Three types of longitudinal oscillations Pre-MECO POGO 1, Pre-MECO POGO 2 and MECO POGO have been observed in the launches of N-II/H-I vehicles. A Nyquist plot of a mathematical POGO model is used to examine stability properties of these oscillations. Pre-MECO POGO 1 and MECO POGO are generated in the LOX feed system installed with an accumulator. Flow fluctuation due to the LOX pump vibration is the main exciting factor for the former, while the fluctuation of LOX tank bottom pressure the latter. Pre-MECO POGO 2, excited in the vicinity of open-pipe resonant frequency of fuel suction line, is affected by fuel flow fluctuation. Frequency, longitudinal structural mode shape and generalized mass related with each POGO are determined from flight data. And the POGO model with these parameters are shown to represent the whole POGO features of N-II/H-I along flight time.
Stability of cylindrical imploding shocks is numerically studied. It is particularly investigated how disturbed imploding waves change the shape of the front as they propagate. Analyses were performed by the ray-shock theory. The finite difference calculation of conservation equations was also made. It is shown that even small disturbances which are generated at the initiation affect the symmetry of the wave to a large extent. The propagation speed of disturbances relative to the imploding speed, that is the propagation speed along the periphery, also affects the change of shape of the imploding wave front. When the shock wave is locally deformed, the propagation direction of the deformed wave also affects the shock stability. The shock fronts disturbed artificially were visualized by shadow photography and compared with numerical results. Both results showed qualitative agreement.
This paper considers optimal rendezvous maneuvers to bring a spacecraft into close proximity of a target in circular orbit (typically a space station). The motion of the spacecraft relative to the target is described by linearized differential equations. Three optimal control problems are formulated using these equations. Optimal maneuvers minimizing energy, fuel, and time required for transfer are investigated and numerical solutions are shown employing SGRA (the sequential gradientrestoration algorithm).