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Aerospace manufacturers are pressing ahead with their plans to develop
the next generation of supersonic jetliners despite renewed uncertainty
about the environmental impact of exhaust emissions that could ground the
high-speed aircraft.

Atmospheric scientists have discovered that the effects of the emissions
on the stratosphere’s ozone layer could be a much bigger problem than they
expected. They now say that a reduction in the emission of nitrogen oxides,
which is something that engine designers have been working towards on the
basis of earlier research, is not enough to make supersonic airliners environmentally
friendly. Nitrogen oxides are one of the main causes of ozone depletion.

Among the most comprehensive work being done is that by NASA under its
High-Speed Research Program (HSRP), which began in 1990. The programme
is designed to relieve the American aerospace industry of the cost of the
basic research for developing a ‘clean’ supersonic airliner, including preliminary
studies of the potential of new materials, for instance.

As part of this work, the administration allocated $36 million over
five years to study the atmospheric effects of supersonic aircraft flying
in the stratosphere. The third report of this Atmospheric Effects of Stratospheric
Aircraft (AESA) programme, still in draft form and unlikely to be published
this year, promises to dampen the optimism of previous research.

NASA used the earlier findings to calculate the impact on the ozone
of a future fleet of 600 supersonic airliners. It found then that the fleet
would deplete less than 1 per cent of the ozone and, according to Louis
Williams, director of the HSRP, would have ‘almost no impact’. His conclusion
was highly publicised at the Paris Air Show in June this year. But if Williams
had been able to quote from the ASEA programme’s latest draft report, he
would have said something like ‘an indication of a possibility of relatively
small effects’.

Until recently, models of the effects of supersonic emissions on the
ozone were similar to those applied to the effects of CFCs. They included
only the chemical effects of nitrogen oxides involving gaseous reactants
and products. Emphasis is now shifting to more complex atmospheric models
that include the impact of particulate soot and condensible gases, such
as sulphate aerosols, on the stratosphere.

Nitrogen oxides react rapidly on sulphate aerosols in the exhaust to
form nitric acid and water vapour. These products are not directly involved
in ozone chemistry and so scientists thought that the reaction probably
reduced the potential impact of nitrogen oxides. They now question that
assumption.

Chemical reactions involving chlorine that deplete the ozone layer are
thought to take place on the surfaces of aerosol particles. For instance,
when Mount Pinatubo erupted in 1991, large amounts of sulphate aerosols
were spewed into the atmosphere, and a marked dip in ozone concentrations
followed. Aircraft exhaust also increases the amount of sulphur aerosols
in the stratosphere, boosting the surface area on which these chlorine
reactions can take place.

¿ìè¶ÌÊÓƵs are also investigating whether the nitric acid and water
vapour formed in heavily used air routes lead to the formation of nitric
acid trihydrate (NAT) clouds, which may trigger ozone depletion through
reactions with chlorine. So aircraft emissions may actually make the ozone
layer more sensitive to chlorine.

But Howard Wesoky, programme manager for propulsion and environmental
compatibility at NASA, says these new findings ‘do not shift the emphasis
strongly’ in assessing the impact of high-speed airplanes on the ozone layer.
He says that reducing the amount of sulphur in fuel is ‘relatively easy’,
and that the levels of chlorine in the stratosphere will decrease because
of efforts to cut CFC emissions. ‘We are still relatively positive towards
the assessment of a high-speed fleet,’ he added.

According to Jose Rodriguez, an ASEA consultant who works for Atmospheric
and Environmental Research in Cambridge, Massachusetts, an ‘amazing’ amount
of progress has been made since the beginning of the programme. He says
that most of the uncertainty will be cleared up within two years.

If ever a fleet of supersonic jets causes a crisis in ozone levels,
the aeroplanes could simply be grounded, says Rodriguez. ‘Everything would
be back too normal within one year whereas the effects of CFCs will be around
for 50 to 100 years.’ He adds: ‘Ultimately the question will be what is
acceptable ozone depletion . . . that is for policy makers to decide. It
is dangerous for scientists to start addressing this issue.’

The final AESA results will be completed in 1995 and are expected to
take into account these indirect effects on the ozone using theoretical
models and data from planned and actual missions to measure the atmosphere.
They will then be assessed by United Nations and other international climate
research bodies. ‘We will then expect an authoritative statement on aircraft
emissions,’ says Wesoky.

Green or grounded

The Montreal-based International Civil Aviation Organization (ICAO),
a UN body that recommends standards for the aviation industry, has set up
a committee for environmental protection. It has recommended that before
future supersonic jets are introduced, ‘appropriate’ emissions standards
should be set.

Both NASA’s AESA programme and the results of the European Community’s
work into the environmental impact of subsonic aircraft flying as high
as the lower stratosphere, will help shape any ICAO recommendations for
future regulations on emissions. The European programme, launched last year,
is known as Aeronox, the impact of NOX emissions from aircraft on the atmosphere
at flight altitudes of 8 to 50 kilometres. According to an ICAO official,
once manufacturers decide what they want to build and provide estimates
about what it will emit and where, the organisation will turn to atmospheric
scientists, working independently and for UN organisations, for an evaluation.
Only then will discussions begin on setting emission standards. Talks are
not expected to start until at least 1998.

