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May 12th, 2007

Supersonic Electric Aircraft

Abstract:
Open Access Article Originally Published: May 11, 2007

Commercial aviation is an essential component of the global economy. The cost of aviation fuel is directly determined by the prevailing world price of oil. It accounts for a major proportion of airplane operating costs. Several airline companies now add a fuel surcharge to the ticket cost of a commercial flight to compensate for the recent rapid rise in fuel costs. World oil prices are expected to remain high for several years. The prospect of sustained high aviation fuel prices could propel airline companies to seek alternative aviation fuels. Seeking alternative fuel could become become paramount for the airline industry should the peak-oil phenomena actually occur. The commercial aviation industry would likely compete for fuel and energy in a market of scarcity and escalating fuel prices.

Breakthroughs and Research
It may become possible for supercooled liquid hydrogen to eventually be used as an alternative fuel for some types of commercial airline service. Extensive research will be needed to resolve the numerous logistical problems that are related to its use as an alternative aviation fuel in supersonic and hypersonic aircraft. Other alternative fuels may include high-density energy-storage technologies that result from breakthroughs in research in the areas of nanotechnology and in high-temperature superconductivity.

Sporadic and significant breakthroughs periodically occur in both fields. High-temperature superconductivity holds great promise for use in high-density energy-storage technology. A coil formed into a torus and made from "high-temperature" superconductive material could theoretically store enough energy to enable a full-sized commercial airliner to undertake an extended trans-oceanic or trans-continental flight. Advances in nanotechnology could enable superconductive materials to eventually be manufactured at a cost that could justify their application in airliner propulsion.

Electrical Storage and Propulsion
Energy stored in a superconductive storage technology could power electric motors that drive the identical propulsion fans that are found at the front-end of modern, "high-bypass" turbo-fan aircraft engines. Such fans provide up to 90% of the propulsive thrust of the turbo-fan engine. Each electrically powered propulsion fan may be driven by multiple (induction) lightweight electric motors during take-off. Some electric motors would "cut-out" under reduced power demand at cruising altitude so that the remaining motors will operate at higher efficiency (electric motors have poor part-load effciency).

Coanda fans may propel subsonic commercial aircraft that use high-density electrical storage technology. Such units were originally developed by physicist Henri Coanda and can operate at comparable efficiency and at comparable flight speeds as turbine-driven propulsion fans. Electrically powered aircraft that use either turbine propulsion fans or Coanda fans could be flown in thinner air at higher altitude (up to 65,000-feet) to reduce energy consumption (less drag on aircraft) on extended flights. The cooler air found at such altitudes could assist in keeping the superconductive energy storage systems functioning properly.

Superconductive energy storage systems used in future commercial aircraft would likely be cooled by liquid nitrogen. Both systems would need to be frequently recharged. Commercial aircraft that operate long-haul service usually undergo cleaning and servicing in hangars after long flights. It is during such service periods when the energy storage and cooling systems could be recharged, a process that would likely be both energy-intensive as well as time consuming.

It may be possible to design the energy storage systems along with their cooling systems to be removed and replaceable during shorts layovers. Such technology may be possible and could help reduce the turn-around time of the aircraft. The introduction of superconductive energy storage systems in commercial aircraft in the long-term future would require that future airport terminals be equipt with power generation technology at or near the premises.

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 that operate routes of under 500-miles would be the most likely candidates to use alternative aviation fuel. Their fleets are mainly powered by turbo-prop or by turbofan engines and may likely have sufficient capacity in the fuel tanks to carry a cheaper fuel with a lower energy content. They may use such fuel if its cost per BTU undersells fossil aviation fuel. Breakthroughs in electrical storage technology could see a future generation of short-haul and commuter aircraft being propelled by electric motors driving propellers or propulsion fans.

Low-Altitude Flight
Ground-effect aircraft use a specialized wing design that generated a cushion of air between the wing and the surface over which it flies. Large and heavy versions of such aircraft could be flown at moderate speed over water and carry passengers and freight between coastal centres of up to 500-miles apart. Eliminating the need for take-off to at least 10,000-feet would cut fuel costs. The performance of such craft can be enhanced by a recent development from Britain that has been successfully tested in a scale model aircraft.

Aeronautical "paddle wheels" are mounted transversely on the topside of aircraft wings to provide propulsion and increase lift at very low flight speeds. Such craft may to be propelled by electrically driven propellers that are the size of helicopter rotors. Such units can move a large mass of air at lower velocity to deliver high thrust (200,000-lbf per propeller) at higher propulsive efficiency. An alternative system could see heated air being ducted through the thick rotor blades to adjustable jets that are built into the tips of the rotors.

