Advanced Propulsion Research Articles

Numerical modeling of hydrogen mixing in a direct-injection engine fueled with gaseous hydrogen

Hydrogen is considered as one of the most promising options to achieve effective decarbonization of the energy and transportation sectors. As such, it has recently been receiving increasing attention because of its promising potential as an energy carrier for advanced energy and propulsion systems. With a focus on internal combustion engines, direct injection (DI) of gaseous hydrogen during the compression stroke offers great potential for high engine efficiency and specific power while reducing the risk of backfiring and pre-ignition issues. Therefore, many experimental and numerical efforts have recently been dedicated to understanding the physical and chemical behaviors of hydrogen in engine during mixing and combustion. This study focuses on computational fluid dynamics (CFD) modeling of the hydrogen DI process in a hydrogen optical research engine. Under the conditions studied, gaseous hydrogen is injected into the combustion chamber via a centrally located single-hole injector at a pressure of 100 bar. Two configurations, namely low- and high-tumble, are investigated to understand the impact of different in-cylinder flow patterns on the fuel-air mixture preparation. Simulations are carried out using the commercial CFD software CONVERGE. The in-cylinder turbulence is modeled with an unsteady Reynolds-averaged Navier-Stokes (URANS) formulation closed by the renormalization group (RNG) k-ε model. Several numerical methods and model constants, including but not limited to turbulent Schmidt number, are evaluated. The numerical results are systematically compared against experimental measurements of velocity and hydrogen concentration fields on the vertical center plane to assess the performance of the CFD model, unveil the physics of hydrogen mixing, and establish best practices for modeling hydrogen DI under relatively high injection pressure conditions.

Refined MagSail thrust model for preliminary mission design and trajectory optimization

A magnetic sail is an advanced propellantless propulsion system concept for deep space missions, proposed by Dana Andrews and Robert Zubrin about 30 years ago. The thrust is generated by the electromagnetic interaction between the solar wind charged particles and an artificial magnetic field obtained through a large current-carrying coil made of superconducting material. This paper discusses an up-to-date single-loop magnetic sail thrust model based on the numerical and experimental results obtained in the last decade. In particular, a particle model is initially used to evaluate the performance of a reference configuration in terms of thrust components in a body reference frame. Then, some scaling relationships are used to estimate the magnetic sail performance as a function of the design parameters. The proposed thrust model is used to analyze the minimum-time transfer problem between two circular and coplanar heliocentric orbits. Minimum flight times necessary to complete ephemeris-free Earth-Venus and Earth-Mars transfers are calculated as a function of the magnetic sail characteristic size in a parametric way.

Data-Driven Modeling of General Fluid Density Under Subcritical and Supercritical Conditions

Advanced propulsion and power-generation systems often operate under extreme conditions, where thermophysical properties of the working fluids undergo complex variations in a wide range of fluid states, where empirical cubic equations of state could yield substantial errors in density prediction. The present work develops data-driven models for accurate density estimation of general fluids across all thermodynamic regimes. The model starts with the cubic equation of state, whose alpha function is calibrated in a data-driven manner with statistical correction accounting for inherent correlations among training data samples. The developed models are examined for the representative pure substances in aerospace propulsion systems, including oxygen, nitrogen, carbon dioxide, and hydrocarbon fuels. Results show that the model with pressure and temperature as input variables provides consistently superior accuracy over wide ranges of temperatures and pressures, especially in the compressed-liquid region, where the Peng–Robinson equation of state significantly underperforms. The corresponding absolute average relative deviation for the studied substances is below 0.65% at different pressures, compared to 7.16% by the Peng–Robinson equation of state. The model is also extended to examine the density calculations of the selected binary and ternary mixtures, and the consistent result is obtained. The data-driven approach can be adopted to evaluate other thermodynamic properties of fluids and fluid mixtures and characteristics of vapor–liquid equilibrium, and further incorporated into large-scale multiphysics simulations where nonideal gas behavior occurs in the future.

