Low Thrust Research Articles

Improved Primary/Secondary Pole Number Combinations for Dual-Armature Linear Switched Flux Permanent Magnet Machines

Due to additional armature windings on the secondary, dual-armature linear switched flux permanent magnet machines (DALSFPMMs) exhibit higher force densities than conventional LSFPMMs. Nevertheless, the existing DALSFPMMs suffer from larger secondary phase numbers because of the prime secondary pole numbers. This article proposes more optional primary/secondary pole number combinations with fewer phase numbers for DALSFPMMs. The machine topology is introduced first, followed by the analysis of phase numbers, which indicates the DALSFPMMs with 8/9 primary/secondary poles (8p/9s) and 9p/8s have fewer phase numbers and need fewer power electronic devices. Then, the operation principle of DALSFPMM is analyzed from the perspective of air-gap field. After that, the electromagnetic performances of four DALSFPMMs with 8p/9s, 9p/8s, 6p/7s, and 6p/5s are investigated by 2-D finite element method, showing the 8p/9s and 9p/8s machines have higher average thrust forces, lower thrust force ripples, and higher thrust force to permanent magnet (PM) volume ratios than the 6p/7s and 6p/5s counterparts. Finally, a prototype machine is manufactured for the experimental validation.

Evaluation of solid-core thermal antimatter propulsion concepts

Energy released during antimatter annihilation can be used to heat a working fluid to directly produce thrust (an “antimatter rocket”) or run a thermodynamic cycle to generate power for conventional electric propulsion systems (antimatter power generation). For a solid reactor core, we evaluate and compare such antimatter propulsion concepts and assess the resulting performance for different propellants (hydrogen, water, methane, and carbon dioxide) and electric propulsion technologies (arcjets, Hall thrusters, and gridded ion thrusters). In contrast to positron–electron annihilation (which only produces gamma rays), complete stopping of relativistic pions released during antiproton–proton annihilation increases the required reactor core size by approximately an order of magnitude, resulting in significantly larger propulsion systems. Because of reactor core material considerations, the maximum temperature and specific impulse achievable in antimatter rockets is limited. By instead operating the core at a lower temperature to drive a closed thermodynamic Brayton cycle, antimatter power generation can run electric propulsion systems with higher specific impulses. Although this reduces the propellant mass needed for a given mission, the combination of thermodynamic cycle efficiency (around 30%) and electric thruster efficiency (typically between 30%–70%) increases the antimatter mass required by one to two orders of magnitude. The lower thrust of electric propulsion systems also leads to longer burn and maneuver times. Considering emerging industry trends, the use of renewable propellants offers large potential benefits to future antimatter propulsion systems that can refuel in space.

Study on the heat transfer characteristics of regenerative cooling for LOX/LCH4 variable thrust rocket engine

It is a challenging task to investigate the regenerative cooling of the variable thrust LOX/LCH4 expander cycle rocket engine. The decreasing methane mass flow rate leads to the two-phase instability in the regenerative cooling channels (RCC) for low engine thrust. In this study, the geometric dimension of RCC with phase-change is developed. Heat transfer cases are studied based on the experimental correlation, which are investigated the heat transfer characteristics of subcritical methane in the RCC. Furthermore, the effect of variable engine thrust on RCC's heat transfer characteristics is analyzed particularly for low engine thrust. The results demonstrate that the gas-side wall temperature (Twg) was stratified due to the different phase-change heat transfer mechanisms. Twg appeared as a local peak value at the throat, which reached a maximum value in the two-phase region. The maximum value of Twg increased from 858.5 K to 863 K with the decrease of the engine thrust in 20–60% RPL. The RCC's temperature rise of 20% RPL was 1.25 times that of 60% RPL (231 K), whereas the pressure drop was 0.72 that of 60% RPL (0.73 MPa). Moreover, the case calculation results could benefit the scheme design and heat transfer analysis of the RCC.

