Periodically, one comes across technology foresight pieces from the past. The one characteristic they have in common is that they are nearly always wrong – not just a little bit wrong, but both wildly optimistic in predicting the future of, for example, supersonic transport aircraft, and completely blind to transformational technologies, especially in the digital world.
Why would this one be different? Well, obviously, it will not. As a personal perspective on the world of aerospace and related defence technologies, there is always so much going on, particularly in the closely held world of classified research, that one cannot assemble a comprehensive picture. So, there are going to be gaps and misjudgements. Also, there is a lot of guesswork involved in extrapolating from objectives, and visible progress in research and early development, to future capabilities. Guesswork along the lines of – if I really wanted to deliver (a new capability) – what technical infrastructure and technologies would be required? Can the gaps be filled? And if so, with what?
My approach has been to think about a couple of major emerging areas, one from defence, and one from civilian aerospace, and to consider what appears to be going on, and where it might be leading, as a means of identifying key technologies to watch. And to be clear here, this is all about opinion, not fact. Moreover, it is a certainty that I do not have the expert knowledge to do more than speculate in many cases.
Defence and the nature of war
As one considers defence technologies, it is important to realise that perceptions of the nature and conduct of war have changed and are continuing to change – noting that differing societies may continue to have differing points of view. Conventional warfare – the non-nuclear contest of wills between major powers – has, at times, seemed to be an enduringly unlikely prospect. However, history does not support this view, given the succession of actions involving the major powers since the Second World War. The current environment seems to me to feature increasing tension in many areas of the world, and, moreover, to be characterised not only by intractable problems that are not easily resolved, but growing, and unhelpful, competition between the major current powers, the USA, Russia, and China.
Adding to this tension is the increasing capabilities of second tier nations, including, but not limited to Israel, India, Iran, North Korea, and others, as well as the ongoing contest of political ideas and religious beliefs that lies behind terrorist activities in Europe, the Middle East, Africa, and Asia.
Terrorism is a label applied generally to the nationalist or religious aspirations of others, characterised by the unconventional use of lethal force outside the so-called rules of war. Just as one man’s terrorist may be another man’s freedom fighter, it is important to recognise how successful such actions have been, and how difficult they have been to counter using conventional armed forces. One only needs to consider Afghanistan, for example, where the British, the Russians, and the Americans and Coalition Forces have all successively been evicted by informal armed groups, despite all having conventional air and arms superiority.
Well, you might find all that a bit confronting, but not all defence is defensive, and not all objectives are benign. Sometimes attack is an option, and sometimes annexation, and invasion are used to effect regime change. And this piece is intended to be a general technology discussion, rather than political analysis – even though it is framed from the perspective of a Western democratic nation.
This piece is focused on peer-to-peer conflict, rather than on counter-terrorism or counter-insurgent operations. Nevertheless, the development of the capabilities outlined will need to recognise the complexities of countering hostile forces that may be indistinguishable, much of the time, from civilian populations. Clear opportunities to counter the activities of such groups may only be fleeting in nature, and consequently require rapid, accurate, decision-making, and the ability to generate reactive, yet proportional responses.
Future Air Operations
In previous articles for @Hush_Kit, I have considered the nature of air combat, and written on fighters for 2030, and in thinking about those topics, the US, China, and Russia, all appear to be converging on a system-of-systems approach to both air combat and strike missions. The discussion that follows outlines the capabilities that appear to be necessary to deliver a ‘system-of-systems’ approach to future air operations, whether these be pre-planned strike missions; reactive missions addressing short-term or fleeting targets; or defensive operations reacting to threat attacks or weapons.
We will start with an assumption that the key purpose of air operations is the prosecution (a less direct way of saying ‘attack’) of hostile targets. Here I am using targets in an extremely broad sense, as our air operations should be able to deal with targets ranging from enduring, hardened, strategic infrastructure, to fleeting targets of opportunity, reactive defence against incoming threat weapons and platforms, or even something as ephemeral as Political opinion.
Such a range of targets is a key driver for our system-of-systems, which must be capable of recognising, locating, and countering the full range of targets, with the flexibility to carry out operations from merely monitoring force deployments at a time of tension, to complex pre-planned attacks against hardened and defended targets.
