RRM3 Fluid Transfer Module and Tool Pedestal (top) 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 extend these demonstrations to include technologies needed to store and transfer super-cold cryogenic fluids.
The mission launched to space station in December 2018. It will have a projected two-year life on station, though NASA intends to accomplish RRM3's objectives within the first year. On April 8, RRM3 experienced an issue that necessitated the venting of the cryogenic fluid contained within the module, but the mission is still projected to carry out the tool operations associated with transferring cryogenic fuel in space, demonstrating the cutting edge technologies needed to make it possible.
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.
Cryogenic fluid is an important consumable for spaceflight.
As a propellant, cryogenic fluids such as liquid methane and liquid hydrogen are useful for exploration missions because they produce enough thrust, or what is essentially acceleration, to launch a rocket from Earth or another planet. 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.
As a coolant, it keeps spacecraft operational, and the ability to replenish it could extend their lifespan by years.
RRM3 is testing the tools and technologies needed to store, transfer and replenish cryogenic fluid so the technology is 'ready for primetime' when needed.
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 be applied to future human exploration missions.
Engineers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, perform a fit check of RRM3's three external tools. After RRM3 is installed to the outside of the International Space Station, the Dextre robotic arm will mount the pedestal and tools, pre-assembled by astronauts on the space station.
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, and and unique RRM3 mission requirements. The three tools are:
Top: RRM3 tools (left to right) — Visual Inspection Poseable Invertebrate Robot 2, Cryogen Servicing Tool, Multi-Function Tool 2 — during ground testing; Bottom: Astronauts Anne McClain and David Saint-Jacques pose with the corresponding RRM3 tools aboard the International Space Station.
RRM3 will demonstrate the tool operations needed to robotically transfer cryogenic fluid, which is critical for propulsion and life support systems in space. The previous phase of the Robotic Refueling Mission (RRM2) demonstrated many preparation tasks leading up to cryogen replenishment such as removing caps/valves, and installing coolant line adapters. RRM3 robotic operations will focus on the final step of connecting, sealing and managing the hoses needed to enable cryogenic fuel transfer on-orbit. Since launch in December 2018, RRM3 demonstrated the first ever long-term storage of cryogenic fluid with zero boil off, having successfully stored cryogenic fluid for four months on station prior to the April venting operation.
Contained within the RRM3 module is a source tank, representing a supply on a servicing spacecraft and a receiver tank, representing a spacecraft with a depleted cryogenic tank.
During RRM3 mission operations, the space station’s Dextre robotic arm will carry out tasks to sireceiver tank using a suite of three primary tools, without using a cryogenic fluid. The task sequence begins with the multi-function tool 2, which operates smaller specialized tools to prepare for the fmulate the transfer of liquid methane from the source tank to the luid transfer. Next, the cryogen servicing tool uses a hose to connect the source tank to the receiver tank. To monitor the process, the Visual Inspection Poseable Invertebrate Robot 2 (VIPIR2) utilizes a state-of-the-art robotic camera to make sure tools are properly positioned.
Storing cryogens in space can be difficult because their extremely low boiling points cause them to boil off over time if the temperature cannot be maintained. Conventional cryogen storage tanks on the ground use passive cooling systems and intentionally vent their contents to the atmosphere to limit tank pressure during warmup. This greatly reduces the life of the stored cryogen.
A critical part of RRM3 is the demonstration of zero boil off storage of its 42 liter liquid methane supply through the use of active cooling. By using cryocoolers and advanced multilayer insulation to balance temperatures, RRM3 successfully stored liquid methane for four months with zero boil off, demonstrating a system which will dramatically lower fluid loss and eliminate the need for oversized tanks and extra propellant.