The turbo-jet gas turbine engine has been a feature of flight for long enough to be the normal experience of perhaps the majority of travellers. It has settled to a design concept that is reproduced by all the major manufacturers and made in substantial quantities. High by-pass ratio all jet engines power the great majority of the 11,000 or so large commercial aircraft that fly today. In their latest form such engines can each deliver thrust well above 120,000 lbs which would have been received with gasps of astonishment only three or four decades ago.
The question posed by this series of ideas is whether the convergence upon this single formula for design bears any relationship to the form of aircraft engine required in the future.We must ask whether this form, suitable for its age, is entering a new age when another form of propulsion engine will climb to the ascendant position the turbo-fan has today.
The question is prompted by the pressure growing upon climate impact. The newspapers seem to have become locked onto a “story” that aviation will become the Number 1 polluting mechanism later this century. This is most unlikely to be true and will certainly not be true if it is managed at all appropriately. Aircraft emissions do have an effect, although we are not yet sure exactly what this effect is or how precisely its mechanisms work. It may be the case that aircraft emissions have relatively greater effect than ground level emissions in their effect up on the sunlight coming through the atmosphere.We do not know.
What does seem to be the case, however, is that the continued growth of current aviation practice will be unhelpful to our planet. And that the relatively easy gains of the early jet years in cleaning up the emissions from engines will not now be so easy to repeat.With today’s technology, it is unlikely to be the case that emissions can be reduced sufficiently. Something new is needed.
At the same time great care must be taken in designing the set of pressures and incentives that will encourage and foster these developments. It is interesting that, in a period when fuel taxation is being promoted by some, we also see serious scientific analysis that warns that fuel taxation could bring about more damage to the planet by encouraging airlines to favour more financially economic, but globally more damaging mission profiles.
1. Nuclear engines
Despite the obvious risks and difficulties, nuclear engines have obvious attractions. They make no airborne emissions and their waste can be safely and securely handled on the ground. The technical engine problems are well on the way to being solved. The concerns for safety and security may be overcome but almost more difficult will be the perceptions of society for airborne nuclear engines and the containment of the unit. So any programme of technical research needs to be accompanied by social research into the acceptability of these engines.
Of course, many technical problems do remain but the nuclear engine offers a path from heat to propulsion that utilises many of the same heat exchange technologies that we have already developed. Many of these might make the nuclear engine very similar to a jet engine without the fuel pumping and combustion mechanisms. A number of suitable prototypes for nuclear engines have been built at various powers. Reactor volume and weight as well as containment has always been the intractable issue. Either the weight of the containment becomes excessive for an air vehicle or the containment is selectively reduced and the vehicle becomes a hazard for those outside it. These considerations brought to a halt the last known major attempts to produce a nuclear powered aircraft.
Perhaps with new containment materials and a growth in the size of the intended air vehicle the equations for the containment can be revisited.
Accident considerations offer another major issue of both reality and perception. The public perception will be that nuclear engines are little better than flying bombs able to devastate vast areas around anywhere that they crash. There is no reason to take that view but the public have become fed with similar stories for so long that they will certainly re-surface. The reality is that careful measures would need to be taken to ensure that the nuclear material was prevented from contaminating surrounding areas in any conceivable accident. This not only includes the reactor chamber but the pipes, valves and routing of any of the nuclear pathways in the whole unit.
2. Plasma technology
These electrical effects could be conceived as having two applications; either direct use for propulsive force by having wing and fuselage surfaces made with the correctly embedded electrodes, or by drag reducing measures influencing air flow over the airfoils.
The science of para-electrics is becoming better understood and practical experiments have demonstrated their effects. The task of sustaining these into large-scale structures with adequately robust controls remains to be completed. So too are the power and weight reconciliation’s that would be convincing against the weight and power budgets. It has been asserted, for example, that the plasma effects equate to little more than pushing energy ahead of the aircraft. Making the medium easier to fly through but with the power needed to make this effective interchangeable with the power needed to propel the craft without the plasma effect. It is known that a blunt body can be made faster by forward facing (i.e. reversed) jet engines that work on a similar effect. But these examples do not explain the complexity of plasma science.Work continues in subsonic, supersonic and hypersonic regimes and there is much that remains to be discovered about the benefits and limitations of plasmas.
