Welcome back! Last time I introduced the mission of the Raven – to explore the decarbonisation of aviation, particularly the under-exploited potential of advanced technologies to reduce emissions.
However, before we dive into the possibilities offered by future tech, it’s worth understanding commercial aircraft as they exist right now, and why aviation pollution is such a tricky problem to tackle. I’m going to focus on commercial aviation – airliners flying passengers and cargo, as well as business jets. These sectors constitute the majority of aviation emissions, and have broadly similar dynamics. I won’t go into huge depth here, but the constraints of aviation are important dynamics that shape the sorts of solutions that are possible. I’m super aware that I’m skipping a lot, but it’s hard to cover everything at once!
As always, I’m still learning about the space, so if there’s anything I’ve missed, things I’ve got wrong, or areas you’d like me to explore more in future articles, let me know!
A look at the modern airliner
Modern airliners are truly incredible feats of engineering, honed over decades into machines that can whisk large amounts of people and cargo huge distances at high speeds, all whilst maintaining incredible reliability and safety. Nothing on an airliner is for show, so let’s have a look at why airliners are the way they are.
Virtually all airliners and transport aircraft in service today are of a ‘tube-and-wing’ design, so named because the fuselage is a tube shape, and they have a single main wing. But why, other than historical legacy, is this design near universal? Ultimately, many of the answers largely come down to one key factor: weight.
Let’s start with the tube and the wing. The primary reason for a tube-shaped fuselage is pressurisation – maintaining a breathable environment for passengers and crew whilst flying in the very thin air of the upper atmosphere. A tube is extremely efficient for holding pressure, using the least amount of material to give the required strength. A tube also presents minimal frontal area, reducing direct drag, and is also easily scalable – a manufacturer can add in extra sections or remove them to make larger or smaller aircraft without having to redesign everything from scratch (a big deal given the expense and complexity of certification).
Aircraft need to generate lift, which they do through wings – specially shaped surfaces that generate an upwards force from the air flowing past the aircraft. Naturally, you need enough lift to balance out the weight of the aircraft, otherwise you’ve just got a streamlined bus! Having a single large wing is the simplest method to generate this lift, and it is placed so the lift acts around the centre of mass, keeping the aircraft stable. For higher speed, the wings can be swept, allowing them to still function effectively nearer the speed of sound.
Wings can be placed at any point vertically, but generally will run either under the fuselage (for most passenger aircraft) or over it (more common on transport aircraft), allowing the structure of the wing to be continuous, and not to interrupt that efficient pressure cylinder. Passenger aircraft almost all use a low wing, as this allows landing gear to be shorter (and thus lighter), and the main landing gear can fold up into the wing rather than needing a separate structure to house them. A positive dihedral (meaning the wings are angled up towards the tips) means that the engines can fit under them more easily, as well as keeping the aircraft stable in flight.
Speaking of engines, virtually all modern commercial aircraft use some form of gas turbine engine, primarily due to their extremely high power-to-weight ratio, unmatched by any other available propulsion source. Short-haul aircraft may use turboprops – turbines driving a propeller – whilst larger, longer distance airliners mostly use turbofans. These are essentially a highly optimised fan in a tube, powered by a jet turbine core. This is more efficient than simply using the thrust from a turbine directly – accelerating a larger volume of air a small amount is more efficient than accelerating a small amount of air a lot.
Modern turbofan engines are incredibly reliable, and this is why almost every airliner has only two of them. Originally long-haul and larger airliners had three or four engines, but modern engines have become extremely powerful and reliable, meaning an airliner can have just two, each of which is more than capable of powering the plane alone. Engine failures are extremely rare, so the increased cost associated with additional engines no longer comes with benefits. Indeed, turbines generally become more efficient with greater size, so fewer, larger engines is preferable to a greater number of smaller units. With the recent retirement of the A380 and upcoming end of production of the 747 in coming years, the era of the 3 or 4 engined airliner may be over for good.
Modern aircraft also tend to follow a straightforward design for the tail (often referred to as the empennage). The vertical rudder keeps the plane heading straight – important for dealing with crosswinds when landing, or maintaining direction if an engine fails. The horizontal elevators serve two functions: they allow the aircraft to pitch up and down; and also trim the aircraft – canceling out any small differences in weight distribution due to fuel, cargo or passengers. Placing these control surfaces as far from the centre of lift and mass means they have the maximum leverage and thus can be smaller and lighter than if they were closer to the centre. Weight is everything!
This tube-and-wing approach to design also has the benefit of, as much as is possible, decoupling the elements of an aircraft. So much of an aircraft’s design is interdependent, with strong links between weight, wing area, thrust, structure and much more. However, the tube-and-wing design minimises the coupling of elements – meaning that the fine details of, say, an engine, won’t affect the design of other parts of the aircraft except in aggregate. This reduces the complexity of what is an immensely involved design process to more manageable levels.
What about the environment, though?
