**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 highspeed transportations [1].

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.

**Figure 1.** *US north American X-15. Courtesy of NASA.*