To achieve these effects, our system-of-systems based air combat capability is going to need to be able to:
• Identify and locate targets, varying from the strategic to the tactical, and from static to moveable, mobile and fleeting, as well as characterising the defence systems for those targets;
• Communicate relevant information on the targets either to the higher command structure, or, for targets that must be addressed in near real-time, to other system-of-system elements;
• Collect, analyse, and disseminate information from other elements across the system-of-systems;
• Strike targets, using appropriate and proportional weapons systems, or neutralise by other means, such as electronic attack, cyber warfare or deception;
• Be survivable and persistent for as long as required; and
• Be responsive to changes in the Commander’s intent; the Rules of Engagement; the achievement (or not) of objectives; and to changes in opposition tactics or force deployments.
These capability requirements lead us into a series of technology topics, which are the defence topics we have selected to watch.
As an aside, in thinking about these defence aspects, I would ask the reader to bear in mind not only the thought ‘that’s cool, we’re doing all that’, but also to reflect on ‘OK … so how do we respond if this gets done to us?’ I remind the reader of this, because I recall making a similar comment years ago in response to an Army paper on ‘Manoeuvre Warfare’. This was very gung-ho about the prospect for the future Army but had not a word of consideration about how to respond to being outflanked, or to having one’s own logistic lines cut. My observations were unwelcome.
The paper in question also took an overtly ‘offensive’ rather than ‘defensive’ perspective to operations, which was perhaps counter to the political will of the time. My observation to that effect was also unwelcome, but I mention this because the ‘system-of-systems’ approach I am considering is also intended to allow unfettered operation over hostile territory, is ‘offensive’ rather than ‘defensive’ in nature and appears to be the direction being taken by the US, Russia, and China.
The civil transport domain has, since the age of airborne mass transport began, after the Second World War, been driven by one consideration – economics. Although some might see the appearance of large turbofan-powered aircraft in the 70s as a response to noise regulations in Europe and elsewhere, this was really a side effect. It just happens that the solution to greatly improved operating economics and reduced noise was the same – the larger mass flow, and lower exhaust velocity of a high-bypass-ratio turbofan happens to reduce noise, as well as increasing efficiency and lowering cost.
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However, we are entering a period where there is, rightly, concern about the impact of human activity on the global climate, and attention is falling on the fossil fuel burnt by air transport, and the effect of exhaust gases on the upper atmosphere. Of course, this is, and will become increasingly, also an economic driver. In the longer-term taxes – in the form of carbon-pricing – and cost increases due to diminishing availability, will both drive up fuel costs, and consequently the search has begun for alternative propulsion systems.
So civil aerospace technology needs might be driven by the following capability needs:
• Further improvements in propulsive efficiency
• Further improvements in aerodynamic efficiency
• Air transport systems, at all scales from the personal to mass transport, which minimise, or remove altogether, their impact on climate change
• Innovative solutions to the growing needs of internet-based businesses
Aerospace Technology Aspects
The discussion above has highlighted two principal problem spaces – military air operations in a system of systems environment, and economical but environmentally responsible civil air transport. As we have seen, both problem spaces will require the development and deployment of multiple technologies. Consequently, it does not make sense to assemble a ‘Top 10’ list, because the requirements are likely to be ‘all of the above, and more’. Instead, I shall look at what I consider to be the key areas needed to deliver the suggested capability, and then try to shed some light on some technology directions and programs of relevance.
Elements of a ‘system of systems’ air combat capability
Having outlined the capability requirements for such a system, we can perhaps identify some of the system elements that will deliver this capability.
Persistent Intelligence will be required. To conduct effective operations, it is first necessary to understand what is happening – in the broadest sense. What can we observe that will inform us about threat capabilities, force structure, deployment of men and materiel, sensor and weapon capability, political will and intent, infrastructure, and Industry? A huge ask, and probably only part of the tasking of worldwide intelligence services. If we narrow the focus, and assume a time of tension between major powers, with a high probability of conflict developing, a key need will be to assess potential key elements which might become targets.
Once the situation nears actual conflict, or conflict is taking place, persistent intelligence is needed to inform Commanders of the changing disposition of threat defence capabilities, force deployment, logistics and electronic activity, as well as updating similar information on friendly forces, and facilitating battle damage assessment. Depending on the nature of the conflict, many fixed elements of infrastructure will already have been located, and assessed for criticality as targets, so the critical need will be to identify and locate mobile and moveable targets, the nature and capability of their physical and electronic defence systems, and the sensors used by those systems.
