A rotor disk assembly with a platform spacer secured to the rotor disk and un-coupled from the rotor blades. Adjacent rotor blades are secured to the rotor disk slots by conventional means. The rotor disk includes a dovetail extending from the rim and positioned substantially midway between adjacent rotor blades. A platform spacer having a size and shape to occupy substantially the entire space formed between the adjacent rotor blades and the rotor disk rim is secured through a slot to the dovetail projecting from the rim. The platform spacer is a solid piece and forms the outer hot gas flow path between the blades. The platform spacer provides damping to the rotor blade assembly as well as eliminates hot gas flow migration below the platform. The platform space also un-couples the platform from the blades so that the load due to the platform is transmitted directly to the rotor disk and not through the blades, allowing for the blades to have improved LCF. Uncoupling the platform from the blade also eliminates the thermal mismatch between the platform and the blade which produces excessive levels of stress at this junction. With the un-coupled platform spacer of the present invention, the rotor blades can be formed from a single crystal material to allow for higher temperature limits and longer LCF of the assembly.