Spacecraft have previously used passive cooling or pumped liquid loops to maintain proper thermal environments. Future large spacecraft, such as the Space Station, will have higher power levels and longer heat transport distances. To better meet these requirements, a two-phase thermal bus concept has evolved which includes a thermal transport loop with controlled temperature and pressure. A two-phase cooling system has the advantages over a single-phase system of lower mass flows, lower pumping power requirements, and operation at nearly isothermal conditions, since heat is transferred by evaporation and condensation.The system employs an electrically-driven pitot pump to supply a controlled flow of nearly saturated liquid to any arrangement of series or parallel evaporators. The rotary fluid management device (RFMD) provides the evaporator flow, centrifugally separates the two-phase flow returning from the evaporator, recirculates flow to the condenser, resaturates subcooled liquid, and separates and collects noncondensible gases. Constant saturation conditions within the RFMD are maintained by a simple backpressure regulator. An external accumulator communicating with the RFMD maintains constant liquid level in the RFMD during system turndown.Test results presented have been obtained with an RFMD, control regulator and accumulator, which are functionally identical to a flight system. Flight prototypic designs for the RFMD and backpressure regulating valve (BPRV) have been generated and are described.