The aircraft ghosts in to land, the loudest sound being the squeal of tyres on touchdown. Engine noise, so long the bane of aircraft and airport developers alike, has been banished to the history books.
What is more, the only significant emission from this generation of powerplants is completely harmless water vapour.
The airliner moves with barely a whisper towards the gate and the few loyal friends and relatives that await the passengers at the unsociable hour of 4am. But this is not a delayed arrival breaking curfew - airports are open 24h a day since the introduction of near-zero and zero-emissions engines and the perfection of powerplant and airframe noise suppression.
The days of noise curfews, emissions taxes and airport gridlock are past.
Today, however, the environment problems facing the industry loom huge. Aviation is the second largest consumer of oil in the transportation sector after cars, and jet fuel consumption is forecast to have the largest percentage growth rate within the US transportation sector. Worldwide, the consumption of oil by aviation is expected to grow in line with the projected 5% annual air traffic growth. Oil consumed by the transport business is expected to rise by 33% from 1995 to 2015, with an annual rate of growth of 2.3% for jet fuel.
The result is increasing levels of NOx emissions. NASA says aircraft contribute up to 4% of mobile-source NOx emissions overall, but at London Heathrow Airport it has been shown that aircraft contribute between 16% and 35% of the ground-level NOx concentration near the UK airport. In the USA, growth at several major airports is restricted because of smog-related emissions from aircraft. These include Atlanta, Boston, Chicago, Houston, Los Angeles, New York, Phoenix, Philadelphia and Washington DC.
Moreover, more than 70 million people in the USA live in areas that exceed federal ozone standards, many of them close to the airports listed above. To make matters worse, these gases have known connections to respiratory diseases which particularly affect the young and aged.
While emissions are fast assuming a higher profile in environmental battles, the problem of aircraft noise has never been far from the headlines since the start of the jet age. Although technology has made major strides towards Stage 3 and soon to be decided Stage 4 limits, the noise impact of subsonic aircraft is still a huge burden on the growth of the industry.
Curfews, noise budgets and limits and slot restrictions have grown in step with passenger figures. Around the world, the number of airports with local noise-related restrictions has grown from 140 in 1980 to 350 in 1998. Around $300 million a year is being spent on noise mitigation projects.
Potential savings from fuel-efficient aircraft and engines are estimated at over $80 billion over the next 25 years.
While tackling emissions with technology is one approach, NASA is also investigating the use of hydrogen as an alternative to hydrocarbon fuels. Initial "hybrid" systems, while drastically reducing NOx, would not totally eliminate it.
They would, however, form the basis of a new hydrogen fuel infrastructure, and provide a bridge to future propulsion systems being studied under the agency's Zero CO2 Emissions project. This is looking at clean-burning hydrogen fuel-cell technologies from space applications. The fuel cell would be used to generate power for electric motors, which would produce the actual thrust, and builds on proton exchange membrane technology under development for the automotive industry.
The weight of the fuel-cell engine system would be close to that of an advanced gas turbine, and between four and 10 times less than current turbine engines, NASA says.
As envisaged, the fuel cell would be buried in the wing, alongside the liquid hydrogen tanks. The oxygen required for the reaction would be recovered from air taken in from the atmosphere. Carbon anodes and cathodes would be immersed in an electrolyte and contain platinum as a catalyst.
The reaction would break down oxygen molecules into atoms, and as the gas bubbles pass the cathodes, oxygen would absorb electrons while combining with hydrogen to form hydroxide ions.
At the same time, hydrogen would give up electrons at the anodes as it combined with the hydroxide ions to form water. The resulting current flowing between the anodes and cathodes would be used to power large electric propfans mounted above the wing. The only byproduct would be water vapour.
NASA's project calls for an assessment of hybrid fuel cells and a liquid hydrogen optimised turbofan by late 2002, and involves scaling up a fuel cell to multimegawatt proportions, at the same time developing methods of controlling NOx production in the hydrogen-fuelled turbofan. Aviation may have reason for thanking the automotive industry if it is to fulfil its potential in the 21st century.
Source: Flight International
