Mass and volume efficient solar arrays are sought by NASA, DoD and commercial space to enable high power missions from 20-30 kW up to 300 kW. Flexible substrate arrays can have higher specific power (W/kg) and specific volume (kW/m3) than conventional arrays. Typical designs for flexible substrate arrays require a stiff boom mechanism to deploy the array and provide the deployed structure. Graphite composite slit-tube booms are thermally stable and can enable next-generation flexible arrays by improving mass, volume, and cost. CTD has developed and demonstrated a 5cm diameter graphite composite slit-tube boom and canister designed for a 23m tether-stiffened solar array. The Stable Tubular Extendible Lock-Out Composite (STELOC) boom proposed here will feature two innovations to the composite slit-tube design that enhance stiffness. Slit-Lock interlocks the edges of the slit and Root-Lock eliminates the open section at the root of the boom when fully deployed. Combined, these innovations enable a 10cm STELOC boom that is much simpler, lighter, and stiffer than a 25cm diameter coilable longeron boom. This Phase I program will demonstrate a full length, 10cm STELOC boom including all innovative features to enhance stiffness. The program will also develop a conceptual design that meets all boom requirements provided by an identified spacecraft prime contractor for their flexible substrate array.