Low Speed (Electrically-powered) Supersonic Flight
An American company called Supersonic Aerospace International (SAI) recently undertook research into reducing the sonic boom of supersonic commercial flight. The result was a unique configuration of supersonic aircraft capable of flying quietly at Mach 1.5. It is theoretically possible to develope an electrically powered engine capable of propelling a commercial aircraft to such a flight speed.

A high-temperature superconductive energy storage system would supply power to 2-sets of electric motors that drive different propulsion systems. A subsonic propulsion system of electrically driven propellers would accelerate the aircraft up to a flight speed of Mach 0.5 when the supersonic engines would engage. These engine would have a cross sectional profile similiar to that of a ramjet.

The electric motor and compressor would be housed in a straight tube intake pipe that would be flowed by a section of gently increasing diameter. A shock wave at the entrance of the pipe would see air speed drop from Mach 1.5 to Mach 0.7. The air temperature would rise from minus 40-degrees F to 95-degrees F. An electrically driven axial flow (single spool) compressor operating at 93% isentropic efficiency and having a pressure ratio of 8 to 1 would further increase air temperature to 580-degrees F.

The heated air would flow into the diffuser (where area quadruples) where air pressure would increase by up to 25% and air temperature would rise to 648-degrees F at the maximum cross section. Air would then flow into a nozzle (smaller cross section) at sonic speed and accelerate into a diverging exhaust section where air would leave at over 2534-feet per second (Mach 1.5 at -40-degrees F is 1507-feet per second). The engine would move a very large volume of air to provide sufficient thrust to maintain flight speed. A pressure ratio of 10:1 on the compressor could raise the exit velocity of the air to 2704-feet per second.

Faster Supersonic Flight
The electrically powered engine that could theoretically propel an aircraft to a flight speed of Mach 1.5 could be modified to operate at higher speed. The engine intake would be modified to an "Oswatitsch" design with variable geometry. That design would generate (weaker) oblique shock waves at the entrance to the engine as well as be able to "dump" excess air or duct in extra air depending on flight conditions.

The faster engine may use a single-spool axial flow compressor that has more pressure ratio (up to 15 to 1 with variable stator blades) to raise air temperature. The aircraft may carry water in special tanks and electrolysis gear to generate hydrogen that may be injected ahead of the nozzle of the engine. The combustion of the hydrogen would increase the air temperature and raise the exit velocity of the gas that leaves the engine. Flight speeds of Mach 2 to Mach 2.4 may be possible.

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 ethanol or by hydrogen while future supersonic aircraft could use liquid hydrogen as fuel. The commercial aviation industry of the future (post peak oil) could likely require vast amounts of electric power to recharge superconductive energy storage systems, recharge liquid nitrogen cooling systems as well as to generate, compress and supercool large amounts of hydrogen.

Modern commercial aircraft are energy intensive during take-off. 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 likely overwhelm the power generation industry of most developed nations.

Major international airports may eventually need to generate electric power on-site to meet the energy needs of future fleets of electrically powered and hydrogen-fueled commercial aircraft. Airport power stations may be nuclear; use hydrogen fusion or be based some other unconventional power generation technology that is still subject to research. The heat that will be rejected by these thermal power stations could be reclaimed and put to productive use that would would include:

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

Energy Storage
The ability to store large amounts of energy at or near major airports could gain importance during a post peak-oil period. Electric power could be purchased from the grid during their off-peak periods and put into short-term storage. Airport power stations that encounter off-peak periods could replenish airport energy storage systems that may include superconductive storage, flow batteries, hydraulic storage in hydroelectric dams in nearby mountains (coastal airports) or off-site pneumatic storage (subterranean salt domes that were emptied). Air that is exhausted from pneumatic storage systems may be sufficiently cold to assist in "replenishing" liquid nitrogen supercooling systems.

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
Future scientific breakthroughs are likely to occur in both nanotechnology and in superconductivity. High-density energy storage technologies could be the likely result and appear in the distant future. Electrically powered commercial aircraft that fly at subsonic speeds could appear in the future irrespective of whether or not peak-oil actually occurs. Alternative liquid fuels that are cost-competitive to fossil oil are also likely to appear and find application in aviation. Large ground-effect aircraft that fly above water and that carry either passengers or freight between coastal cities are also likely appear in the future.