The same system will also collect information from other system elements and disseminate and report as necessary to other friendly force elements on threats, targets, Commander’s Intent, and changes in Rules of Engagement.
To achieve this, the following system elements are likely to be required:
• A stealthy, high-flying, long-endurance autonomous platform
• Sensor packages able to collect the information outlined above, which implies sensor capabilities across a wide range of the electro-magnetic spectrum
• On-board analytical capability to transform sensor data into actionable information, including, for example, the identification and localisation of threats and targets; track information on both air and ground moving targets; and indications of operational mission success or failure
• Covert, robust communication capability enabling two-way communication through satellite or other relay systems to either the tactical or strategic command authority.
• Covert, rapid, robust two-way tactical communication to other system elements.
These capabilities are likely to be supplemented by the future Global Air Dominance System, a stealthy manned platform, which is likely to contribute additional sensors and Battle Management capability.
Effectors will be required to impact on and defeat threat targets and defensive systems. Platforms will be required to deliver kinetic effects (weapons), and non-kinetic effects (electronic attack, countermeasures, deception operations, cyber-attack, and defence. Platforms will require their own sensors for targeting, and to contribute to the overall picture of battle, and covert, robust means of communication with other system elements, and in some cases, the weapons they deploy. Additional support systems may also be required to ensure sufficient reach and persistence, and to supplement tactical intelligence collection. Some additional specialist capabilities will be required, for example to counter submarines, surface combatants and perhaps theatre ballistic missile defence.
System platform elements are likely to include:
• Crewed and un-crewed strike platforms
• Crewed and un-crewed air combat systems
• Crewed and un-crewed Electronic Warfare systems
• Crewed and un-crewed air refuelling tankers
• Crewed and un-crewed tactical surveillance assets
Technologies to develop all these systems are maturing, and programs are already in place addressing some of these needs.
Kinetic Effects. The system-of-systems approach is likely to make a wider range of weapons available to Commanders, as any networked platform within weapon range would be available, allowing a wide range of proportional, timely and well-matched solutions to be deployed.
In considering the systems deployed to create the required kinetic and non-kinetic effects, there are generally three major considerations: target-matching; proportionality; and range. In a non-system-of-systems environment, platform sensor matching was also important since platform target detection before weapons release was generally required. However, in the future considered, target information is expected to be pervasive, so that third-party targeting would be the norm, and target localisation would generally be delivered over the information network. Timeliness of response, however, is an important new driver, as this is a potential benefit from the system-of systems approach.
In addition to the full suite of current kinetic effect weapons capabilities, a range of highly reactive and precise solutions are likely to be sought, adding the following capabilities:
• High-speed Air-to-Surface weapons
• High-speed Air-to-Air weapons
• Long-range high-speed systems
• Directed Energy weapons
Non-Kinetic Effects. Clearly, with a heavy dependence on information systems, communications networks, software enabled capabilities, and sensors exploiting a broad range of the electro-magnetic spectrum, attention is going to need to be paid to both offensive and defensive non-kinetic effects.
This will lead to a need for the system-of systems to include the following capabilities:
• Electronic Attack and Protection
• Cyber Attack and Protection
• Ability to deceive and decoy threat sensors, and to overcome threat deception and decoys
Defence Aerospace Technologies
So, having explored the capabilities which might be needed to deliver system-of-systems air combat, what are the key technology developments required. To quote Arthur C Clarke “Any sufficiently advanced technology is indistinguishable from magic”. This discussion revolves around identifying where the magic lies.
Cognition and decision-making
Having postulated a persistent, stealthy, high-flying autonomous data collector and disseminator, one of the areas where advanced technology will be needed is in the translation of that data into information, and that information into actions. The volume of data which might be collected is prodigious, and the need for accurate and rapid decision making is paramount.
Conversion of sensor measurement data into location, identity, and track is vital if action is to be taken quickly. The volume of such data is too great to rely on passing back to the Command authority for conversion to information, and for decision-making. For this system-of-systems to work, there will have be an automated system capable of not only converting data to information, but also capable of cognition – that is understanding and acting on the information available.
