The cost of liquid combustible fuel would likely escalate during the post peak-oil period. Synthetic fuel that is processed from natural gas and coal via could become an important alternative fuel. The market demand for and prices of coal, natural gas and biofuels such as ethanol and biodiesel would also escalate. Municipal transit systems would likely be using more electrically powered vehicles as the number of (lithium) battery powered cars in service increases. The commercial aviation industry would likely compete for fuel and energy in a market of scarcity and escalating fuel prices.

Short-haul/Commuter Aircraft

Aircraft turbine engines are very flexible in the kind of fuel that they can burn. Short-haul and commuter airline companies would be likely candidates to use alternative aviation fuel as most of them serve short routes of under 500-miles. Most of the short-haul and commuter airline fleet is powered either by turbo-prop or by turbofan engines that burn naptha or other similar fuel. Short-haul aircraft that have sufficient capacity in the fuel tanks could burn cheaper fuel with a lower energy content on short commuter routes. A liquid fuel such as ethanol has 60% to 65% the energy content (BTU's per pound) of aviation fuel and some short-haul and commuter airline operators may switch to using such a fuel for reasons that may include:

• The own aircraft that have the necessary capacity in the fuel tanks.
• The cost per BTU of alternate fuel drops to below that of fossil aviation fuel.
• There would be overall economic benefit from using alternate fuel in service.

Low-Altitude Flight

The aviation world discovered the existence of ground-effect flight in a craft called "the Caspian Sea Monster" following the collapse of Soviet communism. This craft used a specialized wing design that generated a cushion of air between the wing and the surface over which it flew. Though originally conceived for military use, ground effect flight has found civilian applications carrying passengers and freight between population centers that are separated by large bodies of water.

Ground-effect flight technology allows very large and very heavy aircraft to be flown at high speed on less fuel than high-altitude aircraft. A recent development from Britain involves the installation of transversely mounted aeronautical "paddle wheels" on the topside of aircraft wings to provide propulsion and increase lift. A demonstration of this technology involved the use of a radio controlled scale model and proved that the concept works. The technology can be applied to large, heavy ground-effect craft that can be flown across bodies of water and between coastal airports. Paddle wheels that are not used for propulsion can be replaced by rolling hollow cylinders that will use the boundary layer effect to redirect the flow of air over the airfoils so as to increase lift.

The weight advantage of ground-effect aircraft allows them to be powered by unconventional engines such as external-combustion air turbines and burn low-cost fuels that have a low energy density such as coal-water fuel. Large ground-effect aircraft could carry freight on extended trans-oceanic journeys while giant hovercraft may be better suited to service on short routes. Propellers the size of helicopter rotors could propel the latter craft to achieve higher propulsive efficiency since they move a large mass of air at low velocity to deliver the same thrust at higher efficiency than smaller propellers that move a smaller mass of air at high velocity. Both types of giant craft could be powered by similar unconventional power plants and burn the same kinds of (low energy density) fuels at competitive cost in a future energy-constrained market.

Hydrogen Fuel

While the preferred aviation fuel would be a liquid, hydrogen has a high specific energy content of 51,000-BTU per pound. A commuter jet built by Dornier was modified in Europe to carry a bulky, heavy, reinforced fuel tank filled with compressed hydrogen on the roof of its fuselage. Fuel tanks that contain compressed hydrogen are heavy, they will have limited storage capacity, would need to be insulated and be cooled during refueling. However, a tank of compressed hydrogen could offer a higher specific energy density (BTU's per pound of fuel plus weight of fuel tank) than a lighter tank of a liquid fuel that has far fewer BTU's per pound. The cost (per BTU) of liquid aviation fuel could exceed that of hydrogen during a post peak-oil period.

Hydrogen-powered commercial aircraft may be restricted to serving several high-density short-haul and commuter routes where competition from high-speed rail passenger service is absent. However, the peculiarities of geography could offer a market niche to commuter and short-haul airline operators in some nations. During a post peak-oil era, that market niche could require that a substantial amount of hydrogen be produced for aviation purposes. Hydrogen production would increase demand on power companies and require them to develop and implement strategies to meet the growing demand. Power generation installations and electrolysis equipment to produce hydrogen could appear at major airport terminals in the long-term future.

The use of compressed hydrogen as aviation fuel could greatly increase the time durations required for refueling aircraft. Insulated fuel tanks would need to be cooled as they are filled. The reduction in air temperature with increasing altitude could help keep tanks cooler on hydrogen-powered, subsonic jet-powered aircraft. Using supercooled saturated hydrogen as fuel could require that empty fuel tanks be easily and quickly removed from aircraft and easily replaced with full fuel tanks. The use of quick-release couplings and specialized shut-off valves on fuel lines could enable rapid and easy replacement of aircraft fuel tanks at airports.