Plasma-assisted deflagration to detonation transition in a microchannel with fast-frame imaging and hybrid fs/ps coherent anti-Stokes Raman scattering measurements

This study examines kinetic enhancement by nanosecond dielectric barrier discharge (ns-DBD) plasma on fuel-lean, Φ = 0.7, dimethyl ether (DME), oxygen (O2), and argon (Ar) premixture during deflagration to detonation transition (DDT) experiments in a microchannel. Nonequilibrium plasma produces active species and radicals as well as fast and slow heating of a mixture to promote ignition due to energetic electrons, ions, and electronic and vibrational excitations. Experiments are conducted to examine the influence of the plasma discharge on the premixture and on the resultant deflagration to detonation transition (DDT) onset time and distance through the use of high-speed imaging and one-dimensional, two-beam, femtosecond/picosecond, coherent anti-Stokes Raman scattering (CARS). A high-speed camera is used to trace the time histories of flame front position and velocity and to identify the dynamics and onset of DDT. The results show that plasma discharge can nonlinearly affect the onset time and distance of DDT. It is shown that a small number of plasma discharge pulses prior to ignition result in reduced DDT onset time and distance by 60% and 40%, respectively, when compared to the results without pre-excitation by ns discharges. The results also show that an increase of number of the plasma discharge pulses results in an extended DDT onset time and distance of 224% and 94%, respectively. Time history of the deflagration wave speed of DME and the analysis of ignition timescale under the choking condition of the burned gas of the deflagration wave suggest low temperature ignition may play a role for DME near the isobaric choking condition of the burned gas and the DDT. Plasma-induced low temperature oxidation of the reactive mixture is assessed via the CO2 to O2 ratio as measured through fs/ps CARS during the gas excitation in discharges. CARS measurements also confirm negligible vibrational and rotational heating of the gas by discharge. The present experiments demonstrate the ability of nonequilibrium plasma to alter the chemistry of DME/O2/Ar premixtures in order to control DDT for applications in advanced propulsion engines.

Recent concept study for cryogenic fluid management to support opposition class crewed missions to Mars

NASA recently completed a mission concept study to evaluate the feasibility and propulsion technology development requirements for reduced travel duration crewed missions to Mars. A high-level goal of the study was to minimize the health impact on the crew caused by the space environment. This was implemented in the study by limiting the crew to a total of approximately-two years of in-space operations and travel time. For the initial mission, the crew would stay about 30 days on the Martian surface. The propulsive demands of such a mission are immense, and the study identified two advanced propulsion options with the potential to meet the mission requirements—both options rely on nuclear fission to provide efficient propulsive energy. One propulsion option was a nuclear electric propulsion (NEP)/Chem Hybrid, with a reactor and energy conversion system powering xenon propellant ion thrusters to provide an efficient, but lower-thrust, push for most of the mission duration. This concept also relied on a liquid oxygen/liquid methane (LO2/LCH4) chemical propulsion stage to provide high thrust for maneuvers while near the Earth and Mars. The second propulsion option was nuclear thermal propulsion (NTP), in which the reactor heats liquid hydrogen (LH2) propellant to expand through a nozzle for thrust at about twice the efficiency of the best chemical propulsion systems. Both vehicle concepts rely on storing large amounts of cryogenic propellant (either LO2/LCH4 or LH2) for multiple years in space without loss, far exceeding state-of-the-art capability. To enable this new capability, the team assumed the use of several advanced cryogenic fluid management (CFM) technologies and analyzed the integrated system performance. This included considering the vehicle-level effects of the size, mass, and power requirements of these CFM elements. Further, the team evaluated the development required to enable such a mission in the mid-2030 s and determined that it was feasible. The paper elaborates on the assumed CFM technologies, provides key analysis results, and illustrates the feasibility of technology development for the proposed solutions to the CFM challenges for each propulsion concept.

