A dual-mode thermal management loop system (100, 300) configured to operate in either a powered-pump mode or a passive-capillary mode, wherein the dual-mode thermal management loop system comprises: a pump; an evaporator downstream of the pump; a condenser receiving fluid from the evaporator; an accumulator receiving fluid from the evaporator and the condenser; a pump bypass line receiving fluid from the accumulator; a first valve communicating fluidly with the evaporator, wherein said first valve controls whether the evaporator is supplied with liquid from the pump or liquid from the pump bypass line; and a second valve fluidly communicating with the evaporator, wherein the second valve is in a liquid line downstream of the evaporator disposed between the evaporator and the accumulator; wherein in the passive-capillary mode: capillary pressure in the evaporator drives fluid circulation; the first valve prevents fluid circulation through the pump; liquid flows from the accumulator, through the pump bypass line, and to the evaporator; gas exiting the evaporator flows to the condenser; the second valve is closed; liquid exiting the condenser flows the accumulator; wherein in the powered-pump mode: the pump drives fluid circulation; the first valve prevents fluid circulation through the pump bypass line; the pump pumps liquid from the accumulator to the evaporator; gas exiting the evaporator flows to the condenser; liquid exiting the evaporator flows through the second valve to the accumulator; liquid exiting the condenser flows to the accumulator; and the second valve comprises a back pressure valve that controls back pressure in the evaporator. a pump; an evaporator downstream of the pump; a condenser receiving fluid from the evaporator; an accumulator receiving fluid from the evaporator and the condenser; a pump bypass line receiving fluid from the accumulator; a first valve communicating fluidly with the evaporator, wherein said first valve controls whether the evaporator is supplied with liquid from the pump or liquid from the pump bypass line; and a second valve fluidly communicating with the evaporator, wherein the second valve is in a liquid line downstream of the evaporator disposed between the evaporator and the accumulator; wherein in the passive-capillary mode: capillary pressure in the evaporator drives fluid circulation; the first valve prevents fluid circulation through the pump; liquid flows from the accumulator, through the pump bypass line, and to the evaporator; gas exiting the evaporator flows to the condenser; the second valve is closed; liquid exiting the condenser flows the accumulator; capillary pressure in the evaporator drives fluid circulation; the first valve prevents fluid circulation through the pump; liquid flows from the accumulator, through the pump bypass line, and to the evaporator; gas exiting the evaporator flows to the condenser; the second valve is closed; liquid exiting the condenser flows the accumulator; wherein in the powered-pump mode: the pump drives fluid circulation; the first valve prevents fluid circulation through the pump bypass line; the pump pumps liquid from the accumulator to the evaporator; gas exiting the evaporator flows to the condenser; liquid exiting the evaporator flows through the second valve to the accumulator; liquid exiting the condenser flows to the accumulator; and the second valve comprises a back pressure valve that controls back pressure in the evaporator. the pump drives fluid circulation; the first valve prevents fluid circulation through the pump bypass line; the pump pumps liquid from the accumulator to the evaporator; gas exiting the evaporator flows to the condenser; liquid exiting the evaporator flows through the second valve to the accumulator; liquid exiting the condenser flows to the accumulator; and the second valve comprises a back pressure valve that controls back pressure in the evaporator.