Abstract: The development of prospective inner and outer space economies focuses on the use of bunch of small space vehicles operated as a quasi-single artificial organism. Such economies include the need for using swarms of small satellites providing communication and surveillance services, being a distributed materials production plant in space, or performing research expedition to study the resources and environments of the new worlds. The use of multiple space vehicles performing tasks as a quasi-single system makes the execution of such missions resilient by reducing the failure risks that is higher for the single-vehicled mission, especially performed in deep space. The core technology for operating distributed space systems is propulsion. From a variety of propulsion technologies ranging from the use of the pressurized cold gas to the implementation of laser beams destroying the surface of solid propellants to generate thrust, some stands out for small spacecraft applications. In this work, the summary on the space-operated propulsion is provided by highlighting the impetus of more frequent use of one technology over other. The discussion on the trends in propulsion is supported by the discussion on the physical, engineering, production, operational, and societal rationales overview. This review serves as the mean for reevaluating of the global propulsion trends and guiding the future inner and outer space propulsion assisting economies effective development.

Abstract: Traveling wave ultrasonic motors (TWUMs) are critical components in precision systems, their performance is susceptible to degradation under dynamic disturbances in harsh operating environments. This paper presents a monolithic U-shaped rotor designed to intrinsically achieve quasi-zero stiffness (QZS). Unlike conventional QZS systems that rely on assembling discrete positive and negative stiffness elements, the proposed design generates the target mechanical characteristic through the tailored nonlinear response of a unified U-shaped structure, thereby improving preload stability. Through exploring the critical parameters of the rotor cross-section, the finite element method (FEM) is employed to optimize the geometry configuration and characterize the mechanical performances. Simulation results show that the QZS behavior, demonstrating a stable force plateau of 320 ± 10 N across a 0.7 mm displacement range. A maximum von Mises stress of 788 MPa is obtained, well within the material's safety margin, thereby ensuring the structural integrity. Experimental tests validate the effectiveness of the proposed design. This compact, monolithic U-shaped rotor provides a robust and reliable QZS solution, demonstrating significant potential for enhancing the stability of TWUMs in applications prone to harsh environments such as extreme high and low temperatures, thermal cycling conditions, shock environments.