Comparative study of hydrogen and kerosene commercial aircraft with advanced airframe and propulsion technologies for more sustainable aviation

Present work performs a comparative study of two medium-range commercial jets with future airframe and propulsion technologies powered by hydrogen and kerosene fuel to achieve a substantial reduction in overall aviation emissions. The study aims to investigate the cumulative effect of different energy networks combined with airframe efficiency gains achieved by aircraft-related technologies to suggest a better option for the potential next-generation commercial aircraft similar to the Airbus A320. Advanced airframe and engine technologies include laminar flow control, active load alleviation, new materials and structures, and ultra-high bypass ratio turbofan engines. Two aircraft with hydrogen and kerosene propulsion systems were sized to compare their performance characteristics, equivalent CO2 emission, and direct operating costs. The design was performed using a multi-fidelity approach and included the effects of future airframe technologies and the hydrogen propulsion system. The design comparison showed a significant contribution of airframe and propulsion technologies in achieving more environmentally friendly aircraft. The green hydrogen option showed a 41–63% reduction in overall emissions compared to the kerosene aircraft depending on flight conditions while the blue hydrogen variant achieved a 21–26% reduction level. A rather optimistic price scenario shall be met to enable an operational benefit of green hydrogen while the blue hydrogen variant has more potential of being economically acceptable by the market. In circumstances when operating costs drive the decision-making more than emissions, kerosene may be a more favorable option as a compromise between emission and costs, given the positive effect of airframe and propulsion technologies.

Predictive model of cooling system for railway electric propulsion: validation of design choices and last mile analysis

For many years the most advanced railway propulsion systems have been based on the use of electric motors. Compared to the other available solutions, these are characterized by greater operating economy and less environmental pollution. In order to operate as efficiently as possible and to guarantee the high standards of electromechanical reliability, these propulsion technologies need adequate cooling systems to operate in controlled temperature conditions. Considering this, the implementation of increasingly accurate and performing simulation models can represent a significant advantage and provide great support in the optimization phase of cooling systems, in terms of reduction of dimensions and weight, rapid and low-cost experimentation, prediction of critical scenarios and choice of the best solution strategies to be taken. In this regard, in the context of the collaboration between the University of Naples Federico II, University of Sannio and the company Hitachi Rail STS S.p.A., several lumped parameter models of power converter cooling systems for railway electric propulsion were developed in the Simcenter AMESim environment. These models allowed to validate specific design choices from thermal and hydraulic perspectives, aimed at higher energy efficiency of the system, and to perform the so-called Last Mile predictive analysis, in order to evaluate the possibility of removing a vehicle from the railway line as consequence of a fault of specific components.

A Comprehensive Review on Fuel Cell UAV Key Technologies: Propulsion System, Management Strategy, and Design Procedure

Unmanned aerial vehicles (UAVs) can be regarded as one of the most emerging technologies. Compared with piloted aerial vehicles, UAVs qualify for higher safety and lower cost, and they are more suitable for dangerous missions. Recently, fuel cell (FC) UAVs have obtained more and more attention due to their relatively higher energy density. However, their propulsion system still needs to be further improved to meet different requirements of performing difficult tasks. This article aims to give a comprehensive review of the key technologies of FC UAVs. First, various architectures of the propulsion system and corresponding advantages and disadvantages are introduced. Then, three kinds of management strategies for FC UAVs are reviewed. Next, some special applications and considered design procedures are summarized. Compared with other review articles, this article tries to cover more aspects of FC UAVs. Since these aspects have a tight relationship with each other, giving a comprehensive review can be more beneficial to us to understand the state-of-the-art development of propulsion systems for FC UAVs. The conclusions show that advanced propulsion architectures, efficient online energy management strategies, and professional design procedures are still in urgent demand in the commercial process of FC UAVs.

Diversity of Physical Processes: Challenges and Opportunities for Space Electric Propulsion

The renewed interest in space exploration has led to the growth in research efforts pertaining to advanced space propulsion systems, including highly efficient electric propulsion systems. Although already tested in space many decades ago and being currently employed on various space platforms and thousands of satellites, these systems are yet to reach their full potential for applications on orbit and in deep space. One specific feature of space electric propulsion is the large diversity of physical processes used in this technology, which is not typical for many other types of propulsion systems used in transport, such as those used by airplanes or automobiles. Various physical processes and mechanisms underpin different electric propulsion technologies and should be integrated to drive the future science and technology of space electric propulsion systems. This opinion article briefly highlights this feature of space electric propulsion and outlines some challenges and opportunities that follow from this diversity.

