A dual-mode thermal management loop system 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 . <EOS>