As NASA's Parker Solar Probe spacecraft begins its first historic encounter with the sun's corona in late 2018 - flying closer to our star than any other mission in history - a revolutionary cooling system will keep its solar arrays at peak performance, even in extremely hostile conditions.
Every instrument and system on board Parker Solar Probe (with the exception of four antennas and a special particle detector) will be hidden from the sun behind a breakthrough thermal protection system (TPS) - an eight-foot-diameter shield that the spacecraft uses to defend itself against the intense heat and energy of our star.
Every system will be protected, that is, except for the two solar arrays that power the spacecraft. When the spacecraft is closest to the sun, the solar arrays will be receiving 25 times the solar energy they would while orbiting Earth, and the temperature on the TPS will reach more than 2,500°F (1,370°C). The cooling system will keep the arrays at a nominal temperature of 320°F (160°C) or below.
"Our solar arrays are going to operate in an extreme environment that other missions have never operated in before," said the Johns Hopkins Applied Physics Lab's Mary Kae Lockwood, spacecraft system engineer for Parker Solar Probe.
The very outermost edges of the solar arrays are bent upward, and when the spacecraft is closest to the sun, these small slivers of array will be extended beyond the protection of the TPS in order to produce enough power for the spacecraft's systems.
The incredible heat of our star would damage conventional spacecraft arrays. So, like many other technological advances created especially for this mission, a first-of-its-kind actively cooled solar array system was developed by APL, in partnership with United Technologies Aerospace Systems (UTAS) in Windsor Locks, Connecticut (which manufactured the cooling system), and SolAero Technologies of Albuquerque, New Mexico (which produces the solar arrays).
"This is all new," Lockwood said of the innovations related to the actively cooled solar array system. "NASA funded a program for Parker Solar Probe that included technology development of the solar arrays and their cooling system. We worked closely with our partners at UTAS and SolAero to develop these new capabilities, and we came up with a very effective system."
The Parker Solar Probe cooling system has several components: a heated accumulator tank that will hold the water during launch ("If water was in the system, it would freeze," Lockwood said); two-speed pumps; and four radiators made of titanium tubes (which won't corrode) and sporting aluminum fins just two hundredths of an inch thick.
As with all power on the spacecraft, the cooling system is powered by the solar arrays - the very arrays it needs to keep cool to ensure its operation. At nominal operating capacity, the system provides 6,000W of cooling capacity - enough to cool an average-sized living room.
Somewhat surprisingly, the coolant used is nothing more than regular pressurised water - approximately five liters, deionised to remove minerals that could contaminate or harm the system. Analysis showed that, during the mission, the coolant would need to operate between 50°F (10°C) and 257°F (125°C) - and few liquids can handle those ranges like water.
"Part of the NASA technology demonstration funding was used by APL and our partners at UTAS to survey a variety of coolants," said Lockwood. "But for the temperature range we required, and for the mass constraints, water was the solution." The water will be pressurised, which will raise its boiling point above 257°F.
The solar arrays feature their own technical innovations. "We learned a lot about solar array performance from the [APL-built] MESSENGER spacecraft, which was the first to study Mercury," said Lockwood. "In particular, we learned how to design a panel that would mitigate degradation from ultraviolet light."
The cover glass on top of the photovoltaic cells is standard, but the way the heat is transferred from the cells into the substrate of the panel, the platen, is unique. A special ceramic carrier was created and soldered to the bottom of each cell, and then attached to the platen with a specially chosen thermally conductive adhesive to allow the best thermal conduction into the system while providing the needed electrical insulation.