<p>The uncontrolled deployment process of spool-first deployable booms can have adverse effects on spacecraft attitude, therefore it needs to be well studied and understood for applications. In this paper, the deployment of a spool-first deployable boom was analyzed experimentally and verified by a developed dynamics model. The deployment was observed in multiple experiments where the forces exerted on the base of the boom were measured. Additionally the recordings from the slow-motion camera were captured to derive relations for velocities and accelerations governing the spool movement during the deployment. A dynamical model of the deployment was finally developed and verified against the experimental data. It also enabled predicting reaction forces acting on the spacecraft. The large number of experiment series made it possible to determine a mean value of deployment efficiency, which characterizes the amount of energy lost in the process.</p>