There can be few propulsion-development efforts as focused, or as consistent, as the US Integrated High Performance Turbine Engine Technology (IHPTET) programme. Tasked with essentially "doubling propulsion capability" (ie, improving all major performance and maintenance measures by a factor of two) by the start of the 21st century, this vast initiative is just at the halfway mark, with all targets achieved to date.

Yet, as plans are made to complete Phase II and award contracts for the start of Phase III, there are changes in the air. The baseline goals of the programme - a joint effort involving the US Air Force, Navy and Army, NASA, the Defense Advanced Research Projects Agency and industry - remain unaltered. The IHPTET project's focus remains on developing technologies for more affordable, more robust, higher-performance turbine engines.

What is changing is the focus of the end user - be it a fighter manufacturer, a missile maker, an unmanned air-vehicle (UAV) designer, or even an airline.

Some of these changes are already apparent, and the IHPTET has been adjusted in recognition of them. Phase II goals, with a target completion date of 1997, include a 20% cut in maintenance costs. Phase III, with an end date of 2003, envisages a further reduction of 35%.

When plans were drawn up for Phase I in 1988, maintenance costs were not even mentioned. All that mattered was improving thrust-weight ratio, combustor-inlet temperature and fuel consumption. Similarly, the realities of the post-Cold War economic environment have also brought production costs into the frame for the first time. Production cost-reduction targets of 20% have been identified for both turbofan/ turbojet and turboprop/turboshaft areas in Phase II. Further cuts of 35% have also been targeted for Phase III.

"We're also examining development cost as a potential goal," says Richard Quigley, chief of the turbine-engine division at the USAF Wright Laboratory Aero Propulsion and Power Directorate, which is at the heart of the IHPTET effort. "Just as we added cost goals for Phase II and III, we might add durability as another. People expect it to be durable," he says.

While goals are changing, so are applications. Most of the ultimate destinations for IHPTET technology are well known from the start, particularly in the turbofan/turbojet and turboprop/turboshaft areas. The third research focus has been on expendable engines for missiles and UAVs. Now the IHPTET managers are starting to see new potential applications, which appear to be opening up "grey" areas covering one or more areas of research.

Changing applications

UAVs, for example, have traditionally been catered for within the expendable, or XTL series of experimental powerplants. "The goals are the same, but the applications are changing," says Quigley. "For example, you're talking about 500h or more running time for a UAV. It straddles the JTAGG [Joint Turbine Advanced Gas Generator] and the JETEC [Joint Expendable Turbine Engine Concept] areas."

Similarly, the supersonic performance goals for missiles have also become more ambitious, with new expectations of speeds of Mach 4 and above. "We have two potential demonstrators, both focused on supersonic performance," Quigley adds. Another potential driver for more change is the Joint Strike Fighter (JSF) programme. The international nature of the JSF, and particularly the involvement of UK engine-maker Rolls-Royce, creates opportunities for the first significant non-US participation in the IHPTET project.

To date, it has been a strictly US-only effort, with the supply by R-R of some advanced compressor blades to a Pratt & Whitney Advanced Turbine Engine Gas Generator (ATEGG) project the only known exception. Despite some hints of opening up the IHPTET programme, its stated goal remains the development of technology "-to maintain US military-aircraft superiority and continuing US commercial-aircraft pre-eminence".

Programme officials decline to be drawn on the possibility of broadening its scope, but Flight International understands that informal discussions with the UK began in mid-1997.

For the meantime, the focus of IHPTET work is on completing Phase II, beginning Phase III and promoting spin-off engine-technology applications in civil and military powerplants. "We're about done with Phase II and have achieved a 60% improvement in thrust-to-weight ratio," says Quigley, who adds that the final Phase II-build ATEGG core, the XTC76/2 will be run in the first quarter of 1998. The equivalent Joint Technology Demonstrator Engine (JTDE) for Phase II, the XTE76/1 is expected to follow in late 1998, he adds. "We will then take the XTC76/2 core back and that will become XTC76/3, which will run up at NASA Lewis for the critical 'T3' [compressor exit temperature] demonstration," says Propulsion Branch chief Tom Gingrich. "It's an important demo, because it has to do with fuel consumption," explains Gingrich. "T3 is a real challenge: everything depends on compressor discharge, so, when you crack that, you're making real progress."

