Indian Defence Review

The Green Revolution

The subject of ‘Climate Change’ has taken on huge proportions in recent years. Numerous studies, conferences, protocols and awareness campaigns have been conducted worldwide in a bid to save the environment from degradation. Because it involves combustion, aero-engines release carbon dioxide (CO2) and other greenhouse gases into the earth’s atmosphere, thereby contributing to and speeding up global warming. Apart from CO2, other emissions are Nitric Oxide and Nitrogen Dioxide (together termed as oxides of Nitrogen or NOx), water vapour and particulates such as soot. A combination of all these have significant effect on the overall global warming.

While aviation per se, may not be the major contributor, the fact that there has been such a proliferation of aviation assets these past few decades, has increased the awareness and led to efforts to control global warming. The Intergovernmental Panel on Climate Change (IPCC), a body constituted to evaluate and quantify the sources or contributors to global warming, is involved in a series of studies to offer suitable control measures. From increasing efficiencies of aircraft and their operation, to alternative fuels and to actually proposing reducing air travel (ending Frequent Flyer Programs), there is immense pressure being exerted to control global warming which is contributing to climate change.

Trends in Aero-Engine Development

Materials: Aircraft designers continue to look for new sources of power while they refine existing power plants, both jet and internal combustion engines. There is constant effort to develop strong, light, heat-resistant materials to replace some of the metallic components in aircraft engines. Fan blade technology has enhanced performance and efficiency of the turbofan engine. The latest in this innovation is the patented ‘Diffusion-Bonded-Super Plastically-Formed’ (DB-SPF) fan blade introduced by RollsRoyce. Considerably lighter than Pratt & Whitney’s hollow titanium blade and GE’s composite blade, the DB-SPF has high strength and good resistance to fatigue as also to Foreign Object Damage (FOD). Computational modeling techniques have been at the core to developing these modern engine components.

Blisks: ‘Blisks’ have made their impact on military engine technology. Whereas in a conventional compressor the blade is fitted into machined slots on the periphery of compressor discs, in the ‘Blisk’ the blades and disc are integral, resulting in weight savings of almost 30 per cent and contributing to better T/W ratios. Blisk technology, used in the EJ200 engines, is expected to be also used in the proposed Joint Strike Fighter, a trans Atlantic collaboration.

Ultra-High-Bypass-Ratio: Ultra-High-Bypass-Ratio turbofan engines with bypass ratios of 9:1 (likely to go up to 15:1) are in the pipeline. The high bypass design necessitates increased engine size and consequently increased weight and nacelle drag. High pressure ratios require better cooling techniques, increasing complexity and probably weight of heat exchangers.

Ceramics/Plastics: Ceramics and plastics seem to be a probability in the future. While ceramics were thought to provide the solution, their associated problems in the automotive industry have obviated their use in aero-engines. Technologies such as lightweight Polymer Matrix Composites (PMC), Metal Matrix Composites (MMC) such as titanium reinforced with silicon carbide fibres and Ceramic Matrix Composites (CMC) are in the offing. Fibre-reinforced composite materials, more like plastic than steel, are being looked at. Whilst they have the capability to withstand very high temperatures over a short period of time, will they be able to sustain in the ‘most hostile environment of them all’ – the Jet Engine?

ADVENT/AETD: The turbofan engine was a quantum jump over the turbojet and now the US is in the process of developing engine technology which will power the new generation fighters. The three engine ‘biggies’, GE, Rolls Royce and Pratt & Whitney are in collaboration/competition to develop programs to demonstrate such engines. The Adaptive Versatile Engine Technology (ADVENT) program (GE/Rolls Royce) and the subsequent Adaptive Engine Technology Development (AETD) program (GE/Pratt & Whitney) are looking at producing fuel-efficient, high thrust power plants for not only the Lockheed Martin F-35 Upgrade program, but also for the future ‘Sixth Generation’ combat aircraft. The ‘adaptive’ technology strives to improve fuel efficiency by 25 per cent over that existing presently on the F135 engine which powers the F-35, thereby increasing combat radius of the aircraft by 25 to 30 per cent and persistence by 30 to 40 per cent. It is also likely to increase supersonic cruise radius by 50 per cent and reduce aerial tanker burden by as much as 75 per cent! Anti-access and Area Denial mission capability over long distances will become the forte of the USAF.

Joint Strike Fighter. F-35 Lightning II Propulsion. F135 Conventional Take-Off Landing

SABRE: The crystal ball shows the growth of systems like the SABRE hybrid jet/rocket engine, capable of propelling a spacecraft into orbit or an aeroplane in the atmosphere at a velocity of Mach 5 (3800mph/3300kts/6115kph).

Reaction Engines Ltd. is developing the SABRE engine for its SKYLON spacecraft and its airliner derivative, the LAPCAT A2 Hypersonic aircraft. The SKYLON (which will precede the LAPCAT) is intended to take off and land at conventional airports.

It is expected to take off and accelerate to hypersonic speeds under its own power i.e. without the use of boosters. Since it will be required to operate from dead stop to hypersonic speeds, the engine is undergoing some complex technology developments.

Conclusion

The development of aero-engines over the years has followed a path of rising propulsive and thermal efficiency. The design innovation of the ‘by-pass’ concept to enhance performance and efficiency in an engine, was a huge leap. The associated reduction in noise pollution and better combustion efficiency addressed the growing demand for climate control and environment protection. The concordance between the requirement of civil aeroplanes and military transport operations has contributed to dual-use technology which has benefitted both parties immensely.

Combat aircraft requirements for high performance through varied ranges of altitudes, speeds and manoeuvres have been successfully met with. The super cruise and super manoeuvrability elements of fifth generation fighters have been effectively catered for but the quest for even better performance and weight reduction, is far from over. Materials and their usage in aero-engines will remain a daunting task. Man and machine are destined to be shackled together in their thirst for the ultimate dream.