The Environmental Impact of Aviation
This is a post I first published as part of my work with Forge the Future - a weekly climate newsletter occasionally interspersed with deep dives into environmental topics. It was originally published as two parts due to its rather lengthy nature, but here I’ve combined the entire thing into a single missive, with some light editing, and the pictures I didn’t have the space to include in the original.
Aviation has long been in the eye of the climate movement, though particularly so in the past few years. The rise of the youth climate strikes, Extinction Rebellion and Greta Thunberg has led to the phenomenon of ‘flygskam’ - flight shame, where people are made to feel guilty for flying. But how bad is it - does it deserve its bogeyman reputation?
Aviation by the numbers
This is an unusual time to be writing about the aviation industry, as it is one of the sectors hardest hit by the ongoing COVID-19 pandemic and resultant lockdowns and travel restrictions. Nevertheless, pre-pandemic, the aviation industry moved around 4.5bn people in 2019, along with over 60m tons of air freight. There are somewhere north of 25,000 airliners in service, and this is only predicted to grow, coronavirus notwithstanding.
The IATA predicts a 3.5% compound annual growth rate of the industry, with passenger numbers potentially over 8bn by the late 2030s. The full impact of emissions is complex, but aviation emissions are estimated at between 2 and 2.8% of total global emissions, but the full warming effects on the planet could be as much as twice that. More concerningly, emissions themselves grew around 2% a year, despite the energy intensity of aviation decreasing by around 2.8% annually. The difficulty of decarbonising aviation, combined with its rapid growth means that it could make up 15% or more of total emissions by 2050 if action is not taken.
How aviation impacts the environment
Aviation’s full impact on the environment is complex. CO2 is perhaps the most well understood byproduct, and as the climate metric of choice, it gets the most attention, but aircraft produce a number of other emissions that make them particularly problematic. Amongst these other emanations are nitrogen oxides, sulphur oxides, soot and water, each of which have different effects. Nitrogen oxides, for example, reduce methane (a greenhouse gas) producing a cooling effect, but increase ozone, which has a heating effect.
Some of the largest effects are from ‘contrail cirrus’. These are high level clouds that sometimes form from the contrails aircraft leave in their wake. They can persist for up to 18 hours, and whilst they reflect some solar radiation, they also trap heat, resulting in a net warming effect. The impact of this is still being quantified, but studies put the radiative forcing effect of aviation contrails today as greater than that of the CO2 released from all powered aircraft throughout history.
Many of the emissions from jet engines also are produced by ground transport, but as jet airliners spend the majority of their time cruising at high altitude, these emissions are released directly into the upper atmosphere, where their effects are more pronounced.
Impact of COVID-19
Given the severity of the coronavirus pandemic, and particularly its effect on travel, it’s worth briefly looking at how the aviation industry has been affected. International travel has plummeted, and many airlines, which tend to run on razor-thin margins, required bailouts from their respective governments. Largely these bailouts have been condition-free - the US, Germany and the UK all placed minimal or no conditions on airlines receiving handouts. France was a rare exception, making its bailout of AirFrance conditional on the airline improving its environmental footprint, stipulating that it would need to reduce emissions and phase out flights competing with domestic high-speed TGV rail services.
Despite the dire state of passenger travel, air freight has continued, and indeed has played a vital role in keeping vital medical supplies such as PPE flowing across the globe throughout the pandemic. A number of airlines have temporarily repurposed their passenger aircraft for freight, filling seats with parcels in place of people in a bid to keep money flowing as passenger counts fell.
The pandemic has undoubtedly changed how many view air travel, with many businesses reviewing the need for business travel in particular, realising that in fact many interactions can be conducted remotely. Whilst tourism demand is likely to remain low for some time due to concerns from the public and government restrictions, not to mention financial insecurity for many, it will likely rebound in a year or two, and there is little sign that the crisis has dampened general enthusiasm for foreign travel.
Why are planes hard to decarbonise?
The decarbonisation of aviation is a multi-dimensional problem. The technical problem itself is extremely challenging due to the nature of flight, which I'll explore more in part 2, but as with many industries, there are other forces at play.
Politically, aviation is complex - it bridges borders and is a connecting force worldwide. Not only is it almost by nature an international problem, it also drives a huge volume of trade and tourism, both key economic forces. This means any conversation about aviation gets complex, fast. Everyone has an opinion and wants a say, and all it takes is one player not playing ball to scupper an international agreement.
