With the first flight of a low-sounding aircraft imminent, is supersonic travel just around the corner?

The X-59 is a unique experimental aircraft. It is a supersonic aircraft designed to fly at a speed of Mach 1.6-1.8. The key is that it is very "quiet" when breaking the sound barrier. The volume of its sonic boom transmitted to the ground is roughly equivalent to the sound of closing a car door 6 meters away. The X-59 will make its first flight this year. Once it has passed relevant tests, NASA plans to let it fly over selected cities in 2026 to obtain more experimental data. These data may break the limitations of commercial supersonic flight. For the past 50 years, the United States has prohibited civilian aircraft from flying faster than Mach 1 over its land due to the huge interference of sonic booms to ground communities around the routes.

Written by Cui Kai (Director of the Science Communication and Popularization Committee of the Chinese Society of Aeronautics)

On January 12, 2024, NASA officially launched the X-59 low-sonic-boom research aircraft. The aircraft was developed and manufactured by Lockheed Martin under the support of NASA's "Quiet Supersonic Technology (QueSST)" project.

Figure 1: Physical display of X-59 aircraft. Source: NASA

Regarding this aircraft, NASA's press release stated: "The X-59 is a unique experimental aircraft that can not only demonstrate its ability to fly at supersonic speeds, but will also show that it will only produce gentle sounds rather than loud sonic booms." Catherine Bahm, the project leader, said: "The launch of this aircraft is a huge milestone in achieving the overall goal of eliminating sonic booms in the Quesst mission." Bob Pearce, the deputy administrator for aeronautics research at NASA headquarters, declared: "NASA will share the data and technology obtained from this unique mission with regulators and industry. We will also demonstrate its ability to conduct 'quiet' supersonic flights over land and seek to open new commercial markets for American companies to benefit travelers around the world."

With all this publicity, we can't help but ask a series of questions: Why is the X-59 so popular? What can this aircraft do? Why does its name start with X? Does this mean that supersonic aircraft will soon return to commercial aviation after more than 20 years? This article will answer these questions one by one.

1

Why is the X-59 aircraft receiving so much attention?

The X-59 aircraft has rekindled people's memory and yearning for supersonic commercial flight.

Since the Wright brothers invented the first airplane in 1903, the pursuit of flight speed has almost become an eternal theme in the past 100 years. Mach, which is a multiple of the speed of sound, is generally used to measure the speed of an airplane. Currently, the speed of commercial shipping aircraft is lower than the speed of sound (generally between 0.8 and 0.9 Mach). If an aircraft flies faster than the speed of sound, we call it a supersonic aircraft.

The reason why the speed of sound is used as the unit of measurement for flight speed is that during flight, the aircraft will constantly disturb the surrounding airflow, and these disturbances will propagate farther in the air at the speed of sound, so there is a close connection between flight speed and the speed of sound. Therefore, using the speed of sound to measure the speed of an aircraft becomes a reasonable and optimal choice. When the flight speed of an aircraft approaches or exceeds the speed of sound, the air flow around it will change suddenly, and a physical phenomenon called shock wave will occur.

Figure 2 The "diagonal lines" around the plane are shock waves. Source: NASA

The appearance of shock waves will cause a sharp increase in the aircraft's resistance, preventing it from flying faster. Therefore, many people once thought that the speed of an aircraft would never exceed the speed of sound, and even gave this phenomenon a name called "sound barrier."

However, this misconception was soon broken. On October 14, 1947, U.S. Air Force test pilot Chuck Yeager flew the X-1 rocket plane at a speed of Mach 1.06, achieving the first manned aircraft to break the sound barrier.

After that, with the development of jet engines, supersonic flight basically became a standard performance of fighter jets.

In contrast, the speed of civil aircraft has increased much more slowly. So far, there have been only two supersonic civil aircraft in history, one is the Concorde jointly developed by Britain and France, and the other is the Tu-144 developed by the former Soviet Union. The latter is said to have "secretly referred" to many Concorde technologies during its development, so it was also nicknamed "Concordeski". The Tu-144 was a political product in a sense, and there were many problems, so it ended hastily after flying dozens of times. The life of the Concorde was much longer. It was put into commercial service in 1976 and ended its last flight in 2003, with an operating time of 27 years.

