Increasing demand for cheap and reliable access to space has driven development of small scale liquid rocket engines and the turbomachinery that powers them. Leakage losses and cavitation are two of the major avenues to performance degradation found in centrifugal pumps in rocketry applications and were assessed in the context of the LUMEN LOx/Methane demonstrator engine under development at the DLR Lampoldshausen. Firstly, an empirical equation model for pump leakage was developed and validated with experimental test data with adequately accurate results. A detailed numerical model was then developed to assess potential alternative designs for the main pump annular seals with a reduction in total leakage of over 20% generated. The significant influence of manufacturing tolerances was also demonstrated with this model before a full pump simulation was used to characterise the asymmetry of the pump. Lastly, an attempt to include a prediction of cavitation performance was completed with mixed results.