This implies an extraordinary level of artificial intelligence (AI), and of trusted autonomy, and the development of technologies in this area will be a key, if such a system is to be delivered. This is a key area – while at first blush it seems to me unlikely that such a degree of AI yet exists, and further, in such a form that it could be trusted to act autonomously, I am reliably informed that sonar systems can discriminate up to 400 tracks simultaneously. This is certainly a key area a key area to watch. Management of the risks in this area might involve a half-way house, where proposed courses of action are provided to the Command authority for approval before implementation. Alternatively, the Global Dominance Air System might be used to provide battle management and place humans in the decision-making loop.
Communications and Information Dissemination
For our system-of-systems to work, covert, trusted, un-jammable, impenetrable communications links will need to be established. Some of these will need to deliver two-way information transfer to a remote Command authority, presumably by some form of satellite link. Others will be more local in nature, sharing information around the elements of our system, facilitating third-party data, sharing sensor information to enable the detection of stealthy threats and so on.
How might such a system work? Another area where advanced technologies indistinguishable from magic may play a part? But if Elon Musk has 42000 internet relay satellites in orbit by 2027, there should be a great deal of system redundancy available, along with global coverage. Could such a system be covert, secure, and un-jammable? Well, I assume that would depend on the waveforms used if a conventional datalink were used.
Other possibilities exist, including the use of lasers for optical communication, and dedicated LPI datalinks are already in service, catering to the needs of JSF, for example. The Multi-function Advanced Datalink, MADL, provides communication between stealthy platforms, with frequency-hopping and anti-jamming capability using phased Array Antenna Assemblies (AAAs) that send and receive tightly directed radio signals.
Clearly, establishing secure communication of critical information over a wide geographic area is one of the keys to successful implementation of system-of-systems air combat. Ensuring the communications network is managed to avoid information latency, aliasing of targets, disruption, jamming, or penetration by hostile entities is one area to watch, as will be bandwidth, cyber protection, security, and resistance to detection.
This whole area is going to be difficult when operating in coalition, given the sensitive nature of both collection technologies and analysis techniques.
Sensors and sensor co-ordination
Networking sensors together is one of the opportunity areas of a system-of-systems approach. As an example, if our network were able to access Infra-red Seeker Tracker (IRST) measurements from two separate stealthy platforms, while also having real-time knowledge of their locations, any heat source detected by the IRSTs should be able to be tracked, by triangulation of the two IRST bearings. This capability would provide a passive means of tracking stealthy air targets in real time, although it would be necessary to be able to discriminate between friendly and hostile targets, which might require additional information.
Similarly, it is likely that extensive use would be made of third-party targeting (TPT). TPT is useful because it allows there to be geographic separation between the sensor providing targeting data and the platform which uses a weapon to engage the target. That platform can remain passive and difficult to detect and neutralise, using off-board data supplied over the net to localise the target, rather than active and potentially detectable sensors.
Collection and collation of sensor measurements and tracks from other system elements really falls under the earlier discussion of cognition and decision making. However, our high-flying intelligence and information platform will have its own sensor suite. Given the need for the platform to provide persistent coverage over hostile territory, the challenge will be to generate useful sensor data in a covert manner. Passive sensors, including optical and infrared sensors will provide useful data, and multi-spectral analysis of sensor data may be useful in discriminating and identifying targets in a cluttered environment.
Use of an active sensor, such as a low probability of exploitation synthetic aperture radar, is however, likely to be considered because of the utility of such sensors, not only in ground mapping, but also in providing Ground Moving Target Indicator (GMTI) capability, as well as change detection. GMTI can be particularly useful, not only in locating moving vehicles, but also in determining where they have come from and their destination. This information can be extremely useful in building a picture of threat logistics infrastructure, potentially enabling, for example, the location of fuel and ammunition supply depots to be identified. Change detection may be of particular interest in irregular conflicts, o here there is a potential for improvised explosive devices or landmines to be employed.
The technology challenge here is to deploy an active radar or similar sensor system, while preventing this being used as a means of detecting the high-flying platform at the heart of our system.
In attacking targets, and defending against attack, survivability is a key consideration. The degree to which risk is taken in this area may vary across a broad spectrum – a terrorist may have a view where the sacrifice of their own life is considered necessary, or even advantageous, whereas risk to service or civilian lives may be sufficient to prevent some military actions from going ahead. In general, though, if objectives are to be sustained, continuing loss of men and materiel will need to be avoided, and hence combatants will seek survivable options.
