Propulsion Technology Research Articles

Modelling of Boil-Off and Sloshing Relevant to Future Liquid Hydrogen Carriers

This study presents an approach for estimating fuel boil-off behaviour in cryogenic energy carrier ships, such as future liquid hydrogen (LH2) carriers. By relying on thermodynamic modelling and empirical formulas for ship motion and propulsion, the approach can be used to investigate boil-off as a function of tank properties, weather conditions, and operating velocities during a laden voyage. The model is first calibrated against data from a liquefied natural gas (LNG) carrier and is consequently used to investigate various design configurations of an LH2 ship. Results indicate that an LH2 ship with the same tank volume and glass wool insulation thickness as a conventional LNG carrier stores 40% of the fuel energy and is characterised by a boil-off rate nine times higher and twice as sensitive to sloshing. Adding a reliquefaction unit can reduce the LH2 fuel depletion rate by at least 38.7% but can increase its variability regarding velocity and weather conditions. In calm weather, LH2 boil-off rates can only meet LNG carrier standards by utilising at least 6.6 times the insulation thickness. By adopting fuel cell propulsion in an LH2 ship, a 1.1% increase in fuel delivery is expected. An LH2 ship with fuel cells and reliquefaction is required to be at least 1.7 times larger than an existing LNG carrier to deliver the same energy. Further comparison of alternative scenarios indicates that LH2 carriers necessitate significant redesigns if LNG carrier standards are desired. The present approach can assist future feasibility studies featuring other vessels and propulsion technologies, and can be seen as an extendable framework that can predict boil-off in real-time.

Asymmetric Force Effect and Damage Analysis of Unlooped Segment of Large-Diameter Shield under Synchronous Propulsion and Assembly Mode

This article outlines the merits of the proposed synchronous propulsion and assembly method in the construction of a long-distance tunnel. In traditional shield construction, the stress of the unlooped segment during the assembly process is systematically overlooked. However, in the novel tunnel construction method, the advancing force of the shield directly acts on the unlooped segment, so the safety of the unlooped segment is unknown. In this paper, focusing on the safety of the segment assembly process, the segment interactions and stress concentrations under asymmetric force effects are analysed in detail via a suite of finite element models. The results show that in the synchronous propulsion and assembly mode the segments will rotate inward. A clamping effect will gradually appear during the assembly process, which makes segment deflection decrease and the stress distribution more uniform. Under asymmetrical stress, the damage to longitudinal segments is highly correlated with the types of assembly errors. The damage position that is deflected radially inward will change with the deflection angle, and the outer joint of the segment is the largest. Based on the numerical outputs, guidelines for the application of synchronous propulsion and assembly technology in practical engineering are provided.

Optimal Control of Transfer to Vertical Orbits from Lyapunov Orbits Using Low-Thrust Engine

There are many different families of periodic orbits in the Earth-Moon system, such as Lyapunov orbits, halo orbits, vertical orbits, etc. The establishment of a lunar space station requires a spacecraft to be able to transfer among these orbits. Lyapunov orbits have been used by some missions and are well-studied orbits, while periodic vertical orbits can provide large amplitudes of spacecraft motion outside the plane of the Moon’s motion, which makes it possible to avoid shadowing of the orbits and use them as relay satellite in cislunar space. Modern researchers mainly consider the use of high-thrust engines for transfer. With the development of electric propulsion technology, the use of low, but long-acting thrust for deep space exploration has become especially relevant. This is due to the high specific characteristics of the propulsion systems of this type. In this article, an algorithm has been developed for determining the optimal control with a low-thrust engine for a transfer from Lyapunov orbit to a vertical orbit. The minimum time of flight or the minimum costs of the working body are used as criteria for optimality. In the calculation for solving the two-point boundary value problem of the optimal control theory, the parameter continuation algorithm is used, which allows to gradually get the transfer from some simple results to the final transfer trajectory. The results obtained make it possible to assert that the use of intermediate axial orbits allows the use of propulsion systems with lower thrust levels. In this case, the duration of the flight increases slightly with an almost unchanged consumption of the working body. Moreover, the homotopy method makes it possible to reduce the consumption of working body, while the control of the engine throttling becomes discrete. The results of this study and the algorithms proposed in this article can be used to determine the optimal program control and ballistic design of lunar missions.

