A two-phase nitrogen thermosyphon was developed in order to efficiently integrate a cryocooler into an insulated liquid nitrogen filled tank as part of a Protoflight Zero Boil-Off (ZBO) Development Ground Test. NASA Marshall Space Flight Center's (MSFC) Advanced Space Transportation Program supported this test to improve performance of in-space propulsion system concepts. Active cooling was integrated via a thermosyphon, made of copper, 1070 mm (42") in length with an inner diameter of 11 mm (0.436"). It was charged with nitrogen to 1.55 MPa (225 PSIA) at 300 K which provided a fill ratio of 15%. The temperatures and heat flows through the thermosyphon were monitored during the start-up phase of the ZBO test and steady-state tests were conducted over a range of increasing and decreasing heat flows. The results showed that the performance of the thermosyphon exceeded expectations and had a thermal resistance of 0.2 K/W at a heat flow of 8.0 W. The design calculations also showed that the thermal resistance of a thermosyphon can be made relatively constant over a wider range of heat flows by making the ratio of evaporator area to condenser area 3:1.
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