Analysis of Minimal In-Flight Oxygen Collection Cycle for 2 Stage Launchers

Abstract

[Abstract] In the context of an ESA-funded study on an air separation device intended to provide in-flight Oxygen Collection capability to future launchers, experimental and system level investigations are performed. Formerly hosted by the Belgian Royal Military Academy, it is now sustained by the Universite Libre de Bruxelles. Looking at the world wide picture of space launcher studies, oxygen collection is seldom studied by space researchers and engineers. There are few really rational causes to this situation. Aerospace engineers are often reluctant to invest themselves in very different fields, which is strongly required here. Besides, the research projects are often cornered by funding constraints towards very high technology options. This often pushes resulting preliminary studies away from practically and economically viable new applications. To make design trades more accessible over such an heterogeneous set of concepts, simple models are required. The work provided here is the result of simplifying both the cycle, to limit technological constraints while retaining most of the performance, and to provide an approximate modelling while retaining a sensible analysis and sufficiently accurate predictions. The vehicle considered here has been presented in previous articles, but the result has a wider scope. Strong beneficial effects comes from combining the propulsion cycle of the first stage (possibly subsonic) with the separation process, leading to both oxygen collection and even improved propulsion efficiency for the first stage. This approach attempts to draw a reasonable bottom line for the separation plant performance and for the required system complexity. Simplified, but widely accepted, methods are presented for analysing the various aspects of the separation plant performance. Although the model has some drawbacks that can be corrected using a limited set of more accurate predictions, the mass and energy balance can be solved accurately. Options that have a strong impact on performance –mainly expressed by the collection ratioare analysed: use of significant hydrogen pressurisation and use of para-ortho conversion that improves cooling capacity of hydrogen. Although the system is brought to minimal complexity and some hydrogen capacity is wasted for system heat integration simplicity, the retained performance are found well within minimal requirements to sustain operation and are consistent with previous more accurate computations. The analysis therefore allows to assess a viable bottomline for the propulsion system complexity and technological level and to predict performance of oxygen collecting two stage to orbit vehicles with a rather simple analysis.