Thrust measurement and thrust balance development at DLR\u2019s electric propulsion test facility

Electric space propulsion thrusters only produce low thrust forces. For the fulfillment of a space mission this implies long thruster runtimes, and this entails long qualification times on ground. For such long testing times, a ground facility requires a vacuum chamber and a powerful pumping system which can guarantee high vacuum over extended times and under thruster gas load. DLR’s STG-ET is such a ground test facility. It has a high pumping capability for the noble gases typically used as propellants. One basic diagnostic tool is a thrust measurement device, among various other diagnostic systems required for electric propulsion testing, e.g. beam diagnostics. At DLR we operate a thrust balance developed by the company AST with a thrust measurement range of 250 mN and capable of thruster weights up to 40 kg. Adversely, it is a bulky and heavy device and all upgrades and qualification work needs to be done in a large vacuum chamber. In order to have a smaller device at hand a second thrust stand is under development at DLR. The idea is to have a light and compact balance that could also be placed in one of the smaller DLR vacuum chambers. Furthermore, the calibration is more robust and the whole device is equipped with a watercooled housing. First tests are promising and showed a resolution well below 1 mN. In this paper we give background information about the chamber, describe the basics of thrust measurement and the development of a new balance.

Auxiliary air pressure balance mode for EPB shield tunneling in water-rich gravelly sand strata: Feasibility and soil conditioning

Machine loads are always high, or soil conditioning is required to be rigorous when the pressure chamber is full of excavated soil during earth pressure balance (EPB) shield tunneling. This study investigates the feasibility and soil conditioning of the auxiliary air pressure balance mode for EPB shield tunneling in water-rich gravelly sand strata to decrease the machine load based on a project in China. It is indicated that the suitable slump value for the mode to avoid muck spewing should be lower than that for the tunneling mode with the pressure chamber fully of excavated soil. The water pressure monitored in the ground around the tunnel experienced rising, falling and stable stages during the EPB shield tunneling of each lining ring. The peak value of water pressure occurred immediately after the EPB machine stopped and decreased quickly during the first 30 min thereafter, followed by a gradual decrease. The time required for the water pressure to again reach its initial value was 70–90 min and did not influence the tunneling behavior of the following lining ring. The monitored ground displacement was much lower than its allowable value. The monitoring results indicated the success of the application of the auxiliary air pressure balance mode with the discharged muck with a slump value of 2–10 cm in the strata, satisfying the controls of ground displacement and advancing speed. However, the higher soil workability induced a lower thrust force and cutterhead torque, more stable cutterhead torque and chamber pressure, and a higher shield advancing speed. Hence, the ideal slump value was determined to be 8–10 cm for the studied project constructed by the EPB shield with the auxiliary air pressure balance mode in the water-rich gravelly sand strata. Finally, future research is recommended for the application of the tunneling mode.

Experimental Validations of Hybrid Excited Linear Flux Switching Machine

Linear Flux Switching Machines (LFSMs) possess the capability to generate adhesive thrust force, thus problems associated with conventional rotatory electric machines and mechanical conversion assemblies can be eliminated. Additionally, the unique features of high force/power density, efficiency, and a robust secondary structure make LFSMs a suitable candidate for linear motion applications. However, deficiency of controllable air-gap flux, risk of PM demagnetization, and increasing cost of rare earth PM materials in case of PMLFSMs, and inherent low thrust force capability of Field Excited LFSMs compels researchers to investigate new hybrid topologies. In this paper, a novel Double-Sided Hybrid Excited LFSM (DSHELFSM) with all three excitation sources, i.e., PMs, DC, and AC windings confined to short moving primary and segmented secondary providing short flux paths is designed, investigated, and optimized. Secondly, unequal primary tooth width optimization and additional end-teeth at all four corners of the primary equip proposed design with balanced magnetic circuit and reduced end-effect and thrust force ripples. Thirdly, the measured experimental results of the manufactured proposed machine prototype are compared with corresponding simulated model results and shows good agreements, thus validating the theoretical study.