Other effects besides propulsion may prove to be important including the effect of the plasma on the radar reflection of the craft.
The weight budget required for high power plasma physics on-board is also challenging scientists. The plasma generating equipment will demand its own part of the weight budget and it is not known whether the overall systems outcome would be positive or negative.
3. Fuel cells
The fuel cell has captured the enthusiasm and efforts of many engineers around the world. Its ability to take two common gases, hydrogen and oxygen, and to create energy and water offers a clean, climatically sensitive way to generate energy. Small fuel cells are being produced and marketed already and are well beyond the laboratory stage.
Today’s technology contemplates only auxiliary power unit to augment but not to replace the main power plant of the aircraft. To manufacture large, propulsion level fuel cells will r equire advances in the energy density of the stack. Georgia Institute of Technology has experimented with a UAV propelled by an advanced proton exchange membrane but it generates only 500 watts.
One of the fundamental issues in a large fuel cell of this design is the storage of the compressed hydrogen. The pressure vessels for a liquid gas system are large and heavy and represent a major obstacle to deriving practical designs for significant ranges in full size vehicles.
Another approach is the hybrid fuel cell running on energy dense carbon fuels. The Solid Oxide Fuel Cell (SOFC) hybrid being studied by NASA uses liquid methanol as the fuel. This combined system of directly reforming fuel with a gas turbine end stage offers the best balance of weight and power to date.
4. Solar cells
Solar energy has been proposed as an idea for the future of aircraft propulsion, or as an augmentation to it. The idea is attractive, the sun’s energy is free and in the upper altitudes is easily available. The challenges are also great. The sun does not shine at all times and some kind of interface between the sun and the engine is usually necessary, and most commonly as a battery. The weight of the battery used for this purpose on one long range experimental aircraft8 weighed nearly half of the aircraft weight. Whilst the battery fraction may not always need to be as high as on this aircraft, which was intended for sustained night flights, they still represent a formidable obstacle to the use of solar power.
The power generation of the solar cells is also a problem area. The present efficiency of power conversion to power incident is at a low level, somewhere around 20% with a theoretical maximum believed to be in the mid-30’s per cent. The area needed to produce 1Kw of output power is presently about 5 m2. For a medium 150 seat airliner the wing area is around 122 m2 . The solar power production if the wings were fully covered would be in the order of 25 Kw. This compares to a typical fan-jet engine rated at around 10,000 kW. Whilst the figures may not be precise they clearly illustrate the gulf between solar power and today’s carbon fuel engines. Most solar cell power has been applied to auxiliary power units and to very low-drag experimental aircraft designed on a glider-like principle and able to maintain altitude with very small injections of power.
5. Distributed propulsion
The concept of distributing propulsive force over the aircraft instead of having two or four discrete engines has been interesting engineers in several ways. The idea presented in the workshop was that by having multiple engines, in the limit down to covering the surface with tiny engines, these may be integrated better and more flexibly with the mission. The benefits looked for were to reduce the overall fuel burn whilst making for better control.
Several design schemes have already gone some way towards the idea. One scheme from NASA Langley for a BWB has a modest number of core engines embedded in the wing exhausting across a wider span than usual.
This design shows predicted savings on TOGW of about 5%.
Incorporating blown flaps into the design of distributed thrust also brings benefits and new concepts joining these ideas show considerable promise.
Moving from a modest number of engines to many engines represents further complication and it is not yet clear how this would be accomplished in terms of the mechanical design. The particular benefits of having a mass of very small engines contributing to thrust is not entirely clear.The main purpose of any engine is to provide thrust alone, lift alone or some combination of the two. For a thrust alone set of many engines, each has to be aligned and arranged to contribute to the thrust vector – either directly or via inter-c0nnected ducts. For lift alone the placement of the engines would need to allow the lift component to be achieved, again either directly or via ducts. Engines with a combined role would have some additional complexity ion allowing the force of the engine to be directed to one or the other use in selectable proportions. For very many small engines a ducted system would probably present considerable friction losses to the propulsion system.