The Onsager pancake model for the fluid dynamics of the countercurrent flow of gas in a centrifuge is used to study the separation performance of a single-stage gas centrifuge cascade. Based on the fluid dynamic solutions, Cohen-Onsager theory is used to calculate the separative performance. The separation considered is the enrichment of natural U-235 to 3%. The effects of the introduction of the feed gas on the separation performance are studied, and this work is compared to similar work by Ratz who used a much simpler model. The present work agrees well with the simpler model on predictions of separative performance but not on parameters such as the axial location of the feed stream.