Preface

Movement is a critical aspect in the lives of almost all creatures, including humans, birds, and insects. Plants that cannot move by themselves live with the help of movable living creatures. Without movement, creatures would not be able to sustain their species in the future. However, movement requires a certain type of force.

To move from one place to another, work must be performed by the action of a force, and typically, energy is consumed while performing the work.

First, we would like to describe the movement of human beings and materials. Originally, movement was only enabled by the legs and arms of humans; thus, the range of work or moving distance was not significantly large. As time passed, tools and devices for transportation were developed. In the early stages, small vehicles such as bicycles were developed; however, the origin of the force was still attributed to human effort. Later, the application of heat sources as the origin of the force to enable further travel and transport of heavier materials was considered. Steam engines were invented first and then gasoline engines were developed. A considerable amount of effort has been devoted to developing several types of engines, for example, in the category of jet engines. In addition, many types of heat sources have been investigated.

Jet engine types include turboshaft engines and fan engines such as turboprop, propfans, and unducted engines. Propellants as heat sources have grown in size with the conversion of heat energy to mechanical energy. Moreover, the desired travel distance has been increasing, from travel on Earth to travel in space. The large weights of heat sources prevent the realization of travel to the stars. A breakthrough in these difficulties has been eagerly awaited, and a new approach to the realization of travel is expected. Currently, the generation of propulsions is considered a serious issue for long-distance travel.

Propulsion plays an important role in a wide variety of applications. All creatures, such as humans, birds, insects, and fish, are closely related to the movement in their own lives. Additionally, their movement is closely related to the generation of forces for their sustenance. Therefore, the topic of propulsion is crucial when considering the evolution of creatures. However, this topic has a wide range of applications. Thus, it is difficult to cover all the applications of propulsion in such a small book. Accordingly, we have selected typical examples from among many important applications.

Chapter 1 presents a summary of the book, while Chapter 2 explains the advantages of hybrid rockets using solids and liquids. These rockets were developed for ascent vehicles for travel to Mars. This chapter also presents the design of a test setup for hybrid motor firing. Moreover, the proposed hybrid propulsion system is demonstrated to provide sustainable, safe, and low-cost systems for space missions. Chapter 3 presents the proposal of new technology. According to Einstein, inertia and gravitation represent two sides of a coin. An accurate understanding of the role of inertia in general relativity may lead to a breakthrough in advanced propulsion.

Therefore, a rest mass fluctuation can be used for propulsion when inertia is gravitationally induced. Recent work supported by National Innovative Advanced Concepts Phase 1 and Phase 2 NASA grants can be made to work. The most general statement of the principle of relativity is captured in Einstein's equivalence principle and the gravitational induction of inertia. Recent design innovations have significantly increased the thrust produced by these Mach-effect gravitational assist (MEGA) impulse engines. Chapter 4 explains the recent development of an engine using plasma called the Hall thruster. Rocket engines can be of two types depending on the fuel source, namely, chemical and electric rockets. Electrostatic thrusters are useful for launching small satellites in low Earth orbitals and are capable of firing for long time intervals. Space scientists are interested in developing thrusters based on electric propulsion technology from the perspective of cost reduction. This chapter provides a general overview of the technology of electric propulsion and its applications. Plasma-based electric propulsion technology has been used for space missions with regard to spacecraft station keeping, rephrasing, and orbit topping applications. Chapter 5 describes the special low noise engine thrust management at take-off and approach for supersonic civil airplanes. The assessment of noise levels at certification reference points is conducted by considering two engine noise sources, namely, the fan and exhaust jet. Finally, by considering a simple model of a thin plate, Chapter 6 discusses the generation of force caused by unsteady motion. In such unsteady motions, vortex generation plays an important role. This mechanism is considerably different from that of the forces produced by the momentum change based on the combustion of the propellant. Furthermore, "virtual momentum" is vital in understanding the mechanism of force generated by the movements, such as the travel of insects and birds through fluids.

As mentioned, in addition to human travel, insect and bird flying is the minimum movement required for living. For a deeper understanding of the mechanism of force generation, further research is required. Besides the analyses of motion in 2D, an analytical approach towards those of the motion in 3D will be expected to be carried out. Viscous effects on force generation are also a theme in this field. Moreover, we intend to further investigate the development of powerful engines with more efficient devices.

**Kazuo Matsuuchi**

Professor Emeritus, University of Tsukuba, Japan