Design optimization and off-design performance analysis of axisymmetric scramjet intakes for ascent flight

Scramjet engines are one of the most economical and reliable high-speed airbreathing propulsion technologies that can be used for low-cost satellite launchers and hypersonic atmospheric transportation. For access to space, the flight envelope practically comprises varying freestream conditions and altitude during ascent flight, constituting a complex optimization problem subject to various constraints from design and operation perspectives. This paper presents the results and insights obtained from a multi-objective optimization study of three classes of axisymmetric scramjet intakes, i.e., three-ramp, Busemann-based, and generic intakes represented by smooth Bézier curves, with the same intake mass flow rate for all classes. Optimization is conducted by means of surrogate-assisted evolutionary algorithms coupled with computational fluid dynamics simulations using a Reynolds-averaged Navier–Stokes flow solver for steady-state flowfields. It aims to minimize the intake drag and maximize the compression efficiency at Mach 7.7 at an altitude of 30 km simultaneously. It has been found that generic intakes, which offer greater local shape control, can achieve the highest compression efficiency of 94.8%, the highest total pressure recovery, and the highest flow uniformity at the intake exit. The Busemann-based intakes, on the other hand, can produce the highest static pressure ratio while incurring the lowest drag force. The utility of principal component analysis has effectively reduced the dimensions of the solutions, and has identified clusters of non-dominated solutions according to their characteristics such as geometric attributes, exit flow profiles, and wall property distributions, thereby conducing to obtain further physical insights into the optimal configurations.

Navigating stellar wobbles for imaging with the solar gravitational lens

The solar gravitational lens (SGL) offers unique capabilities for direct high-resolution imaging of faint, distant objects, such as exoplanets. For that purpose, in the near future, a spacecraft carrying a meter-class telescope with a solar coronagraph would be placed in the focal region of the SGL. That region begins at $\ensuremath{\sim}547$ astronomical units from the Sun and occupies the immediate vicinity of the target-specific primary optical axis---the line that connects the center of the target and that of the Sun. Clearly, this axis is not at rest. It undergoes complex motion as the exoplanet orbits its host star, as that star moves with respect to the Sun, and even as the Sun itself moves with respect to the Solar System's barycenter due to the gravitational pull of planets in our Solar System. Although less prominent, other motions exist. An image of an extended object is projected by the SGL into an image plane and moves within that plane, responding to the motion of the optical axis. To sample the image, a telescope must always be on the move, following the projection, with precise knowledge of its own position with respect to the image. We consider the dominant motions that determine the position of the focal line as a function of time. We evaluate the needed navigational capability for the telescope to conduct a multiyear exoplanet imaging mission in the focal region for the SGL. We show that even in a rather conservative case, when an Earth-like exoplanet is in our immediate stellar neighborhood at $\ensuremath{\sim}10$ light years, the motion of the image is characterized by a small total acceleration that is driven primarily by the orbital motion of the exoplanet (its effect on the projected image estimated to be at the level of $\ensuremath{\sim}6\text{ }\text{ }\ensuremath{\mu}{\mathrm{m}/\mathrm{s}}^{2}$, decreasing inversely with distance to a target) and by the reflex motion of our Sun (target independent, contributing at $\ensuremath{\sim}0.2\text{ }\text{ }\ensuremath{\mu}{\mathrm{m}/\mathrm{s}}^{2}$). We discuss how the amplified light of the host star allows us to establish a local reference frame that significantly relaxes navigational requirements for the imaging operations. We conclude that the required navigation in the SGL's focal region, although complex, can be accurately modeled, and a $\ensuremath{\sim}10$-year prospective imaging mission is achievable with the already available propulsion technology.

Cross-Validation of Hybrid-Electric Aircraft Sizing Methods

The number of studies on hybrid-electric aircraft is steadily increasing because these configurations can lead to lower operating costs and environmental impact than traditional aircraft. However, due to the lack of reference data of actual hybrid-electric aircraft, the design tools and results are difficult to validate. This paper analyzes the key points that must be validated when developing or implementing a hybrid-electric aircraft design tool by contrasting the assumptions and results of two independently developed sizing methods. An existing 19-seat commuter aircraft is selected as the baseline test case, and both design tools are used to size that aircraft. The aircraft is then resized under consideration of hybrid-electric propulsion technology. This is performed for parallel, serial, and fully electric powertrain architectures. Finally, sensitivity studies are conducted to assess the validity of the basic assumptions and approaches regarding the design of hybrid-electric aircraft. Both methods are found to predict the maximum takeoff mass (MTOM) of the reference aircraft with less than 4% error. The MTOM and payload-range energy efficiency of various (hybrid-) electric configurations are predicted with a maximum difference of approximately 2 and 5%, respectively. The results of this study confirm a correct formulation and implementation of the two methods and provide a reference data set that can be used to benchmark design tools.

