46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

MSL Rear Thermal Characterization

Juan Cepeda-Rizo, 1 Gordy C. Cucullu III, 2 Daniel Zayas, 3and Pat Wu, 4 Shyam G. Sunder5 Jet Propulsion Laboratory, Pasadena, CA, 91109-8099

The on the MSL rover are essential for navigation and mobility. Precaution is taken to operate the cameras below their maximum allowable temperatures. The location of the rear hazcams – on each side of the hot 2000 W MMRTG module – make it susceptible to overheating during extended daytime use. Ground-based prediction tools used to estimate a worst-case temperature of the camera have been shown to be overly conservative and restrict rover operation time. The goal of this study was to characterize the thermal behavior of the camera in situ, using telemetry as a means to improve the prediction tools. A model was also created to allow for predicting temperatures during any given set of changes in environment and orientation.

Nomenclature

AFT = Allowable Flight Temperature ECAM = Engineering Camera HRS = Heat Rejection System Hazcam = Hazard Avoidance Camera LMST = Local Mean Solar Time LST = Local Solar Time MAHLI = Hand Lens Imager MMRTG = Multi-Mission Radioisotope Thermoelectric Generator MSL = = Navigation Camera = Mars solar , 24h 39m 35s

I. Introduction The Rover (Figure 1.) has traversed nearly 10 kilometers and has completed approximately 2.5 - years of operation as of the writing of this paper. In June 23, 2014, Curiosity finished four-seasons and one year of operation.

1 Senior Thermal Systems Engineer, 4800 Oak Grove Dr., Pasadena, Ca 91109, M/S 125-123. 2 Senior Thermal Systems and Hardware Engineer, 4800 Oak Grove Dr., Pasadena, Ca 91109, M/S 125-123. 3 Thermal Systems Engineer, 4800 Oak Grove Dr., Pasadena, Ca 91109, M/S 125-123. 4 Senior Thermal Systems Engineer, 4800 Oak Grove Dr., Pasadena, Ca 91109, M/S 125-123. 5 Intern, Thermal Systems Engineering, 4800 Oak Grove Dr., Pasadena, Ca 91109, M/S 125-123. 46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

Figure 1. Curiosity Self-Portrait at '' Site on This self-portrait of NASA's Curiosity shows the vehicle at the 'Mojave' site, where its drill collected the mission's second taste of Mount Sharp. The scene combines dozens of images taken during January 2015 by the MAHLI camera at the end of the rover's robotic arm [1].

Of the total of seventeen cameras on Curiosity, twelve are redundant pairs of engineering cameras and include the Front and Rear Hazcams, and the Navcam, and are used in part to aid in the navigation and positioning of the rover. The hazcams pairs are mounted on the lower portion of the front and rear of the rover. These black-and- white camera pairs use visible light to produce three-dimensional (3-D) terrain maps. These maps safeguard against the rover inadvertently crashing into obstacles, and works in tandem with software that allows the rover to safely make some navigation choices on board. II. Description

1. Engineering Cameras

A. Description of the Engineering Hazcams The cameras each have a wide field of view of about 120 degrees. The rover uses pairs of hazcam (Figure 2) images to map out the shape of the terrain as far as three meters (ten feet) in front of it, in a "wedge" shape that is over four meters wide (13 feet) at the farthest distance. The cameras need a wide FOV because they cannot move independent of the rover; they are mounted directly to the rover body [2]. The engineering cameras, or ECAMs, are of the same design used on the twin Mars Exploration Rovers, and that landed on Mars in 2004. The camera electronic box contains a heater resistor that warms up the electronics to above the minimum operating temperature of -55 °C. Because the detector head is thermally isolated from the electronics box, the camera electronics can be heated without significantly warming the detector head, which helps to keep thermally induced CCD dark current to a minimum. [3]. 46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

Figure 2. A schematic of the engineering camera and components.