Figure 3 Concorde (left) and Tu-144 (right) | Source: Data

The Concorde can fly at speeds exceeding Mach 2, which is faster than the Earth's rotation speed. It only takes more than three hours to fly from New York to Paris. Due to the time difference, the local time when the plane lands is earlier than the local time when it departs on some flights, giving passengers the feeling that they have "arrived before they even set off." It is this unique high-speed flight experience that made the Concorde popular among high-end people at the time.

However, despite the unique experience, the Concorde had a fatal problem in actual operation, which was loss. At that time, British and French airlines lost $40-50 million a year in operating the Concorde. Such a huge burden forced the Concorde to eventually withdraw from the stage of history. Among the reasons for the loss, the most important one was the unresolved sonic boom problem. This was also the main bottleneck restricting the development of supersonic commercial flight.

The emergence of X-59 will likely change this situation and push supersonic civil aircraft back into history at a faster pace.

2

What functions can X-59 achieve?

The X-59 is a supersonic test aircraft designed with a low sonic boom.

Acoustic boom (also called sonic boom) is a unique phenomenon in supersonic flight. As can be seen from Figure 2 above, when flying at supersonic speed, many parts of the aircraft will generate shock waves when interacting with the air. These shock waves not only propagate outward in the air, but also constantly interact and merge, and eventually converge into two stronger shock waves, called the head shock wave and the tail shock wave. Due to the large pressure difference on both sides of the strong shock wave, when these two strong shock waves reach the ground, they will have an impact on various facilities and organisms on the ground. If it reaches the human ear, it will be reflected as two huge explosions.

The first person to pay attention to this phenomenon was a British graduate student named Gerald B. Whitham. He not only analyzed and explained this phenomenon, but also proposed a calculation prediction method through theoretical analysis. In 1956, the U.S. Air Force conducted a sonic boom flight test at Edwards Air Force Base, which was also the earliest known sonic boom flight test. Although the measurement equipment used at that time was relatively simple, the test results were basically consistent with the theoretical analysis, and subsequent flight tests and ground tests also further verified the theory.

In the 1950s, as the inventory and flight test demand of supersonic fighters continued to increase, a large number of aircraft were tested around bases in the United States, and sonic booms began to attract public attention and disgust. According to records, between 1956 and 1968, the U.S. Air Force received a total of about 38,831 claims about sonic booms, of which 14,006 were approved in full or in part. The reasons for the claims were mainly that the sonic booms caused broken glass, damaged plaster walls, and falling objects. These claims also include some strange reasons, such as residents complaining that sonic booms caused the death of their pets or the madness of their livestock.

The sonic boom problem forced the Federal Aviation Administration (FAA) to implement a ban in 1973, prohibiting civil aircraft from flying faster than Mach 1 over the land in the United States. Under this regulation, the routes of the Concorde, which was about to be put into operation, were greatly restricted, and the blow it brought was almost fatal, because the reduction in route options meant a significant reduction in passenger traffic and a significant increase in operating costs.

Because of the importance of this issue, the U.S. government has conducted a large number of studies on it since the 1960s and has continued to this day. The relevant research involves many aspects, including: (1) conducting flight tests to better quantify the relevant fluid dynamics and atmospheric physics phenomena; (2) conducting community surveys to collect public opinion data from the disturbed sample population; (3) establishing and using acoustic simulators to evaluate the response of people and structures; (4) conducting field studies on the possible impacts on livestock and wildlife; and (5) conducting ground experiments and combining comprehensive analysis with test flight data to improve the theoretical structure and establish a mathematical model.

The core of these studies is the aircraft design technology with low sonic boom, that is, using a relatively reasonable shape to change the shock wave shape and strength to reduce the sonic boom. This research process lasted for decades, and the main conclusions finally reached were to reasonably optimize the shape of the aircraft head (such as designing the lower jaw to be similar to the shape of the pelican's beak) and lengthen the head. Both of these nose designs have undergone a lot of computational simulations and wind tunnel tests. At the beginning of this century, these two technologies were flight tested by modifying the F-5E fighter and the F-15B fighter, respectively, to verify their effectiveness in suppressing sonic booms.