Design of Hybrid-Electric Small Air Transports

The potential benefits of hybrid-electric or full-electric propulsion have led to the proliferation of many concepts over the past decade. The lack of conceptual design methods capable of grasping the effects of the new propulsive technologies is often due to the absence of industrial data supporting the research. In the present work, two objectives are pursued. On the one hand, the introduction of semi-empirical methods for weight estimation based on the data provided by the industrial partners of the ELICA CS2 project. On the other hand, the establishment of generative engineering as a conceptual design tool for propulsive system architecting and fault tolerance analysis. The applications of the proposed conceptual design methods deal with two different years of entrance in service.

Low-Power Laser Sailing for Fast-Transit Space Flight.

Space exploration is of paramount importance to advancing fundamental science and the global economy. However, today's space missions are limited by existing propulsion technologies. Here, we examine the use of laser-driven light sailing for agile Earth orbital maneuvering and for fast-transit exploration of the solar system and interstellar medium. We show that laser propulsion becomes practical at laser powers ≥100 kW and laser array sizes ∼1 m, which are feasible in the near term. Our analysis indicates that lightweight (1-100 g) wafer-scale (∼10 cm) spacecraft may be propelled by lasers to orbits that are beyond the reach of current systems. We discuss material requirements and photonic designs and introduce new figures of merit. We show that lightsails made of silicon nitride and boron nitride are particularly well suited for the discussed applications. Our architecture may pave the way to ubiquitous Earth orbital networks and fast-transit low-cost missions across the solar system.

Computational Analysis of 2D Transition Inlets for Supersonic Capture

Scramjets are the futuristic propulsion technology for supersonic and hypersonic flights. Transition inlets form one of the major technologies to make the Scramjets more utilizable in reality. This paper deals with the 2-dimensional computational analysis of the supersonic capture of the transition inlets using the finite volume approach of ANSYS. The main objective of this paper is to demonstrate the advantage of the transition inlets over the conventional ramp inlets. This study involves sharp-edged ramp, smooth-edged ramp and transition ramp. Simulation of the inlet at supersonic capture has been done using the SST k-omega turbulence model along with a density-based solver with Roe flux. The performance of the inlet is calculated in terms of the total pressure recovery. Along with this the static pressure, Mach number variation through the inlet and the internal reflected shocks at the isolation chamber are studied. The 2D study of the transition inlet is highly important because the effects of the notch can be studied in depth using the 2D models with minimal number of errors. The comparative study of the 2D transition model helps to validate the advantage of the transition ramps over the conventional ramps. Supersonic capture analysis is inevitable as it determines the starting condition of the inlet. In this paper, the analysis is done for the Mach numbers of 2, 3, and 4.

Three-dimensional numerical simulation of multi-physical coupled environment during electromagnetic propulsion

Electromagnetic propulsion technology is a new propulsion technology which uses electromagnetic force to push objects into high or ultra-high speed. However, during propulsion, the armature and launch load are affected by harsh multi-physics environment. The coupled effect was often ignored or over idealized in previous numerical simulations, which leads to large errors between simulation and experiment. In this paper, a three-dimensional multi-physics coupling environment simulation model is established. The distribution of electromagnetic field, structure field and temperature field in armature from static to high speed are effectively calculated. Moreover, the coupled effect between physical fields and the influence of key parameters on the simulation results are revealed. In conclusion, this model can reproduce the physical field distribution of armature and rail during electromagnetic propulsion. The time-varying resistance and inductance and the velocity skin effect (VSE) are the key parameters affecting armature movement. This method reveals a feasible path for the simplification of multi-physical model and the mitigation of extreme environment.

Environmental sustainability of alternative marine propulsion technologies powered by hydrogen - a life cycle assessment approach

Shipping is a very important source of pollution worldwide. In recent years, numerous actions and measures have been developed trying to reduce the levels of greenhouse gases (GHG) from the marine exhaust emissions in the fight against climate change, boosting the Sustainable Development Goal 13. Following this target, the action of hydrogen as energy vector makes it a suitable alternative to be used as fuel, constituting a very promising energy carrier for energy transition and decarbonization in maritime transport. The objective of this study is to develop an ex-ante environmental evaluation of two promising technologies for vessels propulsion, a H2 Polymeric Electrolytic Membrane Fuel Cell (PEMFC), and a H2 Internal Combustion Engine (ICE), in order to determine their viability and eligibility compared to the traditional one, a diesel ICE. The applied methodology follows the Life Cycle Assessment (LCA) guidelines, considering a functional unit of 1 kWh of energy produced. LCA results reveal that both alternatives have great potential to promote the energy transition, particularly the H2 ICE. However, as technologies readiness level is quite low, it was concluded that the assessment has been conducted at a very early stage, so their sustainability and environmental performance may change as they become more widely developed and deployed, which can be only achieved with political and stakeholder's involvement and collaboration.