Расчетные исследования по оптимизации геометрии камеры сгорания ракетного двигателя малой тяги на газообразных компонентах топлива

Due to the tough requirements for the environmental safety of the space objects operation, the use of methane-based fuel together with oxygen is a promising direction in developing a new generation of rocket and space technology, including low-thrust rocket engines. When developing low-thrust rocket engines running on oxygen-methane fuel, a mathematical experiment helps to identify the determining factors that affect the quality of the working process in the combustion chamber and to make a calculated optimization of the parameters for supplying fuel components to the combustion chamber. This contributes to a better understanding of the physics of the ongoing processes and leads to recommendations for the design of individual components of the combustion chamber. The numerical simulation enables us to optimize the geometry of the combustion chamber in order to obtain the maximum value of the chamber coefficient, which for an isobaric combustion chamber can be equal to the coefficient of the flow complex. This approach can significantly reduce the number of expensive bench tests. The paper introduces a physical and mathematical model of the workflow in the combustion chamber of a low-thrust rocket engine and gives a comparative analysis of the calculation results for various modifications of the original geometry of the low thrust rocket chamber. Recommendations are given for changing the initial geometry of the combustion chamber in order to increase the coefficient of the flow complex while maintaining a satisfactory thermal state of this chamber.

A Novel Consequent-Pole Modular-Mover Linear Permanent Magnet Vernier Machine for Thrust Ripple and Cost Reduction

Recently, linear permanent magnet vernier machine (LPMVM) is attracting more and more attention due to its high thrust density, high efficiency, and simple structure at low-speed operation, which makes it a preferable solution for direct-drive linear motion applications. In order to further reduce the thrust ripple and the cost of LPMVM, a novel consequent-pole modular-mover LPMVM (CM-LPMVM) is proposed and analyzed in this article. Due to the dual-flux-modulation effect, the proposed CM-LPMVM can exhibit a low thrust ripple (∼3.2% of the average thrust) and high thrust density (∼387 kN/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) at a low current density of 3 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , as well as reduce nearly half of the PM usage at the same time. The performance improvement of the proposed CM-LPMVM is verified by the comparison to a regular modular-mover LPMVMs, using the finite-element method. Finally, three prototypes are built and tested. The experimental results match well with the analytical data, showing the superiority of the proposed CM-LPMVM.

Many-objective robust trajectory optimisation under epistemic uncertainty and imprecision

This paper proposes a method to generate trajectories that are optimal with respect to multiple objectives and robust against epistemic uncertainty. Epistemic uncertainty is modelled with probability boxes and trajectories are optimised with respect to the lower expectations on cost functions and constraint satisfaction. The paper proposes an approach to the calculation of the lower expectation using Bernstein polynomials, and an efficient many-objective optimisation of the trajectories. A surrogate model of the lower expectation is combined with a dimensionality reduction technique to contain the computational cost and make the optimisation under epistemic uncertainty tractable. This approach is applied to the design of a rendezvous mission to Apophis with a spacecraft equipped with a low thrust engine. The paper presents both the case in which the thrust and specific impulse are affected by a time dependent uncertainty and the case in which the engine is affected by an outage that reduces the level of thrust at a random time along the trajectory.

Remanufacturing for Circular Economy: Understanding the Impact of Manufacturer’s Incentive under Price Competition

Business organizations all around the globe are looking to expand circular models into their supply chains to harness economic and environmental benefits. Moreover, the act of giving incentives to retailers by the manufacturer is also quite prevalent in the present business environment. These incentives are offered to promote the sales of products of a manufacturer. Therefore, this paper examines the optimal decisions for a dual-retailer closed-loop supply chain (CLSC) in which the manufacturer bestows the credit period to the one retailer (a firm that possesses shallow market penetration and has a higher insistence on the usage of the capital venture), and cash discount to the next retailer (a firm that occupies the market to a greater extent and receives lower thrust on the usage of invested capital) under a non-coordinated system and coordinated systems. This study proposes the mathematical model to determine the optimal decisions of the manufacturer in terms of credit period and cash discount and also compute the optimal decisions of the retailers for their retail prices and order quantities to maximize individual’s profit in the CLSC. Moreover, numerical analysis and sensitivity analysis is performed to get insights into the optimal decisions of the manufacturer and retailers. The results of sensitivity analysis show that credit period and cash discount increases with the rise in price elasticity, and decreases with an increase in cross-price elasticity. The findings also confirm that members of dual-retailer CLSC under coordination and manufacture’s incentive scenario generate higher environmental and economic benefits required to attain sustainability in production and consumption.

