This collaborative project between the University of Strathclyde and Reaction Engines Ltd.\ builds on initial research within the Centre for Future Air-Space Transportation Technology (FASTT) at the University of Strathclyde developing an integrated design platform for quickly assessing design parameters based on optimised performance and operations for space access vehicles. This project extends upper atmosphere re-entry models developed at FASTT, targeting at assessing the trajectory from atmospheric entry to the end of the hypersonic phase, to include de-orbiting and gliding phase and to implement a number of numerical tools to improve the convergence rate of the optimisation approach. Reaction Engines' Skylon vehicle is used as a test case to analyse optimised descent trajectories for their standard mission profiles. Reduced order models were used, calibrated by high fidelity simulations, for the vehicle disciplinary models. The final code is able to optimise the descent trajectory, from de-orbiting to final approach. For this project, as well as for the entire design platform, importance was given to developing a consistent and robust software framework that will facilitate the use of the spaceplane integrated design platform as an open source tool in the future.