technological improvements. New airframe technologies have the potential to reduce current fuel consumption by 25 percent, and new engine technologies could provide an additional improvement of 15 percent over the next 15 years.

Improvements in aircraft fuel efficiency have been similar to fuel-efficiency advances demonstrated by the automobile: in 2000, the average new car used 41 percent less fuel per mile than the average new car in 1973; fuel efficiency of new aircraft (per passenger-seat-kilometer) improved about 34 percent over the same time period.

Additional improvements in aircraft fuel efficiency could be achieved by continued advances in the following areas:

  • improvements in airframe aerodynamics from a combination of high-resolution numerical simulations of airflows around aircraft; wind tunnel testing techniques; laminar flow technology; and integrated design of the wing, fuselage, and propulsion system

  • reductions in the weight of airframe and engine structures (such as the nacelle, which supports the engine) from lighter and stronger materials, and high-fidelity finite-element models for more accurate analyses of safety and strength load-factor margins

  • improvements in the aerodynamics of engine nacelle flows and changes in the shape and length of the engine inlet to reduce local drag effects and increase efficiency

  • thrust reversers with higher efficiency to reduce propulsion-system weight

  • fly-by-wire and electrical actuation systems to reduce or eliminate the need for heavy hydraulic systems, and fly-by-light systems to replace electrical wiring with lighter-weight fiber optics

  • advanced engine technology to increase engine bypass ratio (for lower exhaust jet velocity and higher propulsive efficiency) and to increase engine pressure ratio (for higher thermal efficiency)

  • advanced air traffic control and air traffic management systems and procedures to improve operational efficiency—for example, through more direct routing of flights

Even with improvements such as those listed above, most or all new commercial aircraft will not have significantly greater cruise speed, altitude, or range. Large commercial jet aircraft have had cruise speeds of about 500 knots (Mach 0.80 to 0.85) for about 30 years. Typical cruise altitude has also changed little. This trend could change, however, if ongoing Boeing design studies lead to production of a new class of commercial aircraft with cruise speeds of Mach 0.95 or greater. For long-range aircraft, average cruise altitudes have remained fairly constant, at 35,000 to 38,000 feet, over the past 35 years. Although maximum cruise capability has slowly increased and some aircraft can now cruise at altitudes up to approximately 43,000 feet, subsonic aircraft are not expected to see much change in cruise speed or altitude in the foreseeable future. Maximum range is also unlikely to increase significantly, because commercial aircraft can already provide nonstop service between almost any two cities in the world; there is little demand for aircraft with longer ranges.

NASA is the only federal agency with research programs focused on the reduction of emissions from commercial aircraft. NASA’s emissions goals, which are focused on CO2 and NOx, are commendable, but research funding to achieve these goals has been greatly reduced. Figure 3-1 shows the magnitude of emissions research funded by the High Speed Research (HSR) Program, the Advanced Subsonic Technology Program, and the Ultra Efficient Engine Technology Program.

A major thrust of the HSR Program was to develop low-NOx combustor technology for future supersonic aircraft. Component tests demonstrated a reduction of 80 to 90 percent, achieving an NOx emission index of 5 grams per kilogram of fuel (which was the program goal). However, the HSR Program was canceled before the low-NOx technology could be integrated in a test engine to characterize transitory and steady-state performance and demonstrate programmatic goals such as low noise and long life. NASA also conducted extensive combustor emissions research under the Advanced Subsonic Technology Program before it was terminated and replaced with the Ultra Efficient Engine Technology Program. The goals of the latter program are to reduce NOx by 70 percent (with hardware demonstrations at TRL 5) and to reduce CO2 emissions by 15 percent (with hardware demonstrations at TRL 4). Figure 3-2 shows how funds from all three programs have been allocated.

As with any carbon-based fuel, the major combustion products of conventional jet fuel are CO2 and water vapor. Reducing the emission of CO2 and water requires either reduced fuel consumption (through the development of more efficient engines, aircraft, and operational systems and procedures, as discussed above) or the use of alternative fuels. Even though contemporary commercial jet aircraft are designed to operate exclusively with aviation kerosene as a fuel, gas turbine engines can operate with a wide variety of liquid and gaseous fuels. In fact, derivatives of several operational aircraft engines are used in marine and industrial applications using natural gas, diesel fuel, alcohol, and many other fuels. Current and future aircraft engines could also be configured to operate with alternative fuels, such as natural gas or hydrogen. Natural gas would reduce CO2 emissions on the order of 20 percent relative to kerosene. With hydrogen, zero CO2 emissions would result. However, both fuels, especially hydrogen, would increase emissions of water vapor.

Because aircraft have limited volumes available to store fuel, natural gas or hydrogen would have to be in liquefied form. Although the energy density of hydrogen by weight is nearly three times that of conventional aviation fuels, the energy density by volume is one-fourth that of conventional

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