Providing of POGO stability of the Cyclone-4M launch vehicle

1Pylypenko, OV, 2Degtyarev, MA, 1Nikolayev, OD, 2Klimenko, DV, 1Dolgopolov, SI, 1Khoriak, NV, 1Bashliy, ID, 2Silkin, LA
1Institute of Technical Mechanics of the National Academy of Sciences of Ukraine and the State Space Agency of Ukraine, Dnipro, Ukraine
2Yangel Yuzhnoye State Design Office, Dnipro, Ukraine
Space Sci. & Technol. 2020, 25 ;(4):03-20
Publication Language: English
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 interaction of the launch vehicle elastic structure longitudinal vibrations with low-frequency processes in the main propulsion system. The developed mathematical model contains a mathematical description of the low-frequency dynamics of the RD 874 main propulsion system (includes four RD 870 engines with oxidizer-rich staged combustion cycle), the launch vehicle structure, and feed lines. As a result of a theoretical analysis of the POGO stability, based on the developed mathematical model, using the Nyquist criterion, it was found that the “propulsion system — rocket structure” dynamic system is unstable with respect to the first mode of the structure longitudinal vibrations at the initial flight time interval (5 s, 70 s). This instability is caused not only by the convergence of the first oscillation frequency of the liquid in the oxidizer feed line and the natural frequency of the first mode of the longitudinal vibrations of the Cyclone 4M launch vehicle structure but also by a significant increase in the (6 Hz — 9 Hz) frequency range of the dynamic gain of the RD 870 engine. It leads to POGO instability of the Cyclone 4M launch vehicle in the indicated interval of the flight time. This pattern of the POGO-instability development was discovered for the first time, and it can be noted as a characteristic feature of this dynamic phenomenon for rocket engines with an oxidizer-rich staged combustion cycle.
              To provide the launch vehicle POGO stability, it is proposed to install POGO suppressor in the oxidizer feed lines of the main propulsion system. A mathematical model of the low-frequency dynamics of a POGO suppressor with a bellows gas-liquid separation was developed, and the suppressor parameters were determined. The approach to determining the POGO suppressor parameters to provide the POGO stability of liquid propellant LVs was developed: a rational choice of the POGO suppressor design parameters was carried out based on the conditions of the amplitude stabilization of the “propulsion system with POGO suppressors — rocket structure” open dynamic system.
Keywords: bellows type POGO suppressor, cavitation phenomena in pumps, dynamic gains of oxidizer-rich staged rocket engine, Nyquist stability criterion, POGO stability of launch vehicle
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