The industry itself also brings its own challenges to the table. Flight is inherently more risky than ground transport - if something goes wrong the stakes are far higher - so preventing issues is a primary concern. Regulation and certification of aircraft is a hugely complex and involved process - it can take years and hundreds of millions of dollars to certify a new aircraft for flight. These high standards have pushed the safety of air travel to extremely high levels, with accidents not only extremely rare, but far more survivable than ever before.
These safety requirements also push strongly towards the known over the unknown. As an example, if you’ve flown a domestic or short-haul flight, chances are high that you flew on some variant of either a Boeing 737 or an Airbus A320, both of which were first introduced over 30 years ago (the A320 family launched in 1984, and the 737 first flew in 1967!). These aircraft have been regularly updated and improved over the intervening decades, but the core designs and basic airframe have not changed in all that time - certifying an update to an existing airframe is significantly simpler than getting approval for a clean sheet design.
Certification and regulation along with ever more cutting edge engineering has also pushed costs of aircraft development higher and higher. This changes how successful an aircraft programme must be - aircraft need to secure ever larger proportions of the market to pay off the costs of development, meaning there is room for fewer and fewer players. These forces took the market from dozens of airliner manufacturers in the 50s and 60s to just two main players today.
The short- and long-haul airliner market is now essentially a duopoly between Boeing, a vast US aerospace and defence company, and Airbus, a similarly large entity from Europe. Both have close ties to their respective geographic regions, and roughly split the commercial aviation market in two. There are a few manufacturers of smaller aircraft, such as Bombardier and Embraer, but by and large if they compete with the big two, they get bought out - Airbus took a large stake in Bombardier’s C-Series regional airliner program, and Boeing was until recently in talks to buy a majority stake in Embraer’s entire commercial aircraft program. China and Russia have airliner programs, but Russia’s is part of a vast state-owned industrial complex mired in bureaucracy, and China’s is still in its infancy.
Even with the gigantic scale of these companies, the costs of airliner programs continue to spiral upwards. The Boeing 787, one of the few clean-sheet designs in the last couple of decades, cost an estimated $32bn to bring to production, and was delayed for multiple years. Boeing has been very cagey about profits and revenue from the program, but numerous analysts have suggested that the program will likely never make a profit, even after selling a thousand or more aircraft. All this for an aircraft that, whilst innovative, is still remarkably similar to the basic tube-with-wings design that has been the norm for half a century or more.
Finally, it is worth considering the role of airlines in the market - as the primary customer for airliners, their demands in many ways dictate the market. Airlines are squeezed at both ends, facing enormous capital costs - aircraft, fuel, maintenance, crew, flight/landing fees - and a competitive market where ticket prices are being forced ever lower. The famous Richard Branson quote, “If you want to be a millionaire, start with a billion dollars and launch a new airline”, is very apt. Airlines are a very hard business to make money in, and they are thus extremely risk-averse when it comes to new designs - they would almost universally take a safe design over a risky innovation.
All these factors combine to make an industry that is heavily regulated, inherently tied up with geopolitics, immensely expensive, slow moving and incredibly risk averse.
What has been tried
Solutions largely fall into three different camps - technical improvements, offsetting carbon emissions, and avoidance of flying altogether.
Technical improvements have driven significant improvements in aviation. As I mentioned earlier, the energy intensity of aviation has fallen by around 2.8% annually, driven largely by cost and noise factors. Ever tightening noise regulations have forced out older, louder jets, and the hunt for cost savings has driven a push for more fuel efficient planes. Environmental factors are starting to influence technical choices, and may have a larger influence on future new aircraft designs, but for now, cost and noise are the primary drivers for improvement.
Unfortunately, the progress that has been made has been more than offset by huge growth in passenger numbers, meaning that overall, emissions have still increased. There is also concern from some industry figures that efficiency gains are reaching their limits for current aircraft designs, and that further gains will cost more for smaller improvements. The other problem with improving by increasing efficiency is, as Saul Griffith aptly puts it, “You can’t efficiency your way to zero”. To radically reduce or eliminate emissions will require a step-change in technology - something that is not only technically challenging but that the industry is extremely averse to.
Given the lack of achievable and mature technical solutions, industry and regulators have turned to two principal tools to mitigate their emissions - offsetting and sustainable fuels. This suits both airlines and aircraft manufacturers well, as it allows them to make visible efforts to appease environmentalists without fundamental changes to their business models or the aircraft themselves.
