PSI - Issue 71

Sarath Chandran Nair S. et al. / Procedia Structural Integrity 71 (2025) 340–347

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temperature dependent mechanical properties of all the materials of the integrated stage, including the fluid system. Different load cases arising from filling transients till the steady state thermal conditions of the stage with fluid systems are achieved due to ground service conditions of the cryogenic stage are solved. These load cases due to operating conditions of the tanks and fluid lines are arrived at by considering - filling transients of respective tanks, ground, low flow and high flow chilling of feed systems, filling of different fluid systems, steady state thermal conditions, and engine firing conditions. Natural frequency analysis is a prerequisite analysis for carrying out harmonic response analysis. It is carried out for the estimation of natural frequencies, mode shapes, and mass participation for each mode. Results from this analysis give more insight into understanding the dynamic behavior of fluid system. Finite element model of cryogenic stage for a human space mission is shown in fig.2.

Fig. 2. Finite element model of C32 stage used for analysis

All the natural frequencies up to 100 Hz are considered for further harmonic response analysis. The number of natural frequencies below 100 Hz varies from 10 to 30, depending on the fluid system analysed. Harmonic response analysis in all three directions for an input sinusoidal excitation is carried out. It is carried out by assuming the fluid system resonates at its natural frequency and the response/ stress is obtained for that resonating frequency. Number of analysis cases varies depending on the number of natural frequencies below 100 Hz for a fluid system. Vibration levels used for cryogenic upper stage analysis is given in table 1.

Table 1: Sine vibrational level used for harmonic response analysis Frequency Amplitude 5-20 Hz 2.2 mm DA 20-100 Hz 1.8 g

Loads acting on the feed systems are classified into two load categories. First one is ground condition loads, wherein filling transient and steady state conditions of stage and other fluid systems with respect to temperature and pressure are achieved. This ensures the critical load condition of the thermo-structural analysis. The second one is the flight load condition wherein thermal loads correspond to steady state fill conditions of the stage achieved, and flight vibration loads are included. Based on the finite element analysis, finalized the design qualification test (DQT) and flight acceptance test (FAT) stress matrix for cryogenic upper stage for all the polyimide lines as given: Design qualification stress = 2.0 x stress due to thermo-mechanical loads ± stress due to DQT sine vibration level Flight acceptance stress = 1.5 x stress due to thermo-mechanical loads ± stress due to FAT sine vibration level 4. Compliance of polyimide pipes for human space mission As per the system definition and specification of cryogenic upper stage for human-rated space mission compliance criteria for qualification and acceptance of polyimide lines used in cryogenic stage are finailised. Compliance of polyimide pipes for human space mission is given in Table 2.

Table 2: Compliance of Polyimide pipes for human space mission Test condition Factors to be used Qualification stress Higher of

2.0 x stress due to thermo-mechanical loads (ground condition) or 2.0 x stress due to thermo-mechanical loads ± stress due to DQT sine vibration level

Acceptance stress

Higher of