Launching lighter-than-air craft, such as stratospheric balloons, includes filling an envelope with lift gas. Depending on the size and configuration of the envelope, inflation can place excess tension on one or more points along the envelope. This tension can cause undue stress on the material, which can adversely impact the ability of the lighter-than-air craft remaining aloft for long periods of time. Aspects of the technology employ a load cell to measure tension as the envelope is filled with lift gas. Information from the load cell is used to modulate one or more elements of a launch rig to regulate the tension during fill. The load cell may be mounted in proximity to one or more attachment points between the envelope and the launch rig. The load cell readings are cross-referenced with the amount of lift gas in the envelope, and the modulation is adjusted according to this information.