Carbon offsetting is the name for balancing emissions from one source with removal or mitigation of emissions elsewhere. They work on the concept that the atmosphere is a shared resource, so in theory, a ton of carbon is roughly equivalent to any other ton of carbon, no matter the source nor sink. It’s a complex and nuanced topic that is well beyond the scope of this post, but it’s something I’ve written about before, and may look at again in future.
So, efficacy aside, how do offsets work in the world of aviation? Until the last couple of years, the responsibility was almost entirely on consumers, who would have to calculate the emissions of their flights, find an offset provider, and pay themselves. In recent years, tools and top-level aggregators have appeared to make the task easier, but airlines have mostly avoided the blame.
That has changed in the last year or two, with various airlines offering options to offset flight emissions when buying tickets, and this year, Easyjet, the UK budget airline, decided to offset all of its flights from 2020. Others will likely join them soon - more and more airlines are making commitments to reduce or zero out their emissions. As consensus builds internationally on climate action, industry and governmental efforts to reduce emissions will likely grow, and with few tools at their disposal, offsetting is likely to be the weapon of choice.
I’ll explore sustainable aviation fuels (SAFs) more fully later, but there are some options for reduced carbon fuels available already. These are mostly biofuels that have some certified reduction in carbon impact over traditional fossil aviation fuels. Biofuels are another complex topic in themselves, and the subject of heated debate due to the complexities of changing land use from either natural ecosystems or food production to grow the feedstock for the fuels.
Regulators have been cautious in adopting SAFs, approving increasing mixes of SAFs and regular kerosene, and takeup has been limited so far. Much of this has to do with the limited availability and high price of sustainable fuels thus far, but may change with more and more airlines making commitments to purchase and use the fuels as environmental impact becomes a higher priority.
So what about political solutions? The international nature of aviation makes sweeping political agreement complex - 65% of CO2 emissions from aviation are in international airspace, so don’t ‘belong’ to individual nation states. For this and other reasons, aviation was explicitly excluded from the scope of the Paris Agreement. Nevertheless, there have been a number of attempts to regulate aviation’s environmental impact, with varying success.
The EU made probably the first major attempt to regulate aviation emissions as part of its emissions market - the EU Emissions Trading Scheme (ETS). In 2008, the European Parliament voted to include aviation in the ETS from 2012, which would cause all flights into, out of and within Europe to be logged and counted against airlines’ carbon quotas. However, the airline industry, along with countries such as the USA, China, India and Russia reacted strongly against the move. The US even threatened to ban their national carriers from complying with the scheme. The EU eventually put the inclusion of international aviation in the ETS on hold for the indefinite future, although it did still cover flights within the EEA.
In 2016, the UN body responsible for aviation, the ICAO, finalised an agreement between 191 countries to take responsibility for aviation emissions. This scheme is known as CORSIA - the Carbon Offsetting and Reduction Scheme for International Aviation (aviation does like its acronyms!). One of its main aims was to achieve the ICAO goal of making all growth in aviation after 2020 carbon neutral. It enforces measuring and reporting of all aviation emissions by member countries. Airlines must offset all emissions over the baseline year (originally 2020, now renegotiated to 2021 after the precipitous drop in aviation traffic due to COVID-19).
The scheme will start in 2021, and is a voluntary pilot until 2023, covering all airlines with international emissions above 10,000 tons of CO2 per year. There will then be a phased transition to the full scheme, which will be mandatory from 2027. Currently the scheme is only agreed on until 2035 - a point of concern, as if it is not extended further, it will only cover around 6% of projected CO2 emissions from international aviation between 2015 and 2050.
Various countries (mostly in Europe) have implemented some form of aviation tax, often in the form of a distance-based charge on tickets, but approaches are fairly piecemeal at present, and subject to heavy pushback from airlines. The additional financial pressure imposed by the COVID-19 travel restrictions has also given airlines another reason to resist efforts to implement taxes. As with many environmental measures, taxes and policy tools are likely the biggest levers to enact change, but they are slow, and easily stymied by savvy aviation lobbying bodies.
Lastly, there is arguably the simplest solution - fly less. There will always be a need for flying, whether for business, visiting family, goods transport, or countless other reasons, but modern flights are so cheap and easy that many of us indulge more than we should. Flygskam has become a well-recognised phrase in climate circles, meaning ‘flight shame’. Not flying at all is the most radical solution, but as with all such measures, absolutism isn’t as important as reduction - flying less will still have an impact. Frequent Flyer taxes have been touted as a potential solution - in the UK a mere 15% of the population take 70% of all flights.
