NASA and Boeing have announced plans to outfit the International Space Station (ISS) with an upgraded set of six power-producing solar arrays, beginning later this year. It is expected that the new arrays—to be combined with the eight “legacy” Solar Array Wings (SAWs) currently in place on the port and starboard sides of the station’s Integrated Truss Structure (ITS)—will afford a “hike” of electrical power production of 20-30 percent for the sprawling multi-national outpost.
“When it comes to game-changing research and technological development, the Space Station is currently hitting its stride,” said Boeing ISS Vice President and Program Manager John Mulholland. “These arrays, along with other recent upgrades to the station’s power system and data-transfer speed, will ensure that ISS remains an incubator and business model in the commercial space ecosystem for the coming decades.”
In its present form, the ITS possesses eight massive truss segments—P-6 and P-4 on the port side and S-6 and S-4 on the starboard side, together with adjunct structural members—which were launched and installed by successive Space Shuttle crews between November 2000 and March 2009. Each truss houses its own pair of giant SAWs and power channels to contribute to the station’s requirement for electrical energy, power storage and distribution and cooling. Within each truss, 12 nickel-hydrogen batteries stored and routed electrical power from the SAWs to feed on-board systems.
But early last decade, as these batteries approached the end of their functional lifetime, efforts got underway to replace them with new lithium-ion batteries, which are smaller, greatly exceed the storage capacity of their predecessors and can accommodate a larger number of charge/discharge cycles over a full decade. Between January 2017 and last summer’s multi-spacewalk campaign by Expedition 63 crewmen Chris Cassidy and Bob Behnken, 11 sessions of Extravehicular Activity (EVA) totaling over 72 hours saw 48 aging nickel-hydrogen batteries removed from the four trusses and 24 brand-new lithium-ion units fitted in their stead.
Unfortunately, one of the new batteries blew a fuse in April 2019 and was temporarily replaced with a pair of nickel-hydrogen ones. A replacement lithium-ion battery arrived aboard SpaceX’s CRS-19 Dragon in December 2019 and was stowed on the ITS, ahead of its planned installation by Expedition 64 spacewalkers Mike Hopkins and Victor Glover later this month.
Although batteries and other components of the station’s power-generation infrastructure have been regularly upgraded over the years, less attention has been paid to the physical arrays themselves, all of whose hardware has been on-orbit for well over a decade. In June 2017, SpaceX’s CRS-11 Dragon cargo ship ferried the 716-pound (325 kg) Roll-Out Solar Array (ROSA) to the station as a technology demonstrator for more compact electricity-generating arrays. Its hardware was designed to be 20 percent lighter and four times smaller than extant rigid-panel solar array designs.
Equipped with high-strain, one-piece composite slit-tube booms for improved structural stiffness and strength, ROSA helped to characterize deployment loads and kinematics and measured the velocities and accelerations of the array during the unfurling and blanket-tensioning processes.
Although not actually installed onto the station, ROSA was instead grappled by the 57.7-foot-long (17.6-meter) Canadarm2 robotic arm over the weekend of 17-18 June for a week of tests. Unfortunately, efforts to retract ROSA and lock it back into its stowed configuration proved unsuccessful and the experimental array was jettisoned on the 26th. Nevertheless, it returned valuable data in support of the desired concept.
Following the successful ROSA demonstration, plans entered high gear to develop six ISS Roll-Out Solar Arrays (iROSAs) to augment the extant system. In preparation for this mammoth task, last July spacewalkers Chris Cassidy and Bob Behnken removed a pair of “H-Fixtures” from the base of two port-side solar arrays. These fixtures were originally used for ground processing of the arrays and are no longer needed.
Four more H-Fixtures still need to be removed in order for a total of six iROSA modification kits to be installed. The first of these modification kits was delivered to the space station aboard Northrop Grumman Corp.’s NG-14 Cygnus cargo ship last October and will be installed by Expedition 64 spacewalkers Kate Rubins and Victor Glover during an “ISS Upgrades” session of Extravehicular Activity (EVA) in early February. The kit is currently stored in the end-cone of the Permanent Multipurpose Module (PMM).