Figure 4 An F-5E aircraft with a modified head (left) and an F-15B aircraft with a "Quiet Spike" installed on the head (right) | Source: Data

The X59 is 29 meters long and has a wingspan of 9 meters. From the pictures of the real X-59 released by NASA, it should be a combination of the above two technologies, that is, it adopts a slender head design, and also optimizes the shape of its head, especially the lower jaw. In addition to this obvious feature, the X-59 also adopts a smaller wing area and a larger backward tilt angle (sweep angle), as well as placing the engine on the back of the aircraft, which will help improve the shock wave shape and strength, thereby effectively suppressing the sonic boom. It is said that compared with traditional supersonic aircraft, the sonic boom produced by the X-59 will be reduced from "booming thunder" to "gentle door closing sound".

In order to eliminate sonic booms and reduce air resistance, the nose of a supersonic aircraft needs to be very sharp and long, which will seriously interfere with the pilot's field of vision. To solve this problem, the window in front of the X-59 cockpit is replaced with a high-definition panoramic display, which displays images from the front camera in real time during flight to provide a forward view.

Artistic rendering of the X-59 cockpit screen | Source: NASA

3

Why do airplane model numbers begin with "X"?

The X-59 is a research aircraft.

The military aircraft of the United States were originally numbered independently by each branch of the military, that is, the Army, Navy and Air Force all had their own aircraft naming and numbering rules. As the role of aircraft in combat became more and more important, the types and quantities of aircraft purchased and used by each branch of the military increased significantly, so a certain type of aircraft produced by a manufacturer might be provided to different branches of the military at the same time. In the environment at that time, different numbers were often used for the same aircraft in order to adapt to the rules of different branches of the military, which caused great trouble for management.

In order to change this chaotic situation, in 1962, the three U.S. armed forces unified the numbering method of military aircraft according to the order of the U.S. Department of Defense and with reference to previous rules. This work was specifically responsible for the Air Force and was continuously revised later. According to this rule, the number of a military aircraft is mainly composed of the model code, design code and other codes. The model code is mostly one letter, such as A for attack aircraft, B for bomber, C for transport aircraft, F for fighter, etc. The design code is numbered in chronological order of design time and listed after the model code. In addition, additional information such as modification codes are added according to actual conditions. For example, the F-5E and F-15B mentioned above are named according to this rule.

Almost all aircraft within this numbering framework have clear practical combat use directions, such as attack, bombing, transportation, electronic warfare, etc., and the X series aircraft are the only exception. This type of aircraft is developed only for the purpose of exploring or researching a new technology, and the new technology developed based on this type of aircraft will be transplanted to other actual aircraft for use. The letter X comes from the second letter of the English word "explore", and X generally represents an unknown quantity in mathematics. It is not difficult to understand that all X aircraft are responsible for exploring new technologies and researching unknown problems.

The result of the exploration may be a historic breakthrough. For example, the first X-plane is the X-1, the first manned aircraft that broke the "sound barrier" mentioned above. This aircraft directly promoted the development of supersonic aircraft. Another example is the X-15 hypersonic aircraft developed in the 1950s and 1960s, which flew a total of 199 sorties and achieved the hypersonic flight of a manned controlled aircraft for the first time, accumulating a lot of experience and data for the subsequent development of space shuttles. Since the beginning of the new century, the X-43A and X-51A aircraft have achieved hydrogen fuel and hydrocarbon fuel-powered air-breathing hypersonic flight for the first time, respectively, laying a solid foundation for the development of hypersonic aircraft.

The road to exploration is not always smooth. In history, a large number of X-planes have failed or died midway. A typical example is the X-30 aircraft, whose goal is to build a single-stage orbital aircraft that takes off horizontally from a runway and flies all the way into space. Due to over-optimism in the initial demonstration and failure to fully estimate the huge technical difficulties, the project lasted for ten years and cost billions of dollars, and ultimately ended in failure. However, this project directly led to the rapid development of computer simulation technology, air-breathing propulsion technology and new materials, laying the groundwork for the subsequent success of the X-43A and X-51A.

As the X-59 is given the X-series number, we can know that the purpose of this aircraft is to study the sonic boom problem and verify its related design technology, rather than a prototype of a supersonic civilian aircraft. In fact, the X-59 aircraft is only one of the products developed by the QueSST project, and its flight test is not the whole of this project. From the published information, it can be learned that subsequent flight tests will collect sonic boom information of the X-59 by laying a large number of ground test equipment. To this end, researchers will set up 10 microphone stations in the 30-mile desert near NASA's Armstrong Flight Research Center. The research and development of these equipment is also part of this project.

