A thermal management system for operating near a temperature T. The system includes a sheet-like spacer having a plurality of passageways. A degassed working fluid is disposed as a liquid within the passageways of the spacer. A flexible and substantially impermeable sheet-like envelope forms a sealed compartment enclosing the spacer and the degassed working fluid at an absolute pressure that is approximately equal to the vapor pressure of the working fluid at equilibrium with liquid phase of the working fluid at the temperature T. The spacer, the working fluid and the envelope form a sheet-like thermal management system. In some embodiments the spacer is a flexible spacer and the envelope is a flexible envelope and the sheet-like thermal management system is a flexible sheet-like management system. In some embodiments the envelope is a rigid structure.