نشریه علوم، فناوری و کاربردهای فضایی
پیاپی 2 (پاییز و زمستان 1400)

Properties of brazed joints could be affected by some factors like temperature, time, clearance, surface roughness and alloy elements. Actually they influence on the formation of intermetallic compounds in brazed joints. Since intermetallic compounds are brittle, they considerably degrade the mechanical properties of joints. In this study the mechanical strength and microstructural characterization of AISI 316 brazed joints with BNi2 filler metal in different temperature has been investigated. Brazing temperatures changed from 1050 °C, 1100 °C, 1150 °C and 1200 °C for a holding time of 60 min then, the influence of this variable on the brazing strength were examined. Tensile test samples were evaluated at room temperature and metallography samples and fractured tensile sample scanned by a microscope. The results showed that the higher brazing temperature leads to diffusion of boron element into base metal and less volume formation of intermetallic compound phase in the brazing joint and consequently more tensile strength.

The leaf water content is a specific index for the assessment of the physiological status of the plant based on the water content of the vegetation. This research provides an appropriate model based on the reflectance spectra between 400 and 2500 nm to estimate the leaf water content. We examined 53 different species of the well-known Leaf Optical Properties Experiment and a total of 263 spectral curves were employed in a supervised modelling procedure. In doing so, three different linear models were proposed based on the two different indices and their combination. The first index refers to the ratio of reflectance value in two wavelengths and the second one is the ratio of the derivative of the spectral curve slop in two wavelengths. The experimental results indicate the dependence between the water absorption bands and leaf water content. Finally determination of coefficient for hybrid linear model, which is used both indices, resulted to be 87 percent, indicating a very good fit. Also, the 0.06 relative root mean square error represents the aceptable accuracy in the wáter content modelling.

Fault-tolerant control is one of the important issues in automatic control. The reason for this importance is the probability of fault/failure occurrence in control subsystems (sensor-actuator-system). Direct access to spacecraft is not always possible, therefore fault-tolerant control has become even more important in space systems. On the other hand, due to the necessity for weight reduction in these systems, employing hardware redundancy has limitations. Therefore, analytical redundancy has gained much attention in such systems. In this paper, reference inputs are corrected based an open-loop control command adjustment. Using simulation, it is shown that without reference input adjustment, the controller will not be able to satisfy mission requirements when actuator faults occur. Then, the proposed method is used and the desired requirements are satisfied. The advantage of the proposed method is that, there is no need for taking the first and second derivatives of the reference inputs and it is possible to obtain these inputs using integration. This will prevent computational problems associated with differentiation.

This article discusses the need for research, monitoring and forecasting services related to space weather events and the establishment of a national center to achieve these goals. It starts with an introduction to the growing dependence of human civilization on advanced technologies, including space technology, and with an overview of the big investments and costs incurred in building critical infrastructures on Earth and in space. Then, this paper investigates the vulnerability of these infrastructures to space weather events and the need to monitor and predict these phenomena. In the next section, some examples of monitoring and forecasting systems created by different countries and space organizations are introduced. Also, the need to establish a national center in Iran is proposed. The initial organizational structure and mechanism for the activities and cooperation between this center and other national and international institutions are discussed. Finally, different types of actions and the most important activities that can be carried out in this center are presented.

Due to the unique characteristics of the geo-synchronous orbit and the importance of establishing a satellite in this flying corridor, it is necessary to investigate the effect of environmental disturbances on the orbital elements and to maintain the satellite orbital elements in order to increase the longevity and operation of a satellite in this orbit. A satellite in earth orbit is also always exposed to various environmental disturbances such as earth gravity gradient force, gravity of the moon and sun, solar radiation pressure, and so on. For this reason, it is constantly deviating from its original path and needs to study the effect of environmental disturbances on the orbital elements in order to properly correct the disturbed orbital parameters. To achieve the above goals, in this paper, we try to investigate the effect of the environmental perturbations on the orbital characteristics by simulating the satellite translational dynamic behavior in the presence of environmental disturbances. Then, utilizing the genetic algorithm and fuzzy logic approach, an attempt was made to modify the compensation logic of the orbital elements correction, so that, the satellite may be forced to remain in its limited operational orbital window during the mission lifetime. The proposed method could improve the problem-solving operational effectiveness to maintain the position of the satellite with the criterion of minimizing fuel consumption. The case study simulation results may indicate the capability of the proposed approach in satisfying the performance requirements of the satellite station-keeping maneuver.

