Two days after NASA’s Parker Solar Probe flew past Venus
toward its rendezvous with the Sun, the spacecraft had drawn close enough to our star that its power-generating solar array wings began to tilt themselves inward – a task directed by the spacecraft itself, based on the rising temperatures – away from the Sun and behind the sun shield. This is the first time that autonomous, closed-loop solar array angle control based on temperature has taken place on a spacecraft.
This solar array movement, controlled by software within the spacecraft’s main processor, began on Oct. 5 soon after Parker Solar Probe’s distance from the Sun dropped below about 65 million miles (a distance of 0.7 astronomical units, or AU; one AU is 93 million miles, the distance from the Earth to the Sun). The spacecraft – designed, built and operated by the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. – is heading toward its first perihelion, or closest approach to the Sun, on Nov. 5.
To understand how spacecraft use solar arrays for power, and how Parker Solar Probe’s designers had to do things differently because of the mission’s proximity to the Sun, we asked APL’s Lew Roufberg, spacecraft power subsystem lead engineer, to explain:
“Typical space solar arrays either remain in a fixed position relative to the spacecraft body or they track and maintain Sun-pointing with motors. For Parker Solar Probe, we needed to create an alternative method, or the photovoltaic cells would overheat and there would be severe darkening of the cover-glass and the clear adhesive on the cells that are used for optical filtering and radiation protection. The computer-controlled tilting of the wings away from the Sun, when necessary, reduces irradiance – the amount of light that shines onto the array – and that lowers the absorbed heat. As the solar array wings are angled more steeply and closer to the body of the spacecraft, by design they become partially shaded behind the Sun shield (the Thermal Protection System
), and this further reduces irradiance as the spacecraft approaches the Sun. To decrease temperature even more, a cooling system pumps water through channels within the solar array wings, and the heat is transported to radiator panels located in the shadow of the TPS. Those radiators are exposed to cold space and radiate the heat away. During most conditions throughout the mission, the two solar array wings together can produce at least 400 watts of electrical power, about enough to run a kitchen blender – and plenty to operate all of our systems and instruments.”
Parker Solar Probe’s computer contains solar array angle control algorithms developed by APL’s Carson Baisden that uses the temperature of the solar arrays to determine the appropriate tilt angle for the wings. Roufberg explains how the arrays and the software work to manage the heat load and power requirements of the spacecraft:
“As the probe approaches the Sun and temperature increases beyond a set limit – about 150 Fahrenheit, or 65 Celsius – the algorithm transitions to using solar array power to control array angle for improved agility. Solar array power responds faster than temperature changes, as the solar irradiance is modulated by changes to array wing angle. At high tilt angles, where the Sun’s rays are at a grazing angle to the wing, small changes in angle make a large difference. At closest approach to the Sun, if the wings are tilted a couple of degrees too far, there will not be enough power to operate the spacecraft; if the wings are extended a couple of degrees in the other direction, the array will overheat.”
Because of that sensitivity, the actuation (or movement) and commanding of the solar arrays is critical for accurate and reliable pointing. APL’s Sarah Flanigan established an interface with the solar array actuators to ensure the solar array position is known at all times and continuously points to their optimal angle under all conditions. Similarly, the guidance and control system of Parker Solar Probe – lead by the Lab’s Robin Vaughan – is paramount to the health and success of the mission. The solar array angle control algorithms and actuator control logic is integrated within the guidance and control software to work in unison as the spacecraft’s attitude relative to the Sun changes. Due to the harsh environments near the Sun and the unpredictable, ever-flowing solar wind, the conditions can change quickly. The software quickly adjusts the arrays based on the temperature and electrical characteristics observed on the solar array at any given moment.
“In the solar array power control mode, the control algorithm seeks to maintain a solar array output power that is just sufficient to power the spacecraft. This results in the minimum array temperature for a given spacecraft load power. The angle control software measures the solar array power and adjusts wing angle accordingly. The solar array electrical power is proportional to absorbed solar irradiance and is well correlated to array temperature, so controlling array power indirectly limits temperature. However, the close correlation between array power, irradiance, and temperature only exists when the solar array is electrically operated at its peak-power point. This requires active control of the array voltage so that it operates at the single point where maximum power is provided, among all the possible voltage and current combinations that a solar array can produce at any instant.”
Managing the right balance between power and heat is critical for the spacecraft, and because of the distance it will be from Earth and the harsh environment it will be operating in, Parker Solar Probe needs to be able to take care of itself. Roufberg explained that the solar array power control algorithm relies on the spacecraft’s power system electronics (PSE) to extract maximum electrical power from the arrays. APL developed the PSE for high power processing efficiency and low mass for the Parker Solar Probe. The PSE uses an electrical peak power tracking algorithm to extract maximum solar array power.
“The PSE software ensures the best utilization of array power, and also ensures that the measured power correlates to absorbed solar energy and temperature. Higher power extracted from the arrays also reduces wing temperature, as energy is removed from them. The spacecraft power system also includes a Lithium-Ion battery to provide power to loads when the solar arrays are not illuminated, such as during launch and when eclipsed by Venus. The battery operates at an intermediate state of charge to serve as an energy reservoir to accommodate short-term fluctuations in the arrays and load power to ensure that the electrical peak-power tracking can continue without overcharging the battery. The battery state of charge is also used within the angle control algorithm to determine and periodically update the target set-point for optimum array output power.”
Prior to launch, the team developed novel solar array simulators that were tested repeatedly to ensure proper performance over a range of expected and less-than-desired conditions. Some simulations were performed with computer models, some were performed on a power system hardware testbed, and others were conducted with the actual spacecraft while it was in the space-simulation thermal-vacuum chamber at NASA’s Goddard Space Flight Center.
“We’re looking forward to the continued successful operation of the autonomous solar array angle control,” said Roufberg, “It’s one of the many new developments – along with the TPS, solar array cooling system, and on-board autonomy system – that make this historic mission possible.”
Parker Solar Probe will begin its first of 24 solar encounters on Oct. 31.