7.11. Special Military Considerations

Although today's military and civil aircraft projects clearly respond to radically different national requirements, the underlying engine technology of the two types of aircraft has a significant degree of commonality. This situation is not surprising in the case of the military transport aircraft engine: In common with the civil engine requirements, large payloads must be carried over long distances at the lowest possible costs. The similarities are less obvious, however, with regard to combat aircraft (including fighter/bombers), which have no clear civil parallels. Nevertheless, many of the technical advances that have been developed to meet the military challenge have been adopted, in one form or another, in advanced civil engine applications. Indeed, an increasing number of examples of technical advances derived from civil engine research also are relevant to military engines. This two-way exchange justifies the fact that a significant part of today's basic research underpinning aero-engines for the future is supported directly or indirectly through military and civil engine sources (dual use).

Notwithstanding the parallel development paths and mutual objectives mentioned above, limits must be taken into account. In general terms, increases in the fuel efficiency of combat aircraft are consistent with the logistics and operational needs of the military because aircraft with lower fuel consumption rates would be able to remain engaged for longer periods of time, carry additional payload, reach targets from greater distances, or a combination of the three depending on operational requirements. They would also require less in-flight refueling. However, the prescribed altitude for optimum fuel efficiency may not be not be appropriate for military operations. Similarly, some of the restrictions on civil transport ground operations (e.g., engine start, taxi, and take-off procedures) may not be acceptable to military users. Although military procurement officials and operators are now acutely aware of their responsibilities with regard to environmental effects, operational effectiveness will always be the primary requirement. The following sections focus on military aero-engines-in particular on aspects of such engines that, for performance and/or operational reasons, differ from their civil counterparts in ways that might influence emissions. It is important to rationalize these differences in terms of their real impact on the environment by taking proper account of current and likely future proportions of the world's military aircraft fleet compared to the global total of all types and operations of aircraft. Chapter 9 shows numerically how the environmental impact of military operations becomes a diminishing percentage of the total of all aircraft operations as the effects of the anticipated strong growth of civil transport takes effect over the next 50 years or so. Differences between military fuels-F-34 (NATO) and JP-8 (US)-and civil AVTUR/Jet A-1 fuel are covered in Section 7.7.

7.11.1. Differences Resulting from Operational and Design Features

7.11.1.1. Combat Aircraft

Aircraft used in combat operations constitute the largest proportion of the various types of aircraft in the military inventory. They outnumber other aircraft types by roughly 3 to 1; for this reason alone they justify a closer look as the most likely sources of divergence between the emissions of military and civil engines in the future. Figure 7-43 is extracted from some of the same data sources used in Chapter 9 dealing with fuel usage and emissions production, in particular the ANCAT/EC2 report (Gardner, 1998) and the recent NASA report (Mortlock and van Alstyne, 1998).

Clearly, combat aircraft will always be built to respond to quite different mission priorities from those applying to civil aircraft. There are, therefore, some differences in the design features of engines to achieve those priorities. Combat aircraft engines will inevitably be designed to extract maximum performance even though this approach entails accepting a shorter life, particularly of key hot-section components such as turbine blades, and shorter periods between maintenance than civil engines. It is also most likely that military aircraft will be the first to adopt the fruits of the most advanced engine technology in a constant drive to achieve superior performance. Thus, there will always be something of a technology gap between the leading military and civil engines. Of the principal performance requirements, the demand for higher thrust/weight (T/W) ratio engines will continue to be the key driver that will maintain that gap. This consideration inevitably means that T/W targets for new engine cycles will involve higher pressure ratios, higher peak temperatures, and higher fuel/air ratios than the current fleet. The most stringent performance targets today are those of Phase III of the United States' Integrated High Performance Turbine Engine (IHPTET) program (Hill, 1996). This multi-agency/industry initiative has set goals that if fully achieved, will provide important new engine technology levels for adoption in the next century. The principal goal of this third phase of a three-phase program is to achieve +100% in the T/W ratio together with a 40% reduction in fuel burn for a new generation of military engines. These targets are so ambitious that evolutionary improvements in hot-section components of the engine are simply not sufficient.Only radical solutions are likely to achieve the necessary rise in engine cycle temperatures and pressures to meet the T/W targets. Such engine cycle changes imply, as explained in Section 7.4, even higher levels of NOx emissions if conventional engine technology is retained. This approach, however, is unlikely to be wholly acceptable because of associated rising levels of visibility of the brown-tinted NO2 component of NOx gases-which, unchecked, could compromise the stealthiness of the aircraft. It therefore seems inevitable that significant pressure to limit NOx will remain part of the military aims attached to the IHPTET program and that some if not all of these aims will be relevant to environmental and operational performance. In this respect, therefore, the prospects of total divergence of priorities between military and civil engines seems small.

One important difference between military and civil engines is associated with maneuverability. Under certain conditions, for example, the military engine combustor must be able to accommodate the consequences of high-incidence turns during combat maneuvers. These maneuvers can cause unstable internal flow conditions. The combustor must be able to withstand the consequences of such maneuvers over a wide range of flight speeds and altitudes. It must also relight rapidly in the event of a flame-out. In today's climate, the conventional approach of increasing the fuel/air ratio in the combustor to ensure adequate stability is not acceptable unless high-power emissions can be contained using new designs. The pursuit of effective solutions to these important problems, taking full account of emissions, forms an important part of today's military research and development programs.

Reheat/afterburner operations are an important requirement for combat aircraft engines. Although the majority of fuel used during reheat is burned at low altitude, the extra thrust demands associated with combat maneuvers does mean that a proportion of fuel (typically 8%) is burned at altitudes above 2000 km. Evidence (Seto and Lyon, 1994) suggests, however, that this burning within the jet pipe/exhaust system, where the pressure levels are much lower than in the main combustor, does not increase NOx production, although the NO2/NO ratio increases. Thus, it appears that although some operational issues can influence the design and therefore the emissions performance of military fighter engines, the broad technology paths aimed at achieving improvements diverge from those of the civil sector only in detail. Military programs will generally continue to lead in providing technology advances that "spin off" into the civil engine sector. Engine cycle advances that improve the thermal efficiency of the core engine will be particularly important in further improving the fuel efficiency of civil engines.

7.11.1.2. Military Transport

Transport requirements for military operations have many parallels with civil operations. This concordance has led to an increasing trend toward development of military transport and tanker aircraft based closely on civil aircraft designs. The links are even closer with regard to engine development for such aircraft, and they are likely to remain so in the future.

7.11.2. Conclusion

The principal conclusion arising from this brief review of the environmental aspects of military engines is that military operators are already well motivated to demand lower emission levels from new engines for operational as well as environmental reasons. Coupled with the evidence presented in Chapter 9 showing that military aircraft will, proportionately, become an even smaller consumer of the world's aviation fuel, a logical extension of this conclusion would be that military operations will have a negligible effect on global emissions from aircraft for the foreseeable future.

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