The Portland State Aerospace Society's Launch Vehicle 4 is a liquid propellant rocket intended to reach an altitude of 100 km. Our primary goal was to develop the airframe to withstand predicted worst-case loads while minimizing mass. A secondary objective was the sponsor-driven design requirement to divide the rocket into rigidly connected modules along its length. Prior rocket designs used carbon fiber; we argued that machined aluminum isogrid would reduce weight, increase modularity, reduce connection points, and maintain sufficient strength. We developed computationally-efficient FEA modeling procedures to predict ultimate loads. We designed stress testing infrastructure and experiments to demonstrate load bearing capacity and used the test results to validate our FEA models. The material change and reduction of connection points reduced the dry mass of the rocket by 68.5 lbm. This reduction in mass significantly increased our projected altitude in our flight modeling algorithm. The physical testing showed that the isogrid had sufficient strength for current load estimates. Our FEA models aligned with destructive test results after accounting for manufacturing variations. A machined isogrid aluminum airframe offers advantages in design flexibility, specific strength, repeatable manufacturing, and ease of analysis.