MEDLI2 is one of the key technologies on the protective aircraft shell of the rover, which can help record the entry, descent and landing of the spacecraft.
“Tango Delta, landing confirmation, gritty on the surface of Mars, ready to start looking for signs of past lives.” For more than six years, the Mars Entry, Descent and Landing Instrument 2 (MEDLI2) team has been waiting to hear these words.
On February 18, 2021, the Perseverance Rover successfully landed and began a robotic exploration of the Red Planet. MEDLI2 is one of the key technologies on the protective aircraft shell of the rover, which can help record the entry, descent and landing (EDL) of the spacecraft.
The role of MEDLI2 is to collect critical data about the harsh environment during the constant entry into the earth̵
MEDLI2 uses its pressure, temperature, and heat flux measurements to determine the amount of heat generated on the heat shield and rear shell. Together, these two components form the aircraft shell, which houses the Persevering Rover.
MEDLI2 caught fire five hours before the “7 Minutes of Horror” landed. This provides the initial conditions before the input and the time for the MEDLI2 electronics to stabilize the temperature before collecting critical input data. The MEDLI2 team breathed a sigh of relief and received an indication that MEDLI2 has been successfully activated. During the five hours before the entry, the team has been monitoring the entered data.
Most of the MEDLI2 sensors and main electronic equipment are installed on the Mars 2020 heat shield. About 10 seconds after deploying the supersonic parachute, MEDLI2 was powered off for the last time after completing its work. Since it is essential to separate the heat shield to allow the Perseverance Rover to be unplugged from the entering vehicle, MEDLI2 needs to be turned off a few seconds before separation to prevent any power issues. Then, the wire harness connecting the heat insulation board and the rear case is cut off by the ignition of the high temperature cutting machine, and the heat insulation board is put down.
“We found no problems with the separation,” said Henry Wright, project manager for MEDLI2 at NASA’s Langley Research Center in Hampton, Virginia. “The Mars 2020 heat shield is completely separated from the Mars 2020 entry car. When the MEDLI2 hardware falls towards the surface of Mars, it is clearly visible on the Mars 2020 heat shield. Good job!”
After successful landing, all MEDLI2 data are stored in Hengxin in order to return to Earth. Perseverance can also return so-called “critical event data” in real time during EDL. This data includes a subset of the MEDLI2 data, which allows observation of the conditions experienced when entering the vehicle when it occurs. Three days after Hengxin successfully landed, the MEDLI2 data returned to Earth, and the next phase of the project began: data analysis and performance reconstruction.
Wright said: “The returned data is beautiful in terms of depth, complexity and clarity, allowing the MEDLI2 team to make interesting observations immediately.”
The data collected from MEDLI2 also provides measurements that will be used to determine the characteristics of the atmosphere over which Mars will enter the vehicle in 2020. MEDLI2 provides basic EDL observation data to understand how much margin is left on the perseverance item, as well as the data that will be used to improve predictive models and tools for future tasks.
Insight into the data
The insulation temperature of the heat shield was recorded throughout the entry phase, and it was consistent with the entry prediction. During entry, the peak measured temperature of the insulator was 1,830 degrees Fahrenheit or 1,000 degrees Celsius, which correlates with the estimated maximum surface temperature of approximately 2,550 degrees Fahrenheit or 1,400 degrees Celsius.
MEDLI2 also uses its embedded thermocouple to determine how much heat shield protection insulation may have been burned, called ablation. All thermocouples survived after entering the heating pulse, indicating that the ablation of the heat shield was less than expected. This observation can be used to re-evaluate the amount of insulation on the heat shield that can potentially reduce the overall weight of the vehicle.
The surface pressure was also measured throughout the same phase, where the peak surface pressure of the heat shield matched the team’s entry prediction. MEDLI2 focuses on a measurement of the surface pressure of the heat shield to capture the full range of the maximum surface pressure. The remaining six heat shield pressure measurements have a pressure range to emphasize conditions in supersonic flight (from about five times the speed of sound on Mars to the deployment of a supersonic parachute). When the atmospheric density changes and the influence of wind is more obvious, this information can give you a better understanding of the performance of the entering vehicle. MEDLI2 pressure data will be used to improve modeling methods for future EDL tasks.
MEDLI2 includes sensors on the rear shell of the Mars 2020 entry vehicle, and there are very few observations in this area. The rear housing sensor kit includes surface pressure, insulation temperature and direct surface heating measurements. Knowing the surface pressure entering the rear of the vehicle can help reduce the size of the landing footprint. The insulation temperature data of the rear shell is within the initial prediction and can be used to reduce the uncertainty of the modeling. As with heat shield insulation, understanding the temperature performance of the rear case insulation may lead to a reduction in the quality of the rear case insulation. The direct surface heating measurement of the back shell also helps to reduce the uncertainty in the prediction model.
Now that the MEDLI2 team has its flight data and supporting data since Mars 2020, the MEDLI2 team will continue to analyze these data for the next six months.
NASA Langley Research Center, Hampton, Virginia
Jet Propulsion Laboratory in Pasadena, California.