**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 airbreathing 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.