So, let’s talk about emissions. I’ve covered this in a little more depth in my original aviation deep dive, but (perhaps unsurprisingly) the root cause of aircraft emissions is their engines. Jet turbines burn a form of kerosene, a fossil fuel derived substance. Approaches like SAFs may start to reduce the carbon impact of aviation, but with other effects such as cloud and contrail formation potentially an even larger problem, ultimately, a solution which leaves jet turbines burning hydrocarbons is not going to solve the problem.
This is the crux of the clean aviation problem – how do we change fuel or propulsion type?
The power problem
Right now, if we want to majorly reduce or eliminate aircraft environmental impact, we have a few broad options on the propulsion front (with some variations to spice up the mix). The first is electric. It works well in cars, as we’ve seen, but faces a number of major issues in aircraft.
The biggest is, of course, batteries. Current energy density of the best lithium-ion cells is around 40-50x less energy dense per unit weight than kerosene. Then there’s the fact that fuel is burned as a plane flies, decreasing weight. In a highly weight-constrained aircraft, where up to 50% of the take-off mass is fuel, that extra mass is a huge deal, and further complicates the battery problem.
Then we have motors and power electronics. Power to weight ratio is the big problem here, with motors lagging turbines somewhat (though recent developments are making rapid progress in this department). Another issue is scale – as I mentioned, modern aircraft have tended to fewer, larger engines, a decision that makes perfect sense for turbofans, but not so much for motors. To power a modern wide-body airliner, we would need motors capable of producing tens of megawatts of power – these simply do not exist at the requisite power-to-weight ratios. Those immense power loads also require high voltages and currents, which introduces safety issues at high altitudes, where electrical arcing becomes a significant potential hazard.
There is also the charging infrastructure build-out required. Obviously, airports already have power, but as we’ve seen with EVs, a charging network is vital to support roll-out of electric vehicles, and charging an aircraft on the scale of a modern airliner is likely to require some serious hardware.
Hydrogen
Ah yes, the fabled fuel of the future! Hydrogen has been the magic clean fuel of tomorrow for decades, but aviation might actually see an application where it can be of use. As far as aircraft are concerned, hydrogen can be used in either of two main ways – burned like a conventional fuel, or consumed in a fuel cell to generate electricity.
Fuel cells have come on a long way, but are in a similar position to power electronics – they’re not nearly energy dense enough, and again do not exist at the required power ratings. This is being worked on, but it’ll be a while before we see aircraft weight fuel cells operating at multi-megawatt levels. Fuel cells do still potentially have some environmental impact, though this depends how they are employed – something that has simply not been tested enough to fully understand.
Burning hydrogen in turbines is an interesting option, because it avoids the immaturity of an electrical propulsion system. Jet turbines can burn many different fuels, but do need modification for different fuels – flame temperatures, gas expansion ratios and much more vary from fuel to fuel (this is a subject I am still getting my head around). Nevertheless, companies are converting land-based turbines to hydrogen, so the capability is there, even if it hasn’t made it to aviation yet. The disadvantage of burning hydrogen in a turbine versus using a fuel cell is that you don’t completely remove the environmental impact – some nitrogen oxides are still produced, along with water vapour, inducing clouds and contrails. Nevertheless, it could be a 50-75% reduction in overall impact, a huge win from today.
The other complexity of hydrogen is the fuel itself. Firstly, generation. To be low impact, the generation needs to be green. This aspect, fortunately, is being solved already – there’s a vast amount of investment flowing into green hydrogen production, although producing it at sufficient scale at airports is still a work in progress. There are a few promising companies working to smooth out this part of the process significantly.
However, the chemical properties of hydrogen bring their own challenges. Hydrogen is actually extremely energy dense per unit weight – around 3 times more so than jet fuel. However, it is very energy-poor volumetrically, meaning a potential hydrogen aircraft would need far more space for fuel, even if that fuel was lighter than the equivalent Jet A. It can be compressed, either by pressurising or by liquefying it (though neither improves matters a lot), but both pose challenges – high pressure vessels and/or cryogenic fuel storage are complex challenges that can’t be easily dismissed when considering hydrogen.
So, we’re stuck with SAFs then?
Much of the focus of decarbonising aviation has focused on taking a replacement propulsion and inserting it into existing aircraft. This approach makes sense – we know how to make aircraft in their current form, and potentially retrofitting cuts the certification overhead for a new propulsion technology. However, as I’ve noted, both electric and hydrogen have serious shortcomings that limit their effectiveness. Some will be steadily improved with time (power-to-weight ratios, larger motors, high voltage safety) but progress on others is less certain (battery energy density), and in some cases we’re up against physical constraints that cannot be altered (hydrogen density).
If the mountain will not come to Muhammad, then Muhammad must go to the mountain
If we cannot improve these alternative propulsions to the point where they fit current aircraft, can we make aircraft which better fit the new propulsion technologies? Are there ways to change or redesign aircraft to better make use of these new technologies, rather than trying to force them into a design optimised for a different propulsion method? This thought experiment was what set me down this road, and I’ll start to explore it more next time!