Future large spacecraft such as the Space Station will have high power dissipations and long heat transport distances. The combination of these two requirements dictate the need for a new heat transport technology. NASA-JSC has developed the concept of a two phase thermal bus in which the working fluid is evaporated at the heat collection site and is condensed at the heat rejection site. This provides a nearly isothermal system at lower pumping powers than current single phase systems. Boeing has developed a two-phase thermal bus testbed using ammonia working fluid. This testbed uses a Sundstrand rotary fluid management device (RFMD) to provide fluid pumping and liquid-vapor phase management. Overall heat transport capacity is 25 kW. This internally funded testbed is being used for thermal bus heat exchanger characterization and critical component life tests in an ammonia environment. Currently, the testbed has been assembled, proof-pressure tested, leak tested, and checked out. Heat exchanger performance tests are underway.