Aviation and the Global Atmosphere

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9.5. High-Speed Civil Transport (HSCT) Scenarios

The technology for commercial (supersonic) HSCT is being developed in the United States, Europe, and Japan. The goal is to develop an aircraft that can carry approximately 300 passengers, with a 9,260-km range, cruising at Mach 2.0-2.4 at altitudes of 18-20 km. As described in Chapter 7, NASA has an aggressive technology program to develop combustors with NOx emission levels of 5 g NOx (as NO2) per kg fuel burned at supersonic cruise conditions. The HSCT is expected to fly supersonically only over water because of the need to mitigate sonic booms over populated land masses. The potential market for the HSCT is limited by economic and environmental considerations.

9.5.1. Description of Methods

3-D emissions inventories of fuel burned, NOx, CO, and unburned HC for fleets of 500 and 1,000 active (high utilization) HSCTs have been developed based on market penetration models and forecasts of air traffic in 2015 (Baughcum et al., 1994; Baughcum and Henderson, 1995, 1998). Although such large fleets clearly will not be in operation by 2015, the year was chosen as a base year because detailed industry projections of air traffic on a route-by-route basis are available only to that time period. Although the introduction of an economical HSCT may stimulate total traffic growth by an unknown amount, the HSCT will certainly displace some traffic from the subsonic fleet on major long-range intercontinental routes. For this study, possible stimulative effects were ignored to reduce the number of variables, and HSCT-generated RPKs were explicitly substituted for subsonic RPKs on a route-by-route basis.

The most recent set of scenarios based on the NASA technology concept aircraft (TCA) HSCT were used for most of the atmospheric impact calculations presented in Chapter 4. It is not clear when HSCT technology will be mature enough for viable commercial service, so fleet sizes and technology levels are treated parametrically. The projected flight tracks for a fleet of 500 HSCTs above 13-km altitude are shown in Figure 9-23. Because of its speed advantage over subsonic aircraft, the HSCT would likely be used primarily on long intercontinental routes, where that advantage can best be utilized. Because of the sonic boom that trails below the aircraft, the best HSCT routes have a large portion of the flight path over water. These conditions combine to put a majority of HSCT routes at northern mid-latitudes over the North Atlantic and North Pacific.

To project the HSCT fleets and their displacement of subsonic aircraft in the scenarios to 2050, the following procedure was used:

1. 3-D displacement scenarios of subsonic traffic by a fleet of 1,000 active HSCTs was calculated for the year 2015 using differences in the 3-D scenarios calculated for the NASA all-subsonic fleet (Baughcum et al., 1998) and the NASA subsonic fleet in the presence of an HSCT fleet (Baughcum and Henderson, 1998).
2. This subsonic displacement scenario was then scaled for the technology growth factors described in the discussion of the FESG scenario and combined with the HSCT only-scenario (assuming the TCA technology level) and 2050 all-subsonic scenarios.

The 1,000-unit fleet should not be considered a forecast of the actual number of HSCTs that might be in the fleet in 2050. For this sensitivity study, the 1,000-unit value was chosen to represent a fleet that would be the result of a successful HSCT program; this fleet size also was chosen so that previous fleet projections could be used (Baughcum and Henderson, 1998). No changes in fuel efficiency or NOx emissions technology relative to the assumptions used in the reference were assumed for the 2050 HSCT. A detailed description of the route system flown by the 1,000 HSCTs is given by Baughcum and Henderson (1998).

9.5.2. Description of Results

Fleet fuel burned with the HSCT was calculated by assuming that the fuel efficiency and NOx emissions of the subsonic fleet were described by NOx technology scenario 1, the "fuel efficiency" scenario. Table 9-21 gives the total fleet fuel burned and NOx emissions with and without the assumed 1,000-unit HSCT fleet. Fleet fuel burned increases as a result of the substitution of less fuel efficient HSCTs for subsonic airplanes (present HSCT designs have about half the fuel efficiency, measured as RPK per fuel burned, of present subsonic airplanes). However, fleet NOx emissions decrease in spite of the increase in fuel burned because the HSCT is assumed to be designed for very low NOx emissions [cruise EI(NOx) of 5].

A comparison of the altitudinal distributions of fuel use and NOx emissions between the all-subsonic fleet and a fleet containing subsonic and HSCT aircraft is shown in Figures 9-24 and 9-25 for the FESG year 2050 IS92a scenario. The introduction of an HSCT fleet with EI(NOx)=5 combustors would be expected to increase emissions above 12-km altitude and lead to a decrease of NOx emissions below 12, particularly in the 10-12 km band, assuming that the introduction of an HSCT will cause a displacement of subsonic traffic.