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Breakthrough Thermal Protection System Announced
October 12, 2017

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.

According to information provided by Johns Hopkins University’s Applied Physics Lab (APL), 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 (160C) or below.

The solar panels are shown here on this artist rendering of Parker Solar Probe; they are the black squares with gray rectangles on the center of the spacecraft. Image courtesy of NASA/JHUAPL.

“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. However, 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) and SolAero Technologies to mitigate damage from the incredible heat.

“This is all new,” Dr. 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,” Dr. Lockwood explained); 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 system provides 6,000 watts of cooling at nominal operating capacity.

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.

When Parker Solar Probe is hurtling past the sun at some 450,000 miles an hour (724,000 KPH), it will be 90 million miles from mission controllers on Earth—too far for the team to “drive” the spacecraft. This means that adjustments to how the spacecraft is protecting itself with the TPS need to be handled by Parker Solar Probe’s onboard guidance and control systems. These systems use new and effective autonomous software to allow the spacecraft to instantly alter its pointing to maximize protection from the sun. This autonomous capability is critical to the operation of the spacecraft’s solar arrays, which must be constantly adjusted for optimal angle as Parker Solar Probe hurdles through the sun’s harsh, superheated corona.

“During solar encounters, very small changes in the wing angle of the solar array can vastly change cooling capacity needed,” according to Dr. Lockwood, also explaining that a one degree change in the array angle of one wing would require 35 percent more cooling capacity.

“There’s no way to make these adjustments from the ground, which means it has to guide itself,” Dr. Lockwood said. “APL developed a variety of systems—including wing angle control, guidance and control, electrical power system, avionics, fault management, autonomy, and flight software—that are critical parts working with the solar array cooling system.”

That autonomy, along with the new cooling system and pioneering solar array upgrades, will be crucial to ensuring that the Parker Solar Probe can perform the never-before-possible science investigations at the sun.

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