The purpose of this paper is to present experimental data for merging two outlet fluid skirts in a dual pressure-swirl coaxial injector. In this study, a dual pressure-swirl external mixing injector was designed, fabricated. Operational characteristics including discharge coefficient and breakup length with injection pressures difference for internal and external injectors were expressed. Utilizing fast shooting, based on backlighting method, the interaction between the two outlet skirts were investigated and the merge performance map was extracted. The merge performance map results indicated, when the pressure difference of the external injector increases from 0.3 bar to 0.95 bar, the pressure difference of the internal injector for the merge to occur increases. Because the effect of the internal injector injection pressure for merge in this area is greater than the external injector injection pressure, the external injector skirt is pulled toward the internal injector skirt. For injection pressures difference of more than 0.95 bar in the external injector, because the effect of the external injector injection pressure for merge is greater than the internal injector injection pressure, the internal injector injection pressure difference is reduced for the merge to occur and the internal injector skirt is pulled toward the external injector skirt.

Magnetometer is one of the main sensors in Attitude Determination and Control Subsystem (ADCS) of Low Earth Orbit (LEO) satellites and since it is operable in all times during an orbital period, it can be utilized in almost all functional modes like detumbling, nadir pointing and orbit transfer. Consequently, the accuracy of magnetometer data and its calibration have vital roles in mission success. In this paper, regarding to the importance of real-time approaches in practical applications, an Extended Kalman Filter (EKF) is used for magnetometer calibration. Then, in order to study the role of magnetometer calibration in attitude estimation (AE) results, calibrated data is employed in the structure of a Multiplicative Quaternion EKF (MQEKF). Finally, a Hardware in the Loop (HIL) test bed which is equipped with a three axis Helmholtz coil and a three degree of freedom platform is utilized to measure the performance of developed algorithms experimentally. In the calibration process, magnetometer parameters are estimated and used in AE filter. The results show that the attitude error decreases gradually and the final accuracy increases.

Achieving a flawless solid composite propellant requires proper processability of its corresponding filled polymer slurry. In other words, the optimal rheological properties of highly filled suspensions after mixing process, ensure its transfer to the mold and complete filling of complex geometries. In this dynamic study, after adding molybdenum disulfide nanoparticles to the model materials, rheological properties of the obtained suspensions have been investigated.In order to study the effect of molybdenum disulfide nanosheets on the rheological behavior of the suspensions, polyethylene glycol matrix and glass beads, as model, with particle distribution of 60 to 103 microns were employed.Single-layer and multi-layer molybdenum disulfide nanosheets with thickness of 50 to 100 nm was obtained after acid washing, oxidation and heat shock of raw molybdenum disulfide. Then, after preparation of suspensions containing 10-40 vol% glass beads, the effect of obtained molybdenum disulfide (less than 0.1 vol% of the matrix) on the rheological properties of the mixture was studied. Results of frequency sweep and temperature sweep tests showed that complex viscosity of the suspensions had a significant decrease with increasing nanoparticles, while simultaneously, storage modulus was fixed and loss modulus was increased. The test also confirmed this. Finally, the dynamic shear flow test demonstrated that the dynamic viscosity also decreased significantly after addition of nanoparticles.

The monopropellant thrusters of the position control system are a requirement for the development and application of satellites and space capsules in space, which are also high-tech and expensive. In this paper, the design and simulation of a pressure-swirl injector with full-cone spray as the fuel injector of a monopropellant thruster were presented. For this injector, internal flow simulation was performed in order to predict its output flow characteristics. These characteristics include spray cone angle, output velocity distribution, mass flow rate, spray pattern, and etc. For this purpose, the VOF fluid volume method was used and the flow turbulence was simulated using the k-eps model. The results of these studies are presented and discussed in detail in the article. This type of injector is actually a combination of jet injector and swirl injector. Jet straight flow in the center of the injector and swirl flow along the injector wall are flowed. Both flow regimes are combined in the swirl chamber and the spray is formed as a full-cone in the injector discharge nozzle. If the ratio of the outlets is selected correctly, the radial and environmental distribution of the liquid jet will be uniform. This injector is preferred to the capillary type (straight flow) and the swirl type. The pressure-swirl injector spray angle is larger than the capillary type, which improves the coverage of the catalyst bed, at the same time, spray angle is not as large as the swirl injector, which enlarges the radial dimensions of the decomposition chamber.

