The Future of the Aerospace Industry: A Review of Eco-Friendly Advances 

Jaden Ha

Abstract— As aviation advances, society must strive towards a more environmentally-friendly future. Currently, all aviation has contributed to pollution and emissions that are deadly to people and its environment. There are many efforts already that are working to solve this issue such as revolutionary aircraft designs, different types of fuels, and the materials of the aircraft. These technologies, although are not yet fully developed, will help change to a more sustainable world. 

Introduction
Before progressing towards a new path in aviation, it is important to know about the history of aviation and its innovation. On December 17, 1903, Wilbur and Orville Wright flew the first powered airplane where they successfully flew 37 meters and stayed airborne for 12 seconds [1]. Over the years, the Wright Brothers were capable of longer and more controlled flights given larger fuel tanks and engine coolant in order to assist longer operations. Due to the start of World War I, the government demanded aircraft for military purposes that had more speed and range. The first transatlantic flight occurred in 1919 after WW1 John Alcock and Arthur Witten-Brown flew modified Vickers military aircraft from Canada to Ireland for 72 continuous hours [2]. Many countries used the improved aircraft for photography reconnaissance, bombing, and air-to-air combat.

With the popularity of aircraft during the first world war, companies started looking into the possibility of commercial flight. The first passenger service was in 1914, from St. Petersburg to Tampa, and the flight took 20 minutes, leading to a revolutionary change towards propeller-powered aircrafts [3]. The German-built Dornier Do X later launched in 1929, which was a twelve-engine aircraft that could accommodate up to 66 passengers and fly up to 1700 km [4]. The first successful and profitable commercial flight was the Douglas DC-3 according to the Smithsonian, launched in 1936 [5]. It provided a transcontinental US service with only 3 stops. The DC-3 excelled and improved on range, speed, and reliability. Although it was a rather small aircraft, it was able to operate passenger services without any cargo or mail subsidies.

The next revolutionary advancement in aviation was the introduction of the jet engine, which generates power and hot exhaust gasses through burning fuel. The first commercial jet flight was the De Havilland Comet in 1952 [6]. Although it was revolutionary, it had a number of problems, such as the insufficient strength of the fuselage, stress on the square windows that caused the aircraft to break down, and structural failure of pressurization of the cabin [6]. However, the Boeing 707 aircraft was the most successful out of all of the competitors at the time and has since been credited as the start of the jet age. Its first flight was in 1957 and remained in production until 2013 [7]. Boeing added many improved design elements including a wider fuselage, better cargo payload, moving engines to underwing pods for safety during a fire, and changes to flap design for fuselage strengthening [2]. After Boeing’s success with their 707 and 727 models, their 737 design was a new aircraft that beat all competition. The aircraft launched in 1967 [8]. Their design differences include having two engines rather than 3 or 4, placing engines under the wings, providing easier access and a wider cabin and fuselage, and providing an addition of a second deck that was partially used for cargo, and a new high-bypass turbofan engine [2]. This lowered the cost and increased the accessibility and comfort during the flight, allowingThe 747  airlines to offer lower fares and longer routes, as well as providing  flights to more passengers. The extra space gave airlines new options for onboard facilities and cabins, including spacious first class cabins to go inside with the economy cabin and the new business class. 

The peak of jet age possibilities was reached with supersonic aircraft. The sound barrier was first broken in 1947 by an American experimental aircraft, the Bell X-1 [9][10]. The Bell X-1 was powered by a rocket-based engine using liquid oxygen and ethyl alcohol [2]. It was not until the 1960s that supersonic commercial aircraft were developed, with the famous Concorde [11]. They required significant extra power to overcome additional drag at high speed, and a lower wingspan design for more efficiency at high speeds. Supersonic travel, although a very ambitious and exciting development, was ended with the retirement of Concorde in 2003 [12]. The limitations of this aircraft include efficiency and cost. Since the tickets were so expensive many airlines had to cancel these projects. However, there is a possibility of US Boom Supersonic Overture, a Mach 2.2 supersonic passenger aircraft [13].

Advances in Environmental Impact of The Aerospace Industry
Many engineers have focused on improving airplane efficiency the past couple decades. Early achievements were somewhat successful, but most still utilized very fuel-heavy engines, and had high levels of emissions. One of the more significant changes since the 1970s was the improvement in twin-engines, resulting in better performance and more safety [2]. Before this point, twin engines were limited in how far they could travel: no more than 60 minutes away [14]. In the 1980s, the introduction of the Extended-range Twin-engine Operational Performance Standards, or ETOPS, allowed twin-engine aircraft to fly further while having high safety standards [15]. The first twin-flight was about 120 minutes from Trans World Airlines flying a 767 [16]. Since then, ratings have increased significantly. This resulted in a decline of four-engine aircraft and more efficient and cost-effective twin aircraft with lower emissions.

