**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

## *Hypersonic Vehicles - Applications, Recent Advances, and Perspectives*

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].

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 5.** *Typical hypersonic aircraft [5].*

*Introductory Chapter: The Challenge to Fly Faster and Higher DOI: http://dx.doi.org/10.5772/intechopen.103729*

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**.

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.

**Figure 7.** *The X-43 aircraft. Courtesy of NASA [6].*

**Figure 8.** *The X-51 aircraft. Courtesy of NASA [7].*

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].

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].*

**Figure 10.** *Boeing M = 5 concept aircraft. Courtesy of Boeing [11].*

**Figure 11.** *Hermeus M = 5 concept aircraft. Courtesy of Hermeus [8].*

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.

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**.