Not flying in many places doesn’t have to limit travel either. Whilst some journeys will always be out of reach of all but the most determined non-flier, trains are a good alternative in many parts of the world. Europe has significant high-speed rail coverage, and whilst its systems are rather disjointed at present, efforts by retailers like TheTrainLine and RailEurope are helping to bridge these gaps. Night trains in particular are making a come-back in recent years across the continent, offering trips where the journey is part of the experience rather than just a means to travel from A to B. Resources like Seat61 also help to demystify some of the complexity of booking longer journeys by rail across the world.
Looking to the future
That concludes our tour through the current state of aviation. Now, you may be forgiven at this point for thinking that the future of aviation looks rather bleak - political solutions lack teeth, and the only options to reduce the impact of flying are to offset (with all that that entails) or not to fly. However, fortunately, that’s not the case. There are a host of technologies that have the potential to reduce or eliminate the emissions from flight. While the industry might be cautious to adopt them, they do exist, and that’s where we’ll be looking next.
A technical interlude
Firstly, we must take a brief but relevant segue into the mechanics of flight. Flying is dominated by weight - perhaps understandably, given the importance of gravity in our lives, overcoming it takes a lot of energy. This drives how aircraft are designed, and how they have evolved - the need for light weight and huge power.
Whilst small aircraft tend to use piston engines similar to those found on a car, most large aircraft use some variation on jet turbines to propel themselves through the air. Invented in the 30s, jet turbines offered a huge boost in power for the same weight as a piston engine, and were one of the key inventions that made the airliners we fly on today possible. However, that power comes with a hefty fuel cost. Today, most commercial aircraft use either turboprop or turbofan engines - basically jet turbines driving a propeller or a large fan respectively. These improve fuel efficiency significantly, but planes today still need enormous amounts of fuel to fly. Jet airliners generally run on specific aviation fuel that is largely kerosene based - not too far from the gasoline or diesel fuels most cars and trucks run on. However, they burn vast quantities of the stuff - airliners usually carry anywhere from 20-45% of their max take-off weight in fuel for each flight. An Airbus A380 can carry over 200 tons of fuel for a single flight!
These requirements of light weight and high power combined with strict safety standards severely limits the potential solutions. However, there are a surprising number of technologies that will reduce or eliminate aircraft emissions, although each comes with their own collection of drawbacks.
First, let’s look at where our future aircraft gets its power from. Currently, due to the high power and strict weight requirements, aviation-grade kerosene makes up the overwhelming majority of fuel used, but there are a few potential alternatives, both now and looking forward into the future. Let’s look at the three main contenders.
The most straight-forward solution is one we touched upon earlier - SAFs, or Sustainable Aviation Fuels. These are fuels that aim to reduce the climate impact of flying whilst being drop-in replacements for fossil-based fuels used today. These broadly fall into two categories - biofuels and synfuels.
Biofuels are fuels made from biological materials - plants or other biological matter. Not all biofuels are renewable, but those that are aim to sequester carbon within the plant matter which can then be burned as fuel, thus creating (in theory) a carbon neutral cycle - plants absorb carbon, planes burn plant-based fuel and emit the carbon. However, in practice, it gets a lot more tangled than that. Biofuels are a messy and complex topic, as the plants they are made from must compete for space with countless alternatives - for example food crops or forest which was already sequestering carbon. Biofuels are also still conventional fuels, and burning them has all of the climate impacts of fossil fuels - CO2, NOx, contrails.
Synfuels, or synthetic fuels, are a similar concept. Here, the idea is to create an artificial fuel from base chemicals (carbon monoxide and hydrogen) rather than from plants. For renewable synfuels, that usually means using CO2 captured from the air or from carbon capture systems in power plants, along with green hydrogen created using electrolysis with renewable energy. An advantage of synfuels is that the exact chemical composition of the resulting fuel can be precisely controlled, helping both performance and emissions over fossil or bio fuels. For example, synfuels can be engineered with fewer aromatic compounds reducing soot during combustion, reducing NOx emissions and contrail production. Depending upon the source of the carbon, the power used to create the fuels and their chemical makeup, the overall climate impact may be 30-60% less than current fuels.
That improved climate impact does come at a significant energy cost. 1kWh equivalent of synfuel takes between 2.8-4.6kWh of input energy to create, which would massively increase the energy footprint of aviation should the fuel become widespread.
