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 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.
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 will test and hone the ability 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:
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 cryogen 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.
Contained within the RRM3 module is a source tank, filled with liquid methane, and an empty receiver tank, along with various transfer lines.
During RRM3 mission operations, the space station’s Dextre robotic arm will carry out tasks to transfer liquid methane from the source tank to the receiver tank using a suite of three primary tools.The task sequence begins with the multi-function tool 2, which operates smaller specialized tools to prepare for the fluid transfer. Next, the cryogen servicing tool uses a hose to connect the tank filled with liquid methane to the empty 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. The boil off can lead to the formation of one or more large bubbles that increase the pressure in the tank. The bubbles don’t rise to the top like they do on Earth, because of microgravity; if a large bubble settles at the opening to the transfer line, it could interrupt the flow of liquid.
Therefore, a critical part of RRM3 will be storing 42 liters of liquid methane for six months with zero boil off, demonstrating methods to more efficiently use cryogens. By using cryocoolers and advanced multilayer insulation to balance temperatures, fluid loss will be dramatically lowered, eliminating the need for oversized tanks and extra propellant.