Design of multiple space debris removal missions using machine learning

Active debris removal (ADR) allows for the disposal of inactive satellites and larger objects, preventing the build-up of space junk and allowing to replace aging agents in a constellation. To make ADR missions more commercially viable, the removal and disposal of multiple debris objects using a single spacecraft are investigated. This paper proposes the use of artificial neural networks (ANNs) to quickly estimate the cost and duration of the transfers to de-orbit a range of debris objects, so that it is possible to identify the optimal sequence of objects which minimizes the cost and/or the duration of the mission, for the maximum number of de-orbited objects. To this end, the ANN is integrated within a sequence search algorithm based on a tree search. The performance of the proposed methodology is assessed by analyzing three distinctive sequences of multiple space debris removals. A near-term low-thrust propulsion technology enables to dispose of up to 13 debris objects within 10 years, when the optimal design parameters are chosen. The use of ANN allows for this solution to be found 26 times faster than current methods, while enabling the selection of faster and less expensive (being the propellant mass required lower) options.

Spray characteristics of the nanoparticle-containing gel propellants by using an improved single-phase nozzle

Due to the high viscosity and complex rheological properties, the atomization problem continues to be one of the key problems of gel propulsion technology. By analyzing the rheological properties of the gel propellants containing nanoparticles, an improved single-phase nozzle was proposed in this manuscript, which added a cone-like structure inside the nozzle. The equivalent cross-sectional area of propellant passage decreases gradually from inlet to outlet, which ensures that the apparent viscosity of gel propellant decreases continuously until it flows out of the nozzle. A series of experiments were conducted to investigate the atomization characteristics of the improved nozzle. A traditional direct current (DC) nozzle was also used for comparison. The results showed that the improved nozzle proposed in this manuscript can effectively improve the atomization fineness and uniformity of the gel propellant containing nanoparticles by efficiently decreasing Sauter mean diameter (SMD) and also reducing the volume fraction of large droplets in the spray field. Meanwhile, the spray cone angle of the improved nozzle was obviously larger than that of the DC nozzle, which was favorable for mixing and evaporation in the thrust chamber.

Overview of Propulsion Systems for Unmanned Aerial Vehicles

Unmanned Aerial Vehicle (UAV) propulsion technology is significantly related to the flight performance of UAVs, which has become one of the most important development directions of aviation. It should be noted that UAVs have three types of propulsion systems, namely the fuel, hybrid fuel-electric, and pure electric, respectively. This paper presents and discusses the classification, working principles, characteristics, and critical technologies of these three types of propulsion systems. It is helpful to establish the development framework of the UAV propulsion system and provide the essential information on electric propulsion UAVs. Additionally, future technologies and development, including the high-power density motors, converters, power supplies, are discussed for the electric propulsion UAVs. In the near future, the electric propulsion system would be widely used in UAVs. The high-power density system would become the development trend of electric UAVs. Thus, this review article provides comprehensive views and multiple comparisons of propulsion systems for UAVs.

Overview on Digital Twin for Autonomous Electrical Vehicles Propulsion Drive System

The significant progress in the electric automotive industry brought a higher need for new technological innovations. Digital Twin (DT) is one of the hottest trends of the fourth industrial revolution. It allows representing physical assets under various operating conditions in a low-cost and zero-risk environment. DTs are used in many different fields from aerospace to healthcare. However, one of the perspective applications of such technology is the automotive industry. This paper presents an overview of the implementation of DT technology in electric vehicles (EV) propulsion drive systems. A general review of DT technology is supplemented with main applications analysis and comparison between different simulation technologies. Primary attention is given to the adaptation of DT technology for EV propulsion drive systems.

Thermal Management System Architecture for Hydrogen-Powered Propulsion Technologies: Practices, Thematic Clusters, System Architectures, Future Challenges, and Opportunities

The thermal management system architectures proposed for hydrogen-powered propulsion technologies are critically reviewed and assessed. The objectives of this paper are to determine the system-level shortcomings and to recognise the remaining challenges and research questions that need to be sorted out in order to enable this disruptive technology to be utilised by propulsion system manufacturers. Initially, a scientometrics based co-word analysis is conducted to identify the milestones for the literature review as well as to illustrate the connections between relevant ideas by considering the patterns of co-occurrence of words. Then, a historical review of the proposed embodiments and concepts dating back to 1995 is followed. Next, feasible thermal management system architectures are classified into three distinct classes and its components are discussed. These architectures are further extended and adapted for the application of hydrogen-powered fuel cells in aviation. This climaxes with the assessment of the available evidence to verify the reasons why no hydrogen-powered propulsion thermal management system architecture has yet been approved for commercial production. Finally, the remaining research challenges are identified through a systematic examination of the critical areas in thermal management systems for application to hydrogen-powered air vehicles’ engine cooling. The proposed solutions are discussed from weight, cost, complexity, and impact points of view by a system-level assessment of the critical areas in the field.

