Methods and means related to rejecting heat through thermal storage are provided. A heat sink includes internal cavities containing a phase-change material. Heat from a thermal load is rejected by flowing fluid coolant at a normal operating temperature. Failure of the fluid coolant system causes heat storage within the phase-change material at a temperature slightly greater than the normal operating temperature. Restoration of the fluid coolant system results in stored heat rejection and a return to a normal operating temperature. Normal operation of the thermal load can be performed while efforts are made to restore the fluid coolant system.