Date:28 June 2017
The Parker Solar Probe requires some clever engineering to keep the systems cool as it heads into the sun’s atmosphere.
By David Grossman
To better understand solar physics, NASA is sending the Parker Solar Probe spacecraft into orbit around our host star. The probe will fly into the sun’s atmosphere, marking the first time in history that a spacecraft travels into the sun’s corona. As you might imagine, flying so close to the sun requires some special precautions. The probe’s cooling systems and solar arrays are as unique as its journey into the inferno.
On its voyage to the sun, NASA expects the probe to get within 3.7 million miles of the sun’s surface. An eight-foot-diameter shield called the thermal protection system (TPS) will guard most of the spacecraft’s components from the brunt of the heat. However, the craft’s solar panels will be the more exposed and require a robust cooling system.
Interestingly enough, the preferred coolant for the spacecraft’s solar panels is water. “Part of the NASA technology demonstration funding was used by APL and our partners at UTAS to survey a variety of coolants,” said Mary Kae Lockwood, the Parker Solar Probe spacecraft system engineer at the Johns Hopkins Applied Physics Lab (APL). “But for the temperature range we required [about 10° C to 125° C], and for the mass constraints, water was the solution.”
The water will be pressurised, which will raise its boiling point above 257° F, and a deionisation process will strip the water of any minerals that could gum up the system. Although the TPS will get as hot as 2,500° F, the cooling system is designed to keep the solar panels at a functional 360° F or lower. Flying through the sun’s atmosphere, the panels will 25 times the solar energy that panels receive in Earth orbit.
Using a solar array for a craft heading to the sun sounds obvious, but figuring out how to keep the panels from being destroyed in the intense heat is more complicated. There will be a standard cover of glass protecting the photovoltaic cells as well as a special ceramic carrier soldered onto the bottom of each cell. The ceramic substrate, called a platen, will then be glued on with a thermally conductive adhesive.
Letting the water in the cooling system boil away isn’t the only concern. APL researchers also need a way to keep the water from freezing after being launched into space. A heated accumulator tank will hold five litres of water during the launch and keep it from freezing until the spacecraft arrives at its destination, where temperatures will be markedly higher.
The cooling system will also include two-speed pumps and four radiators made of titanium tubes with aluminium fins a mere two hundredths of an inch thick. The system has a cooling capacity of 6,000 watts, enough for an average living room, and will undergo massive temperature swings as it launches into the void of space and toward the corona of the sun.
The complexity of the journey will require the Parker Solar Probe to automatically shift and adjust itself to keep the TPS in the right position to shield the craft. Even a one degree change in the angle of the solar panels relative to the sun would require 35 percent more cooling capacity, according to Lockwood.
“There’s no way to make these adjustments from the ground, which means it has to guide itself,” 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.” The Parker Solar Probe is expected to be one of the most autonomous spacecraft ever launched, if not the most autonomous.
NASA’s new solar spacecraft will trace how energy and heat move through the solar corona and explore the behaviour of solar wind and energetic solar particles. Solar wind can destroy satellites and affect power grids on Earth. “We must understand this space environment just as early seafarers needed to understand the ocean,” NASA states on the Parker Solar Probe website. Our new sun probe will help us get our feet wet.
Source: Johns Hopkins University