Impact of alcohol–diesel fuel blends on soot primary particle size in a compression ignition engine

The use of alternative fuels, such as bio-alcohols, in advanced propulsion systems could a feasible strategy to address several of the current issues associated to the use of internal combustion engines fuelled by carbonaceous fossil fuels. Particularly, soot particles, are one of the key pollutants emitted from compression ignition engines. Therefore, the development of soot formation prediction models providing new understanding on the impact of alternative fuels combustion in compression ignition engines become essential for soot mitigation purposes.This study proposes a new semi-empirical model that predicts in-cylinder soot primary particle growth from an engine fuelled with alcohol–diesel fuel blends. The model uses macroscopic experimental measurements of engine parameters such as instantaneous in-cylinder pressure. Furthermore, an empirical correlation is presented predicting the mean soot primary particle size as a function of alcohol–diesel fuel blend properties and fuel/air ratio. The experimental measurement of primary soot particle mean size are obtained from High Resolution Transmission Electron Microscope (HT-TEM) micrographs obtained from soot particles collected via thermophoresis. Overall, the research findings presented in this work contributes to propose environmentally friendly fuel candidates for transportation.

SiC nanograins stabilized Si–C–B–N fibers with ultrahigh‐temperature resistance

AbstractContinuous ceramic fibers with ultrahigh‐temperature stability are in high demand for applications in advanced space propulsion and thermal protection systems. In this study, SiC nanograins stabilized Si–C–B–N ceramic fibers were prepared using chemically modified polyborosilazane via a polymer‐derived method. The fabricated Si–C–B–N fibers exhibited a rather high tensile strength of approximately 1.8 GPa and a high strength retention of approximately 90% after annealing at 2100°C for 0.5 h under a nitrogen atmosphere. The ultrahigh‐temperature stability can be contributed to the presence of thermodynamically stable SiC nanograins and the encapsulation of SiC nanograins by the BN(C) phase and amorphous Si–C–B–N matrix. Our work offers a convenient strategy for preparing Si‐based ceramic fibers with ultrahigh‐temperature stability at beyond 2000°C.

Beyond the Thermodynamic Limit: Template for Second Law Violators

For 150 years the second law of thermodynamics has been considered inviolable by the general scientific com-munity; however over the last three decades its absolute status has been challenged by dozens of theoretical and experimental counterexamples. This study explores commonalities between some of the most potent of these and reveals a common template that involves broken physical-thermodynamic symmetries and reser-voirs of work-exploitable thermal energy stored at system boundaries. Commercially successful second law devices could disrupt the current energy economy and help support a sustainable energy future. This article expands on a talk presented at Advanced Energy Concepts Challenging the Second Law of Thermodynamics, a symposium hosted as part of the 4th Annual Advanced Propulsion and Energy Workshop (22 January 2022).Keywords: second law of thermodynamics, Maxwell’s demon, sustainable energy

Development of the HeliosX mission analysis code for advanced ICF space propulsion

This work presents some of the first results from a recently developed fusion propulsion code which calculates the performance of an advanced spacecraft design. HeliosX is a system integrated programming design tool written in Fortran 95 for the purpose of calculating spacecraft mission profile and propulsion performance for inertial confinement fusion driven designs. This code uses the vehicle configuration input and then calculates the likely mission profile for a given destination target. The key capability under development is the inclusion of the fusion propulsion system and an adequate modelling of its likely energy outputs and how this integrates into the mission profile. The results of a comparison to a concept from the literature are firstly presented for an interstellar flyby probe and then perturbations of this design to include updates and revisions to its input parameters and expected performance. Although the code is still being developed the early results show good potential to enabling a numerical tool for conducting spacecraft performance assessment of fusion based propulsion systems. The preliminary results of modelling different missions are shown for an interstellar target in ∼37–100 years trip times and ∼0.05–0.12c cruise speeds in flyby and rendezvous modes for orbital insertion to a distance ∼4.3–5.9 ly. A description is given for how the baseline concepts are being evolved since the code has been developed in parallel with the design process which aims to design improved vehicle concepts. Ultimately the calculations shown in this paper are working towards the development of a design tool which can facilitate the proper analysis of a range of different spacecraft designs and place them on a consistent modelling basis so that feasibility assessments can be made.