Additional changes that lower emission levels include changes in aerodynamic and wing design, and an increase in use of composite materials in construction, allowing for lighter aircraft. These changes were present in the past series of the 737 and A320 aircraft [17]. For the 737, there is a 14% efficiency improvement between the Next Generation and 737 MAX series [2]. There is also a fuel burn reduction of 20% and a further 7% improvement in the Next Generation series [2]. Among the widebody aircraft, the Boeing 7E7 program, which the E represents the leap the aircraft would make in efficiency, economy, and environment, is regarded as the most fuel-efficient aircraft on the market [2].

In the future, the Boeing 777x will enter service, which will soon have the largest engines on a civilian aircraft, folding wingtips (allowing larger wings to improve efficiency), and composite wing construction [2]. The A350 is also making more highly fuel-efficient aircraft. They will include having a 53% composite construction, adaptive wings that reduce drag, and an advanced aerodynamic wing design [2].

Despite these efforts, there is currently a major flaw within the aviation industry that negatively affects the environment. This glaring problem is our dependence on fossil fuels and air pollutants they produce. Since the dawn of aviation, kerosene, or petroleum, has been the number one source of fuel, which is not sustainable. The use of kerosene is detrimental to the environment and to human health, as it emits various deadly substances such as carbon monoxide, carbon dioxide, and nitrogen dioxide, etc. Therefore, this fuel leaves a huge carbon footprint as well as contributes to climate change [2]. Looking ahead, as kerosene is increasingly acknowledged as a problem in a carbon-conscious world, there have been proposals for developments that include battery technology and hydrogen power. Battery technology needs to deliver enough power with an acceptable battery size and weight. Hydrogen power, on the other hand, requires extensive engine and technology changes and updates, as well as a large-scale change in fuel storage. These future projects will eventually push the capabilities of technology and aviation to battle real-world problems.

In the following section, I will explore new areas of research into greener fuel options that may change the future of flying. 

Current Research
Air Plasma Jet Propulsion
Currently, although fossil fuels are the world’s primary source of energy, they have been proven to be detrimental to the environment and people [18]. Researchers at the Institute of Technological Sciences at Wuhan Universities however, have developed a prototype that utilizes microwave air plasma for jet propulsion [18]. Researchers generated plasma by compressing air into high pressures and using a microwave to convert the air and resulting steam into ions [18]. The result is that there is no carbon emitted to the atmosphere.

Plasma fuel is a much more environmentally-friendly sustainable source of energy since its main by-products are Hydrogen and Oxygen [18]. This method is believed to also exponentially decrease the cost of aircraft and flight as it is much more cost-efficient compared to fossil fuels [18]. The main question that arises from this concept is how efficient will the plasma fuel be, specifically, will generating plasma be sustainable for longer flights? Additionally, since this topic is quite new in its inception, it will take a long time for aircrafts to implement this technology into their aircraft design. 

Emission Control Systems Technology
In another study, MIT professors and engineers discovered a method of propulsion for aircraft that would eliminate 95% of nitrogen oxide emissions [19]. Nitrogen oxide is harmful as it can lead to respiratory diseases and damage many ecosystems and vegetation. The concept was inspired by an emission-control-system in ground transportation vehicles [19] where the amount of dangerous gasses released into the atmosphere is limited. The emission control system (ECS) is placed in the fuel tank so that it can filter and limit the gas that escapes [19]. Currently, it has been impossible to utilize ECS in the current configuration of airplanes, since it would interfere with the thrust from the engines. That is why the conceptual model of planes with ECS still includes a gas turbine that is integrated into a generator, which produces electricity. 

In this system, the emissions produced would be fed into a system that would clean the exhaust before  entering the atmosphere. This method is quite different from current models as the new configuration allows the emission control system to safely eliminate the emission levels while still allowing the aircraft to function [19]. Although an all-electric system would be ideal, researchers have expressed that a  completely-electric system is not possible without a breakthrough battery [19]. Although there was a consideration of using the concept as an add-on, which would ultimately eliminate the thrust of the plane, the group decided to separate the propellers from the gas turbines, in which the propellers would be powered by a generator. The propellers would instead be powered by an electric generator, so when the system detects unsafe emission levels from the gas turbine, the turbine and its exhaust will be completely isolated from the propellers and instead fed into an emission-control-system, which they researched that would eliminate 95% of NOx emissions [19]. As of right now, the MIT team is currently working on a zero impact configuration that would produce no NOx emissions [19]. Below is a diagram of how the emission control system will be implemented into the new aircraft design [20]. 

This new hybrid-electric plane is a step into the right direction of sustainable aircrafts. Although they still use gas turbines, their model is able to eliminate NOx emissions through their design of an emission-control system. With being able to slowly transition to a reusable energy source, airplanes can become more cost effective, fuel efficient, and environmentally friendly. The only concern is whether this new concept is able to be maintained for a long flight, or else it may take an even longer time to implement into international commercial flight.

