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Aviation History
1991
1991 - 0027.PDF
Manufacturers believe that a new generation of supersonic trans port (SST) could be economi cally viable by the turn of the century. They claim that the designs pro posed would also be environmentally friendly (Flight International, 5-11 Septem ber, 1990, P 150). Many scientists are sceptical, citing studies indicating that SST emissions could be the most potent de stroyer of ozone the world has yet known. In an attempt to make future SSTs "green", hundreds of millions of dollars are being spent by NASA and the engine manufacturers. The little independent re search which does exist indicates, however, that even if future supersonic airliners achieve NASA's ambitious emission goals, they could destroy 2-3% of the remaining stratospheric ozone annually within ten years of their introduction. Funding for research into the effects of SSTs on stratospheric ozone accounts for only a fraction of NASA's $250 million high-speed research programme. The Agency has allocated $26.5 million to inves tigate the atmospheric consequences of global SST travel while the rest of the money is being ploughed into reducing engine noise and sonic boom and developing combustors which industry hopes will reduce emissions. An acknowledged authority on the subject, Professor Harold Johnson of the Department of Chemistry, Uni versity of California-Berkeley, says that NASA is "...spending hundreds of millions of dollars redesigning the engines. It should spend more to understand the effects on the strato sphere." Reviewing research in the field, Johnson adds: "It's very dif ficult to get sharp answers at pre sent. Variables of altitude and season are very difficult to calculate. An swers need to be aggressively pur sued." The impact of aircraft on the atmosphere depends on their altitude. Supersonic pas senger aircraft are a particular threat be cause they fly in the middle and upper stratosphere where the ozone layer is pre sent. Ozone has a molecule comprising three oxygen atoms (03); the ozone layer, a band of atmosphere 15-28km high where 80% of the Earth's ozone is found, protects life on the planet from the Sun's harmful ultra-violet rays. The chemistry of this region of the atmosphere means that emission of nitrogen oxides (NO and N02, collectively known as NOx) from SST exhausts leads, through a catalytic cycle, to the destruction of the ozone layer. The consequences of damaging the ozone layer through chlorine released by the indiscriminate use of chlorofluorocarbons (CFCs) was first realised in the late 1980s. International alarm led the United Nations Environment Programme to establish the Montreal Protocols in 1987. This initiative has helped focus world attention on ozone depletion but as yet has been ineffective in persuading all countries to curtail their use of CFCs or ozone-damaging compounds. Before human activity changed the atmos phere's composition, the natural concentra tion of chlorine in the stratosphere was 0.7 parts/billion (PPB). By the time Dr Joe Farman and his team from the British Antarctic Survey discovered the now- infamous hole in the ozone layer over Antarctica in 1985, the concentration had reached 1.5-2PPB. It now exceeds 3BBP. The contribution to ozone depletion over the past 20 years of the 14 Aerospatiale/ British Aerospace Concordes has yet to be evaluated. Given the massive costs of devel oping and producing a new breed of SST, it is unlikely that manufacturers would consider production of fewer than 300 aircraft worthwhile, and would probably look for nearer 800. Nir*US-?:T0t1S TOTAL OZOhE riftSft'CSFC Images of the Antarctic ozone hole, which shows up as pink, taken as part of NASA's Total Ozone Mapping Spectrometer (TOMS) programme The Pacific Rim is the fastest-growing passenger market and the attraction of more than halving the flying time from Tokyo to New York is obvious. Not surprisingly, Japan is active in SST research and. is anxious to join any international develop ment programme. The next generation of SSTs would seat 250-300 compared with Concorde's 100 and would not be the preserve of first-class passengers. One of the most recent studies to deter mine the global impact on the ozone layer of a new generation of SSTs has been conducted by a team led by Johnson using the Lawrence Livermore National Labora tory's one- and two-dimensional models of the atmosphere. This initial study indicates that nitrogen oxides from aircraft flying in the stratosphere would cause the largest reductions in ozone levels at altitudes of 25-28km and in the tropical latitudes. According to the research team, a fleet of 500 SSTs flying at 22km with NOx- emis sion properties of 1988 subsonic commer cial aircraft would reduce global ozone levels at a steady rate of 20% a year within ten years. By comparison, it would take a century to produce steady-rate reductions of 5% in stratospheric ozone levels through the continued use of CFCs. The study calculated that ozone reduc tions would depend strongly on NOx- injection altitude and magnitude. For a given injection, the largest reduction in the thickness of the ozone layer occurred in the Polar regions. A key variable in any calculation of ozone depletion by SSTs is the nitric-oxide emis sion index which is expressed as grams of nitric oxide produced per kilogram of fuel consumed. Scientists at NASA's Langley Research Center believe that the current nitric-oxide emission index (40-60) for subsonic commercial aircraft could be re duced by a factor of eight or ten by advanced emission-reduction technol ogy. With this in mind, NASA has set an emission-index target of five by 1997. Emission index values for super sonic aircraft have yet to be estab lished. NASA is studying the feasibil ity of flying one of its ER-2 research aircraft behind Concorde or a US Air Force SR-71 to examine emissions from the exhaust plumes. The biggest concern would be the need to fly into the plume while avoiding the turbu lence created by the Concorde or SR-71. Funds for the project have been included within the $6 million allocated by NASA for SST research in 1991. NASA's ER-2s, civilian versions of the U-2 reconnaissance aircraft, were last used to examine the ozone hole over Antarctica. The Agency is backing some studies of environmental effects of SSTs but is pre dominantly "supporting efforts to reduce the emission indices of engines, overall noise and sonic boom in particular. NASA's over sight committee into SST and the atmos phere is funding research in three main areas. Money is being channelled into labo ratory kinetics because scientists are uncer tain about reactions outside the gas phase in the emission plume of an SST. Similar heterogeneous reactions, where nitric acid has combined with water, led to the forma tion of the ozone hole over Antarctica. In the area's winter, stratospheric clouds over ibe continent provide particles on which inactive chlorine types change to active chlorine compounds and destroy ozone when exposed to sunlight. The effect of heterogeneous reactions in the emission plume of SSTs is, for NASA, FLIGHT INTERNATIONAL 2 - 8 January, 1991 25
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