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 and xenon in space.
The mission is scheduled to launch to the space station in early 2018 aboard the SpaceX Commercial Resupply Services Mission 14 (CRS-14). It has a projected two-year life on the space 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. Xenon produces a much lower thrust than cryogenic propellants such liquid as oxygen, methane, and hydrogen. Solar Electric Propulsion (SEP) utilizes xenon as its propellant, and this propulsion technique is low-thrust but tremendously efficient. For long journeys, an efficient propellant like xenon is useful because it means a spacecraft does not need to be burdened by transporting as much fuel, or having as large of a fuel tank. However, xenon does not produce a high enough thrust, or what is essentially acceleration, to successfully leave planetary bodies, because of gravitational pull.
To leave a planetary body like the Moon, or Mars, a chemical or cryogenic propellant is needed to produce a high thrust to allow a rocket to successfully escape the body's orbit. Tug spacecraft that will transport cargo to Mars will operate in a zero-gravity environment away from planetary bodies, so fueling them with an efficient propellant like xenon is ideal for that long journey.
RRM3 will test and hone the ability to transfer these two commodities so they are 'ready for primetime' when needed.
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, tugs powered by Solar Electric Propulsion (SEP) and used to transfer cargo to the Red Planet could be refueled with xenon, and sent on another round trip. Additionally, storing cryogens long-term is a prerequisite for getting to Mars. RRM3 will demonstrate both of these capabilities.
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. Hydrogen and oxygen are types of cryogens, so the ability to transfer and replenish them in space will be critical to this operation.
A suite of three primary tools, designed to be used by space station's Dextre arm, will be employed to conduct mission objectives. This second generation of tools were designed based on operational lessons learned from RRM Phase 1 and 2, unique RRM3 mission requirements, and synergistic requirements/new capability development for the Restore-L project. The three tools are:
RRM3 will demonstrate the transfer of xenon and cryogen, which are 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, the actual transfer of cryogen in orbit will be carried out for the first time with RRM3.
Cryogens in space tend to boil off, which is a loss in fuel. 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, the planned transfer of fluid will be bubble-free 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 a 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 on-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.
The first transfer path (the hard line) is contained internally within the module, and works with a pump mechanism. It will be used to test a transfer system that would activate if a mission encountered an issue with tools or connection methods. The second transfer path (the coupler line) will be used to test 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, and testing fail safe transfer methods in case of unexpected problems, RRM3 will advance these critical technologies for many possible situations and contexts.
For the xenon transfer portion of the mission, RRM3 will demonstrate the transfer of xenon gas from a supply tank to a client tank via a robotic interface in a zero-gravity environment. The transfer will be carried out via a cooperative interface, meaning both the client and the source were designed to interact with each other. It will be the first time the transfer of xenon has occurred in orbit.
Newer satellites and spacecraft are starting to be designed with servicing in mind, particularly for longer missions like the Journey to Mars, which will require refueling for tugs that carry cargo. Missions like these, as well as newer satellites, will likely employ cooperative interfaces. It is important to test the ability to perform operations cooperatively.
Space station's Dextre arm will grasp the Multi-Function Tool 2 (MFT2), to retrieve the Xenon Coupler Adaptor (XCA). The Dextre robot will then maneuver the XCA to interface with, and lock onto, the Cooperative Service Valve (CSV). This union allows high pressure xenon gas to be pumped to the 'client tank.'