Fuel, oxidizer and gas tankages realised for space applications involve fracture mechanics based design aiming at low weight, high strength, and long service life working under hostile environs. Stringent quality control is mandatory to exclude all local defects that may go undetected during nondestructive inspection (NDI) and may grow sufficiently during operation to cause a catastrophic failure. The tankages realised for space applications are charged with gas and corrosive fuels and are intended to work under very low gravity and at extreme temperature environs with a mission life of 7 to 10 years. The fluctuating environ temperatures causes pressure variations exposing the hardwares to pressure cycling situations. This leads to the growth of accepted flaws finally leading to failure at the end of mission life. The raw materials condition and various process adopted however shall lead to the generation of defect which are to be detected with great confidence and reliability. Viewing the above stringent requirements need for utilising complimentary NDI, techniques were felt. X-ray is used as the prime technique for studying gross defects like porosity, etc. However, it is mandatory to establish ultrasonic testing of weldments to detect lack of fusion, tight cracks and other weld defects. The use of ultrasonic testing helps in mapping the defect and characterising the 3 dimensional co-ordinates of the defect. This is essential to carry out analysis of the defect dimensions and hence take decision for reject/rework/acceptance. Ultrasonic testing using immersion technique has been selected as a very reliable methodology in order to establish the repeatability and reliability of the defects being detected. The merits and demerits of the ultrasonic immersion system over the contact type is discussed. Considering the stringent acceptance criteria requirements laid down, immersion techniques using squitter probe and total immersion system have been built. The above technique using auto scan accurately carries out the scan in very small steps where all the motions are stepper motor controlled. It is also possible to carry out 100% scan of the entire weldment or forging surface by feeding the profile co- ordinates and controlling the probe motions. The striking feature of the above automation is that the probe and job motions are controlled and coordinated through the system software. Squitter probe system permits 5 degrees of freedom of motions whereas total immersion system permits two degrees of freedom motions. An attempt has been made in this paper to present the details on the capability and specifications of the system and also the scan data of few hardware weldments. The paper also highlights the procedure for defect mapping and characterising the defect.
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