Introductory Chapter: The Challenge to Fly Faster and Higher

1. Introduction

Starting from 1970s, commercial aviation has increased passengers’ safety, comfort, and wellness. However, nothing has significantly changed about the length of the journeys since then.

A great improvement about travel speed was achieved by the Concorde and Tu-144 aircraft, when they started their commercial operations during the last two decades of the twentieth century, by flying at twice the speed of sound, exactly at M_∞ = 2.04 or about 2180 km/h.

But the supersonic era of commercial flights ended just in 2003 with the last flight of Concorde. Since then, attention to civilian supersonic flight has been lacking for about two decades. Currently, some US companies are showing renewed interest in supersonic transport for both business and common routes due to the availability of a higher technology readiness level (TRL) than in the past.

High TRLs and strategic plans, however, encouraged several research centers and industrials all over the world to look even further ahead toward the hypersonic flight.

Hypersonic flight is the capability to fly at very high speeds—more than Mach 5 or about 6000 km/h.

At hypersonic speed, we would fly over very long intercontinental routes with trip times of about 60 minutes. Therefore, the availability of a hypersonic aircraft will radically revolutionize the future of civilian transportation.

Potential advantages of hypersonic flight are numerous. Apart from reduced travel times, hypersonic flight will also improve access to space. In fact, hypersonic aircraft are expected to fly horizontally in Earth’s atmosphere, like airplanes, and then proceed directly into orbit from a conventional runway, under the thrust of an air-breathing engine.

Today, much excitement and interest regard hypersonic vehicles. But, after about 60 years from the experimental hypersonic flights of the North American X-15 (see Figure 1), sustained hypervelocity travels are still an open issue of high-speed transportations [1].

Figure 1.

US north American X-15. Courtesy of NASA.

Many aerospace agencies, large industries, and several start-ups are involved in many design activities and experimental campaigns both in wind tunnels and in-flight with full-scale experimental flying test beds and prototypes. Therefore, the dream of flying higher and faster with a hypersonic airplane, thus making hypersonic travel almost as easy and convenient as airliner travel, is increasingly becoming a reality.

Nowadays, flying at hypersonic speed is possible just to rocket launchers, in ascent flight, and unpowered re-entry vehicles both manned and unmanned or capsules entered another planet’s atmosphere. However, some countries are going to plan or have claimed that have been already launched propelled hypersonic weapons even though for testing aims.

2. Hypersonic operative scenarios

Generally speaking, hypersonic vehicles would operate within two scenarios, namely space applications and civilian applications.

2.1 Space applications

The space applications scenario belongs to space activities related to in-orbit activities and space exploration. Usually, hypersonic vehicles of this class are referred to as space planes and are unpowered and very blunt to slow down as much as possible while controlling the aerodynamic heating during re-entry or atmospheric entry (i.e. flying on other planets with atmosphere like Mars). So far, only three space planes have ever successfully flown: NASA’s Space Shuttle, Boeing’s X-37B, and the Soviet Buran, as shown in Figure 2.

Figure 2.

US shuttle orbiter, soviet buran, and US X-37. Not to scale. Courtesy of Giuseppe De Chiara.

Further, other countries are developing their own space plane like China and Europe. Few information about research developments of China is available due to reasons of confidentiality and secrecy. But, European Space Agency (ESA) is expected to fly its own in-development space plane, namely Space Rider, whom maiden flight is expected in 2023, see Figure 3 [2, 3].

Figure 3.

Space rider. Courtesy of ESA.

More mature and nearly ready to flight is the two-stage space plane, namely Dream Chaser, under development by the Sierra Nevada Corporation (SNC) [4]. This space plane will support cargo resupply missions to and from the International Space Station (ISS), by 2022, and astronauts transfer by 2025, as well. With the Dream Chaser, the US will have again a commercial vehicle return from the ISS to a runway landing for the first time since the retirement of the NASA’s space shuttle program (Figure 4).

Figure 4.

SNC dream chaser. Courtesy of Giuseppe De Chiara.

2.2 Civilian applications

Civilian applications refer to slender, low drag aeroshapes that can enable sustained maneuvering flight in the atmosphere by exploiting air-breathing propulsion subsystems, like a scramjet or supersonic combustion ramjet engine. In fact, it is worth noting that the difference between supersonic and hypersonic flights is not a question of just giving more gas to the engines but requires different propulsive subsystems, namely ramjets. Fans which compress the air needed to fuel combustion in conventional turbofans engines would disintegrate at hypersonic speeds. Therefore, other types of thrusters without moving parts (i.e., ramjet) are needed.

Thus, rocket propelled aircraft carry their own reserve of liquid oxygen for combustion, thus giving them autonomy outside the atmosphere, but increase weight and volume.

On the contrary, scramjets provide thrust by exploiting atmospheric oxygen, which is compressed in the engine intakes to the aircraft’s own speed, thus avoiding the use of turbines. This means that hypersonic aircraft must be characterized by very aggressive aerodynamic configurations, like waverider aeroshapes. A typical example of such a hypersonic configuration is provided in Figure 5, where the scramjet engine on the belly side of the aircraft is clearly visible [5].

Figure 5.

Typical hypersonic aircraft [5].

A waverider-like configuration allows exploiting the inevitable formation of forebody shock wave (due to the high speed) to enhance its aerodynamic efficiency while properly feeding the air-breathing engine with incoming air through the aircraft bow shock, see Figure 6.

Figure 6.

Typical illustration to explain how air and fuel mix at supersonic speeds inside a scramjet. Courtesy of NASA [6].

This aircraft is expected to perform a variety of civilian (and military) missions, thus making hypersonic and access to space travels almost as easy and convenient as airliner travels. Therefore, the dream of flying higher and faster with a hypersonic airplane could become reality.

