Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
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 NO x 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 NO x production, although the NO 2 /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
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