A Control Moment Gyro (CMG) is a momentum-exchange torque generator. Compared to other momentum-exchange torque generators, such as reaction wheels, this device has advantages of high output torques and rapid responses. Despite these advantages, singularities and the nonlinear nature of the steering logic of this device have hampered its application in small spacecraft control. Although there have been many studies of singularity avoidance, in practice, attitude settling times of maneuvers using CMG have been slow when trajectories produced by this control method pass through or near the singularities. In this works, we applied Receding Horizon (RH) control to CMG steering to overcome such problems described and obtain fast attitude maneuver times. This is achieved by choosing the difference between the current attitude and the target attitude as a performance index with which to determine controller gains for this control method. The results of numerical simulations show that our method achieves superior attitude maneuvering times to singularity avoidance steering laws when singularities are encountered.
Fish have appeared since Precambrian more than 500 million years ago. Yet, there are still much untamed areas for fish propulsion research. The swordfish has evolved a light thin/high crescent tail fin for pushing a large amount of water backward with a small velocity difference. Together with a streamlined forward-enlarged thin/high body and forward-biased dorsal fin enclosing sizable muscles as the power source, the swordfish can thus achieve unimaginably high propulsion efficiency and an awesome maximum speed of 130 km/h as the speed champion at sea. This paper presents the innovative concepts of “kidnapped airfoils” and “circulating horsepower” using a vivid neat-digit model to illustrate the swordfish’s superior swimming strategy. The body and tail work like two nimble deformable airfoils tightly linked to use their lift forces in a mutually beneficial manner. Moreover, they use sensitive rostrum/lateral-line sensors to detect upcoming/ambient water pressure and attain the best attack angle to capture the body lift power aided by the forward-biased dorsal fin to compensate for most of the water resistance power. This strategy can thus enhance the propulsion efficiency greatly to easily exceed an astonishing 500%. Meanwhile, this amazing synergy of force/beauty also solves the perplexity of dolphin’s Gray paradox lasting for more than 70 years and gives revelations for panoramic fascinating future studies.
Flight management systems have greatly reduced cockpit workloads, but are not capable of calculating new flight plans in real time when flight characteristics vary or when flight trajectories become nonstationary. This paper presents a real-time flight trajectory generator (R-FTG) applicable to emergency landing approaches. First, the R-FTG calculates a preliminary flight path, which consists of an initial turn, a straight-line flight, and a terminal turn. The R-FTG then optimizes the preliminary flight path by using a direct collocation method. In order to give the direct collocation method real-time performance, an idea called stage division is incorporated. Combining the direct collocation and stage division enables real-time generation of near optimal flight trajectories. Additionally, wind effects are considered in the generating process. The R-FTG is evaluated by numerical simulations; calculation results of the R-FTG are compared with those of an offline optimization method, and the calculation results under different bank angle constraints are examined. The calculations for the wind effects are also studied. These results show the effectiveness of the proposed real-time flight trajectory generator.
Experiments on air-water two-phase flows under high-G (high-gravity) conditions were conducted on a rotating platform. Data on air-water flows in a 10-mm diameter pipe were obtained for different void fractions, flow rates and test pipeline positions at 0 G to 2.5 G. The two-phase flow characteristics under high-G conditions were obtained by analyzing the effects of acceleration on flow rate, velocity, void fraction, pressures, and flow resistance. The results show that acceleration dramatically impacts air-water two-phase flow in pipes, especially flow rate, pressure, and pressure drop. The interaction between air and water under high-G conditions becomes severe as do the disturbance and instability between phases. Therefore, the changes in flow parameters are more severe in air-water two-phase pipe flow under high-G conditions.
An adhesive bonding structure around a metallic mouthpiece of a cryogenic composite tank was analyzed with fracture mechanics. The energy release rate was formulated analytically by considering difference in strain energies in tension and bending before and after crack growth based on a simplified mathematical model. The analytical results were compared by finite element method calculations for five example tanks; there was a fairly good match between the analytical and numerical results. This analysis provides a guideline for the initial optimal design of a cryogenic composite tank based on fracture mechanics.
Multi-asteroid tour missions require consideration of the visiting sequences and trajectory optimization for each leg, which is a typical global optimization problem. In this paper, the problem is divided into a multi-level optimization problem. Determination of the visiting sequence plays a key part for a tour mission. In this paper, the energy differences between different orbits and phase differences are used to estimate the energy required for a tour mission. First, this paper discusses the relation between fuel cost for transfer and classical orbit elements difference based on the energy relation of Keplerian orbits and the characteristics of low-thrust spacecraft. Second, the phase difference is combined with the energy difference to achieve the rendezvous energy. In addition, the lower and upper bounds of rendezvous time can be estimated by analyzing the phase difference. Very-low-thrust trajectory optimization problems have always been considered difficult problems due to the large time scales. In this paper, a hybrid algorithm of PSO (particle swarm optimization) and DE (differential evolution) is used to achieve a solution to the energy-optimal tour mission. Based on GTOC-3 (Global Trajectory Optimization Competition), this paper determines the exploration sequence and provides the optimal solution.