Multi-satellite reconfiguration of a formation flying around the halo orbit of the Sun-Earth restricted three-body system is investigated using a bi-impulse maneuver. We conclude that the Beginning-Ending (BE) method is the most energy-efficient of all bi-impulse methods assuming that the reconfiguration time is short in comparison to the orbital period. With constraints of energy balance and collision a voidance, the multi-satellite reconfiguration problem is converted to a parameter optimization problem to obtain the optimum solution. Within the constraints, energy consumption is distributed more evenly among satellites and collision is avoided. Several numerical examples validate the conclusions. Both energy and time are optimized by introducing a hybrid cost function and the results show that the weight coefficient between time and energy has a large effect the reconfiguration trajectory greatly.
This work investigates relative motion around the artificial Lagrange points. Two control strategies are used to analyze the stability of relative motion around artificial Lagrange points. The relative motion is strictly unstable and further investigation shows that it diverges very slowly in the vicinity of some artificial Lagrange points. A parameter is defined to quantify instability of the relative motion. Then, the pseudo-stable regions of artificial Lagrange points are given when different values of the parameter are treated as the critical value of pseudo-stability. A large pseudo-stable region exists for one of two control strategies—the passive control strategy. This paper discusses the passive stability design of a sail, and both the attitude and orbital dynamics are considered simultaneously. An artificial Lagrange point in the pseudo-stable region is used to validate the design method. The results show that both the attitude and relative motion are pseudo-stable.
This study presents an analysis that seeks to optimize the performance of a LEO-GEO Orbit Transfer System using tethers. We propose a new rotational tether transfer system with an electrodynamic tether (EDT) for orbit transfer from LEO to GEO. The EDT is used for orbit restoration to the initial orbit after release of the payload. The EDT can be propelled without propellant, so the total system mass is expected to be much lower than that of a conventional system with ion thrusters for orbit restoration. In addition, a balance tether is also used as a rotational tether so that the EDT can be attached to the platform, and the center of mass of a balance-tether system is on the platform when the payload is not attached. For transfer from GTO to GEO, we compared three types of transfer system: chemical propulsion; ion thrusters; and a rotational tether. The results of the mission analysis show that the EDT-balance tether hybrid system can reduce total system mass by almost 30% compared to an ion thruster-balance tether combination for the same mission. The results also show that there is an optimum mission interval for each transfer method from GTO to GEO.
Design and synthesis of a nonlinear non-minimum phase supersonic flight vehicle longitudinal dynamics control for g commands output tracking are presented. The non-minimum nature of the resulting input/output pair necessitates using a modified switching manifold in sliding mode control theory. The dynamic sliding manifold is designed to compensate for unstable internal dynamics of the system associated with coupling between the moment generating actuators and aerodynamic forces on the flight vehicle. The method is simple to implement in practical applications and enables the sliding mode control design to exhibit the desired dynamic properties during the entire output-tracking process independent of matched perturbations and accommodates unmatched perturbations. Simulation results are presented to demonstrate the performance, robustness, and stability of the autopilot.
The dynamic behavior of premixed flames propagating in non-uniform velocity fields was investigated to assess the significance of intrinsic instability in turbulent combustion. Two-dimensional unsteady calculations of reactive flows based on the compressible Navier-Stokes equations including a one-step irreversible chemical reaction were performed. A sinusoidal disturbance was superimposed on the velocity field of the unburned gas, and its wavelength was set equal to the linearly most unstable wavelength, i.e. the critical wavelength, at the Lewis number Le=1.0. To investigate the effects of intrinsic instability on the dynamic behavior of premixed flames, we treated two basic types of intrinsic instability, i.e. hydrodynamic instability and diffusive-thermal instability. The dynamic behavior of cellular flames generated both by intrinsic instability and by velocity disturbances was numerically simulated. When Le=1.0, at the initial evolution, we observed a cellular flame whose cell size was equal to the wavelength of velocity disturbances. After that, cells combined together, and one large cell appeared. When Le=0.5, on the other hand, several cells smaller than the wavelength of velocity disturbances were found, and the combination and division of cells were observed. This is because that the size of cells caused only by intrinsic instability is smaller than the wavelength of velocity disturbances. Thus, the dynamic behavior of premixed flames is drastically affected not only by velocity disturbances but also by intrinsic instability. The burning velocity of cellular flames propagating in non-uniform velocity fields was larger than that of planar flames, since cellular flames had larger surface area. The burning velocity became larger as the intensity of velocity disturbances became higher, and the dependence of the burning velocity on the intensity was strongly affected by the Lewis number. The relative significance of intrinsic instability on the evolution of turbulent premixed flames was identified by comparing the growth rate and production rate of flame fronts due to intrinsic instability and turbulence. In the region where the growth rate due to intrinsic instability was larger than that due to turbulence, the dynamic behavior was strongly affected by intrinsic instability. This region includes the combustion conditions of many industrial combustors, and then intrinsic instability plays a significant role in the characteristics of turbulent combustion.
The structure of the flowfield formed by normal sonic injection into a Mach 1.8 air stream was investigated using acetone planar laser induced fluorescence (acetone PLIF). Parametric calculation of fluorescence intensity indicates that it represents molar concentration within ±2.5% error for the present experimental conditions. For the special situation of isentropic flow without any mixing, the Mach number can be deduced from the fluorescence intensity. Mach number distribution from PLIF data agreed well with that obtained from PIV measurement. The visual images of jet trajectories obtained from acetone PLIF are compared with those obtained from Mie scattering and indicate that the acetone PLIF produces a reasonable trajectory, while Mie scattering overestimates the jet penetration.
An optimum method for design of a liquid hydrogen regenerative cooling combustor for the LOX/hydrogen engine was constructed using the author’s previous empirical correlation of C* efficiency and calculation model for combustion characteristics, and the present calculation model for the heat load characteristics for LOX/hydrogen combustion. Using this method, the atomization characteristics of the injected LOX jet, the combustion performance including combustion stability, and the heat load on the combustor were evaluated for LOX/hydrogen upper-stage engines such as the LE-5, RL-10 and HM-7. This method was then applied to the LE-5B engine, which is the derivative engine of the LE-5 and has been used as the second stage of the H-2A launcher, to improve combustion stability and to optimize configuration of the injector and combustor. A reduction of about 30% in chamber length of it with sufficient combustion performance was achieved by such optimization.
An application of a model predictive control (MPC) for a deflection limiting maneuver of a flexible structure is studied. To limit the deflection of a flexible structure to within an allowable range during maneuver, deflection-limiting control (DLC) is employed as the primary control, and MPC is combined with DLC to overcome the disadvantages of DLC, including low robustness to modeling errors and disturbances. Moreover, a simple adaptive method is used to estimate the first modal frequency that dominates the vibration motion of the flexible structure. The effectiveness of the proposed algorithm is evaluated by numerical simulations.
This paper proposes a new method to predict flutter based on the wavelet transform, which transforms time signals into the localized time-frequency domain. A flutter test was conducted in a low speed-wind tunnel to obtain experimental data in the sub-critical region. The prediction technique was applied to the response signals of a low-aspect-ratio wing model subjected to the wind-tunnel turbulence. It was found that the linear extrapolation of a new index defined in the transformed domain could well predict the flutter speed. Other prediction methods were also examined using the same signals to compare predictability.