The coming disruptive storms in civil aerospace propulsion

Citation

Singh, E.P.R. (2014), "The coming disruptive storms in civil aerospace propulsion", Aircraft Engineering and Aerospace Technology, Vol. 86 No. 1. https://doi.org/10.1108/AEAT-11-2013-0196

The coming disruptive storms in civil aerospace propulsion

Article Type: The coming disruptive storms in civil aerospace propulsion From: Aircraft Engineering and Aerospace Technology: An International Journal, Volume 86, Issue 1

Introduction

The confluence of the growth of civil aviation and the need to limit its impact on climate change is set to bring the aerospace industry to its tryst with destiny. The option of resolving this problem by limiting the growth of civil aerospace carries with it the problem that this would slow down globalization and economic growth within the developing world, leaving large populations at poverty level. Given the connectivity of today’s world via the internet and social media, such actions carry the potential risk of political unrest on a global scale.

Aerospace: the gas turbine as a disruptive technology

The underlying principles of the gas turbine were described in a British Patent No. 1833 “A method for rising inflammable air for the purposes of producing motion […]” This patent was taken out by John Barber in 1791. It took some 150 years before the principles described by Barber were turned into products by Frank Whittle and others, illustrating that the presence of appropriate technology and the need for the product were necessary for the journey from concept, prototype to product to occur.

Frank Whittle’s patent, British Patent No. 347206 was taken out in January 1930, and Whittle’s first jet engine ran in April 1937. The Second World War accelerated the development of the jet engine, principally in Germany and the UK. An important opportunity for the jet engine arose from the fact that its performance at high altitude was much better than that of the then existing internal combustion engines used for aviation. The jet engine allowed aircraft to fly above the weather, offering passengers comfort and safety, a key to the large growth of civil aviation. Prior to, during and in the early years after the Second World War, the internal combustion engine was the prime mover within aviation. Eventually, the gas turbine proved to be a successful disruptive technology, evidenced by the fact that many world-ranking companies that produced piston engines for aircraft are no longer in existence.

From the 1950s onwards, civil aviation has grown at about 5 percent per year. The growth was driven by technology improvements, mainly within the gas turbine, which allowed the emergence of larger, more efficient and longer-range aircraft with increased earning capacity. The wealth of the nations was growing at the same time as unit fares within civil aviation were reducing. These changes provided the capital and income for advancing the technology of engines and airframes for civil aviation, further reducing unit fares and driving a virtuous circle of growth for the benefit of the airline passenger.

The growth has been so large that civil aviation is today recognized for literally having shrunk the world, connecting its peoples and driving globalization. This has resulted in large populations being lifted out of poverty, particularly in the fast developing world. The development of gas turbines from aerospace has resulted in products which find application within oil, gas, power, pharmaceutical and marine industries. Additionally, the very large engineering science investments resulting from the gas turbine industry have had an impact on computational methods, materials and understanding of design, resulting in technology transfer and diffusion, creating an ever-wider impact.

Challenges facing the gas turbine industry

The ability of gas turbines to fly above the weather, offering comfort and safety, has resulted in a very large and still growing aerospace activity. A consequence of this is that there are now concerns that civil aviation will contribute to climate change and global warming.

One view is that society will be able to markedly reduce greenhouse gases, principally carbon dioxide, by energy efficiency initiatives within housing, cities, transport and industry. Additionally, the increasing use of renewable energy and nuclear power will contribute to reduction of greenhouse gases produced at ground level. Leaders within civil aviation may argue that this reduction in ground-level carbon dioxide would be sufficient to allow continued growth in civil aviation, which is currently only contributing about 3 percent of man-made carbon dioxide. If necessary, airline passengers could pay an increasing “green tax”.

Whether such a stance would be acceptable to society at large has yet to be tested. The green community may view the growth of civil aviation as posing an unacceptable risk into the longer term future, and as an indulgent and unnecessary activity for the relatively well-off. There is the possibility that ever-larger proportions of younger members of society will empathise with this. It should be noted that a 5 percent growth over a 100-year period would result in a 20-fold increase in carbon dioxide, even if an assumption were made that improvements in technology and operations would reduce fuel burn per passenger kilometer by as much as 80 percent.

An alternative suggested is the use of bio-fuels. Whilst technically feasible, it remains unclear whether such fuels can be produced in quantity at competitive prices without concerns that arable land and water necessary for agriculture and other uses are being diverted as a fuel to serve the desires of a small rich sector of society. Bio-fuels may well be appropriate “drop-in” bridging fuels for a decade or two, but are unlikely to form the basis of a long term solution.

