Although aircraft only accounted for approximately 9% of U.S. transportation energy consumption, which percentage may seem relatively small, projections by the U.S. Energy Information Administration (EIA) indicate that air travel is expected to experience sustained growth in energy demand, potentially raising its share to 14% by 2050. The US Department of Defense (DoD) is one of the largest consumers of jet fuels globally, at nearly 78 million barrels to power aircraft and combat vehicles with a total cost of $9.2 billion. This significant consumption has a substantial environmental impact.

The aviation industry, however, faces significant challenges in decarbonization, due to the energy density requirements of liquid hydrocarbon fuels used in airplanes, which is difficult to replace by other energy sources such as batteries. A comprehensive, interdisciplinary approach, encompassing sustainable fuels, clean gas turbine technology, electrification, fuel cell and batteries, sustainable aircraft design and manufacturing, optimization of air traffic and fleet management, is required to address this challenge.

Given the current and anticipated high market share of internal combustion engines, it is essential to enhance their efficiency in order to reach the shared goal of reducing carbon emissions. To this end, the transportation industry is looking into pre-chamber jet ignition technology for spark ignition and hybrid engines, as it has the potential to increase combustion efficiency, lessen cycle-to-cycle variability, and enable engines to operate in lean-burn or highly diluted scenarios. The main advantage of pre-chamber ignition compared to spark ignition is that its high-velocity jets create multiple ignition sites throughout the chamber, resulting in a rapid burn rate and quick consumption of the primary charge with minimal cycle variations.

Furthermore, Knock is a major challenge for high load operation of spark ignited gasoline engines with higher compression ratios, because the end-gas undergoes higher temperature and pressure trajectories during combustion. Pre-chamber combustion creates long-reach ignition jets, which have the potential to mitigate knock due to their rapid consumption of the end-gas. Although it has been reported that pre-chamber combustion can mitigate knock as compared to traditional spark ignition and different pre-chamber designs show different knock tolerances, the underlying physics are not fully understood. Due to differences in ignition and combustion behavior between traditional spark ignition and pre-chamber jet ignition, the knock behavior, such as the origin and severity of the knock, may be different, which needs to be understood to further optimize pre-chamber design and improve overall engine performance.

Propulsion technologies for drones and small unmanned aerial vehicles (UAVs) are evolving rapidly, offering a wide range of opportunities to enhance performance, endurance, and environmental sustainability. Fully electric propulsion, powered by lithium-based batteries, is currently the most common approach due to its simplicity, low noise, and minimal emissions. These systems are well-suited for short-range missions and consumer drones. Fuel cells, particularly hydrogen fuel cells, are gaining traction for their higher energy density and longer flight durations compared to batteries. They provide a cleaner alternative to internal combustion (IC) engines and offer the potential for extended missions in surveillance, mapping, and delivery services. Meanwhile, traditional IC engines remain valuable in scenarios requiring high power-to-weight ratios, quick refueling, and rugged operational environments.

Despite these promising developments, each propulsion technology presents significant challenges. Electric systems are limited by battery energy density, which constrains flight time and payload capacity. Fuel cells, while offering longer endurance, require complex onboard storage solutions for hydrogen, along with careful thermal and water management. IC engines, though mature and reliable, generate noise and emissions, making them less suitable for urban or environmentally sensitive missions. Additionally, integrating propulsion systems into compact airframes while optimizing for weight, efficiency, and reliability remains a critical engineering hurdle. Continued research and innovation are needed to overcome these barriers and enable flexible, mission-specific propulsion solutions for the growing UAV market.

Hydrogen and alternative fuels research plays a critical role in shaping the future of sustainable energy, particularly in aerospace applications. As global efforts intensify to reduce carbon emissions and mitigate climate change, the aerospace industry faces growing pressure to transition from conventional fossil fuels to cleaner alternatives. Hydrogen (cryogenic), with its high energy density and zero carbon emissions, stands out as a promising candidate. Its potential to significantly reduce the environmental footprint of aircraft makes it a focal point of research and development efforts across academia, government agencies, and private industry.

However, integrating hydrogen and other alternative fuels such as bio-derived fuels into aerospace systems presents unique challenges. Hydrogen storage at cryogenic temperatures is an enormous challenge and requires technological innovation. Additionally, unlike conventional jet fuels, hydrogen has a wide flammability range and low ignition energy, making it more prone to accidental ignition if not properly managed.  Hydrogen leaks are difficult to detect due to its colorless and odorless nature. These properties demand the development of advanced sensor technologies, leak prevention systems, and emergency response protocols tailored specifically to hydrogen-powered aircraft.  Lastly, integration of the sustainable fuels with existing aircraft to ensure compatibility is an important issue.

Electrification plays a vital role in energy security, with energy storage systems—particularly batteries—being essential for a resilient and sustainable future. Batteries are widely used across various sectors, including automotive, aerospace, consumer electronics, renewable energy, telecommunications, and medical devices. While government agencies have invested billions in developing cleaner, more efficient, and reliable energy storage solutions to improve energy density, safety, longevity, and cost-effectiveness, battery fire safety remains a critical challenge.

In the aerospace industry, the importance of batteries continues to grow with the trend of electrified aircraft. Advanced battery technologies will be essential for powering electric and hybrid aircraft. This transition not only supports environmental goals but also promises significant operational cost savings and new market opportunities in air mobility and regional transportation. Between 1991 and 2018, there were 191 fire incidents involving batteries carried as baggage or cargo on airplanes. As aviation electrification gains momentum, we can expect a significant increase in such incidents if fire safety issues are not addressed. The extreme temperatures and pressure changes during flights make batteries more prone to fires, and aviation regulations are even stricter than those for ground vehicles.

Urgent research is needed to mitigate the risks associated with thermal runaway, a phenomenon in which uncontrolled heat generation leads to hazardous conditions. The danger is particularly severe in large battery systems used in EVs, e-mobility devices and e-aircraft, where uncontrolled thermal runaway can result in catastrophic failures, endangering lives and property. Additionally, the toxic gases released pose serious health risks to firefighters and first responders.