Overall, SAFs make an excellent bridge solution given their drop-in nature, but they are unlikely to reduce environmental impact more than around 60%. Regulators are starting to permit fuel mixes containing SAFs on a number of routes, but thus far take-up has been low. SAFs are still significantly more expensive than conventional fuels, and the volume available is tiny. However, as more and more airlines start to consider their environmental footprint, companies are starting to commit to large orders of SAFs, which in turn spurs more production, and with scale, prices will fall.
Another option on the roster of alternative fuels is hydrogen. Hydrogen’s major ace card is that it is a truly clean fuel - when burned with oxygen, it produces water as the only byproduct (when burned in air, there will be some NOx produced from the nitrogen present). Hydrogen has been touted as the clean fuel of the future for decades, with hydrogen cars, trucks, boats, trains and more proposed over the years. So what benefits does it have, and why has it failed to take off (pun intended) so far?
The clean-burning nature of hydrogen makes it a prime candidate for reducing emissions in any situation involving fossil fuels, but it has a few other tricks up its sleeve. Firstly, it can be generated by splitting water using electricity (a form of electrolysis), allowing it to be generated easily from clean energy sources. Second, it can be used in many conventional fossil fuel-burning engines, from those in cars to jet turbines, removing the need for a new propulsion method. The ace card is the hydrogen fuel cell - a device that essentially reverses electrolysis, combining hydrogen and oxygen to produce electricity at a high efficiency. All this makes hydrogen clean and very versatile - it can be used in most conventional engines as well as a power source for electric propulsion.
However, there’s a ‘but’ - all these features do not come without downsides. Hydrogen is the lightest element in the periodic table, and that brings some unique quirks. The molecules are so small that they leak out of many pipes and tanks, and can cause metals to become brittle. It also has a very low density, meaning that gaseous hydrogen must be stored at very high pressures to avoid it taking up vast volumes. Liquid hydrogen is more dense, though even then it still has a third of the volumetric energy of kerosene (though 3-4x the energy per unit weight!). However, hydrogen boils at a chilly −252.87°C, which means that storing liquid hydrogen runs into a host of cryogenic complexities.
Then there’s generation of hydrogen. Currently, the vast majority of hydrogen is generated from steam cracking of natural gas, and is therefore not emissions-free. Green hydrogen is possible, but requires large amounts of clean energy - something that is in relatively short supply currently. As such, prices for green hydrogen are 4-7x those of kerosene currently, though the EU is one of a number of regions making a big push to lower prices by ramping up production in the next decade.
Hydrogen for cars and ground transport has faced an uphill struggle - the fuel is currently expensive, difficult to store, and in short supply. Fuel cells are also still a relatively undeveloped technology, and as such are only around 70-80% efficient, and suffer from poor power-to-weight ratios. When weighed up against batteries and electric motors, which are efficient, simple and can hook into the existing power grid, there was no contest.
However, in aircraft, the conditions are significantly different. Aircraft travel huge distances, and require an energy source that is as light as possible. Cost is less of a factor, and as such hydrogen looks like one of the most promising routes to almost completely decarbonise aviation. Switching from conventional fuel to hydrogen (burnt in jet turbines) would reduce overall climate impact by 50-75%, and using fuel cells could push that to 90-95%.
There are still major barriers however. Fuel cells need to become much lighter and more powerful, and the insulated tanks required for liquid hydrogen also must become much lighter. There is the not insignificant burden of regulation and certification to overcome, not to mention building out a hydrogen supply at every major airport worldwide. Nevertheless, it’s enough of a serious contender that Airbus is making a multi-billion dollar push to develop hydrogen aircraft, calling the fuel ‘the most important transition this industry has ever seen’.
The final energy source to consider is, of course, batteries. Batteries have come on a huge way in the last two decades, steadily increasing in energy density and dropping vastly in price. However, compared to kerosene, batteries have 40-50x less energy per unit weight - a gulf that will not be bridged by anything other than a revolutionary new battery technology in the next couple of decades.
Batteries also have another weight disadvantage to overcome - unlike fuel, they aren’t consumed during the journey. The huge amounts of fuel required by conventional aircraft means that by the end of a flight, they are significantly lighter, and thus require less energy to propel through the air. Batteries are sadly the same weight when fully charged as when they’re flat as a pancake.
Does that mean we won’t ever see battery-electric aircraft? Not at all, but they will dominate in smaller, lighter aircraft that cover short distances - realms where the advantages of electric propulsion outweigh the disadvantages of heavy batteries. A number of companies are looking to introduce electric versions of pilot trainer aircraft - small, lightweight two seat aircraft that fly for an hour or so at a time. Operators of short distance propeller aircraft have also prototyped conversions of small passenger planes, such as the de Havilland Beaver and Cessna Caravan, which can carry 10 or so passengers a few hundred miles at most.
