Low-frequency longitudinal (POGO) oscillations of liquid launch vehicles is a phenomenon inherent to almost all liquid rockets. POGO oscillations of launch vehicles can lead to various emergencies: damages of the rocket structure and liquid propellant propulsion system, unacceptable malfunctions of the rocket control system. The use of liquid-propellant rocket engines with an oxidizer-rich staged combustion cycle for the first stage of launch vehicles can introduce a number of features into the POGO stability analysis. First of all, in this case, longitudinal vibrations of launch vehicles can occur due to the low-frequency instability of a liquid propulsion system at frequencies associated with the dynamics of the circuit of the turbopump-gas generator-gas duct. Another feature of these engines is the manifestation of a significant maximum of the module of the engine dynamic pressure gain in the low frequency range (up to 10 Hz), which can lead to POGO instability of the launch vehicle even in the initial part of its flight with significant values of the rocket structure generalized masses for the lower modes of launch vehicle natural vibrations. To predict the POGO stability of the currently designed Cyclone-4M two-staged launch vehicle, the mathematical model of the low-frequency dynamics of the "propulsion system-rocket structure" system has been developed. The model describes the interac
An approach is developed for operability diagnostic of liquid propulsion feeding system of launch vehicle space stages in microgravity conditions in active and passive flight, including multiple start-ups of main engines with minimal levels of tank filling. The approach is based on the method of finite elements, liquid volume method (VOF), technologies of 3D-computer analysis (CAE-systems), and impedance method. As a part of the proposed approach based on mathematical modeling of dynamic processes in the stage propulsion feeding system in microgravity conditions, the slosh motion parameters of the «gas-liquid» interface in propellant tanks are determined together with the free gas bubbles' parameters. At the same time, the effectiveness of the propellant management device is evaluated during stage propulsion system operation. Along the way, the parameters of transient processes in the propulsion feeding system of main engines are calculated during its multiple start-ups and shutdowns, and amplitudes and frequencies of propulsion feeding system are determined. The computed fluid motion parameters and the liquid propellant-free surface shapes showed a good agreement with data obtained in test studies of the motion of the experimental model of the «Centaur» upper stage propellant tank in a «drop tower». The transient process parameters of the space stage feed system showed satisfactory agreement with experimental data obtained in water testing. The developed approach will reduce the amount of testing of developed and upgraded launch vehicle space stages.
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