Numerical investigation of wind turbine wakes under high thrust coefficient

AbstractWe study wind turbine wakes of rotors operating at high thrust coefficients (CT &gt; 24/25) using large‐eddy simulations with a rotating actuator disk model. Wind turbine wakes at high thrust coefficients are different from wakes at low thrust coefficients. Wakes behave differently at high thrust, with increased turbulence and faster recovery. Lower induction in the wake is achieved because wakes in high‐thrust conditions recover much faster than in normal operating conditions. This enhanced recovery is possible thanks to the turbulence generated in the near wake. We explore the mechanism behind this behavior and propose a simple model to reproduce it. We also propose a Gaussian fit for the wakes under high‐thrust conditions and use it use it to initialize an Ainslie type model within the FAST.Farm framework.

Unique Practice Technique: Introducing the Tarsal Twist Manipulation

Background: High-velocity low-amplitude thrust joint manipulations are interventions with a growing body of evidence supporting their use to manage spine pain; however, evidence is lacking to support joint manipulation to treat lower extremity pain. Cuboid manipulations, the most researched foot manipulation, have been included in the successful management of cuboid syndrome, posterior tibial tendinopathy, and iliotibial band pain. Yet, there are no clear reasons for how or why patients improve after a cuboid manipulation. The proposed mechanisms include biomechanical, neurophysiological, and/or placebo.Technique Summary: This Unique Practice Technique presents a novel tarsal manipulation, the Tarsal Twist, that can improve outcomes through a combination of mechanisms and be provided to patients in the acute, subacute, or chronic phases of healing. The Tarsal Twist is performed with the patient supine. The clinician performs a high-velocity, low amplitude thrust through the calcaneus and first metatarsal in the direction of forefoot inversion, calcaneal eversion, and forefoot abduction.Outcome Measures: While anecdotal evidence from the authors suggests this tarsal manipulation is effective, additional research needs to be performed to identify the true efficacy; therefore, this novel tarsal manipulation should be used in conjunction with the standard of practice.Conclusion: The Tarsal Twist provides clinicians with an additional option for treating patients with foot and ankle dysfunction and challenges the traditional cuboid manipulation technique.

Efficacy of manual therapy interventions in management of lumbar prolapsed intervertebral disc: A pilot randomized controlled trial

Background. This pilot trial reports the initial estimates of the efficacy of manual therapy interventions in lumbar prolapsed intervertebral disc and determines the feasibility and acceptability of full powered “randomized controlled trial” on efficacy of “spinal mobilization with leg movement (SMWLM)”, high velocity low amplitude thrust (HVLA) and neural mobilization (NM) in lumbar PIVD (Prolapsed Inter-Vertebral Disc) and pilot data will be used to perform sample size calculation for full trial. Material and methods. 48 subjects diagnosed lumbar PIVD were randomly distributed into 4 groups. The primary outcomes were feasibility, assessment procedure, retention rate, adherence and acceptability to the intervention. The secondary outcomes measures were pain, disability and straight leg raise (SLR) range of motion. Results. 90 subjects were screened based on selection criteria. Out of them, 50 (55.55%) were eligible. 48(96%) subjects accepted to participate in study. Baseline data of all the groups was similar but post-intervention score were significant when compared the data between the groups. Highest mean change for visual analog scale (VAS), oswestry disability index (ODI) and SLR were found in SMWLM group. No adverse effects were reported by subjects. Results also suggest that the outcome measures were feasible and acceptable and the treatment considered as the beneficial approach. Conclusions. Present study suggests that it is feasible and acceptable to do a fully powered “randomized controlled trial (RCT)” to evaluate the efficacy of manual therapy interventions in management of lumbar PIVD. This study also reveals that manual therapy interventions are effective in management of lumbar PIVD.

