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HIGH-TECH ENGINES FOR THE NEW FIGHTERS

In recent years the aspects of fighter aircraft development which have attracted all the attention have been electronics, improved sensors and stealth technology, but the engine will always be a critical factor in the performance of a fighter. The powerplant, including fuel, accounts for between 40 and 60% of the take-off weight and around one-third of the life cycle costs of the weapons system. Above all, development of a new engine is a costly and long drawn out process. This makes it all the more important that the design should be optimised for the requirements of present and future fighter aircraft.

Eurojet EJ200

For engines such as the Eurojet EJ200, the Snecma M88 or the Pratt & Whitney F119 this means that the design is primarily geared towards deployment in the air-to-air role. Rapid take-off and powerful performance during intercept missions are just as important here as trouble-free operation, including at high angles of attack or high angles of sideslip. Instantaneous response to movements of the throttle lever is taken for granted, as is sufficient power in the dry thrust region to be able to fly faster than Mach 1 without afterburner (supercruise).

On top of all this higher reliability and lower maintenance requirements are increasingly stipulated these days, coupled with cost-saving spare parts provisioning. Last but not least the engine must have a 15 to 30% growth potential, for the one thing you can count on in this business is that there will be calls for more thrust.

When it comes to technical realisation of these requirements, leading engine manufacturers in the USA, and Europe and Russia all follow a similar route. The standard approach today is a twin-shaft turbofan with a low bypass ratio, compact dimensions and low weight. Key requirements are a thrust-weight ratio of around nine, an overall pressure ratio of over 25 and turbine inlet temperatures in the region of 1800-oK.

As the engineers try to get by with ever fewer fan, compressor and turbine stages, the amount of work each of those remaining has to perform is increased. This is only possible with improved aerodynamics. Today's high-performance computers allow the highly complex flow processes to be calculated in three dimensions so that the blade profiles can be optimised.

Again, losses can be avoided by better sealing, and today brush seals are customary. New manufacturing techniques are also making an impact. Bladed discs or "blisks", in which the blade and disc are fabricated from a single piece of material, permit better aerodynamic forms. Finally, significant gains can be made with a digital electronic engine control system.

New, heat-resistant materials are also making a critical contribution to higher performance. Nickel- and titanium-based alloys dominate the scene, with single-crystal blades the norm in the highly stressed turbine area. Another feature is ceramic-based surface coatings. Despite this, the components would not survive without adequate cooling by engine bleed air from the compressor. This naturally is another area in which progress has been achieved through refinement of the shape of the cooling air ducts.

Today the preference in the afterburner is for a radial design, as this is simpler to maintain and offers problem-free response characteristics. Finally, the moving jet nozzle with both convergent and divergent parts permits accurate adjustment of the exhaust cross section to the prevailing pressure and speed conditions.

Items such as Full Authority Digital Engine Control (FADEC), fuel lines and gearboxes are generally fitted to the underside of the engine for improved accessibility. Casing parts such as the bypass duct are already partly manufactured from composite materials. It is standard practice for the engine to have a modular design so that parts can be replaced without then requiring time-consuming calibration tests.

Naturally the manufacturers are not content to stop at what they have already achieved. For example, Munich-based MTU, which is responsible on the EJ200 for the low and high pressure compressors and the Digital Engine Control Unit (DECU), is conducting research in four aspects of fighter aircraft powerplants.

• On the fan/compressor, research is aimed at increasing the pressure ratio still further. Aerodynamic improvements should make it possible to raise the pressure ratio in a three-stage fan from four as at present to over five, which would make MTU a world leader. Such a design, which amongst other features contains the biggest blisk ever manufactured by MTU, has been on the test rig since December 2000.
• New materials and construction processes should help to reduce weight. One interesting approach here is the bladed-ring concept, in which the blades are not attached to a heavy disc but to a titanium ring reinforced with silicon fibres. This would enable the weight to be reduced by a quarter, albeit at present at the cost of a very expensive manufacturing process.
  They are also very interested in Munich in the use of fibre composite materials. It should be feasible to manufacture the inlet guide vane and stators in the compressor part from composites, and perhaps even fan blades as well. To protect against erosion, MTU has developed patented surface coating technology. In the hot part of the engine titanium aluminide alloys appear promising. Efforts are aimed at turbine inlet temperatures of 2000-oK.
• In the area of thrust vector control MTU has developed the control system for the EJ200 while its Spanish partner ITP is responsible for the nozzle. This technology should not only enable completely new manoeuvres in close in air combat but also bring performance gains of seven to eight per cent in the "normal" flight envelope. For example, this would increase the supercruise speed of the Eurofighter to Mach 1.3. Having completed around 75 hours of tests on the test bench with an EJ200 over the period 1998-2000, the company is now trying to obtain funding for a flight test programme. There are plans to install the engine in the X-31.
• The generic term "more intelligent engine" covers a number of studies aimed at improved monitoring of operational status and even more sensitive automatic control. In particular, it should be easier to predict failures, and hence to optimise maintenance effort and logistics.

Since it is inconceivable that a future fighter aircraft and its engines should be developed other than in a collaborative European programme, it is appropriate that the research work should be co-ordinated among the companies. Under the European Technology Acquisition Programme (ETAP), MTU, Rolls-Royce, Snecma and Volvo and the relevant defence ministries are therefore currently attempting to set up a framework agreement. This would be driven by the requirement for a potential successor to the Tornado (FAWS/FOAS) in the period after 2015. Possible objectives, for example, might be a thrust-weight ratio of 15 and a 50% reduction in life-cycle costs.

The Americans, who in any case have far more money available, are naturally not sitting back either. Following on from the successful Integrated High Performance Turbine Engine Technology (IHPTET) programme, the Versatile Affordable Advanced Turbine Engine (VAATE) programme is planned for the next few years. As the name suggests, it is not just a matter of increasing performance but also of reducing costs - to be precise, by up to 65% for a large fighter engine. The aim is to obtain a threefold improvement in the thrust-weight ratio and reduce fuel consumption by one-quarter - ambitious goals which only go to show that there is still plenty of potential lurking in engine design.

From page 100 of FLUG REVUE 2/2001