When the Concorde was built, questions about its environmental impact
were never raised, recalls Claude Lenseigne, chief engineer of supersonic
transport at the French company Aerospatiale, a member of the Airbus consortium.
The Anglo-French aeroplane produces the equivalent of 40 grams of nitrogen
oxides per kilogram of fuel burned, which gives it a so-called emission
index of 40. NASA says the next generation of supersonic aircraft will produce
no more than 5 grams.

To meet this constraint, American manufacturers are studying two types
of engine design. In both, combustion is designed to occur where nitrogen
dioxide formation is at a minimum. This is done by controlling the mixture
of fuel and air to avoid peak flame temperatures when the fuel/air ratio
is one.

But there are safety concerns attached to these technologies, notably
the chances of losing the flame, called a blowout, or causing combustion
where it should not occur. ‘We still have a long way to go,’ says Richard
Hines, programme manager at Pratt & Whitney, one of the US companies
getting funds under the HSRP.

Noise pollution is also a central concern. The next generation of supersonic
aircraft must be three times, or 20 decibels, quieter than the Concorde
if it is to be able to land at all major airports and comply with current
noise regulations for subsonic aircraft.

Using current technology, the noise requirements could be met, but with
heavy weight and performance penalties. A low noise exhaust system for supersonic
aircraft, more than 2 metres wide and 4 metres long, could weigh as much
as the main engine unless lighter materials are developed.

If manufacturers are concerned that noise rules for takeoff and landing
will be tightened, they are worried about the issue of sonic boom. Research
into the impact of slight changes in the shape of the jet to soften the
boom has tended to make the airplane heavier and noisier at takeoff. It
is generally felt that supersonic flight may never be acceptable over land
so the new generation of aircraft must also fly efficiently subsonically.

Built with 1990 technology, a polluting and noisy supersonic jet would
weigh more than 500 000 kilograms. Williams says the 2005 cleaner and quieter
version will weigh around 300 000 kilograms because it will be made out
of new materials. Lightweight composite materials will probably be used
for up to half of the supersonic structure compared to only about 5 per
cent of today’s jetliners. New materials will also have to be developed
to withstand the high engine operating temperatures.

The dream of building these po-tentially lucrative, and hugely expensive
jets has created some rather surprising bedfellows in the cutthroat aeronautics
industry. NASA money has brought together Boeing, McDonnell Douglas, Pratt
& Whitney and General Electric, and has even tempted Deutsche Aerospace.

But most manufacturers admit the launch of a new supersonic will depend
on getting the assurance that air and noise pollution standards will not
change just as production begins.

* * *

Competing for the demand for supersonic travel

Despite the uncertainty about the impact of exhaust emissions and the
prospect that regulators could ground an aircraft that has cost around $15
billion to develop if it fails to meet environmental standards, airline
manufacturers are eager to prepare for a boom in air travel expected at
the turn of the century.

NASA has asked the US government for a 63 per cent increase next year
in its HSRP budget, which would bring the total for 1994 to $187 million.
Commercial airline manufacturers in the US and Europe refuse to divulge
exactly how much they are currently spending on supersonic research.

European spending is certainly dwarfed by NASA’s budget, admits Claude
Len-seigne, chief engineer of supersonic transport at the French company
Aerospatiale, a member of the Airbus consortium. The supersonic market is
big enough for only one design and, according to Lenseigne, that design
will be star-spangled unless European governments begin investing in the
future of commercial aviation.

In the minds of many American supersonic proponents, however, it is
a matter of national pride that the aircraft be of US design. The SST,
the US equivalent of Concorde, never got off the drawing board. The Russian
equivalent, the TU-144, flew only briefly.

Over the 10 years to 2015, the aerospace industry forecasts a market
for between 500 and 1000 new supersonic jets carrying passengers on long-haul
flights over oceans where the sonic booms produced by high-speed flight
will not bother anyone. By 2005, more than 300 000 passengers will fly almost
5000 kilometres daily, predominantly over water and mainly across the Pacific
Ocean. Within a decade, this number will double.

The aircraft will have twice the range of the Concorde, or about 11
000 kilometres, and will carry up to three times as many passengers, or
about 300. And yet, insists the industry, fares will be no more than 20
per cent higher than for subsonic travel. A transatlantic round trip on
Concorde is currently nearly two and a half times as expensive as standard
business class on a Boeing 747.

American and European versions of the new supersonic will fly at roughly
the same speed as the Concorde: up to two and a half times the speed of
sound. In practical terms this would reduce the flight time between Los
Angeles and Tokyo from more than 10 hours currently to just over four hours.
According to Louis Williams, director of NASA’s High-Speed Research Program,
‘the world is too small to go much faster because too much time would be
spent accelerating and decelerating.’