Development of Biofuels
Another exciting avenue of current research is that of bio-fuels for greener, more sustainable options to power aircraft. A group from Washington State University recently developed a jet fuel based on lignin, or a type of organic polymer that makes plants sturdy, for fueling planes [21]. During the team’s tests, the researchers discovered that this bio-fuel can fully replace petroleum-derived fuel [21]. Bin Yang, the professor of WSU's Department of Biological Systems Engineering, describes how this can be a real solution in the future to aromatics, a problematic organic compound in the aviation industry [21]. In fact, according to Joshua Heyne, the current co-director of the WSU-Pacific Northwest National Laboratory Bioproducts Institute, aromatics have contributed more to climate change than carbon dioxide [21]. The way Yang developed this lignin fuel was by transforming lignin from agricultural waste into a bio-based lignin fuel [21]. 

The lignin fuel is certainly a possible choice for sustainable fuel in the future since it is extremely efficient, it is relatively cheap, increases the fuel performance, and reduces greenhouse gas emissions [21]. Now that agriculture has been becoming more sustainable due to lignin fuel, companies can trust this new kind of fuel even more. However, the only real mystery is whether climate change will impact this lignin fuel from lasting longer. As climate change becomes a growing threat to the world, agricultural practices must continue to change in order to satisfy having conditions impacted by climate change, such as extreme droughts. With agriculture becoming sustainable for a longer span of time, manufacturers can not develop the lignin jet fuel.

New Polymer Technology could save lives and help the Environment 
Although up to this point I have focused on biofuels, there are other areas of aerospace engineering that could reduce their environmental impact. Researchers from ETC Zurich, a public research university in Switzerland, discovered a type of laminate that can change colors when the material is worn down [22]. The laminate  consists  of alternating layers of a plastic polymer and artificial nacre, or mother of pearl. The formulation of this polymer makes it very break-resistant and able to  increase its fluorescence the more the polymer is stretched [22]. Over time, researchers can identify overstressed parts of the material.

This concept is extremely helpful for aircrafts since people can easily detect failure of the material of the plane before it breaks down during flight. This new concept could reduce the risk of mechanical failure and crashes, therefore saving human lives and property damage as well as reducing the amount of harmful waste a plane crash can cause. Crashes not only can cause many deaths directly, but also the fuel that escapes into the environment, causes new pollutants that harm the environment. In current aircraft, the parts of the plane that wear out the fastest are more prone to result in catastrophic failures, including the propellers, cylinder fins, and the battery box [22]. Therefore, incorporating laminate on these areas could vastly improve oversight into airplane part replacements.

Future Challenges
Recent research suggests that the aviation industry will progress towards a more environmentally friendly path as there have been many recent strides in reducing harmful emissions and making aerospace more eco-friendly.  As aircraft have advanced significantly since their inception, we are getting closer and closer to the possibility of an airplane that runs on electricity, or is completely emission free/neutral. However, this vision may take decades to fully be completed. As renewable energy is becoming the main source for aviation, engineers can come even closer than ever to creating a more perfect, reliable, and efficient eco-friendly model, from the fuel, to the engine, and to the materials used.

However, there are still many roadblocks ahead in order to achieve this kind of innovation. For one, current eco-friendly approaches covered here are still in their infancy. Additionally, the engineers and aviators have to completely change their designs of current planes to create something that will satisfy our environmental-friendly goals. Furthermore, without a battery capable enough for the plane to function long-term, these kinds of adjustments are really not ready for commercial flight and will, for now, only be for special occasions and testing. Finally, the current most realistic concepts are only hybrids, meaning that there will still be emissions, even if they are significantly reduced.

Conclusions
If we were to look ahead, over the next 100 years, it is very likely that commercial flight with these new innovations will be inevitable. There will also likely be technology advances that we cannot currently foresee. During this stretch of time, flight will likely include much longer flights as energy sources become more efficient and closer to nearly unlimited. This may lead to one day having the potential for a complete autonomous flight.

After all, the history of aviation proves that there is certainly a possibility of a revolutionary innovation that will benefit the environment. Institute of Technological Sciences at Wuhan Universities demonstrated that plasma is a possible choice for a new kind of fuel. MIT researchers and professors developed a new concept for a hybrid-electric model for an aircraft. Researchers from ETC Zurich in Switzerland discovered a type of laminate that can change colors if the material is worn down, which is very helpful to see if the plane needs a remodel. Although these various conceptual upgrades will take a long time to fully achieve and incorporate into popular designs, they are steps that will lead to a final product that will benefit society and the environment.

Acknowledgment
I would like to thank Heather Killeen, Ph.D. for her help in providing feedback while preparing this manuscript.

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