From Coal to Sunlight

As Allied bombers pounded Nazi Germany in 1945, the Third Reich’s synfuel plants were high-priority targets. Germany was oil-poor, so much of the gasoline that powered its planes, tanks, and jeeps was synthetic, manufactured from its ample coal supplies. Destroying those fuel plants was a central Allied strategy.

Trying to keep planes in the air, the Nazis developed a test model, the Lippisch P.13a, that would be shot into the air by catapult or booster rocket and kept aloft by burning pulverized coal in a ramjet engine. Germany surrendered before a prototype was ever completed.

After World War II, nuclear-powered aircraft had obvious appeal for the United States and the Soviet Union. Uranium contained so much energy that bombers theoretically would have larger ranges than conventional planes and need less support infrastructure in far-flung locations—an airborne deterrent, constantly ready to deliver atomic bombs.

A major obstacle was the need to shield flight crews from radiation. In the 1950s the Air Force carried out 47 test flights of a B-36 bomber that ran on conventional fuel but had been modified to carry a working, one-megawatt reactor. The reactor weighed nearly 18 tons and required an 11-ton, rubber-and lead-lined cockpit plus a water barrier behind it to protect the crew. Even for a 200-ton bomber, the load was a challenge. “To make it safe and make it fly is the Air Force’s problem,” a 1956 news article observed.

The Air Force was also sensitive to the risks of flying a plane carrying a nuclear reactor. Test flights were conducted in remote parts of Texas and New Mexico and were shadowed by planes carrying technicians, nicknamed the “glow in the dark squad,” who, in case of disaster, would parachute to crash sites and block off the area.

U.S. military engineers could never solve the shielding or safety issues. In 1961, President Kennedy canceled the program, stating that “the possibility of achieving a militarily useful aircraft in the foreseeable future is still very remote.” The Soviet Union tried similar modifications to a Tupolev Tu-95 bomber with similar frustration.

The superpowers also experimented with military planes fueled with liquid hydrogen. In this form hydrogen contains nearly three times more energy per kilogram than jet fuel, although it occupies a volume four times larger. It can be either burned directly in a turbine or used to generate electricity in a fuel cell. And its main emission is water vapor.

But hydrogen is highly flammable, making it more hazardous to transport and handle than standard jet fuel. When engineers at Lockheed’s secret Skunk Works facility were working on a hydrogen-powered replacement for the U-2 spy plane in the 1950s, they built a liquid hydrogen storage facility a few hundred yards from the municipal airport in Burbank, California. “My God in heaven, you’re gonna blow up Burbank,” a visiting expert from the National Bureau of Standards declared. The research, codenamed Project Suntan, was ultimately scrapped.

The Soviets got slightly further. In 1988, they flew a modified commercial airliner with one of its three engines powered by liquid hydrogen. The hydrogen tank was so large it filled most of the passenger cabin. Three years later, the Soviet Union broke apart, triggering an economic collapse that ended most Russian advanced aviation research.

In the 1970s and 1980s, model airplane builders in the United States and Europe started experimenting with designs that used photovoltaic cells to power electric motors. The models initially spent a few seconds in the air, increasing gradually to minutes, and then hours.

On July 7, 1981, AeroVironment, a California company funded by NASA, flew a piloted plane called Solar Challenger 163 miles across the English Channel, staying aloft for five hours without the aid of reserve batteries. Physicist Paul MacCready, who led the team that designed the 217-pound plane, told the New York Times that he couldn’t “think of anything practical” about flying a plane on solar power, but hoped the flight would spur interest and investment in solar energy more broadly.

Until the 2000s, major airlines and aircraft manufacturers focused on making jet fuel and engines ever more efficient rather than finding alternative power sources. Roger Connor, a curator at the Smithsonian National Air and Space Museum and licensed commercial pilot, says that serious conversations about alternative fuels, especially for large passenger jets, have only developed in the past decade or so, alongside rising concern about climate change. “Practical progress is still a long way off,” he contends.

Drop-in Solutions

Today biofuels, known in the industry as sustainable aviation fuels, or SAFs, are the most straightforward option for a power shift. They can be blended with conventional jet fuel and don’t require redesigning engines, planes, or airports. You may already have flown on a plane powered partly by SAF. But that part would have been very small.

In 2025, global SAF output was about 1.9 million metric tons, which represented 0.6% of total jet fuel consumption and cost up to five times more than petroleum-based jet fuel. Still, major airlines are quick to tout their role in boosting SAF production and reducing costs.

In November 2023, Virgin Atlantic flew a Boeing 787 powered with 100% SAF from London to New York City. Boeing and Airbus, the world’s largest passenger jet manufacturers, have pledged to make their planes capable of running entirely on SAF by 2030. (Commercial planes are typically certified to use blends up to 50% SAF.)

Airlines and manufacturers claim that SAF, if they can just get enough of it more cheaply, will cut carbon dioxide emissions from aviation by up to 80%. But environmentalists and many energy experts are skeptical. Analysis by Project Drawdown, a U.S. nonprofit that promotes science-based solutions to climate change, points to issues that are challenges for biofuels more generally: the climate benefits vary depending on feedstocks and production methods, and it’s hard to scale up production to levels that would have a real impact.

“Sustainable aviation fuels derived from feedstocks like corn and soybeans don’t reduce emissions significantly,” says environmental scientist Emily Cassidy, a coauthor of the Drawdown analysis. Growing, fertilizing, and harvesting those crops, which are the main biomass feedstocks in the United States, generate greenhouse gases. So does converting prairie grassland to plant more corn and soybeans. A 2021 study calculated that land-use changes and the fertilizer used to produce corn ethanol made it 24% more carbon-intensive than the automobile gasoline it was blended into.