The other realm seeing huge attention at the moment is the eVTOL air taxi market, often referred to as Urban Air Mobility (UAM). There are hundreds of companies trying their hand at some form of small, vertical take-off short range aircraft, though only a few have actually flown anything. This regime fits electric flight well - the distances are small, and electric motors are as well suited as any to the power requirements of vertical propulsion. However, the whole market is new, meaning it faces vast regulatory and certification hurdles, not to mention pricing challenges. It also does next to nothing to decarbonise existing aviation, seeking instead to move commuters from cars on the ground to cars in the air. In a very Silicon Valley move, it seeks to solve congestion with a vastly complex, over-engineered solution when better public transit and improved city planning would likely solve the core issue far more effectively.
The next area of aircraft to look at is propulsion. Modern aircraft largely either use turboprop or turbofan engines - essentially these are jet turbines with either a propeller or a big fan on the front respectively. Each works best in different domains, which determines which aircraft use what. Turboprops have high propulsive efficiency, but become rapidly less efficient above around 0.6-0.7 Mach. Turbofans are only efficient at much higher speeds - 0.8 Mach and above, hence turboprops dominate on slower, shorter distance aircraft, whilst most airliners use turbofans for their improved performance at higher speed.
It used to be common for longer range jet aircraft to have 3 or 4 engines, principally for redundancy. There are no lay-bys in the sky, and especially on long oceanic flights, the aircraft must be able to keep flying long enough to reach safety should an engine failure occur. Multiple engines add a safety margin, but introduce significant cost and maintenance overhead - jet turbines are complex machines, and more engines means higher running expenses. In addition, turbine engines become more efficient with size, so two larger engines will generally be more fuel-efficient than four smaller turbines.
However, with time twin engined airliners have become reliable enough that rules governing how far they could fly from a diversion airport (known as ETOPS) have been increased significantly. Modern engines are reliable enough that the recent Airbus A350 received an ETOPS-370 rating, meaning it can fly for 370 minutes on only a single engine. This means even very large aircraft can use only two engines without sacrificing range or route coverage. Larger jet turbines are generally more efficient, giving another boost to twin-jets. But what does the future hold for propulsion?
One possibility is to combine electric motors with jet turbines for still further improved efficiency, much as in hybrid cars. There are a host of different ways of combining the two modes, each with different benefits and drawbacks.
A parallel hybrid would see motors and engines mechanically linked, with either or both combined able to power the aircraft. In this way, engines could be made smaller, and use the motors as a booster for high-power periods of flight such as take-off. A bonus is that the motor could work in reverse as a generator for in-flight power. An alternative is the series hybrid. Here, the aircraft is propelled by electric motors, but the electricity is provided by a jet turbine running a generator. This decouples the propulsion from the turbine, which creates more flexibility in design, but it relies on each stage of the powertrain being efficient - the benefits of electric propulsion must outweigh converting engine power to electric and back to motive force again.
Hybrids are generally assumed to have a battery, but given their high weight, an alternative is to avoid batteries altogether - an architecture known as turbo-electric. This is a well proven approach in trains and ships, where it avoids the complexity of heavy mechanical gearboxes, but whether it can translate effectively to aircraft remains to be seen.
But why would you want a partially or fully electric propulsion system? Pure electric propulsion is highly energy efficient versus combustion engines, but if an engine is involved, then there must be other benefits to justify the conversion from jet turbine to electric motors.
Engines are large, heavy, and require lots of maintenance. This in turn limits where they can be placed on an aircraft. Electric motors, however, come in all shapes and sizes, are mechanically simple, and only require a power cable. This means they can be located all across the airframe to reduce drag or increase power.
This can take a whole variety of forms. Wingtip motors (see the Eviation Alice) can reduce wingtip vortex drag. A tail motor can re-energise the boundary layer of air slowed down by the fuselage, reducing skin drag. Motors can be used for blown flaps, increasing take-off performance and reducing mechanical complexity. Scattering motors across the wing can increase lift, reducing the wing area required. As a handy side effect, having more motors gives redundancy, which means less power overhead. A downside of modern aircraft having just two engines is that they need to fly safely should an engine fail, meaning that each engine provides a lot more power than is needed. Electric motors can reduce that overhead by having a high level of redundancy, without the maintenance and efficiency disadvantages that come with many jet turbines.