NASA has noted that, in spite of their age, the current arrays continue to perform well, although they are showing anticipated signs of physical degradation and wear. “To ensure a sufficient power supply is maintained for NASA’s exploration technology demonstrations for Artemis and beyond, as well as utilization and commercialization,” the agency highlighted, “NASA will be augmenting six of the eight existing power channels of the space station with new solar arrays.”
Under an extension to its ISS Vehicle Sustaining Engineering contract with the station’s prime contractor Boeing, the six new iROSA arrays represent larger and more capable versions of ROSA. The high-efficiency XTJ Prime Space-Qualified Triple-Junction Solar Cell Arrays—which utilize the selfsame design as will fly aboard the CST-100 Starliner—are being provided by Boeing’s subsidiary, Sylmar, Calif.-headquartered Spectrolab. And major supplier Deployable Space Systems (DSS) of Goleta, Calif., is responsible for providing the housing canisters, solar array frames and “blankets” for the new system.
“The XTJ Prime space solar cells are much more efficient than any of their predecessors and are fit to support the cutting-edge research being done aboard the International Space Station,” said Spectrolab President Tony Mueller.
NASA previously reported that although the replacement of all eight power channels “would provide the most operational flexibility for the program”, the plan to upgrade only six channels through iROSA represents “the minimum amount required to avoid negatively impacting ISS operations”.
The six iROSA arrays will cover (and “shadow”) about two-thirds of the “legacy” arrays, with the “unshadowed” portion remaining active. “The two arrays are electrically combined,” NASA explained, “and both provide power to the ISS, resulting in an increase in power performance compared to the legacy ISS solar array.”
In its NASA Technical Reports Server (NTRS) documentation and imagery, NASA indicated that the first pair of iROSA arrays will cover the oldest set of legacy arrays—Power Channels 2B and 4B—on the P-6 truss, which was installed by the STS-97 shuttle crew in late 2000. “The first modification kit is planned to be assembled on the 2B power channel,” NASA’s Gary Jordan told AmericaSpace. “The 4B mod kit is scheduled to be completed on a following spacewalk.”
This will be followed by the second iROSA pair, which will cover one power channel each on the P-4 and S-4 trusses. The third iROSA pair will then cover the second S-4 power channel and one channel on the S-6 truss. Under this planning scenario, in its eventual configuration, iROSAs will cover both power channels on the P-6 and S-4 trusses and one power channel apiece on P-4 and S-6.
“This specific iROSA configuration was selected after careful consideration to balance observed degradation in the legacy solar arrays with the anticipated demand on each channel,” Mr. Jordan told AmericaSpace.
“The combination of the eight original, larger arrays and the smaller, more efficient new arrays will restore the power generation of each augmented array to approximately the amount generated when the original arrays were first installed,” NASA explained, “providing a 20-percent to 30-percent increase in power for space station research and operations.” And positioning them in front of six legacy arrays and mounting them onto the existing Beta Gimbal Assemblies (BGAs) will allow the new system to tap into existing Sun-tracking, power distribution and channelization functionality.
At present, the eight legacy arrays provide up to 160 kilowatts of electrical power, with the six new iROSA arrays expected to yield an additional 120 kilowatts during orbital “daytime”. Additionally, the “uncovered” legacy array will continue to generate an estimated 95 kilowatts for a combined total of 215 kilowatts. Physically, the new arrays measure 63 feet (19 meters) in length and 20 feet (6 meters) in diameter and are expected to be delivered to the station in three pairs aboard SpaceX Dragon cargo ships on May 2021’s CRS-22 mission and also aboard the CRS-25 and CRS-26 flights, targeted for April and September 2022. Following the arrival of the first set of iROSA arrays in May, the first EVA to install the hardware may occur as early as June.