Cassidy notes that European regulations, by contrast, require virtually all SAFs to come from sources other than energy crops, such as waste fats, used cooking oils, garbage, and stalks from food crops. But the aviation industry argues that those sources are limited and expensive and there isn’t enough capacity to refine them.

Planes Full of Batteries

The biggest shift toward clean transportation in recent decades has occurred on the ground, not in the air. Electric and hybrid cars are projected to make up 29% of global new car purchases in 2026. But electrifying flight is more challenging.

Getting planes or helicopters off the ground demands enormous battery power. And producing that energy quickly for takeoff and landing generates huge quantities of heat, which requires special cooling equipment. All of this greatly increases weight and takes up space that could be used for passengers and cargo.

“Electrification is not an ideal solution today for aircraft,” says University of Michigan professor Viswanathan, but with the technology maturing faster than SAFs, he predicts it’s “going to win long term.”

Viswanathan points to what he calls an emerging low-altitude economy—drones and small electric aircraft moving packages and, eventually, passengers locally and regionally at altitudes of roughly 3,000 feet or lower. Progress in this sphere, he says, is challenging reservations about costs and technological possibilities.

Connor, of the Smithsonian, concurs. He identifies three converging technologies influencing this field, known as advanced air mobility aviation: automation; distributed propulsion systems (picture lots of small rotors on a helicopter instead of one large one); and newer, high-density batteries that can efficiently power small fixed-wing planes or helicopter-style designs, called VTOL (vertical takeoff and landing) aircraft. “You can take about a thousand pounds of payload maybe 50 miles with the existing technology,” he says.

Ukraine’s success using drones in its war against Russia is boosting military and civilian interest in these devices. Domestic applications could include autonomous delivery of goods to remote places like Alaska or to offshore oil platforms. And of course, people who can afford it will pay to reach their destinations faster and avoid traffic hassles. In large, congested cities like Seoul, São Paulo, Mexico City, and Los Angeles, helicopters are an established way of getting around for the ultrarich.

Boeing subsidiary Wisk Aero has developed six generations of electric-powered VTOL (eVTOL) aircraft since 2011. Currently it is testing a pilotless air taxi, similar to a driverless car, that would carry four passengers. In March the Federal Aviation Administration approved a three-year initiative that will allow eight companies to test eVTOL aircraft across 26 states. Some of the manufacturers are focused on shuttling VIPs around Los Angeles during the 2028 Olympic Games; others are targeting cargo delivery or support of emergency medical responses.

Skeptics argue that electric air taxis will just be inefficient helicopter replacements for the rich. But in a 2021 study, Viswanathan and a graduate student calculated that with improving battery designs, air taxis could be almost as energy-efficient as land-based electric cars. And as these vehicles improve, Viswanathan predicts, costs will decline and make flights more widely affordable.

According to eVTOL.Travel, an industry platform for planning trips and booking rides, air taxi rides through about 2027 are projected to cost $3 to $6 per mile and then decline as the industry scales. If eVTOLs are actually certified and hit this target, a trip from Heathrow Airport to central London would cost $90 to $160—comparable to a premium Uber or Lyft.

New Planes, New Airports?

As cleaner flight infiltrates small-scale aviation, prospects for greening large-scale passenger travel are much murkier. “What happens to larger aircraft, even in a 20-year time frame, is anybody’s guess,” says Viswanathan.

Hybrid systems, with gas turbine engines and electric motors working in tandem, could be one path. At the Farnborough International Airshow in July, a modified Saab 340B turbo propeller plane, a 34-seater widely used for regional flights, became the first hybrid-electric aircraft to exceed 30,000 feet.

A different concept under study is a design called the blended-wing body, which makes flight more efficient by making planes more aerodynamic. Instead of a tube with wings sticking out horizontally, a blended-wing plane looks more like a manta ray—one unified shape with wings swept back.

This design reduces the drag that pushes against an aircraft as it move through the air. In certain versions, it can enable the entire plane to generate lift, the upward force created by air flowing at different rates over and under a conventional aircraft’s wings.

Designers have experimented with blended-wing shapes since the 1920s, and the concept’s influence is obvious in Northrop Grumman’s B-2 “stealth bomber” and its successor, the B-21. Blended-wing passenger planes could use 20% to 30% less fuel than conventional aircrafts, and their wider bodies could carry more passengers and cargo.

California start-up JetZero has designed a 250-passenger, blended-wing plane called the Z4 that it says will have international range and reduce fuel use compared to conventional planes by up to 50%. The company hopes to fly a prototype by the end of 2027 and has announced investments and partnerships with United, Delta, Alaska Airlines, Japan Airlines, and Gulf Air.

But there are drawbacks to the design. While wider bodies mean cabins could be larger, many passengers wouldn’t be seated near windows, which could increase air sickness. Passengers in the wings might experience uncomfortable forces when the planes bank. Different cabin configurations also would require new boarding and evacuation protocols.

Connor is skeptical that blended-wing planes could be widely adopted without redesigning airport terminals and gates. For the near term, he says, “It’s definitely the low end where the greatest progress has been, and will probably continue to be, at least for the forthcoming decade.”

Put another way, you’ll probably buckle up on an electric air taxi long before you board a green transatlantic flight. If there’s a pilot, don’t forget to tip.