Our system-of-systems approach has, as one of its objectives, reducing the risk to human operators. It does this by essentially postulating three forms of vehicle – un-crewed, autonomous, survivable, and persistent platforms, like our intelligence, sensor and command and control platform; un-crewed, autonomous and ‘attritable’ platforms that the operators are prepared to lose if necessary, when attacking strongly defended targets; and crewed, survivable platforms, used only where a human in-the-loop and on-the-spot is critical.
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We can examine what is reported about current projects and programs and see this thinking in action. Alongside crewed aircraft concepts and programs such as the B-21, Global Air Dominance System and F-35, we can see the Unmanned Wingman, the XQ-58A Valkyrie un-crewed strike platform, Neuron, Taranis, as un-crewed and potentially ‘attritable’ platforms. We have the US Navy experimenting with autonomous air-to-air refuelling using the X-47B and no doubt un-crewed electronic warfare, jammer, and decoy projects already in hand. We have at least early attempts in high-flying, difficult to detect autonomous sensor and communication systems like the RQ-170 Sentinel and speculation about the RQ-180. Autonomous manoeuvring air combat is perhaps a little further away, although some might argue that Surface-to-Air Missiles and Cruise Missiles are simply ‘fully-attritable’ un-crewed autonomous air combat and strike systems.
The detailed roles and implementation of the US Global Air Dominance System (GDAS) remains an area for speculation. This is expected to be a manned, stealthy platform, with long endurance, and is likely to feature sensors, command and control and battle management systems. Given the developments in autonomous adjuncts, it is possible that the weapons capability of the GDAS may be limited, although there have been recent suggestions that it may employ directed energy weapons.
Otherwise, rapid development of manned and unmanned aerospace systems of all sorts is proceeding apace in China, and I speculate that the J-20 will be used as a major area-denial asset in the future, with a range of long range air-to-air and air-to-surface weapons. In Russia, a replacement for the MiG-31 strategic defence fighter is awaited, with some ambitious performance targets being suggested. Meanwhile the Su-57 is maturing, and a range of unmanned systems are also in development.
Where are the magic technologies here? Most of these capabilities already exist or are in demonstration, although manoeuvring autonomous air combat, and persistent but undetected platform presence, are perhaps at the challenging end of the spectrum. Developing manned penetrating and survivable platforms is going to continue to push the boundaries of signature reduction technologies, and the consequential impacts on aircraft configuration will demand sophisticated, but achievable flight control technologies.
Weapons – Kinetic and Non-kinetic Effectors
The use of air assets in conflict encompasses a wide range of activities as we have seen. Sensor and networking platforms informing the command and control of the air domain, with electronic protection and attack and decoy and disinformation operations helping to shape the battlespace. Stand-off capabilities like AAR and AEW platforms help in providing reach, persistence, and situational awareness, but the deployment and use of weapons to prosecute targets is the end point of all this activity.
From the earlier discussion, we have seen that a wide variety of weapons are likely to be required to match weapons to the wide variety of targets, and, as far as possible, generate precision effects. Consequently, there is a great deal of activity going on looking to develop weapons that can be deployed in the networked environment of the future.
What capabilities are being sought for future weapons? Some suggestions are:
• Rapid action – short time between a decision to strike a target and the required effect. This is a key attribute for tackling fleeting targets
• Re-programmable – the ability to optimise the fusing, or the nature of kinetic effect, allowing a broader range of targets to be prosecuted with the same system
• Non-kinetic – weapons that can disable a hostile response without collateral damage, for example by nullifying opposition sensors, disrupting information systems, or corrupting software
• Precision – able to produce very local effects
• Long stand-off – a response to operating against very capable air defence systems
• Small – weapons for deployment from stealthy platforms will be internally carried, and internal volume is likely to be at a premium for many platforms
• Networked – data-linked to allow third-party targeting and an indication of effect
Some of the capabilities above are routinely available with current weapons systems, but there are plenty of technology areas to watch in the general area of effectors.
Active programs to develop hypersonic weapons are underway in at least the US, Russia, and China, and this is clearly a technology area to watch. Most of the hypersonic weapons being discussed appear to be aimed at air-to-surface applications, including anti-ship weapons. However, it seems plausible that a long-range anti-air hypersonic system could be useful as an area denial weapon, for example against high-value targets such as AAR and AEW&C platforms.