4

When will supersonic civilian aircraft "return"?

Even if the X-59's flight test is a huge success, it does not mean that the return of supersonic civil aviation is just around the corner.

First of all, the launch of the X-59 only indicates that the development of the aircraft has been completed. According to the development plan of the QueSST project, this only completes the first step of the four phases contained in the project. The remaining three phases of research will be carried out in sequence, including flight verification of sonic boom performance (Phase II); flying over selected American cities and asking residents to share their reactions to the sound made by the X-59 to understand the public acceptance of this technology (Phase III); and providing a complete analysis of community reaction data to US and international regulators for reference in the formulation of new rules for quiet supersonic flight over land (Phase IV). The entire project is expected to be completed in 2030.

Secondly, from the positioning of the X-59 aircraft, it can be known that its function is mainly to verify the low sonic boom design technology, rather than the prototype of a supersonic civil aircraft. According to NASA's plan, it is expected that new noise standards will be proposed at the ICAO Committee on Aviation Environmental Protection (CAEP) meeting in 2031. If approved, it will take about 10 years to develop practical aircraft, and supersonic civil aircraft may be launched as early as around 2040. This will be a relatively long process. In addition, although private companies represented by Boom in the United States have begun to formally develop supersonic civil aircraft and have completed the first flight of its XB-1 aircraft on March 22, 2024 local time, from its appearance, the typical low sonic boom characteristics cannot be seen, and the sonic boom characteristics are not mentioned in its public publicity information. Although the company claims that it has received 130 orders and will officially start commercial operations in 2029, it is still unknown whether it can obtain airworthiness certification by then.

Third, the operation of supersonic civil aircraft involves not only technical and economic aspects, but also a social issue. The resistance of supersonic flight increases dramatically, so fuel consumption also increases accordingly, which will lead to an increase in exhaust emissions. In addition, the flight altitude of such aircraft is generally around 18 kilometers above sea level, which is close to the area with a high density of the ozone layer. Whether it will bring new environmental pollution problems will also be the focus of subsequent research. In the context of increasing attention to environmental protection worldwide, this issue will also have a great impact on the development of supersonic aircraft.

Figure 5 Boom's XB-1 (above) and supersonic civilian aircraft renderings | Source: Boom Technology

However, in addition to the United States, China is also developing supersonic civil aircraft technology. Among the 20 major scientific issues and engineering technical problems officially released by the China Association for Science and Technology in 2019, one of them is "green supersonic civil aircraft design technology." According to relevant experts from the China Academy of Aeronautics, China has made breakthroughs in the core key technologies of green supersonic civil aircraft, such as high-precision prediction of sonic booms and low-resistance low-boom design. Therefore, although it is not possible to give a specific timetable for the return of supersonic civil aircraft, it is believed that in the near future, green supersonic civil aircraft will return to the blue sky and provide humans with faster, more comfortable, and greener travel tools.

References

[1] https://www.nasa.gov/news-release/nasa-lockheed-martin-reveal-x-59-quiet-supersonic-aircraft/.

[2] https://www.space.com/nasa-x-59-quesst-supersonic-jet-unveiled.

[3] Domenic J. Maglieri, Percy J. Bobbitt, Kenneth J. Plotkin, et al. SONIC BOOM, Six Decades of Research. NASA/SP-2014-622, 2014.

[4] Dennis R. Jenkins, Tony Landis, and Jay Miller. AMERICAN X-VEHICLES An Inventory—X-1 to X-50. NASA/SP-2003-4531, 2003.

[5] Zhu Ziqiang, Lan Shilong. Research on supersonic civil aircraft and sonic boom reduction. Acta Aeronautica Sinica, 2015, 36(8): 2507-2528.

[6] Qian Zhansen, Han Zhonghua. Current status and challenges of sonic boom research. Acta Aerodynamica Sinica, 2019, 37(4): 601-619.

[7] https://news.cri.cn/uc-eco/20190701/6eaede1a-6e64-dd98-4ea8-906a43b92a7a.html

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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