PSI - Issue 71

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

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hydrogen and liquid oxygen are used as propellants. These propellants are filled and stored in separate tanks. Liquid hydrogen – the fuel - is stored at 20 K, whereas the liquid oxygen – the oxidiser – is stored at 77 K. The propellants flow to the stage propellant tanks, and engine and other systems are managed by means of various fluid systems like propellant filling system, ullage gases vent system, tanks pressurisation system, etc. These fluid systems consist of pipelines of different diameters and lengths with proper support brackets. The cryogenic fluids cause the propellant tank to shrink significantly, which in turn loads the pipeline and brackets. To prevent from these excessive loading, flexibility in the system is provided by using pipelines made of special material called polyimide, which has high coefficient of thermal expansion and low modulus of elasticity compared to conventional stainless-steel material. Polyimide is a composite material used in cryogenic applications along with stainless steel end adaptors. Polyimide pipe is a layered structure consisting of multiple layers of polyimide film. Metallic end adaptors are provided at the both ends of the pipe for welding the pipes to the tanks and other lines. A detailed literature survey was carried out for polyimide material, its process, and qualification/acceptance for cryogenic rocket application. The mechanical and thermal properties of polyimide material are given by Schwartzberg et al. (1970). Physical and thermal properties, mechanical properties, dimensional stability, thermal aging, electrical properties, chemical properties, nominal construction, safety and handling are given in detail by Kapton (2022), who is one of the manufacturers and suppliers of polyimide material. Construction of polyimide pipes for cryogenic rocket application and its NDT methods used for qualification of polyimide pipes for rocket launch vehicle application are explained by Harish Reddy et.al (2016), Sekhar et. al (2016), Madheswaran et. al (2014). Ultrasonic inspection and radiography inspection techniques are used to evaluate the quality of polyimide pipes. Constitutive modelling of polyimide pipeline for cryogenic application is explained by Niyas Ali (2014). Finite element analysis technique used to evaluate the structural adequacy a polyimide pipe is explained by the author. Development and recent progress of hoses for cryogenic liquid transportation are given by Qiang Chen et.al (2024). Qualification/acceptance testing of polyimide pipes for cryogenic application or cryogenic rocket application are explained in these literatures. For human space programme of ISRO, cryogenic stage is planned to use as upper stage. Polyimide lines are used in different fluid systems of this cryogenic stage. These polyimide pipes are to undergo flight acceptance test for their acceptance before being inducted into the stage for human space mission. Finite element simulation was carried out for all the polyimide lines, considering the stress/strain to be simulated based on integrated structural analysis of the cryogenic stage. Pressure test method is used for the acceptance test of polyimide pipes for generic cryogenic stages. Test pressure is finalized on the acceptance stress to be simulated for each pipe. But considering the safety actor for human space missions, the acceptance test pressure will be higher and many of the polyimide pipes will not withstand the high pressure. This is due to the failure at polyimide – metallic adaptor interface. On the other hand, these polyimide lines will not experience such huge pressure during the operating condition. Hence, these polyimide pipes are selected for acceptance test by deflection test method. Based on finite element analysis, acceptance test parameters were finalised and prepared deflection test plan was prepared. Flight acceptance tests were done through deflection tests by monitoring load and deflection at the control locations predicted through analysis. Deflection tests were completed successfully, test data processed and compared with predictions for all the lines, and these details are explained in this paper. 2. Requirement of Polyimide pipes in cryogenic rockets Polyimide pipelines are used in various fluid systems of the cryogenic stage, like fill & drain system, feed system, thermal conditioning system, propellant tank vent & relief system, etc.. Propellant tanks and fluid circuits operating at cryogenic temperatures need thermal compensation to take care of differential contraction/expansion occurring in fluid lines and tanks. These polyimide pipelines are used to reduce the loads at the support location due to thermal contraction of the propellant tanks. The advantages of polyimide material are lower stiffness, possesses flexibility to absorb differential contraction, low density, and low Young’s modulus at room temperature and cryogenic temperatures. Polyimide pipelines have the following advantages over metallic pipelines: ● The density of the polyimide material is 1670 kg/m³ , which reduces the pipeline weight considerably compared to the metallic materials. ● Polyimide pipeline retains its higher percentage of elongation even at cryo temperatures. Hence, it acts as a thermal compensator during propellant chilling/filling for the pipelines. ● Polyimide pipelines have low stiffness due to low Young’s modulus , which results in reduced loads on the support brackets under thermal loads, compared to metallic pipelines. Polyimide pipe is manufactured by winding polyimide tape around a solid core of the required diameter and