Heat Transfer Enhancement Methods Applied in Energy Conversion, Storage and Propulsion Systems

With the development of energy storage and conversion or advanced propulsion systems, heat transfer enhancement methods have become widely applied [...]

Development of an anisokinetic particle sampling probe for use in a gas turbine engine compressor

Sand and dust particle ingestion is an inevitability for aircraft operating in arid environments. For conventional takeoff and landing aircraft, significant dust can be ingested into the gas turbine powerplant. Helicopters and vertical takeoff and landing (VTOL) aircraft are at especially high risk due to their tendency to blow significant debris into the air during takeoff and landing operations. The present study highlights the development of an anisokinetic particle sampling probe for use in aircraft engines to obtain real-time measurement of ingested particles often present in these harsh environments. Offtake of particles during engine operation in dusty conditions will provide researchers with an improved understanding of particle breakage tendency and component erosion susceptibility. Three foundational studies were conducted to establish a baseline understanding of probe performance: an aerodynamic study, a particle tracking study, and a particle sampling study. These studies were performed using the Free Jet rig at the Advanced Propulsion and Power Laboratory at Virginia Tech. Particles as large as 1.3 mm were sampled at Mach numbers where M = (0.25, 0.70) and yaw angles ranging from 0° to 45° relative to freestream, conditions not previously investigated. Results indicate that the probe operates sub-isokinetically throughout the full range of test conditions and that probe aerodynamic capture efficiency is inversely proportional to both Mach number and yaw angle. However, this efficiency limitation does not notably influence the sampling probe’s ability to capture the test dust of interest. While the presence of the probe in the flow does result in an airflow velocity reduction of up to 55%, due to their relatively large Stokes numbers the particles of interest only experience a decrease of roughly 5%. These results indicate that this probe is capable of providing researchers with valuable particle size and shape information through effective particle sampling at particle sizes (100 μm ≤ dp ≤ 1,300 μm) and speeds (M ≥ 0.25) not previously investigated.

Interstellar Propulsion Using Laser-Driven Inertial Confinement Fusion Physics

To transport a spacecraft to distances far beyond the solar heliosphere and around the planets of other stars will require advanced space propulsion systems that go beyond the existing technological state of the art. The release of fusion energy from the interaction of two low mass atomic nuclei that are able to overcome the Coulomb barrier offers the potential for ∼1011J/g specific energy release and implies that robotic missions to the nearby stars to distances of ∼5–10 ly may be possible in trip durations of the order of ∼50–100 years, travelling at cruise speeds of the order of ∼0.05–0.15 c. Such missions would be characterised with ∼kN-MN thrust levels, ∼GW-TW jet powers, ∼kW/kg-MW/kg specific powers. One of the innovative methods by which fusion reactions can be ignited is via the impingement of laser beams onto an inertial confinement fusion capsule, imploding it to a thermonuclear state. This paper gives an overview of the physics of inertial confinement fusion and the interaction of a laser beam with a capsule to include the simulation of a 1D particle-in-cell code calculation to illustrate the effects. In the application to deep space missions, various spacecraft concepts from the literature are discussed, and the range of values assumed for the pulse frequency, burn fraction and areal density appropriate for the mission are presented. It is concluded that advanced space propulsion via inertial confinement fusion is a plausible part of our future, provided that experimental validation of ignition is on the horizon and numerical models for feasibility concepts are developed to high fidelity and on a consistent basis.

Designing lightweight dual-phase refractory VNbTiSi-based eutectic high-entropy alloys for use at elevated temperatures

