Pre-conceptual staging trade-offs of reusable launch vehicles

Abstract

Reusable launch systems have revolutionized the space transportation industry during the last decade. The established success of Falcon 9 (SpaceX) played a pivotal role for many private companies and space agencies, pushing them to invest consistent resources in reusable launch vehicles (RLV) oriented on re-covery for both main and upper stages. In this study, a novel pre-conceptual methodology for the sizing of reusable main stages based on optimal staging, structural index curves and rocket engine characteristics is introduced. The approach can serve to develop initial guesses and bounds for further detailed RLV sizing. It extends conventional launcher design methodologies, based on optimal staging with velocity budgets, to reusable trajectories with recovery hardware. This is used for a conceptual analysis of the suitability of design alternatives in terms of performance and parametric cost metrics and including different recovery solutions based on VTVL and VTHL rocket concepts. The study also explores the differences in using hydrogen, methane, propane, ammonia and kerosene as fuel. The results show the importance of modeling the recovery propellant and a sensitivity to the number of expected reuses for cost optimal designs differing from minimum take off mass solutions. Although it provides fast results which can be suitable to initialize the conceptual design of RLVs, its validity is limited as a consequence of the difficulty in conceptually modeling additional design parameters and their impact the vehicle trajectory and dry mass. Therefore, its results have to be carefully used within subsequent design phases, although it can be useful as an initialization strategy. Particularly, the velocity analysis discipline can produce initial guesses for detailed trajectory optimization, and the optimal staging routine for Multi-disciplinary Design Analysis and Optimization (MDAO) assessments. The performed trade-offs will be further complemented with future detailed studies on structural design and high fidelity MDAO.