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Transient performance analysis is essential for the successful design of a spacecraft thermal management system. This paper presents three dynamic models for analyzing system-level transient responses of a loop heat pipe (LHP)-micro-electromechanical-system (MEMS) thermal management system consisting of a LHP and a variable emittance radiator employing MEMS technologies. The recommended hybrid mathematical model, which is deduced from the fundamental 3-node and 4-node thermal networks, provides an efficient closed-form equation enabling direct calculation of the fluid mass flowrate in the LHP from the nodal temperatures of the thermal management system, and a set of state equations that independently compute the temperature responses of the cooled object, LHP evaporator, LHP condenser, and MEMS radiator. The temperature transients of a 50 W LHP-MEMS thermal management system, as well as the hydraulic responses inside the employed LHP, were numerically investigated and discussed in detail.