Breakthroughs and Research

While compressed hydrogen gas may become an alternate aviation fuel for short-haul service, supercooled liquid hydrogen could also become an alternative fuel for other types of commercial airline service in the future. However, until numerous logistical problems that are related to its use are resolved, liquid hydrogen may see limited service as an alternative aviation fuel. Other alternative fuels may include high-density energy-storage technologies that could result from research breakthroughs that are likely to occur in such diverse fields as nanotechnology and high-temperature superconductivity.

Sporadic and significant breakthroughs have occurred in both fields over the past few decades. The field of high-temperature superconductivity holds great promise for use in high-density energy-storage technology. Breakthroughs in these fields are likely to occur more frequently in the future as more scientists and engineers are expected to graduate from educational institutions in India and China. Theoretically, a coil formed into a torus and made from "high-temperature" superconductive material could store enough energy to enable a full-sized commercial airliner to undertake an extended trans-oceanic or trans-continental flight. Advances in nanotechnology will ultimately enable superconductive materials to even be manufactured and then produced at a cost that could be justified in applications like airliner propulsion.

Energy stored in a superconductive storage technology could be supplied to electric motors that drive the identical propulsion fans that are found at the front-end of modern, "high-bypass" turbo-fan aircraft engines. The propulsion fans in such engines can provide up to 90% of the propulsive thrust. Each propulsion fan may be driven by multiple (induction) electric motors at low altitude. Electric motors have poor part-load efficiency. Some motors could "cut-out" under reduced demand at cruising altitude and the few motors that operate will do so at higher efficiency.

Subsonic commercial aircraft that will use high-density electrical storage technology in the long-term future may use Coanda fans for propulsion. Coanda fans were originally developed by a physicist named Henri Coanda. They are able to operate at comparable efficiency and at comparable flight speeds as turbine-driven propulsion fans. Certain designs of electrically powered aircraft could be designed to be flown in thinner air at higher altitude (up to 65,000-feet) as a way to reduce energy consumption on extended flights. The cooler air found at high altitudes could assist in keeping the superconductive energy storage systems functioning properly.

The superconductive energy storage systems and their (liquid nitrogen) cooling systems would need to be designed to be easily and quickly replaced during layovers at airports. After completing a long flight, aircraft using such energy storage systems would undergo cleaning and servicing in hangars before returning to service. Rapidly recharging the energy storage systems would be an energy-intensive process whereas a slow recharge would otherwise keep the aircraft out of service. The introduction of superconductive energy storage to power aircraft would require that airport terminals be equipped with power generation technology and energy storage systems in the future.

Low-speed Supersonic Flight

The Concorde SST flew at a speed of Mach-2 (1960-feet/second or 1337-miles per hour) at 65,000-feet. The temperature of the air at this altitude is typically near minus 60-degrees F. The atmospheric pressure is 0.00099-lb per cubic foot or 1% the value that is found at sea level. Most commercial aircraft fly at 35,000-feet and in air that has over 12-times the density of the air in which the Concorde flew. Thin air at high altitude reduces drag on the aircraft that fly that high and can assist in reducing overall energy consumption depending on flight speed and engine performance.

Modified Rolls Royce Olympus engines that were equipped with afterburners powered the SST Concorde. These engines had originally been developed for use in military aircraft. At a flight speed of Mach 2, the shockwave at the intake (buckets) to the afterburner could yield a pressure-rise of between 4.5 to 1 and 5.5 to 1 in the air passing through it. The energy efficiency of jet engines (the TSFC or thrust specific fuel consumption) increases as engine pressure ratio rises along with the engine combustion temperature. Most commercial airliners fly at Mach 0.8 and are powered by turbo-fan engines that typically have pressure ratios in the vicinity of 12 to 1.

Future aircraft that are equipped with superconductive energy storage could use electrically driven propulsion fans for subsonic flight (Mach 0.8) and separate engines for low-speed supersonic flight (up to Mach 1.5). These supersonic engines would be activated at high subsonic speed (Mach 0.8) and at high altitude. Each engine would use an Oswatitsch intake that would generate a series of oblique (weak field) shockwaves at supersonic speed and cause air to flow downstream of it at high subsonic speed (Mach 0.95) and into a section that contains pilot-activated air scoops and a "dump door". This section would be located ahead of a set of cones that would be designed to raise engine pressure-ratio (at low supersonic flight speed). The combustion chamber and jet pipe would be located downstream of the last cone.

The cross-sectional area of each cone increases along its length and causes air speed to decrease and air pressure to increase. Sets of cones were originally used in the ejectors of many designs of steam locomotives and could generate pressure ratios in excess of 10 to 1. Some ejectors could convert a portion of the low-pressure (25-psig), high-speed flow of exhaust steam to a low-speed flow with sufficiently high-pressure to force its way past a one-way valve and into the boiler (250-psig). A set of rotating cylinders with axially fluted surfaces may be located ahead of the entrance to the first cone to cause the flow of air to pulsate through the engine and possibly raise efficiency.