B. The Rear Hazcam Description The rear hazcams are located behind the rover on each side of the MMRTG, which powers the rover and sees temperatures near 200°C. The hot plates (in blue in Figure 4) typically see temperatures of 70 °C, and provide much environmental heating to the rear hazcams, so much so that they are typically 20 °C warmer than the Navcam or front hazcams.

Figure 4. There are two rear hazcams on each side of the MMRTG heat exchanger plates (blue tubing). Their proximity to the hot plates causes them to be much warmer than the other cameras.

2. Rear Hazcam Environment One of the benefits of exposure to the hot MMRTG environment the rear hazcams experience, is that they can be operated much earlier in the morning and much later in the evening than any other cameras or actuators. A downside of the extra heating is that they are more susceptible to overheating, especially during the summer season. Figure 5 shows a description of the rear hazcam environment. It is worth noting that the purpose of the cold plate is to reject excess heat from inside the rover. 46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

Figure 5. Description of the environmental heating loads that the rear hazcam experiences.

The cameras have two circuit boards in the electronics box that are thermally isolated from the chassis. There is thermal radiation between each board, but little thermal conduction, and some thermal convection from the CO2 gas from the Mars atmosphere. In general, the rover receives heating and cooling from solar, atmosphere, ground, and sky, as well as self-heating from usage. However, a large portion of the heating comes from thermal radiation from the MMRTG as previously described.

3. Model Description A model was created using Thermal Desktop illustrated in Figure 6. The model represents the heating environment described for the rear hazcam.

Figure 6. Thermal Desktop Model

46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

Model Construction Enclosure Represented by six rectangular Thermal Desktop surfaces, with nodes combined at the interface • Material is Al 7075, eighteen nodes total • Mounting boss features for electronics represented by diffuse nodes of equivalent thermal mass (four nodes total). • Each electronics board represented by sixteen node Thermal Desktop surface boards joined together by “ribbon cable”. • Node-node contactor at each corner to represent fastener attaching boards to mounting bosses. • Internal Radiation and Conduction through CO2 was also included. • Power-on heatload split evenly between boards. • Flexures modeled as three node-node contactors from the enclosure to a boundary node representing the rover chassis. • Flexures are Ti-6Al-4V • Flexure resistance values from ANSYS thermal FE model are 413 K/W • Actual fasteners used: – NA0070 Screw (2.8 mm ⌀), G10 Washers • Modeled as a parallel thermal resistance between: – G10 Washer • ID = 2.4mm, OD = 5mm • Thickness = 1.6mm on bottom washer, and 2.8mm on top washer • k = 0.25 W/(m K) – Stainless Steel Bolt Shank • k = 0.11 W/(m K) • Shank’s Conductive Length = thickness of washer • Cross-sectional area = 6.15 sq. mm

Figure 7. Results of the Thermal Desktop Model

46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

III. Testing of the Rear Hazcam on Mars

Hazcam Thermal Characterization and the ECAM Calculator In order to avoid overheating the engineering cameras and to aid the planning of the camera usage, an Excel based tool called the ECAM calculator was created that contains a data set of pre-launch rover model prediction results. The tool allows us to input time of usage and duration of camera-on times and makes a maximum temperature prediction. The calculator assumes worst-case conditions and is generally expected to produce overly conservative estimates.

In order reduce the conservative margins within the ECAM tool, an in situ thermal characterization plan for the rear hazcam was created. The plan for the rover was to incrementally increase the duration of camera use in small increments, and track the temperature of the rear hazcam, in their maximum image acquiring state, all the while avoiding the rear hazcam maximum temperature limit of 50 °C. In the first run the rear hazcam operated for 11:50 minutes, and subsequent runs added approximately 5-minute increments to the runs. A total of six runs were performed with on-times roughly ranging from 11:50 minutes to 30 minutes as shown in Figure 8.

Figure 8. NASA-JPL Streams query tool showing the six separate flight measurements overlapped on top of each other. The maximum peak occurred during the 25-minute camera-on run on Sol 752.

The maximum temperature occurred following 30-minutes of the camera use on Sol 752. The temperature achieved on Sol 752 was 48.5 °C, only 1.5 °C below the AFT maximum of 50 °C.