Shape-Based Approach for Low-Thrust Earth–Moon Trajectories Initial Design

A robust finite Fourier series (R-FFS) approach is developed for fast generation of Earth–moon trajectories using continuous low thrust. Each component of the position vector is approximated using a finite Fourier series as a function of time; these approximations are then used to design a trajectory that satisfies the equations of motion and the constraints, at discrete points, as well as the problem boundary conditions. The R-FFS method leverages the three-body problem characteristics to achieve all the required plane change without the use of propulsion. The trajectory is divided into phases: escape, intermediate, and capture. The escape phase is further divided into segments. The phase of the trajectory near the L1 Lagrange point is designed first and is always a thrust-free phase. This thrust-free phase is optimized to achieve the required plane change, enabling planar trajectories in the other phases. The initial guess needed by the solver, in the escape and capture phases, is generated using an analytic approximation developed in this paper. The numerical results show that the R-FFS can generate three-dimensional transfers to high lunar orbits, low lunar orbits, and halo orbits, while meeting constraints on the maximum thrust level of the engine.

Initial research on thermal decomposition of 98% concentrated hydrogen peroxide in thruster-like conditions

The current research is focused on investigating the possibility of harnessing the thermal decomposition of highly concentrated Hydrogen Peroxide 98% i.e., utilizing a decomposition process initiated and maintained solely by a temperature source for low thrust propulsion system applications. This early study investigates the influence of the initial temperature of the heater, power input condition and chamber pressure on the behaviour and stability of the prototype thruster. While it is still too early to conclude on the practicality of the research it demonstrates that a stable configuration with good transient start-ups times is indeed achievable.

Low-Thrust Solid Rocket Motors for Small, Fast Aircraft Propulsion: Design and Development

Small, low-thrust, long-burn-time solid propellant rocket motors could provide propulsion for a new class of kilogram-scale, transonic, uncrewed aerial vehicles (UAVs). This paper investigates technological challenges of small, low-thrust solid rocket motors: slow-burn solid propellants, motors that have low thrust relative to their size (and thus have low chamber pressure), thermal protection for the motor case, and small nozzles that can withstand long burn times. Slow-burn propellants were developed using ammonium perchlorate and 0–20% oxamide (burn-rate suppressant), with burn rates of at 1 MPa. Using these propellants, a low-thrust motor successfully operated at a thrust/burn area ratio 10 times less than that of typical solid rocket motors. This kilogram-scale motor can provide 5–10 N of thrust for 1–3 min. An ablative thermal protection liner was tested in these firings, and a new ceramic-insulated nozzle was demonstrated. This paper shows that small, low-thrust solid motors are feasible and presents a baseline design for the integration of such a motor into a small UAV.

Conceptual Study on Low-Melting-Point Thermoplastic Fuel/Nitrous Oxide Hybrid Rockoon

A hybrid rockoon, which is a hybrid rocket launched from a high-altitude balloon, is proposed. The rocket system configuration was studied for single-stage or multiple-stage rockets, and the performance and range safety requirements were considered. The propellants of the hybrid-rocket motor are a low-melting-point thermoplastic fuel and nitrous oxide, which are beneficial, owing to their mechanical properties at cold temperatures experienced in high-altitude environments. Three-dimensional launch trajectory analyses were performed for science missions aimed at sampling cosmic dust levitating in the upper stratosphere. The apogee altitude can be significantly increased by elevating the altitude of the launch point because the problem of low thrust level of the hybrid rocket is solved by increasing the nozzle expansion ratio. Suborbital trajectories with multiple apogee points and orbital missions to extremely low Earth orbits are presented, which provide a long flight path in the upper atmosphere. The sequence of events and flight characteristics of the proposed hybrid rockoon system are discussed, and necessary technological improvements in the structure and propulsion systems are presented.