A jet engine using an innovatively designed set of cones could achieve higher engine pressure ratio and higher engine efficiency at low supersonic flight speed (Mach 1.2 to Mach 1.5) than the engines on the SST Concorde could achieve at Mach 2. The combination of lower supersonic flight speed, lower air density at higher altitude, higher engine pressure ratio and higher engine efficiency could enable a low-speed supersonic aircraft flying at Mach 1.5 to become cost competitive (fuel costs) against subsonic commercial flights (Mach 0.8) that fly at lower altitude (35,000-feet). If such aircraft could carry over 200-passengers on extended trans-oceanic routes, they would compete against subsonic commercial aircraft in terms of travel duration and ticket prices.

Hypersonic Flight

Hypersonic flight (flight speed of Mach 20 at altitude of 200-miles) has been researched worldwide. Such aircraft would likely use supercooled liquid hydrogen as fuel. It is uncertain as to whether a market for hypersonic air travel services could actually be developed, however, such a market would require that power generation capacity be expanded near airports that offer hypersonic flight services. Hypersonic aircraft may need to be equipped with rapidly interchangeable modules that contain the fuel tanks contain supercooled fuel, the supercooling systems and the superconductive energy storage systems that provide power for the cooling systems and subsonic flight engines. The fuel tanks and the superconductive storage systems would be recharged in specialized facilities away from the aircraft.

Power Generation

The number of electrically powered and hydrogen powered road and railway vehicles would likely increase during a post peak-oil period. Commuter aircraft that operate short-haul service could be powered by hydrogen. Additional breakthroughs that are likely to occur in high-temperature, superconductive energy storage technology could lead to the development of electrically powered long-haul aircraft. A future commercial aviation industry could need vast amounts of electric power to recharge superconductive energy storage systems, recharge aircraft super-cooling systems, generate hydrogen as well as compress and supercool large amounts of the gas.

Modern commercial aircraft use a tremendous amount of energy to become airborne and undertake long-haul flights. The power output of an engine of a long-haul commercial aircraft (15 to 20-Mw at 180-miles per hour during take-off) is equivalent to the power requirements of a city of 20,000-people. Airports that serve metropolitan areas presently process continual processions of large long-distance aircraft during peak periods. Such aircraft could require between 300-Mw-hr and 1000-Mw-hr of power to undertake trans-oceanic flights at subsonic speed. The power requirements of a future electrically based commercial aviation industry could overwhelm the power generation industry of most developed nations.

Major international airports around the world would need to generate electric power from on-site power stations to meet the energy needs of fleets of electrically powered and hydrogen-fueled commercial aircraft. The airport power stations may be nuclear; they may operate on hydrogen fusion or be based an unconventional power generation technology that is still being researched. Large amounts of heat will be rejected by these thermal power stations, by the generation and compression of hydrogen as well by replenishing aircraft supercooling systems. Most of the rejected heat could be reclaimed and put to productive use. The options would include:

• Heating buildings (district heating) during winter.
• Putting heat into geothermal storage during summer.
• Powering absorption air-conditioning systems during summer.
• Energizing low-grade heat engines to generate electricity during winter.

Energy Storage

The ability to store large amounts on energy at or near major airports would gain importance should electrically powered aircraft be developed during a post peak-oil period. Power could be purchased from the grid during their off-peak periods and put into such short-term storage. Airport power stations could encounter off-peak periods and could replenish airport energy storage systems. These technologies could include superconductive storage, flow batteries, off-site hydraulic storage at nearby mountains (coastal airports) or off-site pneumatic storage (subterranean salt domes that were emptied). Exhaust air from pneumatic storage systems would produce cold air that could assist in replenishing supercooling systems on some types of future aircraft. The availability of energy storage would assist in recharging aircraft during peak periods.

Power Regulation (Airports)

Power stations that provide energy for air transportation use may have to be excluded from the regulatory framework. Most of the electrically powered airliners that will be recharged would be "foreign" owned, that is, the owners would be domiciled in a different jurisdiction to where the aircraft would be recharged. The idea of regulators in one jurisdiction looking after the interests of parties who live, do business and pay taxes in another jurisdiction is quite ludicrous. Power stations that supply a future airline industry with electric power would need to be regulatory-free despite the "foreign" airline owners being "captive" customers. It would be possible for power to be supplied to a single airport by several small providers who compete against each other. Power providers and airline companies could negotiate deals including on a daily basis.

Conclusions

There is the possibility that oil prices will rise in the future and that peak-oil may actually occur. There is also ongoing research being undertaken in areas that at present may be unrelated to the energy industry or the airline industry. Several energy options and technological alternatives could become available to the commercial airline industry after peak-oil occurs. Breakthroughs in nanotechnology, superconductivity, new power generation technologies (hydrogen fusion power stations, more cost-effective nuclear power stations that produce less waste material) could offer a future airline industry several alternatives by which to remain operational should future oil prices escalate and future oil production declines.

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