Another topic which has been examined for a considerable period, at least in the US, is the possible application of boost-glide vehicles. These are powered systems which are (hypothetically) air-launched and climb to near-space altitude, before gliding at high speed to strike their desired targets. An alternative approach is a conventional rocket launch, followed by a series of re-entry skips into the upper atmosphere to achieve extended range. BGV capability would appear to be well within reach from a technology perspective. The technologies required have much in common with air-launched anti-satellite systems, which might also be available to some countries. The capability on offer is essentially short notice global reach.
A third area to watch is directed energy weapons. These are generally in the form of lasers, but other forms of energy may be available. Several issues have been encountered in the development of such weapons, but there is clearly a potential application to air weapons. The provision of sufficient electrical power to achieve a worthwhile effect has been an issue, but there has been a recent suggestion that such weapons might be one possibility for the US Global Air Dominance System. Directed energy weapons are, by their nature, precision weapons, and do not have a conventional kinetic effect. They are already in widespread use at lower power, as counters to imaging infra-red seekers, in the form of Laser DIRCM (Directed Infra-red Counter Measures).
Another form of novel non-kinetic weapon which might appear is the non-nuclear electro-magnetic pulse (EMP) generator – this seeks to disrupt sensors and communication systems by generating an extremely large electro-magnetic signal. While the technology exists, many military systems are hardened against EMP, but significant effects might be expected in EMP is directed against civilian infrastructure or other non-hardened systems.
Where is the magic required for Defence Aerospace?
• Stealthy, persistent, surveillance and communications platforms
• Covert sensors
• Information management systems to turn sensor data into actionable intelligence
• Multi-way communication and information dissemination system
• Unmanned strike and air combat systems
• Unmanned support systems
o Cooperative sensors
• Reactive rapid response weapons
o Hypersonic systems
o Directed Energy
o Boost-glide vehicles
Technologies for economical but environmentally responsible civil air transport
While much of the discussion in this area is going to be about propulsion systems, because of a widely perceived need to reduce carbon emissions to the atmosphere to reduce the rate of human-induced climate change, it is important to remember that economic operation remains an enduring driver for commercial air transport. Consequently, there will be continuing pressure for innovations that can improve both aerodynamic and propulsive efficiency. Just as the same economic pressures resulted in the adoption of high-bypass-ratio turbofan engines, and coincidentally dramatically reduced aircraft noise, so improved aircraft and engine efficiency helps to reduce the power required, and the impact on the environment.
Aerodynamic efficiency: biplanes, flying-wings and the return of other zombies
In the cruise, airliners fly at high altitude to maximise propulsive efficiency and minimise fuel burn. Two major routes to aerodynamic efficiency are to increase the aspect ratio of the wing (the slenderness of the planform – defined as (span squared)/wing area) which reduces the drag due to lift, and to reduce the wing area, which reduces zero lift, or profile, drag.
Increasing aspect ratio has been a trend in the evolution of jet airliners. The de Havilland Comet had an aspect ratio of 5.65, and the aspect ratio of the latest variant of the Boeing 777 is 9.96. This increase in aspect ratio has largely been enabled by using advanced composite materials for the wing structure. Reducing wing area demands the use of efficient high-lift systems to meet demanding take-off, landing and single engine climb requirements. While further incremental improvements are possible, dramatic change in aerodynamic efficiency will require a new approach.
Technologies to watch in this space include braced wing structures, reduced or zero wing sweep, and blended wing-body designs. Use of a braced wing design, where a high wing is structurally braced by another lifting surface acting as a strut, might appear old-fashioned and unlikely, but would allow significantly higher aspect ratio wings to be used without attracting a weight penalty. An approach which might facilitate this, is to reduce or eliminate wing sweep. While this would undoubtedly require a reduction in cruise speed, it would not only result in a lighter wing design, but it would also enable laminar flow wing profiles to be used, which have the potential of substantially reducing profile drag.
The first proposals for a jet-powered blended wing body airliner design came not from NASA, nor from Airbus, but from Armstrong Whitworth, and date back to 1943. The proposal materialised at half-scale as the AW52 but was dropped because the swept wing was unable to deliver the benefits of laminar flow. However, with modern design methods and flight control systems, aircraft like the B-2 have shown that aerodynamic efficiencies are achievable, and research efforts to apply blended wing body configurations to commercial aircraft continue. The benefits being sought are reduced drag, through the carriage of passengers in the wing rather than in a separate fuselage, and structural efficiencies because distributing the load across the wing allows a lighter design.
