A gas turbine engine includes a turbomachine and a thermal management system. The thermal management system includes a heat source heat exchanger configured to collect heat from the turbomachine during operation; a heat sink heat exchanger; and a thermal transport bus having a heat exchange fluid configured to flow therethrough at a pressure within an operational pressure range. The thermal management system defines an operational temperature range for the heat exchange fluid, the operational temperature range having a lower temperature limit less than about zero degrees Fahrenheit at a pressure within the operational pressure range and an upper temperature limit of at least about 1000 degrees Fahrenheit at a pressure within the operational pressure range.