نشریه فناوری در مهندسی هوافضا
سال پنجم شماره 1 (پیاپی 16، بهار 1400)

In this paper, the potential capacity of asteroids to provide minerals is investigated from technical, economic, and legal points of view. To this end, Near-Earth Asteroids, which are more accessible, are considered as asteroid mining candidates. Reviewing technical issues, as well as economic evaluations for plutonium and gold, are presented. The results prove the economic feasibility of the asteroid mining industry under the assumed conditions. The most critical challenges to realize asteroid mining are not technical or economic, but legal issues. These issues raise uncertainties and hence increase investment risks. Still, Near-Earth Asteroid exploitation is technically possible. With space technology’s maturation in the near future, the demand for minerals on the one hand and their scarcity and increasing excavation costs on Earth, on the other hand, will make many governments and corporates interested in asteroid mining.

This paper investigates the heat transfers and ablation of thermal insulators used in motors and nozzles. The heat and mass transfer equations are considered in two dimensions in a convergent-divergent nozzle. The finite volume method and the implicit method for time dependence have been used to solve these equations. We used the equations finite volume method with implicit formulation for time dependency to solve equations. The reaction equation, which is written in the form of Arrhenius, is solved using the Runge-Kutta method, and the density and the flux of the gas produced at each step are obtained. Also, we represent a model for the rate of recession. The validation of the model is compared with the experimental results in silica-phenolic ablation insulators and shows a good agreement of the simulation results with the experimental observations. After validation of the obtained results. The effect of the blowing correction coefficient has been investigated, the results show that this coefficient is very important for the accurate design of ablation insulators. The blowing correction factor reduces the convection heat transfer coefficient, reduces the surface recession, and thus, reduces the back temperature of the insulation. Therefore, in cases where the thickness of the insulation is significant or the heat of ablation is low. This parameter can be important and affect the final results and thus the final design.

The main purpose of this paper is to provide a regular design process of drag sail for deorbiting a satellite in LEO orbit concerning the mission requirements. In this regard, the design requirements and manufacturing considerations of the drag sail have been studied at first. The paper continued with different atmospheric models and the necessary equations for the initial design of the drag sail. Then, with the help of the Life-Time (LT) tool in STK software, the deorbiting time of several satellites with different mass to surface ratios from two initial altitudes of 700 and 800 km was obtained. In this calculation, three models of static and dynamic atmospheric density: Harris-Priester, Jacchia- Roberts, and MSISE2000, have been used. Usually, these data and pieces of information are using to design the suitable sail specifications (size, shape and, mass) to remove satellites from the Leo orbit. Finally, using the presented equations and provided diagrams, a drag sail is designed to deorbit a 20 kg satellite from an initial height of 800 km with a mass to surface ratio of 0.01 kg/m. The geometry of this sail is selected in a square shape and based on four beams. This geometry is chosen by compromising between the two elements of reliability and flexibility. The simulation results show that the designed sail can meet the mission requirements of the deorbiting system and take the satellite out of orbit within 3.6 years.

This paper presents a study concerning active vibration control of a smart, flexible spacecraft during attitude maneuver using thrusters/ a reaction wheel and piezoelectric patches. The large-angle maneuver and residual vibration of the spacecraft are controlled utilizing an extended Lyapunov-based design (ELD) and strain rate feedback (SRF) theory. The single-axis fully coupled rigid-flexible dynamic of the system is derived applying a Lagrangian approach and Assumed Mode Method (AMM). The system's overall stability, including energetic terms covering a hub, two flexible appendages, PZT sensor/actuator, RW dynamics, and torsional spring, has been proved, and the control law has been derived accordingly. A pulse-width pulse-frequency (PWPF) modulation is used to alleviate the excitations of high-frequency flexible modes. However, due to the fast maneuver, there are still residual vibrations in the system. Hence, the SRF algorithm using PZT is applied to prepare further vibration suppression. The performance of the proposed extended controller is compared to the conventional Lyapunov and pole placement control algorithms. The numerical results for simultaneously large angle attitude and vibration control of a flexible spacecraft through a comparative study verify the merits of the proposed approach.

Experimental and numerical analysis of a gas turbine model combustor is performed for two cases of regular operation, and attached flow (Coanda jet). The flame shape is visualized experimentally to reveal the differences in the flame structure of the two casesThe combustor sound frequency is measured experimentally and is close to the plenum's resonance frequency. Analysis of the measured resonance sound in frequency space reveals two fundamental frequencies for the normal operation and only one for the case of attached flow. After verifying the numerical results for the cold flow of the burner under normal operation, numerical modeling of both cases is performed using a large eddy simulation. Numerical results predict the attachment of the flow (in the form of a Coanda jet) to the dump plane of the burner for a change in a split ratio of flow between the inner and outer nozzles. In the case of flow attachment, it is shown that less negative axial velocities exist at the chamber entrance and outer nozzle instability ceases. In this way, the plenum acts as a normal single neck Helmholtz resonator with one fundamental frequency for resonance when flow adheres to the burner head. ‌‌

In this work, the force-displacement equation and non-linear threedimensional oscillations of a pendant drop are investigated numerically. The presented novel force-displacement function allows following the dynamics of a pendant drop and realizing its elastic behavior. The growth and detachment of drop, which is pending due to gravity from a capillary tip, is considered (assuming high density and high viscosity ratios and immiscible two-phase flows). The three-dimensional multirelaxation time lattice Boltzmann method (MRT-LBM) was used to simulate the growth, detachment, and oscillations of the drop. It was realized that the growing drop shows three different elastic behaviors simultaneously (hardening, linear, and softening). It was shown that the growing drop has oscillatory acceleration (in the direction of flow). The acceleration had a maximum dimensionless amplitude of about 35 and a dimensionless frequency of about 40. It was concluded that drop generates acoustic waves upstream while growing. Although the flow was in the gravity direction, low amplitude transverse oscillations with the dimensionless frequency of 500 to 800 were detected during the drop growth.