There are a few hurdles to cross before we see large electric or hybrid aircraft however. Weight and power, as always, are critical. A typical wide-body aircraft may need as much as 60MW of propulsive power, and creating motors with that power at a similar weight to jet turbines is a significant challenge. Similarly, other pieces in the electric power train such as inverters are also not light nor powerful enough yet, though rapid progress is being made on both fronts. A further challenge is making light-weight cables that can safely transmit incredibly high voltages and/or currents without introducing a fire hazard.
Boundary layer ingestion
Another engine-related efficiency measure proposed is boundary layer ingestion. The boundary layer is the layer of air immediately next to a surface such as an aircraft’s skin. This layer is slowed by viscous effects, and adds a dragging force to the aircraft. Boundary Layer Ingestion places propulsors to suck up this boundary layer air, speeding it up and negating much of the drag. This technique has been proposed in a number of efficient concept aircraft such as the Aurora D8 and the NASA STARC-ABL, but not yet on any production aircraft. Despite the efficiency gains, the idea faces difficulties with managing the complex structural stresses inherent in running fans in slowed or turbulent air.
Ultra-high bypass engines
A turbofan engine uses a big fan at the front of the engine to pull air both through and around the engine. The ratio of air going through the turbine in the middle versus around the outside is known as the bypass ratio, and generally, a higher bypass ratio (i.e. more air going around rather than through) provides lower fuel consumption for the same thrust (less air is being combusted to move the same overall volume of air). This drive has seen turbofan engines improve from bypass ratios of 0.5-1:1 up to 10-12:1 on the latest jetliners. This trend is likely to continue, with major engine manufacturers chasing 15:1 for their newest designs.
Another option being considered is the propfan - a hybrid of a turboprop and turbofan which in theory captures the fuel efficiency of a turbofan but with the speed of a turbofan. First proposed in the depths of the oil crisis in the 1970s, a few designs were developed in the ‘80s, but as oil prices dropped, development largely stalled. The designs looked promising, offering considerable fuel efficiency gains, but had major issues with noise that will need solving before they see wider acceptance.
More Electric Aircraft
Current aircraft have a plethora of different systems all running in parallel to ensure everything runs smoothly. In addition to the engines and their fuel system, there’s also an electrical system, for running everything from lights to computers and more, pneumatics for brakes and landing gear doors, hydraulics for flight surfaces and landing gear, and more. Given the strict safety requirements, these systems are often replicated for redundancy, adding a lot of weight and complexity.
Significant efficiency savings can be had from moving most if not all of these systems to electric equivalents, simplifying the architecture of the aircraft significantly. In addition, electric equivalents to hydraulic or pneumatic systems are often lighter, allowing the plane to consume less fuel through weight reductions. These could either be run from the engines or via a dedicated fuel cell or similar electrical generation system.
This was one of the major innovations in the Boeing 787. Whilst not every system is electric, much more of the aircraft is powered this way than its predecessors, giving an estimated 3% fuel savings. However, even this partial shift was not simple - the aircraft was plagued by battery issues, which led to the plane being grounded after a series of fires. Nevertheless, now that the teething issues have mostly been worked out, this looks like a likely option for many new aircraft, although it’s not generally considered a cost-effective option to retrofit old aircraft.
Other future technologies
Blended surfaces and laminar flow
Smooth surfaces are crucial for reducing drag on aircraft flying at high speeds. Modern aircraft still have a number of abrupt edges, from wingtips to flight control surfaces, all of which can induce drag. The gains from smoothing an individual part may be small, but added up across an entire airframe, this can yield a few percent of additional efficiency. NASA has performed testing on a business jet fitted with ‘seamless’ flaps that eliminate the gaps created by conventional moving flight surfaces.
Another area that has seen much research is laminar flow. To oversimplify massively (aero experts, I’m sorry!), moving air largely falls into two regimes - laminar flow, which is smooth and stable, and turbulent flow, which is chaotic and unstable. Turbulence around an object moving through air will cause drag, increasing the energy required to propel it forwards. For aircraft, therefore, it is desirable to keep flow laminar around the vehicle for as long as possible. There have been many efforts to improve the levels of laminar flow around wings and other surfaces, both through passive design features and more active approaches.
