MSL EDL Instrument (MEDLI) Suite
PI: Dr. F. McNeil Cheatwood, NASA Langley Research Center
Deputy PI: Dr. Michael J. Wright, NASA Ames Research Center
The Mars Science Laboratory Entry, Descent, and Landing Instrument (MEDLI) Suite is a set of engineering sensors designed to measure the atmospheric conditions and performance of the MSL heatshield during entry and descent at Mars. While not part of the core MSL scientific payload, it will provide important information for the design of entry systems for future planetary missions. The instrument suite was designed and developed by NASA Langley Research Center, in partnership with NASA Ames Research Center.
The MEDLI suite consists of seven MEDLI Integrated Sensor Plugs (MISP) and seven Mars Entry Atmospheric Data System (MEADS) pressure sensors, located on the heatshield of the spacecraft. In addition, a Sensor Support Electronics (SSE) box is mounted inside the heatshield to provide power, signal conditioning, and analog to digital conversion. The instrument suite starts acquiring data after the entry system separates from the spacecraft bus (about 10 minutes prior to entry) and continues to take data at approximately 8 Hz until after the main parachute is deployed (about 4 minutes after entry). A limited amount of the data collected may be included in the real time telemetry stream during entry, but the full dataset will be transferred to the Rover Compute Element and will be transmitted to Earth within the first month after landing.
MEDLI Integrated Sensor Plug (MISP).
Mars Entry Atmospheric Data System (MEADS) pressure sensor.
Sensor Support Electronics mounted on vibration isolators.
The MSL spacecraft will enter the Martian atmosphere at approximately 6.1 km/s, making it the second fastest NASA entry to Mars (Pathfinder entered at 7.3 km/s in 1997). However, the MSL aeroshell is much larger than Pathfinder’s (4.5 vs 2.65 meters), and MSL is also much heavier. As a consequence, the flow around the MSL spacecraft is expected to become turbulent early during the entry, and the resulting heat flux and shear stress on the heatshield will be the highest ever encountered at Mars. In addition, MSL is flying a guided lifting trajectory, a first for Mars entry. The design of the entry system to withstand such environments relies primarily on simulation tools, such as computational fluid dynamics (CFD). Because the Martian atmosphere is mainly composed of CO2 (as opposed to air), it is very difficult to conduct experiments on Earth that simulate all of the aspects of a Martian entry. As a consequence, the uncertainties in the engineering models for the heating encountered and the aerodynamic performance of the spacecraft are high, with the result that the spacecraft was designed with large margins (which come at the cost of mass). Including these margins, the heatshield is designed to withstand 216 W/cm2 of heating, 540 Pa of shear, and 0.37 atmosphere of pressure.
The only way to reduce these margins on future Mars missions is to obtain data on the performance of the system during the entry. These data can be compared with the pre-flight predictions to evaluate the assumed level of uncertainty and to identify places where the current models require improvement. The MEDLI suite will provide these data, and will in fact return the largest EDL dataset ever obtained during a non-Earth entry. The data collected from the MEADS sensors will be combined with data from the Inertial Measurement Unit (IMU) to provide data on surface pressure distribution, vehicle orientation, dynamic pressure, Mach number, and the atmospheric density and winds as a function of altitude. The data collected from the MISP sensors will be used to evaluate the peak heat flux, distribution of heating on the vehicle, map transition to turbulence, and evaluate the thermal protection system (TPS) surface and in-depth material performance.
Each of the seven MISP plugs includes four type-K thermocouples at different depths in the TPS material (nominally 0.1, 0.2, 0.45, and 0.7 inches below the surface), and a single Hollow aErothermal Ablation and Temperature (HEAT) sensor. The thermocouples measure the temperature of the TPS as a function of time during entry, while the HEAT sensor measures the propagation of a single isotherm (~700°C) through the material. These two measurements, taken together, provide a detailed time history of the performance of the TPS material, at a frequency of 8 Hz (2 Hz for some of the deeper sensors). In addition, the HEAT isotherm measurement can be calibrated to provide an assessment of the recession, or loss of surface material due to ablation, as a function of time. Although all of the thermocouples are placed below the surface of the TPS, it will be possible to reconstruct the aerothermal environment by modeling the anticipated performance of the various sensors in relation to each other. The seven MISP plugs are concentrated primarily in the high heating, turbulent flow portion of the aeroshell. The 1.30-inch diameter plugs are constructed from the same TPS material used on the heatshield, Phenolic Impregnated Carbon Ablator (PICA) in this case, and are installed by drilling a 1.31-inch diameter hole into the heatshield PICA and smaller holes through the aeroshell structure for the wires to pass through. The entire assembly is secured into place using RTV-560, the same adhesive material used as a gap filler between the PICA tiles. The final design was verified via arc jet testing at NASA Ames, using identical conditions to those used in the qualification of the MSL heatshield.
Location of the seven MISP plugs (T1-T7) and seven MEADS pressure ports (P1-P7) on the MSL heatshield. The colors indicate predicted heat flux, with white the hottest and blue the coolest areas.
Each of the seven MEADS sensors includes a pressure transducer mounted on the interior of the heatshield structure. A 0.1” hole is drilled into the TPS to allow an accurate surface pressure reading to be made. This diameter is large enough to reduce measurement lag to an acceptable level and testing verified that a hole this size would not adversely impact TPS performance. A metallic tube extends from the TPS bondline at the bottom of the hole to the pressure transducer body. Each tube is designed to withstand launch and entry vibration loads by the inclusion of a strain relief loop. The pressure transducers are a split design which separates the measurement device from the electronics. These electronics are incorporated into the SSE to keep them warm during the cruise phase between Earth and Mars. The seven pressure locations form a cross pattern in the low-heating, high-pressure portion of the aeroshell. The pattern allows engineers to evaluate vehicle orientation by looking at the pressure difference between two locations as compared to predictions. The final design was verified via arc jet testing at the Boeing Large Core Arc Tunnel (LCAT) facility, using worst-case predictions of heating, shear and pressure at the port locations.
This document was prepared by Dr. F. McNeil Cheatwood/NASA LaRC et. al. The content has not been approved or adopted by JPL or the California Institute of Technology. Any views and opinions expressed herein do not necessarily state or reflect those of JPL, or the California Institute of Technology.