Towards better correlation between optical and commercial spark ignition engines through quasi-dimensional modeling of cycle-to-cycle variability

Internal combustion engines are still the main choice when considering propulsion technology in the transport sector. Spark ignition units offer the advantage of good efficiency with simpler after-treatment systems. Lean operation is a promising strategy that would further improve efficiency, but requires mitigation of cycle-to-cycle variability. Within this context, and given the increasing trend of using simulation based evaluations during engine development, the current work investigated combustion in an optical spark ignition engine through measurements and quasi-dimensional simulation. The possibility of visualizing in-cylinder processes provides unique insight, but also introduces complications with respect to commercial engines. For this reason, quasi-dimensional simulation was applied so as to better understand the factors that induce cycle-to-cycle variability. For the specific case of the investigated engine, cycle-to-cycle measured exhaust air-fuel ratio was found to be directly correlated to variations of engine output. Several routes of incorporating these effects into simulations were evaluated. Introducing a random component in the period of laminar-turbulent flame transition was found to ensure good grounds for simulating peak pressure variability. Indicated mean effective pressure on the other hand was found to depend less on the initial stages of combustion and was strongly correlated to aforementioned variability of exhaust air-fuel ratio.

TECHNOLOGY OF PROPULSION SYSTEM FOR UNMANNED COMBAT AERIAL VEHICLE (UCAV) – A REVIEW

Unmanned Combat Aerial Vehicle (UCAV) is an unmanned aerial vehicle (UAV) that is used for intelligence, surveillance, target acquisition, and reconnaissance and carries aircraft ordnance such as missiles, ATGMs, and/or bombs in hardpoints for drone strikes. These drones are usually under realtime human control, with varying levels of autonomy. Unlike unmanned surveillance and reconnaissance aerial vehicles, UCAVs are used for both drone strikes and battlefield intelligence. Unmanned Combat Aerial Vehicle (UCAV) propulsion technology is significantly related to the flight performance of UCAVs, which has become one of the most important development directions of aviation. It should be noted that UCAVs have three types of propulsion systems, namely the fuel, hybrid fuel-electric, and pure electric, respectively. This paper presents and discusses the classification, working principles, characteristics, and critical technologies of these three types of propulsion systems. It is helpful to establish the development framework of the UCAV propulsion system and provide the essential information on electric propulsion UCAVs. Additionally, future technologies and development, including the high-power density motors, converters, power supplies, are discussed for the electric propulsion UCAVs. In the near future, the electric propulsion system would be widely used in UCAVs. The high-power density system would become the development trend of electric UCAVs. Thus, this review article provides comprehensive views and multiple comparisons of propulsion systems for UCAVs.

Research on Key Technologies of Hypersonic Vehicle

Hypersonic technology is an interdisciplinary research field, which has become the commanding point of new technology in the current aerospace field. Hypersonic vehicle is an important weapon to deal with future war and break through the missile defense system. It has great strategic deterrence significance in the military. This paper introduces the key technologies of hypersonic vehicle, such as integrated aerodynamic configuration, propulsion technology, thermal protection technology, navigation and control technology. The research status, main problems and development trends of various key technologies are summarized.

Analysis on the Development of Wind-assisted Ship Propulsion Technology and Contribution to Emission Reduction

With the issue of peaking carbon dioxide emissions and carbon neutrality increasingly becoming the forefront of international public opinion, changes in the shipping industry are imminent and destined to cause long-term transformation across the entire industry chain, from ship design to maritime operations. Wind energy as a clean energy source has attracted the increasing attention of experts in the shipping field. This paper summarizes the application and development of wind-aided navigation technology for ships represented by rotors, towing kites, wing sails and soft sails, and analyses the constraints of wind-aided navigation technology. The contribution to emission reduction of typical wind-aided navigation methods such as rotors, towing kites, wing sails and soft sails is summarized. Finally, the development suggestions of wind-assisted ship propulsion (WASP) technology are summarized and proposed, which can provide reference for further research and application of this technology.