An industry notorious for lacking the highly sought after “green badge”, commercial flight has been one of the guilty pleasures of the present required to rely on the questionable effectiveness of carbon credits to maintain face. The typical one-way transatlantic flight generates around 1.2 tonnes of carbon dioxide per passenger, the equivalent to 4,000 miles of driving in a 35 mpg car[i]. Despite this ugly figure, aviation makes up 12%[ii] of CO2 emissions for transport and only 2-3% of the total[iii]. Pressure is building up for a greener image based on greener credentials. There is no one golden ticket to this end, but improvements can be made with a combination of new and upcoming technologies.
In a world with physical limits, the miracle of modern flight is made possible with a delicate balance of capacity vs. mass, pressure vs. friction, speed vs. structural strength, and distance vs. energy storage. If we seek a greener form of air travel, we are in effect tilting the balance in favour of new materials combining greater strength, lower friction and density, with forms of higher energy storage with less waste products (at least while airborne). This combination will result in aircraft that can carry a greater number of passengers further, faster and with fewer pollutants.
Material science has been developing at a staggering rate in the last century and is only getting faster. The most noteworthy of recent discoveries is the fabled carbon nanotube. Nothing more than a rearrangement of the fourth most common element in our universe[iv] and chemically identical to graphite and diamond, this substance can offer much to aviation. The combination of very high electrical conductivity with strength of around 100 times that of steel[v], it offers a lot in weight reduction. If the 135 miles and two tons of copper wiring in a Boeing 747 were replaced by carbon nanotube cables (nanoribbons) an 80% weight reduction could be achieved[vi].
If nanotube composites are used for the structural components, even greater weight savings can be made. Bayer MaterialScience[vii] have developed an aluminium/carbon nanotube composite with tensile strength comparable to steel at less than half the weight. This would considerably lower the 200-300 ton weight[viii] of a Boeing 747, providing huge fuel savings. In seeking to increase the current hull strength of an aircraft we need look no further than MIT. Engineers there have pioneered a process now known as nanostiching[ix]which can create materials 10 times stronger than current aerospace materials with more than one million times their original electrical conductivity, thereby mitigating much of the danger from airborne lightning strikes.
A lighter, stronger hull will definitely improve efficiency of flight. The real problem however lies with how the aircraft is powered, namely the form of propulsion and energy storage. To date only chemical forms of energy storage have a high enough energy density to sustain commercial flight. This comes at the cost of emitting huge amounts of waste gases at cruising altitudes. This is a trend that will not be broken without some impressive breakthroughs in battery technology, nuclear energy generation or wireless energy transmission.
[i] http://www.timesonline.co.uk/tol/travel/holiday_type/green_travel/article673044.ece [ii] Stern Report Annex 7 [iii] Working Group III Report, IPCC May 2007 [iv] http://en.wikipedia.org/wiki/Carbon [v] http://en.wikipedia.org/wiki/Tensile_strength [vi] http://www.xconomy.com/boston/2008/03/26/nanocomp-wins-air-force-grant-to-make-carbon-nanotube-wiring-for-aircraft/ [vii] http://www.bayermaterialsciencenafta.com/news/index.cfm?mode=detail&id=ABE84C28-A44C-DF70-B3CC3344CC3CE224 [viii] http://en.wikipedia.org/wiki/Boeing_747-8 [ix] http://www.eurekalert.org/pub_releases/2009-03/miot-mc030409.php