Preliminary Results of a Hybrid Thermoelectric Propulsion System for a Multirotor UAS with Active Rectifying, Electronic Throttle Control and Supercapacitors

The main drawback of unmanned aerial systems (UAS) is that almost their entire field of application is autonomous in terms of energy. Flights beyond 50 min are nearly impossible when using conventional energy storage systems (lithium-ion polymer or lithium-ion batteries). Several commercial products have been developed using hybrid systems (H-UAS). Although the improvement they have provided is undeniable, H-UAS in the present market are strongly limited by their low thrust vs. weight ratio, which is caused by limited electrical power generation and a non-optimal energy conversion with relatively low efficiencies. This paper reviews these systems to show the preliminary results of a prototype of hybrid generator which state-of-the-art electronics as well as a new approach using a supercapacitor (SC) array are used to save fuel, increase the thrust vs. weight ratio, optimize losses during conversion and prevent the overheating of the internal combustion unit (ICU). Whereas current generators mostly operate with the ICU at a constant speed, delivering maximum power, the presented prototype includes a throttle control system, and the engine works with a variable regime according to the power demand. Thus, fuel consumption is reduced, as well as heating and wear. The lifespan of the engine is also increased, and the time between maintenance operations is lengthened. The designed system provides almost twice the power of the hybrid current generators. The reduction in the RPM regime of the engine is achieved by means of a supercapacitor array that provides the necessary energy to keep the DC output power constant during the engine acceleration when the flight envelope experiences a perturbation or a sudden manoeuvre is performed by the pilot. To obtain maximum efficiency, the diode rectifiers and conventional converters used in the reviewed products are replaced by synchronous converters and rectifiers. The whole system is controlled by means of a FPGA where a specific control loop has been implemented for every device: ICU’s throttle, DC bus converter, charge and discharge of the SC’s array, cooling and monitoring of temperature for the cylinders heads, and on-line transmission, by means of a XBEE™ module, of all the monitored data to the flight ground station.

Root Cause Analysis of In-Situ AP1000 Reactor Coolant Pump Lock Cup Failure

The failure of the large-scale canned motor reactor coolant pump (RCP) of Sanmen Unit 2 caused the unit to shut down in the first cycle of operation. In order to analyze the causes of the RCP fault, based on the fault characteristics and cause analysis methodology, this paper develops the RCP fault root cause analysis method. Through the RCP manufacturing record investigation, on-site operation data evaluation, disassembly and inspection forensics analysis, design analysis and test verification, root cause analysis and evaluation, the root cause of the RCP failure is confirmed that the lower thrust runner lock cup is affected by the resonance flow induced vibration. The initial flaw continues to expand under the action of the resonance and eventually separates. The lock cup fragment makes two holes on the stator can and eventually leads to the RCP failure. The root cause analysis method established in this study can be used for the analysis and treatment of similar complex problems.

An on-line deep learning framework for low-thrust trajectory optimisation

In the preliminary interplanetary mission design stage, a fast low-thrust (LT) transfer cost approximator will improve the mission design efficiency and enable us to design more complex missions. In this study, we propose a Deep Neural Network (DNN) based on-line framework for training approximators for fast low-thrust trajectory optimisation. The online characteristic refers to the ability to continuously adjust the trained DNNs using new LT transfer data from newly found asteroids, which avoids repeated and costly re-training and is particularly useful for mission scenarios where new data are obtained regularly. The framework contains a DNN-classifier and an online-DNN regressor. The Bayesian optimisation (BO) technique is adapted to determine the network structure as well as the feature selection and processing methods. The proposed DNN-classifier significantly improves the LT optimisation convergence rate from 7% to over 98%. The proposed on-line DNN regressor proves to have better generalization ability and scalability comparing to series network structure, giving a mean relative error (MRE) of approximately 0.6% in the test Near-Earth Asteroid (NEA) rendezvous mission scenario. The proposed on-line DNN framework can also be extended to solve other trajectory optimisation problems in other mission scenarios, such as Main-Belt Asteroid (MBA) missions, active debris removal mission (ADR), etc.