A system for launching an unmanned aerial vehicle (UAV) payload includes a launch tube, liquid rocket, and launch control assembly. The rocket is positioned in the launch tube and contains the UAV payload. A booster assembly may include a canister partially filled with liquid. A gas cylinder is filled with compressed gas. The liquid is pre-pressurized by the gas or mixed with the gas right before launch such that, upon launch, liquid and gaseous thrust stages launch the rocket to a threshold altitude. The UAV payload deploys after reaching the threshold altitude. Optional stability tubes may be connected to the launch tube, which may be buoyant for water-based operations. An optional tether may be connected to the liquid rocket for arresting its flight prior to reaching apogee. The UAV payload is not launched directly by the gas/liquid mix. A method of launching the UAV payload is also disclosed.