Clearly, reduced fuel burn is a continuing aim, driven by the operating economics of commercial aircraft, and like aerodynamic efficiency, any improvement in this area will also reduce the impact of air transport on climate change.
Just as improvements in aerodynamic efficiency might take us back to unswept or even strut-braced wings, the search for ultra-high bypass ratio (UHB) engines may lead in the direction of open-rotors – otherwise known as propellers. Solutions involving UHB engines will need to be carefully integrated with the airframe design if maximum aerodynamic and propulsive efficiencies are to be gained while noise in the cabin and outside the aircraft is minimised.
Internal technology changes to gas turbine engines may also increase efficiency, with both geared fans and variable cycle engines being considered as a means of increasing efficiencies. Variable cycle engines provide a means, through internal variable geometry, of altering the by-pass ratio to optimise efficiency under varying flight conditions.
Variable-cycle engines have primarily been proposed for application to military aircraft operating at supersonic speeds. However, a possible future application could be in hybrid powerplants, where, for example, turbofan propulsion could be supplemented by use of an electrically powered propulsor, which would use electrical power from batteries in combination with power generated by the turbofan engines.
Geared fans as an approach which may allow a higher by-pass ratio to be used. By slowing down the fan rotation speed compared to the core, the fan can operate at high thrust while retaining a tip speed that is both efficient and quiet. The technology is not new but represents a trade-off – lower noise and more efficient fan and turbine operation is somewhat offset by the additional weight and complexity of the gearing required.
Alternative energy sources
The bulk of modern commercial transport aircraft are powered by gas turbine engines, burning hydrocarbon fuels. As concern has grown regarding the impact of human activities on climate, attention has also turned to commercial aviation, and the impact of carbon dioxide and other exhaust products on the climate. The presence of increasing quantities of carbon dioxide in the atmosphere has been linked to climate change, and this is leading to pressure to reduce the burning of hydrocarbon fuels, and hence to reduce the release of carbon dioxide and other ‘greenhouse gases’ into the atmosphere.
There are several ways in which this might be achieved, and each of the technologies involved certainly falls into the ‘one to watch’ category which is the subject of this article. There are three areas of interest:
• Use of alternative fuels, such as Hydrogen;
• Use of electrical power, delivered by alternative sources, with batteries and fuel cells being of principal interest; and
• Hybrid solutions, where gas turbine powerplants are combined with electric power.
The big advantage of using hydrogen directly as a fuel is that the by-product of combustion is water, and hence no greenhouse gas emissions are produced.
Disadvantages of Hydrogen include its low energy density compared to hydrocarbon fuels. This means that, for a given energy content, liquid Hydrogen occupies about four times the volume of current fuels, with significant implications for aircraft design. It does, however, only weigh about one third as much, although this is partially offset by the size and weight of the tanks required to contain the fuel. Because of the increase in volume required to store the fuel, aircraft would need to be designed with greater volume to accommodate fuel, generally either in an enlarged fuselage, or in external tankage, resulting in increased drag.
A further difficulty, which is common to this type of technology transition, is that the necessary infrastructure for bulk storage, transport and distribution does not exist. This is really a matter of the market, and Government intent – if hydrocarbon fuel rises in cost due to carbon pricing, at some point the investment in the required infrastructure to switch to a cheaper alternative will follow. Part of the infrastructure required would be to enable the cheaper, and lower carbon footprint, generation of large quantities of Hydrogen, possibly using wind-generated or solar power.
Demonstrations of current gas turbine engines running on hydrogen have been conducted, and the adoption of this technology is dependent on whether aircraft integration and infrastructure issues can be resolved and can result in a product which is economically competitive.
Use of electrical power
A wide variety of experimentation and demonstration activities have been taking place looking at the use of electrical power for aircraft propulsion. There appear, at present to be three approaches to power generation for this purpose, which are suited to different aircraft sizes:
• Batteries – suitable for light aircraft;
• Fuel Cells – demonstration activities proposed or in place for medium-sized aircraft; and
• Hybrid – Proposed for larger aircraft.
The biggest technical challenge to the use of battery electric power for aircraft is the weight of batteries required to generate adequate power and endurance, although aggressive efforts driven by automotive applications are improving battery volume and weight. As a result, although there are many battery-powered projects in demonstration, they are, in general, targeted at small aircraft application. The most popular envisaged application appears to be for autonomous air taxis, which raise several other issues which are discussed below.
