Since the beginning of powered flight, black gold and the kerosene that is refined from it has been intertwined with the development of aviation. Other power sources, such as steam, have been tried, but the specific and complex requirements of an aircraft have meant that the internal combustion engine was the only practical way forward.
However, 104 years after the Wright brothers' first flight we are now at a crossroads: how will aviation survive in the face of growing concerns over its environmental impact, coupled with the increasing cost of oil as a result of its scarcity?
Fuel bills continue to grow, conflict and uncertainty in the Middle East have added to the price pressure on airlines and fall-out from Hurricane Katrina has affected US production in the Gulf of Mexico. These pressures, along with the soaring demand for oil due to the rapid industrialisation of China and India have pushed oil prices even higher.
Environmental factors have also come to a head recently. With increased awareness of the dangers of climate change, the large amounts of carbon used in aviation have meant that the industry has been targeted as a major contributor to global warming.
Unlike other industries, the problem is compounded by the specific needs of any alternative or supplemental fuel for air travel. Ground-based industries can tap into solar, wind, tide and other renewable sources, while car and vehicle transportation can make use of batteries and electric fuel cells. However, the fuel cell technology required to generate enough power to fly a plane is still some way off. The unique requirements needed by planes - such as high energy density, low volume, high flashpoint temperature and low freezing point - make it extremely difficult to identify a suitable replacement for kerosene. This is despite the fact that, since the dawn of the jet engine 60 years ago, fuel consumption has decreased dramatically as engine manufacturers have developed ever more efficient engines.
Alternative fuels to oil break down into two groups: true alternatives and supplements that can be blended or mixed with existing carbon-based fuel to make it go further.
One promising way forward is through biofuel or converting "recent" carbon (such as that locked in plants) into aviation fuel. This can use crops such as rapeseed, soybean or even algae as the base. It can then be turned into a synthetic fuel using the Fischer-Tropsch process, which was used as far back as the Forties by Germany to create synthetic fuel from coal. As a practical example, in late 2006 the US Air Force conducted three trial flights using a 50/50 mix of synthetic fuel and normal jet fuel in a B-52. However, using fields of crops that would otherwise be used for food has its own obvious problems.
Another base for synthetic fuels is gas or coal. This is already used by South African Airways in a 50/50 blend of coal to liquid (CTL) which is again produced using the Fischer-Tropsch process to create a synthetic fuel. Like synthetic fuels created from crops, gas to liquid (GTL) or CTL are extremely attractive to aircraft manufacturers as they involve minimum alterations to the aircraft's systems and minimum changes to the aircraft's configuration (unlike hydrogen or an ethanol-powered airliner). Gas is also attractive from another viewpoint, as some of it is currently burnt off on the top of oil wells as waste product to get to the oil reserves. Using this in a GTL fuel would conserve precious oil even longer. Coal, although it is carbon-based, has another advantage in that coal reserves are expected to last another 200 years compared to oil.
Ethanol (alcohol) is another alternative fuel. In Brazil it has been used in cars and other vehicles for over 20 years, and the country has the largest ethanol-based economy in the world. In 2004, Embraer/Neiva's Ipanema agricultural aircraft was certificated by the Brazilian aviation regulation agency Centro Técnico Aeroespacial as the first production aircraft in the world to be powered by ethanol. The fuel is three to four times cheaper than aviation gas and is more environmentally friendly in emissions too.
Hydrogen may also be used as an alternative fuel; it has the advantage of an extremely high energy content for its weight. It's also a near-perfect fuel in terms of waste products, water vapor being its main by-product. However, the biggest obstacles to using hydrogen are the changes in aircraft design configuration and infrastructure to support it, something that would prove a complex and costly process.
Another alternative could be electrical power. Boeing is working on a small, all-electric demonstrator aircraft to validate some of the technology. The Helios UAV (NASA's prototype solar-powered, unmanned aircraft) has demonstrated solar-powered flight. While Bertrand Piccard (above, centre) is currently working on an attempt to fly around the world in a manned aircraft, the Solar Impulse, using only solar energy.
Finding a solution to the industry's dependence on oil will be one of the toughest challenges since the dawn of flight, and is attracting some of the best engineers, scientists and academics to solving it. The survival of the planet depends on their success.
Tim Robinson is deputy editor of 'Aerospace International' magazine