A simplified trajectory model for the evaluation of the performance of planetary ascent vehicles is presented. The model is based on a “linear-sine steering to orbit” approximation. The ascent trajectory is divided in two phases. In the first phase, characterized by a given steering law referred to a ground-fixed coordinate frame, the influence of atmospheric drag can be relevant. The second phase is an elliptical coasting orbit referred to a planet-centered inertial frame, starting from the burn-out point of the first phase and tangent to the final parking orbit, which is assumed to be circular. The paper shows the main results of the application of the proposed model to a sample Mars Ascent Vehicle case. The results obtained are in good accordance with the ones presented in the open literature for similar cases, showing a maximum error in the estimation of the total velocity change not higher than 10%. The effectiveness of the proposed model, despite its simplicity and ease of use, is thus demonstrated.