This effort seeks to advance TA 126.96.36.199 “In-Space Tank-to-Tank Propellant Transfer”. Tank-to-tank cryogenic propellant transfer in zero-gravity (0g) is needed for Human Mars Surface Missions. As noted in the NASA SSTIP, storage and transfer of cryogens in space is critical for deep space human exploration, so much so that it is one of the six NRC high priorities within the “Launch and In-Space Propulsion” Core Technology Investment. When transferring propellant from a supply tank to a receiver tank, the transfer pipe and receiver tank are at a very high temperature compared to the propellant. Chilldown is the process of introducing the cryogenic propellant into the system, forcing the hardware to cool down to the liquid temperature. The propellant boils off as it cools the line and tank until they reach the liquid temperature. Since the propellant is only storable and useful to the engine in pure liquid form, this vaporized propellant is vented overboard and considered lost. Minimal boil-off loss and fast chilldown of the transfer pipe and the receiver tank are needed to save propellant and extend mission flexibility This effort is focused on 0g chilldown of the transfer pipe, although this project is also considered a stepping stone to eventually performing 0g chilldown of a receiver tank. Efficiency for pipe chilldown is very low in 0g, much lower than chilldown in normal gravity due to the different flow patterns that occur. Only 8% of the cooling capability of cryogen is used and 25% of the propellant is spent during propellant transfer. Therefore new methods are needed to increase the efficiency in 0g. Additionally, 0g chilldown data, which is lacking, is needed to develop accurate two-phase cryogenic pipe flow heat transfer models. Chilldown is such a long process because a majority of the time is spent in “film boiling”. Film boiling is when the wall temperature is much hotter than the propellant so that any liquid that approaches the wall is evaporated before touching the wall. This leaves a film of vapor against the solid surface that insulates the surface due to the low thermal conductivity of vapor. In 0g, this vapor film builds up much more easily due to the lack of buoyancy-induced liquid-vapor interface disturbances. The key to increasing chilldown efficiency in 0g is to reduce or even eliminate film boiling The concept we are proposing is a transfer pipe with a nanoparticle surface coating to attract the liquid towards the wall and drastically increase the 0g heat transfer. This will create faster chilldown and minimize propellant loss. As a secondary objective, two-phase flows in 0g will be visually analyzed to build fluid/heat transfer models for a normal, non-treated pipe. The experiment will be carried out on a parabolic aircraft. Liquid nitrogen will be transferred through several pipe test sections equipped with thermocouples and pressure sensors to measure chilldown heat transfer and fluid properties. The objectives of the experiment are to 1) measure increased chilldown efficiency for a nanoparticle-coated stainless steel pipe in 0g, 2) complete the database from our previous flights to measure the effect of the control parameters on efficiency in 0g for a non-modified stainless steel pipe, 3) record video of the two-phase flow structure evolution during 0g chilldown of a clear pipe and measure vapor mass fraction as a function of system variables to create flow pattern maps. The nanoparticle-coated pipes will advance the technology by reducing/eliminating low efficiency 0g film boiling, reducing propellant loss significantly. This makes them an enabling technology for NASA DRM 9 (Human Mars Surface Mission), where they can be implemented in fuel depots that transfer LH2 and/or LOX to the Earth Departure Stage or Nuclear Thermal Rocket. The new 0g fluid/heat transfer models will be used to design future in-space tank-to-tank transfer systems, which will enable accurate flight hardware design.