Solving the Aviation Problem
Flying is hard to decarbonize, but we need a connected world to solve climate change
One of my favorite book genres is biographies about aviation pioneers. Aerospace engineering is ridiculously complicated, and has attracted some of the most singularly brilliant people in American history1. The modern jet engine is such a marvel of engineering that Simon Winchester devotes an entire chapter to it in The Perfectionists, a wonderful book about the development of modern precision engineering.
But powered flight was really only made possible by the development of the internal combustion engine2 (and later, the jet engine), meaning that modern aviation is inextricably linked with fossil fuels. The reason for this can be most easily understood by looking at a very handy visualization known as a Ragone plot.
The Ragone plot lets us compare the amount of energy you can store per weight on the X-axis, vs the power per weight on the Y-axis. Looking at the graph above, you can see that I’ve highlighted lithium ion batteries, hydrogen, and gasoline. So, imagine that we are trying to power an airplane. We need a way to power the airplane, and it has to be carried on board the airplane. The heavier our power source, the more energy we have to expend lifting it, and shorter the range of our airplane. The Ragone plot above shows that a 1 kg battery can only hold about 250 watt-hours of energy, but an internal combustion engine powered by gasoline can generate around 2500 watt-hours for a single kg of fuel, 10 times as much. And jet engines, which are much more efficient than internal combustion engines, can travel close to 100 times as an electric powered plane, currently.
This means that there’s a large gap between where technology exists right now, and where we need to be to in order to replace fossil fuels for long-distance flight. So what are the options?
For starters, in spite of the limitations above, electric-powered flight is an ongoing area of development. Joby Aviation is one company working in this space3. Due to the limitations on the energy density of batteries discussed above, their current range is about 100 miles. But in the next 10 - 20 years we should expect to see new battery chemistries that may make electric flight more feasible for longer distances.
A different sort of way to make aviation carbon neutral is to make the fuel itself carbon neutral, by creating it from captured CO₂. Carbon capture is energetically expensive, as is conversion to jet fuel. But if it is done using renewable energy, then effectively what we are doing is taking renewable energy and “condensing” it into a high density, high value form (jet fuel), using a waste product (CO₂) as the input, with zero net CO₂ emitted into the atmosphere. Such fuels, known as “e-fuels” or “sustainable aviation fuels” (SAF), are being actively pursued by a number of companies, including Air Company and Twelve. These processes require the development of new catalysts and chemical engineering processes to improve their efficiency. But, they may provide an important mechanism to decarbonize many difficult sectors, including ground transportation, where the internal combustion engine vehicles that are being produced today will still be in operation 20 years from now.
Looking at the Ragone plot again, we can see that there’s a lot of potential for hydrogen as a carbon neutral aviation fuel, because it can be made energy dense by compressing it up to 10,000 PSI. Right now, very little of our hydrogen supply is made renewably. But because hydrogen is such an attractive alternative to fossil fuels for aviation, as well as industrial applications requiring high heat, there is a lot of work going into developing greener sources of hydrogen. Universal Hydrogen is one company developing an end-to-end hydrogen solution for commercial aviation, and they are working to make it possible to retrofit existing planes to run on hydrogen.
Of course, at the far end of the energy density spectrum, well off the right side of the Ragone plot, is nuclear power. Nuclear submarines can travel for months without having to refuel! But for pretty obvious safety reasons (imagine dropping an operating nuclear reactor from 10,000 feet onto a city), it’s unlikely that we’ll see nuclear aviation anytime soon4.
Aviation poses a significant problem to near-term decarbonization: it is very hard to decarbonize, but it accounts for about 2.5% of all CO₂ emissions. It’s tempting to offer solutions like “People should just fly less,” but climate change is a global problem that requires coordinated action on everybody’s part, and cannot be solved by individual choice. A global crisis, requiring a global solution, requires more interconnection and international cooperation of the type that air travel supports, not less. If we want to solve the aviation problem, we’ll have to do it together.
A short list of my favorite books in this milieu includes:
The Wright Brothers, by David McCullough. Just an all around amazing story of two brothers and bicycle mechanics from Dayton, OH who decided to build the first airplane. Because, why not?
Skunk Works, the memoir of Ben Rich, president of the famed Lockheed Martin Skunk Works division that developed some of the most amazing airplanes of the 20th century, including the U2 spy plane, the iconic SR71 Blackbird, and the first prototype stealth bomber, Have Blue.
Boyd: The Fighter Pilot Who Changed the Art of War, by Robert Coram. John Boyd revolutionized aviation early in his career by developing a quantitative framework for air to air combat. But even more fascinating were his attempts to fundamentally change how US military thought about warfare and technology, by fighting Pentagon politics with bravado, stubbornness, and even dirty tricks when necessary. It’s a must read for anyone who wants to understand how to affect change of large organizations,
The Wright brothers were unable to source an engine from any of the existing manufacturers, and so they built one themselves, with the help of their cigar-chomping machinist, Charlie Taylor. As an amateur metal worker and machinist, this still blows my mind.
In classic Silicon Valley fashion, Joby Aviation did many of their early tests at the bottom of a limestone quarry, in part to skirt FAA regulations about above-ground test flights.
A story from Richard Feynman’s memoir Surely You’re Joking, Mr. Feynman recounts that he once received a phone call from a group that wanted to hire him as CEO of their nuclear airplane company. He couldn’t understand why they were calling him, a physicist, until he remembered: When he was in his 20s and working in Los Alamos on the first nuclear bomb, the Army asked everyone to submit ideas for patents for nuclear power. Feynman pointed out that the possibilities were limitless: nuclear submarines, nuclear rockets, nuclear airplanes…the officer said to him “Nuclear submarine’s been taken, you get nuclear airplane.”