay, and by the time of the decision at the end of 1996, the core will have built up significantly more running time. It is the best, lowest-risk solution".

It is hoped that the prospects of a Gripen contract will drive the decision to go ahead with the M88-3, which would then be offered to the French air force for future, heavier, versions of the Rafale. "One thing is certain", says Snecma's military engines vice-president Alain Habrard, "the Rafale will get heavier, and the air force will demand more power". He stresses that while the current M88-2 is "good enough for all perceived requirements", the M88-3 "will definitely be needed one day", pointing out that 80% of the development funding for the engine will come from the French Government.

The M88-3 thrust increase comes from a new, increased-flow, low-pressure compressor now being developed, on which tests began in Snecma's Villaroche plant, near Paris, on 17 February. Technology for the compressor comes from the French CENTOR programme, which incorporates research from the Onera research agency, French universities, and Snecma itself. "The aim is to widen the range of operability," says Masson, "so that the engine is more efficient in intermediate and low-power regimes."

The three stages of the development compressor now in test each feature integrally bladed discs and compressor-discs, known as "blisks". These are lighter than the current units (in which the blades are inserted individually into each disc), while the three dimensional blade design, contributes to the significantly improved performance, namely a higher pressure ratio (4.3 versus 3.8 on the M88-2) and extra airflow (72kg/s versus 65kg/s).

"We don't know at this stage if we will be using blisks throughout the low-pressure compressor", says Massot. "Although we've done a lot of mechanical tests, we have only just begun aerodynamic testing". Much depends on finding the right solution to the blade vibration, which can affect LP compressor blisks in which the blades are relatively long, and there is no natural damping through the movement of the blades in the root. "The trick is to compromise between mechanical strength and aerodynamic performance", says Massot.

The M88-3 will also feature a new nozzle developed under the DRAC programme, which aims to reduce exhaust signature. Massot declines to describe the system, saying only that it will be "very efficient in reheat, with a shorter reheat pipe, and will have a considerably reduced infrared signature". Rig tests of the new nozzle are now under way. "It will be ready at the same time as the rest of the the end of 1996", says Massot.

If Snecma and Rolls-Royce are competing on the JAS39 requirement, then, on the next -generation combat-aircraft engine project their futures are intimately linked.

Anglo-French cooperation on combat aircraft is back in vogue, in part propelled by galloping consolidation amongst US airframe competitors.

British Aerospace and Dassault are tentatively examining a future strike aircraft, to meet their respective air force's emerging future requirements. Rolls-Royce and Snecma are looking to a next-generation power plant for this aircraft under the Advanced Military Engine Technology (AMET) initiative.

An initial agreement on collaboration was signed as far back as November 1990. While hardly a fast-moving programme, its goals are ambitious: to develop jointly core technology for an engine that would have a 15:1 thrust-to-weight ratio by the year 2001, with a possible increase to 18:1 further ahead.

"We're still at the paper study stage", says advanced designs programme manager Jacques Maunand. "We had a kickoff meeting last November, and discussed how we would work together". The two teams will not have any particular responsibility for specific areas of the core demonstrator. "There is no division of discipline", says Maunand. "We will work in parallel, and choose the best approach for the design of the most efficient components".

Acquisition of technologies for core parts is under way and Snecma and R-R are evaluating metal matrix composites, for the high-pressure compressor, with some funding from the European Commission's Euclid programme. In the HP turbine area: "We are looking for more advanced cooling technology, improved coatings, and better nickel alloy-based single crystal materials to bring turbine entry temperatures up to 2,100K", says Maunand, which he adds is "about 260K higher than it is for the M88".

A decision on whether to run a core demonstrator will need agreement between the two companies, and will be crucial to whether the programme goes forward into the next century. Other factors will include the status of UK/French plans to collaborate on a future combat-aircraft programme.

While both industrial partners and the UK Government have publicly backed the AMET programme, the French government has yet to publicly commit to it. This is in part because of the politics and the costs surrounding the Dassault Rafale.

Sources involved in the AMET programme point out that the Rafale has been sold to the French public as an "all-singing, all dancing multi-role combat aircraft."

While the French air force has a tentative requirement for a dedicated replacement for the Mirage 2000D/N strike aircraft, other than the Rafale, sources indicate that "selling" this idea to the French public at the just at the moment could be "difficult."

Germany's MTU is also keen to join the AMET programme. While both the present partners have no objection to this in principle they would like to firm up their own relationship before introducing another member.

Given the German farago over Eurofighter, some sources also indicate that the UK would like to see Germany commit to this programme fully, before entering into further military collaborative agreements.

While in Europe, with the M88, EJ200 and AMET, and the in US with IHPTET, engine developments are continuing apace, Russian programmes appear to have been seriously hit by the near collapse of military funding.

The first flight of Mikoyan's next-generation air-superiority fighter, Object 1.42, has been repeatedly delayed primarily because of problems with the Saturn/Lyulka AL-41 engine.