In-orbit refueling with highly efficient cryogenic propellants is a key technology to fully utilize In-situ resource utilization (ISRU) architectures and facilitate large-scale exploration missions. Cryogenic propellants are characterized by their low saturation temperature and a small enthalpy of evaporation. This has caused the maximum operation time for cryogenically fueled spacecraft to remain in the order of hours due to the need to regularly vent the storage tanks. To avoid the loss of propellant through venting and enable the longer times in orbit needed for refueling operations, new technologies have to be developed. These focus on the active thermodynamic control of the propellants both for storage and transfer.
The development of a liquid nitrogen ground demonstrator system will be used to mature the relevant technologies and components. This encompasses the storage, as well as the conditioning for the transfer itself. The demonstrator will consist of a storage tank, a transfer line with a conditioning system and a coupling interface as well as a receiver tank.The zero boil-off storage of liquid nitrogen in a spaceflight derived tank will be achieved by applying an active cooling system to the storage tank. The active cooling system will intercept heat flowing into the tank. The sizing of the cooling loop and the application to the tank itself will be key points of validation for modelling efforts.The demonstration of a pump driven transfer will aim at mass transfer rates comparable to the rates envisioned for future in-orbit depot transfer operations.In order to avoid any losses of propellant the filling of the receiver tank shall be accomplished without venting. Under these conditions, the filling of the receiver tank will cause a rise in pressure caused by the incoming liquid. In order not to stall the transfer, the compression of the receiver side ullage has to be counteracted by the condensation of vapor in the receiver tank. The rate of condensation depends on the thermodynamic state of the fluid leaving the storage tank. Testing will therefore also focus on mapping the allowable initial conditions for the receiver tank with respect to temperature and fill level.
The demonstrator will operate in a thermal vacuum (TVAC) chamber to ensure environment heat load and pressure boundary are representative. The tanks are sized so that the heat and mass transport processes are representative of future applications. This will allow for a representative demonstration of the active cooling system. The aim of the demonstration is in the maturation of the relevant technologies for future orbital demonstrations of cryogenic refueling as well as the improvement of modelling tools and physical understanding of the system level behavior of the storage and refueling processes.

Keywords
Cryogenic Propellant Management|Cryogenic Transfer|Zero Boil-off|Refueling