Aircraft have used aluminium alloys as their material of choice since the 1920s, and whilst there have been significant advances in airframe design since then, the materials have remained largely the same. However, in recent years, composite materials have progressed in leaps and bounds, offering greater strength and lighter weight. They are more complex to design and work with, and they behave very differently to metals, with vastly different failure modes. This complexity combined with conservative safety regulations slowed their adoption, but recent aircraft programs such as the Boeing 787 and Airbus A350 have embraced composites to a large degree. Future aircraft will likely see a further development of this trend, with composites offering simpler, lighter airframes.
Novel aircraft designs
One of the most interesting areas for me as an aviation enthusiast is new, innovative aircraft designs. After some interesting experimental flourishes in the early decades of flight, aircraft have largely not changed in 70 or more years, sticking to a tried-and-tested approach of tube-and-wings. There are good reasons for this design - a tube efficiently carries the load from the pressurisation needed to carry passengers safely at high altitude. It has a narrow frontal area, reducing drag, and is also scalable, allowing manufacturers to resize an aircraft easily to cater to different markets. The wing, engine and fuselage are also aerodynamically fairly distinct, allowing them to be optimised without interfering with one another. However, researchers in search of more efficiency have come up with many alternative ideas, a few of which we’ll take a brief look at here.
The ‘double bubble’
This concept is not too dissimilar to current aircraft, but widens the fuselage significantly by joining two tubes together - hence ‘double bubble’. This allows the body to add some lift, meaning the wings can be smaller, and also allows for boundary layer ingestion, further improving efficiency. This design was proposed for a NASA next generation aircraft competition some years back, but unfortunately has seen little progress since.
Laminar flow aircraft
We’ve already covered the benefits of improving laminar flow over surfaces, but what if you designed an entire aircraft to optimise for this? In that case, you end up with an unusual-looking, but incredibly efficient aircraft like the Otto Aviation Celera, which performs like a business jet, but cuts fuel consumption massively. It is unfortunately limited to relatively small aircraft - the design can only be scaled up a small amount before the benefits fade away - so it’s unlikely we’ll see such designs for airliners any time soon.
Blended wing bodies
Whilst most efficient aircraft technologies assume a similar design to current aircraft, with a roughly tubular fuselage and separate wings, some researchers have proposed more radical designs. One of these is the blended wing body (BWB), sometimes known as a hybrid wing body. This is a design where fuselage and wings are smoothly blended together, somewhere between a flying wing and a conventional aircraft.
This design reduces the ‘wetted area’ of the aircraft - the surface area exposed to airflow - and thus drag. Some designs also give the fuselage an airfoil shape, allowing the entire aircraft to generate lift. Different studies have shown varying levels of improvement, but the design could yield 20-30% or more efficiency improvements in cruise. Most variants also place the engines on top, which also cuts noise massively over current aircraft. BWB designs also integrate well with hybrid and distributed propulsion designs, as these can utilise boundary layer ingestion, offering further efficiency gains and noise reductions.
Whilst a huge amount of research effort has flowed into the designs from NASA, Boeing and others, nothing more than scale aircraft have flown so far. As the design is so different, there are a host of technical challenges to overcome, from pressurising a cabin that is not round, flight stability, cabin layout, and more. Blended wing body designs offer some of the biggest potential gains, but with so little in common with conventional designs, also some of the highest development risks.
So given all these technologies (and more I didn’t mention), what does the future of aircraft look like? Some of the technologies covered are fairly certain - ultra high-bypass engines, composite airframes and more electric technologies are already starting to be seen on the newest airliners - but beyond that the picture becomes less certain. The heavy burden of regulation, and the domination of the industry by a couple of enormous incumbents means that more radical solutions such as hybrid-electric aircraft or novel airframe designs are unlikely to come from within. However, the barriers to entry for a new entrant are huge. The costs to develop a new aircraft from the ground up are astronomical, and the complexity is similarly huge. Nevertheless, I think disruption is needed if the industry is to radically change. Similar to Tesla in the car industry, a radical new player would likely force the industry giants to innovate or lose market share, moving the field forward even if the new company ultimately did not succeed.
I remain hopeful that in at least some markets, other forms of transport such as high-speed rail could supplant much of the need for shorter flights, but longer distance journeys will be hard to replace. Many airlines will start using SAFs, particularly in areas where carbon taxes on aviation are implemented. Electric and hybrid flight seems likely, but will be a decade away at least, even for short haul applications. Long haul flights may never be fully electrified, but new aircraft designs, fuels and turbo-electric propulsion could massively reduce the impact of these journeys, although there too such changes may take 10-20 years to realise.