The Central Thermal Bus design of the TacSat 4 spacecraft thermal control system (TCS) utilized an ammonia Loop Heat Pipe (LHP) to acquire/transport waste heat from the onboard electronics and reject it to space via eight radiator panels. The TCS underwent flight qualification at the U.S. Naval Research Laboratory (NRL) including the LHP. The thermal performance tests in vacuum demonstrated that the TCS was capable of maintaining the electronics boxes between 0 o C and +40 o C with maximum heat dissipation of 650W under anticipated orbital conditions. TacSat 4 was successfully launched on September 27, 2011 but, soon thereafter, it was apparent that the TCS did not perform as well as designed when the payload was activated for the first time in space. Nevertheless, the TCS was still able to keep the electronics within temperature limits. In early November of 2011, TacSat 4 entered the first eclipse season and then, in the middle of the month, the LHP heat leak seemed to get worse. The payload encountered an “overheat” whenever the heat dissipation was greater than 300W for more than 2 hours. Investigations of the TCS performance anomaly are being conducted independently by the manufacturer and NRL. Hence, not to influence the on-going investigations one way or the other, the authors will refrain from making comments regarding possible causes of the TCS deficiency in this paper.