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MEDLI2 was one of the crucial technologies onboard the rover’s protective aeroshell that helped document the entry, descent, and landing (EDL) of the spacecraft.
MEDLI2’s role was to collect critical data about the harsh environment during Perseverance’s entry through the planet’s atmosphere. It included three types of sensors – thermocouples, heat flux sensors, and pressure transducers – that measured extreme heat and pressure during entry. It also contained electronics and hardware for recording the thermal and pressure loads experienced during entry and through the parachute deployment.
MEDLI2 used its pressure, temperature, and heat flux measurements to determine the heating that occurred on the heat shield and back shell. Together, these two components make up the aeroshell, which housed the Perseverance rover.
MEDLI2 was powered on five hours prior to the “7 Minutes of Terror” to land on Mars. This provided the initial conditions prior to entry as well as time for the MEDLI2 electronics to stabilize temperature prior to collecting the critical entry data. The MEDLI2 team was relieved to receive the indication that MEDLI2 was successfully activated. The team continued to monitor the incoming data throughout this five-hour coast phase prior to entry.
The majority of the MEDLI2 sensors and the primary electronics were mounted on the Mars 2020 heat shield. About 10 seconds after the supersonic parachute was deployed, MEDLI2 was powered down for the last time as it had completed its job. Since it is critical for the heat shield to be separated to allow the Perseverance rover to be extracted from the entry vehicle, it was required that MEDLI2 be turned off a few seconds before the separation to prevent any electrical power issues. The harness connecting the heat shield and back shell was then severed by the firing of a pyro-cutter, and the heat shield was dropped.
“We didn’t find any issues with the separation,” said Henry Wright, MEDLI2 project manager at NASA’s Langley Research Center in Hampton, Virginia. “The Mars 2020 heat shield was cleanly separated from the Mars 2020 entry vehicle. The MEDLI2 hardware is clearly visible on the Mars 2020 heat shield as it falls toward the surface of Mars. Job well done!”
All of the MEDLI2 data was stored on Perseverance for return to Earth after a successful landing. Perseverance also returned what is called “critical event data” in real time during the EDL. This data included a subset of the MEDLI2 data which allowed observations into what the entry vehicle was experiencing as the entry was happening. Three days after Perseverance’s successful landing, the MEDLI2 data was returned to Earth, and the next phase of the project began: data analysis and performance reconstruction.
“The returned data was beautiful in its depth, complexity, and clarity, allowing the MEDLI2 team to make some immediate and interesting observations,” Wright said.
Data collected from MEDLI2 also provides measurements that will be used to determine the properties of the atmosphere the Mars 2020 entry vehicle flew through. MEDLI2 provides essential EDL observation data to understand how much margin remained on the Perseverance entry along with data that will be used to improve prediction models and tools for future missions.
Taking a Deeper Look Into the Data
Heat shield insulation temperatures were recorded throughout the complete entry phase and were consistent with the entry predictions. The peak measured temperature of the insulation during the entry was 1,830 degrees Fahrenheit, or 1,000 degrees Celsius, which correlates to an estimated maximum surface temperature of about 2,550 degrees Fahrenheit, or 1,400 degrees Celsius.
MEDLI2 also used its embedded thermocouples to determine how much of the heat shield protective insulation may have burned away, known as ablation. All of the thermocouples survived the entry heating pulse, indicating the heat shield ablation was less than predicted. This observation could be used to reevaluate the amount of insulation that is needed on a heat shield to potentially reduce the overall entry vehicle mass.
Surface pressures were also measured throughout the same phase with a peak surface pressure of the heat shield matching the team’s entry predictions. MEDLI2 focused on one heat shield surface pressure measurement to capture the complete range of the maximum surface pressures. The remaining six heat shield pressure measurements had a pressure range to emphasize the conditions during the supersonic flight regime (from about five times the speed of sound on Mars through the deployment of the supersonic parachute). This information allows additional insight into how the entry vehicle performs when the impacts of atmospheric density variations, winds, etc., are more pronounced. The MEDLI2 pressure data will be used to improve the modeling approach for future EDL missions.
MEDLI2 included sensors on the back shell of the Mars 2020 entry vehicle, an area that has had limited observations. Surface pressure, insulation temperature, and direct surface heating measurements comprised the back shell sensor suite. Knowledge of the surface pressure on the back of the entry vehicle contributes to a reduction in the size of the landing footprint. Back shell insulation temperature data was within the initial predictions, which can be used to reduce the modeling uncertainty. As with the heat shield insulation, understanding the back shell insulation temperature performance could lead to a reduction in the back shell insulation mass. Direct surface heating measurements of the back shell also contribute to reducing the uncertainty in the predictive models.
Now that the MEDLI2 team has its flight data and the supporting data from Mars 2020, the MEDLI2 team will continue to analyze this data for the next six months.