In this paper, the results of the process of analyzing potential failure situations on the operational product of the reaction wheel condition control operator are discussed and the effects of the identified failure situations are eliminated or reduced. The technique of analyzing failure modes and their effects is the first technique in meeting the requirements of reliability in design. In this regard, the block diagram of the functional flow of the reaction wheel is presented for the first time and the dependence of the functions is presented statically in the form of a matrix. To achieve this goal, the different parts of this operation are identified and their failure modes and the cause of failures of each part are determined. Also, the effects of failure of different levels will be determined locally, at the equipment level, at the subsystem level and at the system level. In addition, the way to diagnose failure and deal with the effect of failure is presented and related analysis is performed, which is a quantitative analysis and will determine the parameters of severity of error effect, probability number and criticality number, calculation and critical items. Took. Then, based on the identified critical sections, a list of critical items is also extracted. The information extracted from the analysis of failure modes and their effects, while helping to improve the reliability of the design of the reaction wheel operator, will provide the designer with important data for fault and error management during the test and mission stages.

In this research, the performance of a monopropellant hydrazine thruster in atmospheric conditions has been investigated experimentally. For this purpose, after designing and constructing the thruster according to the functional requirements of the thruster, a test was designed and after that, the desired thruster was tested in atmospheric conditions. The test results show that the tested thruster can generate 2000 pulses with a width of 0.5 seconds and a periodicity of one second with reproducibility. It was shown that the life of this thruster is more than 2000 pulses and the thruster was able to produce very small beats of 3 mNS in reproducibility. Also, comparing the results of the current thruster sample with the experimental results of other thrusters showed how by selecting the appropriate dimensions for the injector, catalyst chamber and nozzle, the characteristics of pressure rise time, minimum impulse, pulse centroid and pressure drop time in the Thruster can be well controlled. Reducing the injector diameter (by keeping the flow rate constant by increasing the injection pressure) reduces the impulse (within a constant pulse width) and increases the pressure rise time. Reducing the dimensions of the catalyst chamber also reduces the increase and decrease time of the pressure, resulting in a smaller pulse centroid.

In this paper, we investigate the orbital characteristics of a constellation consisting of 24 LEO satellites. All the configurations are based on the Walker Delta model, which include single-regulated (one constellation) and double-regulated (intertwined two constellations) models. For the sake of comparability, it is assumed that the orbital period of all satellites is 127 minutes and regarding the maximum altitude of 2000 km, the three eccentricities of zero (circle), 0.19 (with an apogee of 2000 km) and 0.097 (=0.19/2) with the longest presence time on the mission area, are considered. In order to limit the search space, the inclination of all orbits is assumed to be 40 degrees and three values of 320, 340 and 360 degrees are considered for the argument of perigee in elliptical orbits. Assuming the need for at least two satellites observable, basically, it can be found that for the argument of perigee of 320 degrees pertains the best coverage beside the least standard deviation. As expected, the higher eccentric orbits perform better regardless of changes in signal strength. Double-regulated configurations, on the other hand, can be associated with a larger number of satellites in view, which generally is accompanied by higher standard deviations. In essence, a configuration may be chosen only based on the pertaining mission requirement and no one should be regarded as the absolute optimum.

This paper studies the time-optimal 6 degrees of freedom (6DOF) orbital rendezvous maneuver problem for an inertially asymmetric rigid spacecraft with independent attitude and position control actuators. It is assumed that the spacecraft equipped with the thruster actuators and the control forces and torques are generated along the three principal axes of the spacecraft. In order to obtain the time-optimal 6DOF maneuver state and control trajectories, at first, the relative translational and rotational dynamics of the spacecraft are described. Then, the Gauss pseudospectral method is used to solve the time-optimal control problem in the presence of constraints on control forces and torques. Also, the costates are estimated to first-order optimality proof of the obtained solutions. Numerical simulation results show that for the assumed time-optimal 6DOF maneuver problem, the control structure for all of the control forces and torques is ‘bang-bang’. Eventually, the optimality of the obtained solutions is verified by checking the fulfillment of Pontrygain’s minimum principle.

In the present study, the effect of pre-evacuation on starting process of a second throat exhaust diffuser has been investigated experimentally by examining a thrust optimized parabolic nozzle. An experimental setup called high-altitude test facility has been used with compressed air as operating fluid. According to the importance of area ratio parameter (Ad/Ast) of a second throat diffuser, the effect of this parameter variation has been examined on the start-up performance of the nozzle and diffuser. In all of the diffuser geometries, to evaluate the instantaneous performances, the pressure in the nozzle chamber has charged instantly in two modes; with and without pre-evacuation. then, the vacuum chamber pressure and static pressure distribution along the diffuser are measured by a data acquisition system. The results show that pre-evacuation in the test chamber reduces the start-up time of the diffuser by 50 to 60%. In addition, pre-evacuating the test chamber eliminates the destructive transition phenomenon from the flow separation pattern during start-up of the nozzle and diffuser. Also, It is observed that with the narrowing of the diffuser’s second throat duct, the minimum starting pressure of the diffuser increases and eventually flow chocks at the second throat in a certain area ratio.