## 2 Literature Review
### Hydrogen as an Aviation Fuel: Promise and Challenges  
Studies cite decarbonization goals more often than the other anticipated benefits of hydrogen fuel as the primary incentive for investing in its production to power aircraft [13]. However, if oil shortages, global price increases, and increased carbon abatement costs make jet fuel more expensive, the economic case for hydrogen aircraft strengthens dramatically [14]. As hydrogen production scales and the industry gains experience, costs will expectedly decline. Consequently, airlines will start buying hydrogen-powered aircraft because they make business sense, and not just for a green image. Analysts estimate that by 2030, hydrogen produced with renewable energy will become cost-competitive with traditional fossil fuels, with significant cost declines thereafter [15]. Here are some expected additional benefits of hydrogen-powered aviation that its zero-emission benefit often overshadows:  
1. Fossil fuel reserves are finite and unevenly distributed among the countries extracting it, whereas hydrogen is uniformly abundant. Therefore, all countries can competitively convert air or water to produce hydrogen. Henceforth, hydrogen better promotes energy security that can extend aviation well into the future [16]. 2. Compared with fossil fuels, trade flows of hydrogen are more difficult to tariff or weaponize for geopolitical influence because nations can produce hydrogen locally from abundant natural resources [17]. 3. Investing in the development and integration of hydrogen storage and delivery systems can create a \"hydrogen economy\" with new jobs while reducing cost and price volatility over time [18]. 4. Off-grid hydrogen production can supplement or back up conventional energy grid and natural gas infrastructure rather than stressing their capacity [19]. 5. Powered equipment can store and utilize hydrogen in locations where electrification or transporting fuel is difficult or costly, such as remote, resource-scarce, and contested areas [20]. 6. Clean hydrogen production can avoid carbon abatement costs while progressively contributing to cost reductions over using fossil fuels [21]. 7. Hydrogen has a high gravimetric energy density of 120 MJ/kg, which is almost three times that of jet fuel [22]. This means that hydrogen can extend flight endurance for the same weight of jet fuel. However, the caveat is that the volumetric energy density of liquid hydrogen is about one-quarter that of jet fuel [23]. This means that aircraft must increase the onboard fuel storage volume by four times to achieve the same flight endurance. 8. The operating cost and noise of a hydrogen-powered aircraft are lower than for aircraft powered with conventional jet fuel [24]. Hence, residents will prefer quiet hydrogen-powered aircraft for integration with the living environment. 9. Hydrogen-powered aircraft systems have fewer moving parts, which can lower maintenance costs [25]
10. Swappable hydrogen fuel capsules can significantly reduce the time that aircraft must be on the ground, boosting aircraft utilization and lowering costs associated with airport parking fees [26]. 11. Hydrogen is the lightest element, so if it leaks, the gas rises in open air and disperses quickly, unlike heavier gasoline or jet fuel vapors that pool [27]. Hence, a design thatvents the gas into the atmosphere reduces the risk of leaks forming large flammable vapor clouds. 12. Unlike batteries, hydrogen does not degrade, so it can remain in storage for reliable utilization when needed [28]. There has been a seven-fold increase in hydrogen energy investments within the four years prior to 2024 [29]. Europe, North America, and China currently have the largest number of projects with capital commitments, and hydrogen production capacity in China has been outpacing the rest of the world [29]. Hydrogen propulsion can be implemented as either combustion in a modified gas turbine engine or as a fuel cell generator providing electricity to power propeller motors.