Flight Acceptance Testing of Polyimide Composite Pipelines in a Cryogenic Stage for Human Space Mission

Liquid hydrogen and liquid oxygen are used as fuel and oxidizer for propulsion in cryogenic stages. These propellants are stored in separate propellant tanks at cryogenic temperatures – 20 K and 77 K, respectively. Polyimide pipelines are used in various fluid systems of the cryogenic stage, like fill & drain system, feed system, thermal conditioning system, etc. Polyimide composite material is used for fabrication of pipelines in cryogenic stages of launch vehicles of ISRO missions. 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. Structural analysis of the entire stage, along with all the pipeline lines made of metallic and polyimide composite, has been carried out to evaluate loads/stresses in metallic and polyimide line segments. Different loads during ground servicing and flight conditions are considered for the analysis. Polyimide lines are required to undergo a flight acceptance test for their acceptance before being inducted into the stage for a human space mission. In general, flight acceptance tests were done through a pressure test, but the end adaptors were not capable to withstand high stress during the pressure test, with factors considered for human space mission. Hence, these pipes were identified for deflection test through deflection tests by monitoring load and deflection at the control locations, which are simulated through finite element analysis. Dismantlable test closures with split flanges were designed through structural analysis and were challengeable to ensure the leak tightness during the flight acceptance test with cryogenic fluid. Test plans and predictions were prepared for the deflection test based on the structural analysis. Deflection tests were completed successfully, test data processed, and compared with predictions for all the lines. Accordingly, all the polyimide lines were undergone flight acceptance tests and were inducted into the cryogenic stage for human space mission.