1:41 PM on Oct 24, 2011
Nuclear-powered aircraft have been looked at from time to time over the history of aviation because of their potential for extremely long endurance and, more recently, their independence from fossil fuels. But they always founder on the real and perceived issues of nuclear safety.
What if there was potential for green nuclear power? Writing in Aviation Week & Space Technology's Imagining the Future issue, NASA Langley energy expert Joe Zawodny says experimental evidence indicates low energy nuclear reaction (LENR) technology could be an extremely clean energy source.
Recent LENR research dates back to the late-1980s 'cold fusion' debacle of Pons and Fleischmann but, Zawodny says "a growing body of increasingly repeatable experimental evidence indicates the LENR effect is real and is likely not fusion, cold or otherwise." An LENR power source would have enormous energy density, but the ionizing radition produced would be extremely low.
NB-36H reactor testbed (foreground) (Photo: US Air Force)What would such a clean, green power source make possible? Nuclear power was first looked at for strategic bombers, where the attraction was the ability to stay on station for long periods, then penetrate Soviet airspace at high speed and low level to deliver nuclear weapons, all without refueling. Research went as far as flying a small air-cooled nuclear reactor in a modified Convair B-36 bomber (above) to investigate how to shield the crew from radiation.
Times have changed and more recent interest in nuclear-powered aircraft is exemplified by the Airborne Metro concept developed under the 2008 Out Of The Box study conducted for the Advisory Council for Aeronautics Research in Europe (ACARE). The concept envisages very large "Air Cruisers" (below) - ultimately nuclear-powered - that would stay aloft semi-permanently.
Cruiser concept: ACARE
As these long-range cruisers continually fly looping tracks that cross oceans and take them over major population centers, shorter-range aircraft would bring up passengers and cargo that would ride on the cruiser until they reach their destinations, where they would transfer to other short-range aircraft and fly down to land.
Bhupendra Khandelwal, a propulsion researcher at Cranfield University's School of Engineering in the UK, has taken a closer look at this concept, developing an air transport model (below) that combines nuclear-powered cruisers with chemical-powered short-range aircraft.
Graphics: Bhupendra Khandelwal, Cranfield UniversityTaking off from conventional airports, flying to and landing on the cruiser, the chemical-powered transport would be optimized for take-off, climb and landing, with no need to cruise. This would reduce emissions, says Khandelwal, as the nuclear-powered transport would carry the aircraft to its destination, where it would detach from the cruiser to descend and land normally.
The cruiser, meanwhile, would have taken off from a remote site. This and its extreme endurance, which significantly reduces the number of take-offs and landings, would minimize the risk of a crash leading to a nuclear incident. Also the cruiser would be unmanned, he says, improving safety and avoiding the risk to a crew of prolonged radiation exposure during the cruiser's extended voyage. For further safety, the cruiser would have back-up chemical propulsion.
To allow the conventional chemical-powered aircraft to land on and take off from a nuclear-powered cruiser in flight, Khandelwal proposes using a moving-belt runway (shades of Gerry Anderson). Alternatively, aircraft could hook up to the cruiser using a method similar to aerial refueling, he suggests.
Khandelwal calculates this air-transport model could produce a fuel saving over conventional point-to-point flights of 40% for a 1,000km mission, rising to 85-90% for a 10,000km mission, where the chemical-fueled flights to and from the cruiser would be a smaller fraction of the total.
Nuclear propulsion could be either direct or indirect cycle. In direct cycle, air flows through the compresser, into the reactor where it is heated, and out through the turbine. The risk here is radiation in the exhaust gases. In an indirect cycle a heat exchanger transfers energy from the reactor to the airflow. Radiation risk is reduced, but so is thrust. This is where LENR could come in, providing high energy with low emissions.
LENR represents such an enormous energy density (gigajoules per gram of fuel), and fuel consumption would be so low - the energy from the hydrogen in 40 litres of water could power a 747 half way round the world - that aircraft could be thought of taking off and landing at the same weight, says Zawodny.
But before we get too excited, a huge amount remains to be done before LENR can become a reality. Current devices have extremely low efficiencies and, Zawodny says, "there has not been a demonstration of an LENR apparatus that can reliably be turned on and off at will...When LENR devices work they consume themselves."
Back to the drawing board then?
awt, awimagine, propulsion