Aerospace: new disruptive technologies

The generation of power at ground level needs to be undertaken such that the level of carbon dioxide produced is significantly reduced, and indeed, that some of the carbon dioxide produced is captured, transmitted and stored underground (carbon sequestration) to ensure that industrial activity can continue whilst managing climate change. Much of the power generation will therefore require solar, wind and nuclear power plants. These, along with carbon sequestration, are not suited to load-following. Therefore, if enough power is generated to meet societal needs during working hours, there is likely to be an excess of power during night-time. One use of this excess power would be to generate hydrogen, which could then be separately used as a fuel, which when burnt, would not produce carbon dioxide. The juxtaposition of changes in power generation at ground level, resulting in the production of hydrogen, and the problem associated with the growth of civil aerospace suggest that there may be an opportunity here. The first possibility would be to replace kerosene as a fuel by hydrogen. Such a change would mean that civil aviation would not produce carbon dioxide. As hydrogen has the ability to burn at leaner mixtures and hence lower flame temperatures, the use of hydrogen could also result in the elimination of oxides of nitrogen, which are formed at high temperatures in reactions between dissociated oxygen and nitrogen. Such hydrogen burning “cryo-planes” would have less impact on climate change, and are technologically feasible.

The last decade or so has seen a move towards “more electric” aircraft. During this same period, studies have shown that super-conductivity may be possible at temperatures associated with liquid hydrogen. High-speed generators and electric motors, possibly cooled with liquid hydrogen, also appear both possible and attractive. The combination of high-speed super-cooled generators, electrical power transmission systems and high-speed motors driving small fans creates the opportunity to begin to look in greater depth at turbo-electric propulsion systems and their integration. The separation of the conversion of the chemical energy within the fuel to shaft power, the transmission of this power electrically, and then the use of it to create propulsive force allows the optimization separately of thermal efficiency, transmission efficiency and propulsive efficiency.

Figure 1

These changes have created an environment where significant studies are being undertaken to consider deeply integrated future turbo-electric powered aircraft. The best known of these is NASA’s N+3 studies. Such aircraft, using high-lift airframes, for example, blended wing body shapes, may be powered by turbo-electric propulsion systems (Figure 1). The engines converting the chemical energy stored within the fuel to shaft power could be optimized solely for this purpose, and therefore advanced cycles such as inter-cooled, recuperated and beyond could be considered. The cooling, to achieve super-conductivity, could use hydrogen cryo-coolers. The use of hydrogen in the cryo-cooler effectively gets hydrogen “on board” the aircraft. This opens the possibility of such aircraft will use hydrogen as a fuel.

The power produced would be delivered to a number of small propulsive fans, whose pressure ratio would be designed to maximize propulsive efficiency. The placement of these fans on the upper surface would ensure that propulsion system noise at ground level would be markedly reduced. The possibility exists that the fans could be relatively small and of sufficient number to allow control of the aircraft. This could be achieved by varying the power delivered to individual fans, and possibly also allow the tilting of the fans’ nacelles, such that aircraft control surfaces were minimized or eliminated.

Industrial changes

There are several disruptions implied here. First is the production, storage and handling of hydrogen and the safety aspects involved in the context of aviation. Next are super-conductivity, high-speed electric generators and motors and power management, followed by the opportunity to deeply integrate air frames and engines, including possible advantages by boundary layer suction. Other opportunities exist by using stored electrical energy for emergency or transients, and possibly fuel cells. If undertaken, these changes will create a number of engineering science opportunities, which can be expected to have implications for power transmission and management within ships, trains, and larger land vehicles, among others.

The route to market for turbo-electric propulsion systems might imply first the use of such concepts in UAVs, smaller aircraft, freight carriers and/or fuel tankers, before large civil aircraft enter service with these new technologies. This may be necessary to manage product development and investment risk. Just as the introduction of the gas turbine removed the major suppliers of internal combustion engines which powered civil aviation, the shift described above is of similar or larger magnitude, creating new opportunities for established players and importantly, opportunities for new players to enter civil aviation.

The wider impact

Whilst the engineering science, product and financial challenges are very large, the alternatives of either ignoring climate change concerns or curtailing civil aviation are equally unpalatable. In particular, curtailing the growth of civil aviation may trigger political instabilities because this would not allow significant populations to be moved out of poverty.

Global warming and global cooling at will

The challenges for turbo-electric distributed propulsion are large, but the opportunities for these major disruptive technologies are even more inviting. A further important disruptive technology is the management of climate change, made possible by the use of hydrogen fuel in this context.

The use of hydrogen as a fuel would result in civil aviation producing no carbon dioxide or oxides of nitrogen. The water produced by the combustion of hydrogen could be condensed at the engine exhaust. This water could be emitted at managed droplet size, such that contrails and contrail-induced cirrus clouds resulted, leading to global warming. Alternately, the managed droplet size emitted could be smaller than the wave length of light. At altitude, these droplets would defract the sun’s radiation away from the earth, resulting in global cooling. This would allow the same aircraft to be operated to produce global warming or global cooling at will, and in the same flight, to manage climate. This would result in aviation contributing to managed climate stability, rather than being a potential climate hazard, particularly in terms of global warming. Therefore, the case to limit the growth of civil aviation because of climate concerns would be removed.

Emeritus Professor Riti Singh

Cranfield University, Cranfield, UK