This story was originally published in our Nov/Dec 2022 issue as "Greener Skies Ahead." Click here to subscribe to read more stories like this one.
There’s a reason all passenger jets are shaped like tubes. As an aircraft climbs in altitude, outside air pressure naturally decreases along with oxygen levels. To keep passengers from passing out, planes pump air into the cabin. This increases the pressure and the amount of breathable oxygen inside.
But filling the enclosed space with air is a bit like blowing up a balloon. The challenge, explains R. John Hansman, a pilot and director of the MIT International Center for Air Travel, is designing a cabin that is strong enough to resist rupturing. “The most efficient shape for that is actually a tube,” he says.
This is why airplanes on tarmacs around the world have looked unmistakably similar since the first jet airliner took off in 1949. The iconic tube shape plus two swept-back wings, each supporting one or more engines, are the hallmarks of the modern jet. This tried-and-true model has propelled humans far in the world of flight — both literally and figuratively.
Despite what you see on the surface, however, airplanes have actually changed a lot over the past 70 years. Fuel efficiency and noise reduction, for instance, have consistently been improved in the industry. And newer challenges posed by climate change are pushing engineers to get creative and rethink the future of aviation.
Roughly 60 years ago, the airline industry was cruising into the Jet Age. Gone were the days where passengers rode on small, propeller powered planes such as the Douglas DC-3, the world’s first commercial airliner, which debuted in 1935 with a max capacity of 28 passengers. The demand for air travel — and its increasing affordability and safety — pushed airplane manufacturers to build larger commercial airliners, such as the Boeing 707, which could carry up to 181 passengers on a single flight.
But the first generation of big jets, which came out from the late 1940s through the 1960s, was “very, very loud,” says Janet Bednarek, an aviation historian at the University of Dayton. The constant sound of roaring engines caused tension between the airline industry and people who lived around airports.
The noise problem influenced the next wave of jet aircraft design. In the 1970s, Boeing unveiled the 747, which was two and a half times bigger than the 707 and could seat 366 passengers. The increase in passenger seats drove down the cost for tickets, while new innovations in engine design also reduced noise.
“Even though it was much, much larger, it was quieter than the original 707,” Bednarek says.
Larger planes also helped airlines become more fuel- and cost-efficient on a per-passenger basis. “Airlines are very sensitive to cost,” Bednarek says. “So having an aircraft that can fly higher, farther, faster on less fuel is very important to them.”
Finding ways to improve efficiency and noise both have a lot to do with engine architecture. One tactic is to increase a gas turbine engine’s bypass ratio so that more air flows around the combustion chamber instead of through it. That increases the engine’s efficiency, with the added benefit that the jet also makes less noise.
The larger bypass ratio has increased engine size over time. “If you go back and look at pictures of airplanes from the ’60s, they had little tiny jet engines,” says Hansman. “If you look at airplanes today, they kind of look similar, but the engines are much bigger.”
But, he adds, a bypass ratio can only get so high before the material around the engine gets too large and creates excess drag, making the plane less efficient. “We’re sort of at a point where … you have to do something different. There are proposals to do very different configurations,” he says — meaning we could start to see planes that don’t have the standard tube-and-wing design.
For example, there’s the “double bubble” aircraft, an idea from MIT, NASA, and Boeing subsidiary Aurora Flight Sciences. The design would double the width of the plane body and place engines at the very back, instead of under the wings. It was first proposed in 2010, and in 2017, NASA awarded a contract for research into the design for its new subsonic X-plane.
A more recent push for innovation comes from climate change and pressure to lower emissions, a challenge that both Hansman and Bednarek describe as “existential” for the industry.
Aircraft transportation produces “roughly 2 to 2.5 percent of global carbon emissions,” says Sean Newsum, Director of Environmental Strategy at Boeing Commercial Airplanes. “And while that’s a small percentage, it’s a significant one.”
Searching for Sustainability
Making air travel greener, experts say, will entail more than a one-size-fits-all solution like a new shape for pressurized cabins. Boeing and other aerospace companies are investigating several applications at once to target different parts of the industry.
Sustainable aviation fuels (SAFs) are one promising innovation, and the most scalable at present. These alternatives to fossil-based jet fuels are typically made from natural materials, including cooking oils, animal fat, typical farm crops or algae. And some SAFs are already burning in today’s jet engines, mixed with traditional jet fuels. Researchers are working on increasing the ratio of SAFs that can be used in current and future aircraft. But the ready-to-operate infrastructure is why Newsum says SAFs are the most immediate solution for decarbonizing airplanes.
Hydrogen has also emerged as a promising fuel alternative, since it leaves behind only water vapor. But storing it on planes poses challenges, since it has to stay below minus 423 degrees Fahrenheit to be used as a liquid. So far, no commercial jets are operating with this fuel source. “It’s going to be a while before that can enter the market,” Newsum says — which is why Boeing is focused on SAFs as a short-term solution as they develop hydrogen for long-term use.
On the other hand, a few recent efforts have birthed small aircraft with electric engines. Rolls-Royce completed a test flight of the single-passenger Spirit of Innovation in 2021, and a nine-seater passenger jet developed by Israeli aerospace company Eviation, called Alice, is in the testing stage. The hurdle here, however, is the sheer weight and energy density needed to power larger electric aircraft. The relatively small Spirit of Innovation required 6,000 battery cells — enough energy to power 250 homes.
“The potential for those small airplanes is pretty exciting,” Newsum says. “But it’s going to be a long time, decades perhaps, before that is mature enough to power an airplane the size of a 737.”
For now, Boeing and other companies are building prototypes of small electric planes called eVTOLs, or electric vertical take-off and landing vehicles. Most are designed to shuttle a handful of people on short commuter flights. One startup, Kelekona, has unveiled plans for a 40-passenger “minibus” that looks straight out of a sci-fi film. Some eVTOLs may also fly autonomously.
The money is certainly there — Boeing invested another $450 million this year in its joint eVTOL venture with the Kitty Hawk Corporation, called Wisk — but the road to the runway remains long for these kinds of aircraft.
“Any new technology is going to have its challenges — that’s inherent in the innovation process,” Newsum says. “If there were no challenges, we would already be doing it.
Spaceflight Needs an Upgrade
It’s no secret that launching a rocket into space isn’t great for the environment. Many traditional rocket fuels are dirty, toxic, and blast soot particles into the highest levels of the atmosphere when they breach Earth’s orbit. And humans are launching more rockets these days than ever before.
In the face of climate change, more efficient and sustainable rocket fuels are being brought into the spotlight. A combo of hydrogen and oxygen, which leaves behind only water vapor, powered Jeff Bezos’ New Shepard joyride last summer. And SpaceX uses methane, which burns cleaner and more efficiently than traditional rocket fuels, to power its Starship rocket.
Startups like blueshift Aerospace are even investigating the viability of biofuels made from plant matter to get rockets off the ground. But in practice, that remains a heavy lift in the booming industry of space travel — where frequent and farther travel will require reliable, capable power.