IV. Results

A. Hazcam Telemetry vs. ECAM Calculator The goal of the experiment was to characterize the self-heating profile in hot conditions near the maximum AFT of the camera, which was achieved on the Sol 752 run. Figure 9 shows the last two thermal characterization runs that occurred on Sols 745 and 752, compared to the plot predicted from the ECAM calculator tool.

46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

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0" 0" 5" 10" 15" 20" 25" Minutes% Figure 9. Rear hazcam usage time vs. temperature of the last two runs of the thermal characterization test, compared to the ECAM calculator prediction. The data was normalized to start at 0°C so that the results could be plotted against each other.

The last thermal characterization run on Sol 752 was plotted on top of the diurnal temperature plot of the rear hazcam electronics box and compared with the ECAM calculator results (Figures 10). The plots show the ECAM calculator tool to predict hotter temperatures, between 5 to 10 °C higher than the measured values during the thermal characterization test. ActID#333## 60.00#

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0.00# 13:40:48# 13:55:12# 14:09:36# 14:24:00# 14:38:24# 14:52:48# 15:07:12# 15:21:36# 15:36:00# 15:50:24# 16:04:48# 16:19:12# LMST#

Figure 10. Rear hazcam temperature vs. usage time plotted along side the rear hazcam “cold soak” temperature. The cold soak indicates the start temperature of the camera before powering on.

46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

During the test of Sol 752, the rover was in an orientation that put the sun directly on the rear hazcam and caused the temperature to rise higher than anticipated. Heading of the rover played a significant role in the temperature variation of the cameras.

B. Model Prediction vs. Telemetry The Thermal Desktop model was configured to replicate the conditions of the thermal characterization run of Sol 752, where the camera was turned with full power on at LMST 15:29 for approximately 24 minutes. Figure 11 compares the results of the thermal model with the telemetry during Sol 752.

Figure 11. Rear hazcam temperature during Sol 752 thermal characterization test vs. prediction model. The model was configured with the camera on for 24 minutes, then off.

V. Conclusion

The thermal characterization test of the rear hazcam was successfully run on the surface of Mars and allowed us to determine how long the camera could be used without exceeding the AFT. The data from the test were plotted against and compared to the values predicted by the ECAM calculator tool, and the amount of margin that the tool over predicted was measured. The ECAM calculator tool assumed hot case conditions and orientation relative to the sun. However, it does accommodate seasonal changes and the fact that the environment is much colder during the winter than vs. the summer.

VI. Future Work

Future work involves creation and testing of a dedicated thermal model of the rear hazcam, front, hazcam, and Navcam to aid in the optimal usage of the cameras in the diverse environment on Mars. Focus is also being placed on creation of the lessons learned documentation that will aid the planned 2020 Mars Rover.

46th International Conference on Environmental Systems ICES-2016-311 10-14 July 2016, Vienna, Austria

The next leg of the hazcam thermal characterization will be to run the camera for an extended period of time early in the morning when the danger of overheating is at a minimum. A complete profile of the cooling characteristics during the cold morning is desired so as to aid in fine-tuning our ecam tools.

VII. Acknowledgement

We like to extend our gratitude to Matthew Stumbo for spearheading the characterization testing,and to Amy Culver for planning and execution of the tests.

VIII. References

[1] MSL Press Release 02.24.2015, mars.jpl..gov News Archives.

[2] “Eyes and Other Senses”, Curiosity Rover camera descriptions, (http://mars.nasa.gov/msl/mission/rover/eyesandother/)

[3] Engineering Cameras Journal of Geophysical Research: Planets (1991–2012) Volume 108, Issue E12, December 2003, J. Maki, et al.

[4] A Curious Year on Mars—Long-Term Thermal Trends for Mars Science Laboratory Rover’s First Martian Year 44th International Conference on Environmental Systems, 13-17 July 2014, Tucson, Arizona, G. Cucullu, et al.