Final Phase II tests were due to have begun in 1997, but some hardware deliveries have been late, causing the first substantial slide in the IHPTET schedule since the project began. "We're just a few months behind - maybe up to eight months - and we are ten years into the programme, so that's pretty good," says Quigley.

He says that the problems do not cause him concern, and he remains confident that the full programme goals will be met in 2003.

The final part of the parallel Phase II JTAGG core-demonstrator effort is due to be completed at AlliedSignal early in 1998. The second-build Phase II JTAGG, the XTC56/2, is running "right now", says Quigley. General Electric and AlliedSignal are teamed to compete for the Phase III contract, which is expected to be awarded later this year.

JETEC activity will spool up in the third quarter with test runs of the two Phase II demonstrators at Allison Advanced Development and Williams. The Allison engine, the XTL16-1, follows in the footsteps of the previous demonstrator, which achieved the expendable engine Phase I specific-thrust goal and Phase II turbine-temperature objective. The Williams engine, the XTLE6-2, is a supersonic expendable demonstrator focused on meeting the specific-thrust goal with an emphasis on low cost.

Validating technology

The purpose of the demonstrators is to validate the technology of the critical components developed under the IHPTET programme. In the case of the ATEGG, for example, it is the components related to the high-pressure core of the engine, including the compressor, turbine and combustor hardware. The JTDE, on the other hand, has the advanced core combined with a new low-pressure spool, for testing the majority of the thrust-producing components such as the fan, low-pressure compressor and turbine, as well as afterburner and fuel control.

The focus of the IHPTET effort is therefore as much on components as it is on demonstrators. Two prime areas of immediate research interest are those of combustors and compressors, with recent contracts awarded to GE and P&W respectively.

GE, for example, is working on a combustor concept which uses a "trapped vortex" to enhance fuel/air mixing and optimise thermal patterns. P&W is continuing earlier development work on "splittered" rotors (in which extra reduced-span blades are interspersed between each pair of full-span blades) to test a high-stage-loading compressor design. An earlier splittered rotor developed in the IHPTET effort allowed researchers to produce a single-stage fan with the same performance as that of a baseline three-stage fan. The manufacturers' support of the IHPTET effort is reinforced by the feedback of new engine technology into current and planned civil and military powerplants. Its GE90, for example, uses a variety of IHPTET spin-off developments, including the third-generation dual-dome combustor, film-cooled combustor liner, single-crystal high-pressure-turbine (HPT) blades and vanes, "boltless" blade retention, low-pressure-turbine brush seals, composite fan blades, thermal-barrier-coated HPT nozzle and advanced electronic control. P&W's PW4084 similarly uses features also developed with some IHPTET input. These include the hollow titanium fan blades, advanced single-crystal turbine blades, "floatwall" combustor and airblast fuel nozzles.

Commercial fruits

Other US-made commercial engines enjoying some of the IHPTET fruit include the Williams-Rolls FJ44, with its swept-blade fan. Allison's AE 3007 also incorporates IHPTET-derived technologies in areas such as abrasive turbine-blade tips, brush seals, single-crystal turbine blades and bypass ducts. In the military arena, the improved engine-control logic developed through the programme has already found its way on to the GE F110 and F414 turbofans. The latter also makes use of a wide-chord hollow fan developed with IHPTET help, while the advanced axisymmetric nozzle fitted to some F110s shares the same heritage. P&W's F100 and F119 engines have also benefited from the advanced aerodynamic fan work, combustor and turbine technology of IHPTET.

Compared to baseline engines in 1988, new turbofans and turbojets developed in the USA with IHPTET-developed technology have a 30% increase in thrust-to-weight ratio and a 20% cut in fuel consumption. For turboshafts and turboprops, the potential is for a 40% gain in power-to-weight ratio and a 20% improvement in fuel consumption. Air-breathing missiles can also have a 35% thrust-to-weight increase, consume 20% less fuel and cost 30% less.

"It's a process to pull the country together," says Quigley. "The key thing is the process and the long-range goals for getting there." With so much achieved, there seems every reason to believe that the long-range targets for Phase III, and even for the embryonic Phase IV, which are now being sketched out, will be as achievable.

Source: Flight International