RRM3 Fluid Transfer Module before launch
RRM3 builds on the first two phases of International Space Station technology demonstrations that tested tools, technologies, and techniques to refuel and repair satellites in orbit. RRM3 will demonstrate innovative methods to store, transfer and freeze standard cryogenic fluid in space.
The mission is scheduled to launch to the space station in 2018. It has a projected two-year life on station, though NASA intends to accomplish RRM3's objectives within the first year. RRM3 is developed and operated by the Satellite Servicing Projects Division at NASA's Goddard Space Flight Center in Greenbelt, Maryland, under direction of NASA's Space Technology Mission Directorate.
A cryogenic fluid can be used to keep critical optical equipment cold and operational, or be used as a potent, high-thrust propellant. In its application as a coolant, cryogenic fluid is useful in satellite servicing in that it can be replenished to extend a satellite's lifespan.
As a propellant, cryogenic fluids such as liquid methane and liquid hydrogen are useful for long distance journeys because they produce enough thrust, or what is essentially acceleration, to successfully leave planetary bodies. Without a powerful enough propellant, a planetary body's gravitational pull would prevent a rocket from successfully escaping the body's orbit.
Liquid oxygen, another cryogenic fluid, is also used to maintain life support systems for astronauts, since it is a more efficient way to transport and store oxygen than in its gaseous form.
RRM3 will test and hone the ability to transfer and replenish cryogenic fluid so the technology is 'ready for primetime' when needed.
Artist's concept of Restore-L. Credit: NASA
RRM3 demonstrations will also further develop twelve individual technology elements that are directly applicable to Restore-L, a project that aims to prove the combination of technologies needed to robotically service a satellite in orbit. These technologies range from robotic tools, to fluid transfer systems, to a high-speed processor called SpaceCube 2.0.
The ability to resupply propellant and coolant enables longer journeys than a single tank of propellant would allow. The capabilities NASA will develop through RRM3 can thus be applied to future human exploration missions.
In the context of the Journey to Mars, RRM3's cryogen replenishment techniques could also be used to refuel spacecraft that arrive at Mars through In-Situ Resource Utilization (ISRU). Using this method, the carbon dioxide in Mars' atmosphere could be converted into liquid methane (a cryogenic propellant) and used to refuel a Mars departure rocket. Propellant saved using RRM3's established replenishment method would allow future manned missions to use that cargo mass for other necessary supplies.
Capabilities developed through RRM3 may also be applied to future lunar missions. When mining the moon for water, the hydrogen and oxygen can be separated, then used as fuel, meaning less fuel is required to be launched from Earth. Liquid hydrogen and oxygen are types of cryogens, so the ability to transfer and replenish them in space will be critical to this operation.
Finally, for long duration space travel, spacecraft transporting astronauts will need to store liquid oxygen to maintain life support systems. Having the ability to replenish these supplies frees up cargo mass for other supplies, making longer journeys possible. Additionally, any future habitats for humans will also require large and replenishable supplies of liquid oxygen.
A suite of three primary tools, designed to be used by space station's Dextre arm, will be employed to conduct mission objectives. These second generation tools were designed based on operational lessons learned from RRM Phase 1and 2, unique RRM3 mission requirements, and synergistic requirements/new capability development for the Restore-L project. The three tools are:
Cryogen Servicing Tool before launch
RRM3 will demonstrate the transfer of cryogenic fluid, which is critical for propulsion and life support systems in space. While the previous phase of the Robotic Refueling Mission (RRM2) demonstrated tasks leading up to coolant replenishment such as removing caps/valves, and installing cryogen line adapters, the actual transfer of cryogen in orbit will be carried out for the first time with RRM3 using liquid methane.
Cryogens in space tend to slowly warm up and boil off, which results in less cryogen available for replenishment. In a zero-gravity environment, the bubbles that form as a result of this boil off do not simply shift to the surface of a tank as they would on Earth. Instead, the bubbles are present throughout the tank in a liquid/gas slurry. Bubbles can cause problems when using cryogen as a coolant or propellant, since they interrupt the flow of liquid to the necessary systems and reduce effective transfer rates. For this reason, RRM3 is testing and perfecting bubble-free transfer of fluid via novel technology. In addition to transferring cryogens, the storing of cryogenic fluid for six months while maintaining fluid mass via zero boil off is another key objective of RRM3. By using cryocoolers and advanced multilayer insulation to balance temperatures, propellant loss should be near or at zero, eliminating the need for oversized tanks and extra propellant.
Within the module there is a source dewar, or tank, that holds the cryogen pre-launch and in-orbit prior to transfer. There is also a receiver dewar, which is connected to the source dewar by three different lines or transfer paths. As its name implies, this vessel will be receiving the transferred cryogen.
Between the two dewars are three different transfer paths. The first transfer path (the hard line) is contained internally within the module. It will be used to test the innovative zero-gravity fluid management transfer system without the need for any robotic tools or hoses. The second transfer path (the coupler line) will be used to test the robotic cryogen transfer for a satellite or spacecraft designed to be serviced; this particular test is important for future spacecraft, since they will be designed to be replenished with supplies of consumables like cryogen. The last transfer path (the flex line) will be used to test replenishment techniques for a satellite not designed to be serviced, which is the case for the vast majority of existing satellites. RRM3 will affix to the outside of the International Space Station, and the station's Dextre robotic arm will attach to the Cryogenic Servicing Tool and Cryogen Coupling Adaptor to carry out the cryogenic transfer for both of the external transfer paths.
By testing cryogenic transfer methods for present-day non-cooperative and future cooperative satellites, RRM3 will advance these critical technologies for many possible situations and contexts.