The transition towards more-electric aircraft (MEA) has intensified the need for energy-efficient environmental control systems (ECSs) that can maintain cabin pressurization, thermal comfort, air quality, and avionics cooling while reducing fuel consumption and emissions. This paper critically reviews studies on bleed and bleedless air cycle systems (ACSs), vapor compression refrigeration systems (VCRSs), low global warming potential (GWP) refrigerants, thermodynamic modelling, and optimization strategies for aircraft ECSs. The reviewed studies indicate that ACS performance is strongly influenced by wheel configuration, operating conditions, and component parameters. Genetic algorithm (GA) based optimization reported in the literature has proven effective in improving ECS performance while reducing experimental effort during system design. In addition, VCRSs using low-GWP refrigerants, such as R1234yf, demonstrate potential as viable alternatives to conventional ACS-based ECSs. VCRS studies have primarily emphasized component validation and ground-based testing, while ACS remains the dominant technology in operational service. Future research should prioritize integrated system-level models that combine ACS and VCRS architectures with advanced control strategies to reduce electric-compressor power consumption. These findings provide guidance for researchers and manufacturers developing next-generation energy-efficient ECS technologies.
As modern launch missions demand ever higher performance, reliability, and reusability, health monitoring of liquid-propellant rocket engines (LREs) has become a cornerstone of propulsion-system safety and efficiency. This paper provides a systematic review of the theoretical foundations, core technologies, and engineering applications of fault detection and diagnosis (FDD) within LRE health monitoring systems (HMS). The discussion spans system architecture; advanced sensing and data acquisition; model-based and data-driven diagnostic algorithms; and the integration of digital twins and artificial intelligence. Recent advances highlight a rapid transition from threshold-based monitoring and post-event analysis toward intelligent, real-time, life-cycle health management, featuring multimodal sensor fusion, hybrid physics–data modeling, and predictive-maintenance frameworks. By examining documented engineering practices and representative research efforts in the United States, China, and Russia, the paper distinguishes demonstrated applications from emerging approaches and assesses their reported benefits and remaining engineering challenges. The paper also analyses current challenges — namely data quality, model generalization, interpretability, and engineering validation — and outlines future directions in autonomous, trustworthy, and adaptive health management enabled by large-scale models and digital twins. These developments are expected to markedly enhance the safety, reliability, and operational economics of next-generation liquid rocket engines.
Rarefied gas flows exhibit complex nonequilibrium and nonlinear transport phenomena that challenge the validity of classical continuum models such as the Navier-Stokes equations with linear constitutive relations obtained from Newton's law of viscosity and Fourier's law of heat conduction. This has motivated the development of various macroscopic nonlinear constitutive models through the moment method, which are regarded as approximate solutions of kinetic equations, offering a physically meaningful and numerically efficient approach at moderate Knudsen numbers. This paper presents a comprehensive review of recent research advancements and ongoing efforts in generalized hydrodynamics theory and the corresponding macroscopic nonlinear constitutive models for high-speed rarefied gas flows, spanning theoretical foundations to engineering applications. First, the theoretical foundations of generalized hydrodynamic equations (GHE) are systematically introduced, followed by a detailed discussion of GHE-based macroscopic nonlinear constitutive models within the generalized hydrodynamics framework. Subsequently, the development of numerical methods for solving these models is briefly summarized, along with the extensions of these models to thermodynamic nonequilibrium and thermochemical nonequilibrium flows. In addition, experimental validation and engineering-oriented computational implementations are also highlighted. Finally, the key challenges and prospects of GHE-based macroscopic nonlinear constitutive models are outlined. The review aims to provide promising alternatives and essential support for researchers to investigate the high-speed rarefied gas flows and to develop next-generation high-speed vehicles.
Inspired by the prevalence of flexibility in natural systems, flexible membrane structures are increasingly integrated into engineering applications due to their lightweight construction, compact deployable designs, large deformability, and ability to interact dynamically with surrounding fluid flows. In bluff-body applications, flexible membranes have emerged as promising passive flow-control devices for mitigating flow separation, modifying vortex shedding, and reducing vortex-induced vibrations (VIV). This review systematically examines the role of flexible membranes in bluff-body flows, with particular emphasis on passive flow-control applications. A brief overview of aerospace systems employing flexible membranes is first presented to establish the aerospace context and highlight the advantages of membrane structures, including lightweight construction, compact stowage, deployability, and large deformability. This is followed by a focused discussion of three major passive flow-control strategies investigated primarily in simplified bluff-body configurations: pressurized membrane afterbodies, flexible splitter filaments, and hairy surface coatings. The existing literature is classified according to geometric configuration and boundary conditions, governing parameters and performance metrics, and reported experimental and numerical findings, enabling direct comparison across different systems. Collectively, the reviewed studies demonstrate that membrane flexibility can substantially alter wake dynamics, suppress or modify vortex shedding, reduce unsteady aerodynamic loading, and improve flow stability without external energy input. Finally, experimental techniques and numerical methods used to investigate fluid–structure interaction (FSI) in flexible bluff-body systems are reviewed, providing a comprehensive overview of current research approaches.
The active control of dynamic stall on helicopter blades, wind turbine blades, fixed-wing aircraft, and flapping-wing flyers is reviewed. Different dynamic stall problems are identified, stall types are defined, control metrics are established, and the parameter space is delineated. For helicopter and wind turbine blades, as well as small unmanned aerial vehicles, the primary objective is generally to eliminate or mitigate the effects of dynamic stall. However, in certain fixed-wing, flapping-wing, and energy-generation applications, the goal is to utilize or exploit dynamic stall effects.A major emphasis is placed on nominally incompressible flows, including dynamic stall management, passive control devices, steady and unsteady active control techniques, scheduled control, iterative learning control, closed-loop control, and the role of dynamic stall control in flapping-wing flight. Significant control authority can be achieved by either suppressing or inducing dynamic stall through low-frequency leading-edge slot blowing on thick airfoils, which is particularly suitable for wind turbine blade load control. High-frequency unsteady dynamic stall control—typically achieved using fluidic, combustion-based, or plasma-based devices—is effective because the resulting stall-controlling leading-edge vortices are generated more than an order of magnitude faster than dynamic stall vortices.Simple reactive control can be achieved through the adaptation of stall warning methods, and a feedforward/feedback control architecture is shown to be both practical and effective for gust alleviation in small vehicles. In flapping-wing flyers, the role of dynamic stall control differs between propulsive and hovering flight. In propulsive flight, active flow control may enhance propulsive efficiency. In hovering flight, a relationship exists between Strouhal numbers associated with insect flight, leading-edge forcing on flat-plate airfoils and low-aspect-ratio wings, and vortex shedding.Under subsonic compressible conditions, dynamic stall is driven by shock-induced separation. In such cases, leading-edge modifications—including vortex generators, discrete wall-normal jets or micro-jet blowing, and dielectric barrier discharge actuation—demonstrate significant control potential. Computational methods play a vital role in advancing the application and evaluation of active control strategies by enabling system-level studies that assess aerodynamic, aeromechanical, energetic, and structural benefits.
