Future Fuels for Flight

The Challenge of Aviation Growth

Today’s global commercial aircraft fleet burns more than 100 billion gallons of fuel annually. While this accounts for less than 3% of anthropogenic CO2 emissions, aviation faces a unique decarbonization challenge. Ground-based power generation is shifting toward wind and solar, and surface transportation is increasingly electrified; meanwhile, global air travel continues to grow. With over 30,000 commercial airplanes currently in service—a number projected to double every 15 years—developing safe and compatible SAF is critical.

The Jet Fuel Standard and SAF

Modern aircraft rely on jet fuel, a highly refined variation of kerosene. This fuel boasts exceptional properties: it remains stable across extreme conditions and over time, all while packing nearly 45 megajoules of energy into every kilogram. Because weight is at a premium in aviation, current battery technology—which is up to 40 times heavier for the same energy output—makes the electrification of long-haul flights unviable.

As an alternative, we can produce SAF from waste streams like garbage and sewage, or from agricultural cover crops like jatropha and camelina. However, producing SAF requires a different infrastructure than traditional refineries, making it two to five times more expensive per gallon. And while production is making progress, it still meets less than 1% of global demand. Currently, there are eight approved pathways for blending SAF up to 50% with conventional jet fuel.

Exploring Alternative Carriers: Hydrogen and Ammonia

Hydrogen presents another compelling option. It contains zero carbon atoms and offers three times the energy per kilogram compared to jet fuel. However, it requires four times the volume and must be stored as a liquid at a frigid -253°C. Additionally, combusting hydrogen can increase emissions of nitrogen oxides and water vapor, presenting new environmental challenges.

Fundamentally, jet fuel acts as a dense hydrogen carrier where the carbon simply comes along for the ride. Ammonia, composed entirely of nitrogen and hydrogen, could theoretically serve as a carbon-free aviation fuel. Burning it (under the right conditions) would release benign nitrogen gas, which already makes up 80% of the air we breathe. Unfortunately, ammonia is highly toxic and contains only one-third the energy of jet fuel by both mass and volume. Other abundant hydrogen carriers, like natural gas, also require cold liquid storage on board and pose severe environmental risks if leaked or vented.

The Path Forward

Despite these challenges, the aerospace industry is advancing. Aircraft and engine efficiency continue to improve by an average of over 1% annually—a trend sustained for nearly a century—steadily reducing the fuel required per flight. Looking ahead, renewable electricity could be used to synthesize fuels from waste CO2 and water, manufacturing clean hydrocarbons to carry hydrogen onboard. As SAF production expands, the proximity of major airports to urban population centers will help drive circular economies and waste recycling. Together, these innovations are converging to sustain the growth of aviation and keep the global fleet flying responsibly.

Michael Winter

RTX Chief Scientist