Anyway, the pioneering hypersonic flight was that of the North American X-15 even though it was achieved by exploiting a rocket engine. In November 1961, the X-15 flew at speeds over Mach 6, while on 3 October 1967, in California, an X-15 reached Mach 6.7. In the 1960s, this research program returned with valuable data that is still used in the development of spacecraft and aircraft today [1].

The first prove of a scramjet-powered flight was within the experimental X-43 scramjet program, namely Hyper-X Program, with a successful flight of the Boeing X-43A test bed in March 2004 [6]. The X-43 aircraft is shown in Figure 7.

Figure 7.

The X-43 aircraft. Courtesy of NASA [6].

During the experimental flight, the vehicle flew under its own scramjet power at an airspeed of Mach 6.8, or about 8046 km/h, for about 11 seconds. Then, on November 16, another scramjet-powered X-43A did it again, this time reaching hypersonic speeds above Mach 9.6, or about 10,943 km/h, in the final flight of the X-43A project. Both flights set world airspeed records for an aircraft powered by an air-breathing engine and proved that scramjet propulsion is a viable technology for powering future space-access vehicles and hypersonic aircraft.

Another successful experimental hypersonic vehicle, the Boeing X-51, underwent two successful tests from 2010 to2013. It completed its first unmanned power flight in May 2010, flying at a maximum speed of roughly Mach 5.1 (approximately) at 70,000 feet (Figure 8) [8].

Figure 8.

The X-51 aircraft. Courtesy of NASA [7].

An example of a research program on hypersonic aircraft carried out in Europe is LAPCAT (Long-Term Advanced Propulsion Concepts and Technologies)[9]. It is funded by the European Union and aimed at exploring the path to a hypersonic aircraft for transporting passengers at Mach 8 for antipodal flights, capable of traveling from Brussels to Sydney in 4 hours [6]. LAPCAT has been continued in other initiatives such as STRATOFLY (stratospheric flying opportunities for high-speed propulsion concepts), founded within the H2020 research and innovation program [10]. STRATOFLY investigates the feasibility of high-speed passenger stratospheric flight by taking into account for technological, environmental, and economic factors that allow the sustainability of new air space’s exploitation, drastically reducing transfer time, emissions, and noise, and guaranteeing the required safety levels. In addition, STRATOFLY represents the first step toward future reusable launchers (Figure 9) [10].

Figure 9.

The LAPCAT-MR2 hypersonic cruiser concept [9].

Another example of hypersonic aircraft is provided by Boeing in Figure 10 [11]. It was presented in June 2018 as a concept for a Mach 5 aircraft, a speed chosen because it would allow the use of conventional materials such as titanium. At high speeds, in fact, the aircraft forebody can reach very high temperature due to aerodynamic heating, thus requiring the use of a ceramic heat shield.

Figure 10.

Boeing M = 5 concept aircraft. Courtesy of Boeing [11].

Another promising hypersonic project is that of Hermeus company [8]. It is involved in the development of the Mach 5 aircraft, as shown in Figure 11.

Figure 11.

Hermeus M = 5 concept aircraft. Courtesy of Hermeus [8].

3. Hypersonic flight and vehicle design

Accelerating an aircraft to hypersonic speeds demands huge efforts from both scientific and engineering points of view. Several design issues must be addressed including, for instance, multidisciplinary design optimization, advanced air-breathing propulsion, and thermal control. Therefore, flying at hypersonic speeds demands advanced vehicle design and testing, and creating critical technologies to overcome many related technical challenges, as well.

Several aerospace disciplines are involved in the hypersonic flight. Among others, we have avionics, aerodynamics, thermodynamics, materials technology, guidance and control systems, and propulsion.

One of the most important TRLs relies on the availability of high-temperature materials needed to shield the hypervelocity aircraft against the large aerodynamic heating the vehicle must withstand during flight. Friction with the atmosphere, in fact, exposes the aircraft aeroshape, and especially the aircraft forebody and leading edges, to extremely high temperatures of the order of 1000 K. Therefore, materials and strategies to cope with the high temperature, which threaten to melt and warp the structure, are fundamental.

Another important technology to make hypersonic flight reality is related to advanced propulsive subsystems. To counter the huge wave drag at high speed, reliable, effective, and efficient air-breathing propulsion systems are needed.

Then, also vehicle guidance, navigation, and control (GN&C) is fundamental provided that motion at hypersonic speed is extremely fast and the vehicle behavior in this flight regime must be accurately predicted.

As a result, the hostile environment hypersonic vehicles see in flight suggests that the challenge to routinely fly faster and higher for civilian transportation is too long to overcome.

To address these difficulties, a lot of research efforts are carried out by scientific and industrial international community to ensure that high-speed flight tests are successful. Some of these efforts are summarized in the present book, in terms of numerical and on-ground experimental activities.

Numerical tools, like computational fluid dynamics (CFD), prove to mathematically describe the hypersonic environment and predict the vehicle performance in flight, while on-ground facilities attempt to replicate the extreme environment in order to expose vehicles to the real expected flight conditions. But, even though CFD has made tremendous strides, it is not there yet as a viable replacement for real-world tests. Therefore, the more effective application of computational tools and experimental test facilities takes place when both are applied synergically on the same design analysis in a complementary fashion. For instance, the vehicle behavior is extensively investigated through a lot of cheaper numerical simulations, and the accuracy of numerical results is proved by means of a few expensive experimental test campaigns to compliment computer-based data collecting and to set realistic expectations on the technology.

This is a winning approach to progress toward the demonstration of readiness level of high-speed flight technologies.

Leveraging information and scientific investigation gather in the present book will deliver great benefit and help anyone involved in hypersonics to pursue his/her research aims both as a Ph.D. student and engineer.