The European Space Agency (ESA) is preparing the ExoMars mission for a flight in March 2016. One of the main objectives of the mission is the demonstration of a successful Entry, Descent and Landing on Mars. Gathering scientific data during these flight phases is a further key element of this mission and would provide very important data for future missions. This data could for example be used for an optimization of the heat shield, as the design of the back cover heat shield was carried out with relatively high safety margins. This is due to the fact that the prediction of the aerothermal and radiative loads on the back cover using existing experimental and numerical tools still has big uncertainties.
During entry into the Martian atmosphere, the convective heating of the back cover region of the capsule is significantly lower than the front cover heating but has an instationary behaviour which is difficult to simulate numerically. Beside the convective heating a further driving parameter for the back cover design safety margins is the radiative heating created by the hot and excited carbon-dioxide molecules in the capsule shoulder region behind the bow shock. In some areas of the back cover the predicted radiative heat flux is significantly higher than the convective heating for most of the critical trajectory points. But especially this radiative heat flux on the back cover heat shield during the entry phase cannot be accurately simulated by existing numerical and experimental tools. Therefore the COMARS+ payload has been developed by DLR to measure the aerothermal and radiative loads at different positions on the Schiaparelli capsule back cover.
The back cover instrumentation COMARS+ consists of three combined aerothermal sensors, one broadband radiometer sensor and an electronic box. The aerothermal sensors combine four discrete sensors measuring static pressure, total heat flux, temperature and radiative heat flux at two specific spectral bands. For the pressure measurement a very small Pirani-type pressure sensor is used and the total heat flux is measured by a commercial heat flux microsensor which also includes a temperature signal. The radiative heat flux at two different spectral bands is measured by two sensors called ICOTOM contributed by the French space agency CNES. The infrared radiation in a broadband spectral range is measured by the separate broadband radiometer sensor. The sensor signals are recorded by the onboard data acquisition system using three analogue channels. Hence the overall 23 sensor and 8 housekeeping signals of the payload have to be amplified to the needed input voltage range and multiplexed to the three analogue acquisition channels. This is done using an electronic box which is also part of the COMARS+ payload. In addition to amplification and multiplexing, a sensor signal conditioning is also integrated in the electronic layout.
The ambitious low mass and low power design ended at a total mass of 1.73 kg and power consumption of 4.5 Watt for the complete COMARS+ payload. The following figure shows the complete payload including interconnecting harness.
The COMARS+ sensors are mounted to the honeycomb substructure of the capsule below the back cover heat shield. The cylindrical part of the sensor thereby penetrates the heat shield panels, so that the front end of the sensor is mounted flush with the heat shield surface. The detailed design of COMARS and broadband radiometer sensor is shown in Figure 2 and Figure 3.
The electronic box is fixed to the inner surface of the honeycomb substructure using four honeycomb inserts. The connection between sensors and electronic box is done via the shielded payload harness which is shown in Figure 1.
The honeycomb substructure and the heat shield panels were equipped with corresponding boreholes during the manufacturing process to accommodate the COMARS+ payload components.
Figure 4 shows the four COMARS+ sensors integrated into the Schiaparelli landing module back cover of the ExoMars 2016 mission. In Figure 5 a close-up view of COMARS and radiometer sensor located at the shoulder of the back cover heat shield is shown. All boreholes are covered with Kapton tape until the capsule is fully assembled to prevent intrusion of particles.
Before the COMARS+ payload was integrated into the back cover structure all payload components had to pass an extensive qualification and acceptance test campaign. Mechanical vibration and shock tests were conducted to simulate all mechanical loads that occur during flight like launch loads and stage separation shocks.
Thermal cycling tests under vacuum condition were executed to simulate the thermal environment of the capsule. Different temperature levels were used for the individual payload components depending on their location on the back cover. The test temperatures ranged from -110°C up to 100°C which are the minimum and maximum calculated temperatures for the payload parts during cruise to Mars and Mars entry.
Electromagnetic compatibility tests were used to check that the payload is compatible with the electromagnetic environment of the capsule and does not emit electromagnetic energy that could cause electromagnetic interference in other devices.
To test the sensors in a realistic environment, wind tunnel tests were conducted in the arc heated facility L2K at DLR Cologne using the qualification models of COMARS and radiometer sensor. To simulate Martian atmosphere a mixture of carbon dioxide and nitrogen (97% CO2 and 3% N2) was used for the tests.
The sensors were integrated into a flat plate model with water-cooled cooper nose as shown in Figure 6, with the hot gas flow passing over the model surface from left to right.
Due to the perpendicular orientation of the sensors related to the flow direction the complete sensor surface was surrounded with the high enthalpy flow and the sensors were exposed directly to the boundary layer flow and radiation coming from the shock layer of the bow shock at the model nose. This set-up is very similar to the actual condition at the ExoMars back cover. By varying the distance between model and nozzle exit and the model angle of attack, heat flux and pressure were adjustable in a certain range. But the facility is not able to completely reproduce the heat flux loads and pressures that will be present during the actual flight through Martian atmosphere. The qualification model of the COMARS+ electronic box was also used for the tests to incorporate the whole measurement chain.
After the acceptance tests and before the payload was integrated into the back cover it was sterilized by dry heat microbial reduction in an oven at 125°C for several hours. This is necessary to reduce the payload bioburden according to the planetary protection requirements to prevent contamination of the Mars environment with terrestrial microorganisms. To verify the planetary protection requirements over 30 bioburden assays were performed and analyzed after 72 hours of incubation. After the sterilization process the payload parts were sealed in sterile bags and stored in a cleanroom until integration into the back cover.
The integration took place in January 2015 at the ExoMars integration site at Thales Alenia Space in Turin after a successful flight acceptance review. Functional tests, which were conducted after the mechanical and thermal/vacuum tests at system level (including the complete capsule), and final functional checks in Baikonur were also performed successfully and the payload in now ready for launch in March 2016 from the launch site in Baikonur.
 A. Guelhan, F. Siebe, T. Thiele, Combined Sensor Assembly COMARS+ for ExoMars EDM Demonstrator, 7th European Workshop on Thermal Protection Systems and Hot Structures, Noordwijk, The Netherlands, 3.-5.April 2013.
 P. Omaly, Narrow band radiometer ICOTOM, IPPW-9, Toulouse, France, June 2009.