A gas turbine engine (10, 200, 300, 400, 500, 600, 700, 800, 900), comprising: a shaft (208, 306, 406, 506, 608, 808) comprising a first compressor (210, 516, 612, 812), a fan assembly (212, 302, 402, 502, 806), and a power circuit (214, 304, 404, 504, 610, 810, 902) that provides power to the shaft (208, 306, 406, 506, 608, 808) in a closed-loop system; an inner housing (216, 904) that houses: at least a portion of the shaft (208, 306, 406, 506, 608, 808); the first compressor (210, 516, 612, 812) for compressing a core stream of air (202, 312, 412, 602, 802); and a combustor (218, 424, 514, 614, 814); a baffle (220, 326, 428) that encloses a portion of the inner housing (216, 904) and forms a first air passageway therebetween; a nacelle (224, 328, 430) that encloses a portion of the baffle (220, 326, 428) and forms a second air passageway therebetween; a heat exchanger (228, 318, 418, 524, 618, 702, 818) positioned in the second air passageway that rejects heat from the power circuit (214, 304, 404, 504, 610, 810, 902) into a heat rejection stream of air (206, 316, 416, 508, 604, 804) passing through the second air passageway; wherein air is accelerated as streams in parallel and via the fan assembly (212, 302, 402, 502, 806): as the core stream (202, 312, 412, 602, 802) into the inner housing (216, 904); as a bypass flow stream of air (204, 314, 414) through a first volume; and as the heat rejection stream (206, 316, 416, 508, 604, 804). a shaft (208, 306, 406, 506, 608, 808) comprising a first compressor (210, 516, 612, 812), a fan assembly (212, 302, 402, 502, 806), and a power circuit (214, 304, 404, 504, 610, 810, 902) that provides power to the shaft (208, 306, 406, 506, 608, 808) in a closed-loop system; an inner housing (216, 904) that houses: at least a portion of the shaft (208, 306, 406, 506, 608, 808); the first compressor (210, 516, 612, 812) for compressing a core stream of air (202, 312, 412, 602, 802); and a combustor (218, 424, 514, 614, 814); at least a portion of the shaft (208, 306, 406, 506, 608, 808); the first compressor (210, 516, 612, 812) for compressing a core stream of air (202, 312, 412, 602, 802); and a combustor (218, 424, 514, 614, 814); a baffle (220, 326, 428) that encloses a portion of the inner housing (216, 904) and forms a first air passageway therebetween; a nacelle (224, 328, 430) that encloses a portion of the baffle (220, 326, 428) and forms a second air passageway therebetween; a heat exchanger (228, 318, 418, 524, 618, 702, 818) positioned in the second air passageway that rejects heat from the power circuit (214, 304, 404, 504, 610, 810, 902) into a heat rejection stream of air (206, 316, 416, 508, 604, 804) passing through the second air passageway; wherein air is accelerated as streams in parallel and via the fan assembly (212, 302, 402, 502, 806): as the core stream (202, 312, 412, 602, 802) into the inner housing (216, 904); as a bypass flow stream of air (204, 314, 414) through a first volume; and as the heat rejection stream (206, 316, 416, 508, 604, 804). as the core stream (202, 312, 412, 602, 802) into the inner housing (216, 904); as a bypass flow stream of air (204, 314, 414) through a first volume; and as the heat rejection stream (206, 316, 416, 508, 604, 804).