The emergence of high-entropy alloys (HEAs) provides a vast compositional space for designing high temperature materials for next generations of advanced propulsion systems. In this study, novel lightweight (about 6.0 g/cm3 in density), refractory (melting point higher than 1630 °C) VNbTiSi-based dual-phase eutectic high entropy alloys (EHEAs) that consist of body-centered cubic (BCC) solid solution and tetragonal-structured silicide phase are designed. The as-cast compressive strength at room temperature (RT) can be as high as 2.4 GPa and the RT fracture toughness achieves 24.58 MPa∙m1/2, higher than many of the as-cast and even directionally solidified Nb–Si based alloys. The tensile yield strength and elongation at elevated temperature (1100 °C) can be above 300 MPa and 10%, respectively. The anomalous eutectic structure with rod-like silicide phase dispersed in the continuous BCC network is revealed, and two kinds of orientation relationships between the two phases are identified: {100}BCC//{110}Silicide (or <100>BCC//<110>Silicide), and <111>BCC//[011]Silicide or <111>BCC//[101]Silicide with no paralleled planes observed. The BCC phase provides ductility both at room and elevated temperatures, where plastic deformation is dominated by the multiplanar glide of long straight screw dislocations, whereas the silicide phase undergoes no plastic deformation and acts as the strengthening phase at elevated temperatures. This study evidences the feasibility of phase-constituent simplification in a complex composition space based on the high entropy strategy and the possibility of optimizing materials on demand.

A method for verifying the model of tank operational pressurization process during development of propellant supply system in liquid rocket engines of launch vehicle propulsion systems

During development of propulsion systems for launch vehicles, in order to verify the design results, standalone developmental tests of propellant supply system components were run in specially designed test setups outfitted with the required instrumentation. As an example of such work, this paper describes the experience of standalone developmental testing of operational pressurization of propellant tanks. During tests in the constructed test setups, heat and mass exchange processes were tried out with simulation of design modes of operational pressurization of propellant tanks filled with oxygen and hydrogen. As a result of these tests, a rationale was provided for selection of working mediums as the pressurization gas, design and characteristics of gas injection into propellant tanks, a method for evaluating the efficiency of the pressurization process. Empirical dependences were obtained that were used as the boundary conditions in the models for practical calculations for refining the modes and evaluating the efficiency of the propellant supply subsystem of propulsion systems. The paper calls attention to the need to use for the purpose of development of the propellant supply systems that involves computer simulations the results of such earlier work when designing advanced rocket propulsion systems. Key words: propulsion system, propellant supply system, development of pressurization process, test setup, pressurization gas, propellant components, heat and mass transfer intensity, process efficiency, liquid surface sloshing, computer simulations.

Pulsed plasma rocket- developing a dynamic fission process for high specific impulse and high thrust propulsion

The Pulsed Plasma Rocket (PPR) is an advanced nuclear propulsion concept that uses a fission-based nuclear power system to rapidly cause a phase change in a fuel projectile from solid to plasma during a pulsed cycle. To create the plasma bursts that provide thrust, a highly moderated Low Enriched Uranium (LEU) projectile can be used in combination with an unmoderated LEU barrel to preferentially heat the projectile. A short section of High Enriched Uranium (HEU) at the barrel base, along with a novel control drum mechanism, allows for controlled and rapid neutron population growth to transition into a plasma state in a fraction of a second. The PPR concept is intended to provide fast transit to and from locations such as Mars, with the engine capable of providing 100,000 N of thrust and 5,000s of specific impulse.The pulsed plasma rocket is a revolutionary new technology that is much more efficient than traditional rocket engines. It creates a plasma field that can be used to generate thrust, allowing spacecraft to travel much faster and farther than traditional rockets. This technology is essential for future space exploration, as it will allow us to travel to other planets and moons in the solar system.

Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Methods based on laser-induced fluorescence spectroscopy are widely used for spatially resolved non-intrusive diagnostics of atomic or molecular densities and velocity distributions in plasma applications. With regard to electric space propulsion, one focus is on the investigation of rare gases such as xenon or krypton, which are currently the favored propellants in gridded ion- and Hall-effect thrusters. For gridded ion engines, diagnostics of neutral atoms is of interest since charge-exchange processes between neutrals and ions are the main driver of accelerator grid erosion, which limits the lifetime of a gridded ion thruster. Extending the capabilities of the advanced electric propulsion diagnostics platform which has been developed by the IOM and partners, single- and two-photon absorption laser-induced fluorescence diagnostics have been set-up recently at our institute. Both experimental set-ups, and as a series of first applications, measurements of krypton neutrals in the plume of the radiofrequency ion thruster RIT-10 (ArianeGroup GmbH), and xenon neutrals within the discharge chamber of a gridded radiofrequency ion source developed at IOM, are presented.