Passenger-carrying battery-powered electric aircraft have been successfully flown, but it remains to be seen how far this technology can be pushed in terms of operating weight and endurance. There is, however, substantial progress being made in battery and inverter technology, and it appears only a matter of time before practical applications could be developed, at least for small aircraft.
Certification may perhaps offer greater challenges, particularly for autonomous air taxis, an area of greatest apparent market optimism.
Three issues present themselves. Firstly, if the air taxis are autonomous, the mechanisms for ensuring they are travelling to valid landing places; for controlling and deconflicting traffic at popular destinations; and for ensuring safety of flight in the event of power failure or battery exhaustion will all need to be defined, legislated, and implemented.
Secondly, if the vehicles are not autonomous, but require piloting, then the whole question of licensing and training of operators will need to be addressed in addition to the preceding points.
Finally, operation in an urban environment, with all the clutter of bridges, buildings, powerlines, and perhaps numerous other air vehicles, raises widespread issues of safety and liability which will need to be addressed before such operations could commence.
One area where battery-powered electric aviation does appear to have taken off is the use of small drones to deliver parcels and products as diverse as on-line purchases and fast food. This model is intricately linked to the business transformation resulting from pervasive internet usage, allowing local door-to-door autonomous deliveries to anyone with a mobile phone.
A fuel cell is a battery-like device which brings together Hydrogen and Oxygen (although other fuels are possible) to generate electricity. The fuel for the fuel cell is stored separately, and in some cases, oxygen is simply extracted from the air. The technology is relatively mature and has seen extensive use in power generation for space vehicles. It has also been used for fixed power installations, and to power electric vehicles and buses.
Several demonstration aircraft have been flown using fuel cells, and a recent proposal is focused on using a Britten-Norman Islander, with external tanks for hydrogen fuel. A number of technology challenges exist, including managing heat generation and achieving sufficient power at a reasonable weight. As for hydrogen as a fuel, the necessary infrastructure investment appears to be a substantial issue, particularly given the relative success of battery-powered vehicles, which has drawn attention and investment away from fuel cell powered electric vehicles.
Hybrid propulsion solutions appear to represent the most promising path towards larger size commercial aircraft which are at least partly-electric-powered. The concept is that the aircraft powerplant will be supplemented by an electric propulsor at take off and during the climb, allowing the gas turbine engines to be smaller in size, and a closer match to the power required when in cruising flight. The electric motor would be powered by batteries for take-off and climb and would be recharged by excess power available when the aircraft descends to land.
The viability of this concept depends on whether the combination of gas turbine engines, electrical motor, batteries, generators inverter and propellor can be competitive with simply fitting two larger gas turbine engines.
At present, this seems unlikely, but a hybrid solution will offer much more range than a purely electric battery powered aircraft and will offer some fuel savings compared to a conventional solution. One question that may need a little thought is how to meet engine failure after take-off requirements. Unless the gas turbine engines are large enough to meet the climb gradient requirements with the electric motor inoperative it is hard to see how the aircraft could be certified.
Whether or not any of the alternative powered aircraft offers better economics than a hydrocarbon-fuelled alternative may eventually become a moot point, but at present, there seems no solution likely to economically compete with the highly efficient large long-haul airliners of today.
Air Traffic Management
One area where substantial fuel savings could perhaps be delivered is in air traffic management. The bulk of commercial air traffic is generally confined to operating along rigidly constrained airways, largely to ensure separation from other traffic. But with pervasive and worldwide internet coverage now becoming possible, this constraint appears somewhat counter-productive.
If commercial carriers could select the optimum route and height for each journey based on both geography and predicted weather patterns, substantial savings should be possible. To do this safely global monitoring of aircraft transponder information would be required, together with an automated conflict alert system. This might sound difficult, but it also sounds like a problem ripe for solution giving the potential fuel savings, and the efforts being made to provide the required internet coverage.
Where is the magic required for civil aerospace?
• Advanced efficiency measures
o Propulsion – hydrogen fuel
o Aerodynamic – Blended Wing Body configuration
• Electric power
o Fuel Cells
• Global Air Traffic Management
o Urban area traffic management for UAVs and air taxis
o Exploit global internet capability to free aircraft from prescribed airways
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