Establishing a fundamental calculation model for the Reliability of reusable space launch vehicle upper stages starting from equipment level

Abstract

As the number of space missions increased in the last years and decades, so did the number of launch providers. The growing number of players also intensified the competition between different providers of space launch systems and therefore the need to reduce the cost per launched kilogram, to stay competitive. One of many ways to achieve cost reduction of launches is to pursue reusability of systems, allowing to split production expenses over several launches and thereby reduce the cost per launch. Reusability comes with many challenges towards design and operation, arising from additional phases in the launchers use and an in general longer lifetime, new approaches have to be defined to tackle these. Especially the longer lifetime also proves challenging for the Reliability of the launcher and its systems. The decrease of Reliability over the service life can be counteracted by test and maintenance activities. These activities, not possible and therefore not accounted for in expendable launchers’ Reliability assessments, have to be included in calculations, to assess the Reliability of parts and systems accurately. The development and properties of a new calculation model for the Reliability of reusable launcher systems, starting on equipment level, is described in this thesis. The major important phases, as well as characteristics of reusable systems for the calculation of Reliability are explained, with a focus on the effect of test, repair and replacement activities in scheduled as well as corrective maintenance cycles. From the definition of major model requirements and the properties of different Reliability assessment methods a Stepwise approach of calculation is selected. Splitting the service life of the launcher in several time steps, the selected approach allows changes of the parameters between steps, defined at the point of test and maintenance. The developed model formulas and structure are presented, as well as testing, verification and validation results. By the example of a pressurization system valve, the effect of different maintenance strategies is presented and the results are compared. This proves the fulfillment of the requirements towards the model and its capability to calculate the Reliability with various types of test and maintenance strategies. Furthermore, possibilities for the evolution and improvement of the model are presented, including the transition to system level calculations. Last, chances resulting from the developed calculation model, for RAMS and reusability in general, are described.