Thermo-structural analysis of cryogenic tanks with common bulkhead configuration

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In launch vehicles, cryogenic propulsion stages store liquid oxygen (LOX) at 76 K and liquid hydrogen (LH2) at 20 K, generally in two separate insulated tanks connected through tubular truss components. Consequently, load transfer from the LH2 tank to the LOX tank is very much localized, resulting in a nonoptimal design. This article presents an alternative single tankage design using a common bulkhead (CBH) to enhance the payload capability, which enables maintaining LH2 temperature within a specified temperature when exposed to a temperature gradient. A sandwich insulator using aramid honeycomb embedded with polyimide foam keeps the LH2 temperature within 20 ± 1 K is proposed, based on transient heat transfer analysis for 1000 s. The foam-filled honeycomb core is treated as equivalent foam in the analysis as the thermal conductivity of the core and the foam is quite close. The efficacy of the insulator is established by an experiment to measure the back wall temperature when liquid nitrogen is loaded on the top skin of the panel, and the insulator maintains a temperature gradient of 123 K for 1000 s. A good agreement is obtained between the transient finite element analysis results with experimental data. An externally insulated LOX tank configuration with an optimum length of the skirt–cylinder where the temperature reaches 80 K is arrived at based on slosh, buckling, and thermal analyses. No thermal gradient is found across the thickness of the skirt, while the thermal gradient is observed along the length of the skirt as anticipated. An integrated thermo-structural analysis of the cryo-system is carried out considering temperature-dependent material properties. A positive margin for the skirt is obtained. A payload gain of 366 kg is estimated based on the present study for the new design option with a CBH and skirt as compared to the traditional tubular truss arrangements.