<p>Presently operational flex nozzles systems have flex seal located in a submerged region within the motor. Unsubmerged flex nozzle system (USFN) is a new type of flex nozzle configuration which avoids the submergence in solid rocket nozzles. The advantages of USFN includes increased Specific impulse (<i>I</i>_sp), thermal protection optimisation due to non-accumulation of slag, reduction in actuation force on account of flex seal stiffness optimization for low-pressure region and higher volume of propellant loading in solid motor. It can be adopted in any operating solid motor without major change in the propellant system. Since the splitline is located in the supersonic region, careful design assessment of 4D carbon–carbon (C–C) material under severe thermo-mechanical environment is a challenge. The liner system and design was demonstrated using Ø560mm solid rocket motor. Design check and structural analysis of the hardware was carried out for the pressure test condition. Thermo-structural analysis of the system assembly especially the structural assessment of the 4D C–C throat and split line cavity liner for the static test condition was carried out. This paper covers the interface details, finite element analysis details, hot static test results and its comparison with pre-test prediction.</p>