Poster Session

Multi Nodal Wireless TPS Sensor System Integration Greg Swanson(1), Justin Schlee(2), David Atkinson(3) (1)

University of Idaho, 1000 Park Ln. Eagle, ID 83616 U.S.A., University of Idaho, 1629 S. Mercer Ave. Moscow, ID 83843 U.S.A., (3) University of Idaho, Department of Electrical and Computer Engineering, Buchannan Engineering Building Room W2-1, Moscow, ID 83844 U.S.A., (2)

ABSTRACT The initial development of a system of wireless Thermal Protection System (TPS) sensors has brought about many questions pertaining to the requirements and benefits compared to the wired technology presently used for flight. The University of Idaho and NASA Ames Research Center have prototyped and tested a wireless multi nodal sensor system to be embedded in planetary entry probe TPS. A flight system integration analysis has been conducted to compare the integration of a wireless TPS sensor network to a wired system. A case study of the Lunar reEntry eXperiment (L-EX) sensor suite integrated with the Mars Science Laboratory (MSL) TPS has been completed to compare and contrast the mass, power, cost and data requirements between each topology. Three different sensor topologies are explored, each with benefits and disadvantages. For each topology calculations and equations were derived to give an approximate mass for each system. The results show that implementing a wireless topology can reduce the sensor suite mass approximately 38 percent or 2.85 kg.

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1.0 Background .

1.1. Instrumentation Overview Every spacecraft entering a planetary atmosphere requires a Thermal Protection System (TPS). The TPS must endure severe heat loads, which requires an understanding of atmospheric properties, vehicle aerodynamics, TPS material properties, and the physics of the entry environment. NASA and other space agencies would like to collect temperature, pressure, heat flux, radiation, and recession measurements on flight tests and flight missions in order to verify TPS design and to aid in the characterization of physical and chemical phenomena in the entry environment.

1.1.1 Pressure Transducer Pressure measurements are used to calculate the angle of attack. Shear pressures can also be calculated during entry from the pressure measurements. The total pressure is measured by small inlet and connecting tubes in the TPS material. The main sensor used for the entry pressure measurement is the pressure transducer. Pressure transducers utilize an elastic diaphragm and strain gauges to measure pressure by means of an air intake which is drilled through the TPS material. Air flow produces pressure on the diaphragm. The deformation of the diaphragm is measured by the strain gauges. The strain gauges then create a resistance which is then correlated with pressure. The pressure transducer itself is mounted to the carrier structure behind the TPS. Each transducer has a sampling period of 20 ms which correlates to 50 readings per second with an error of 0.01 %. The mass of each transducer is 300 grams and has a current draw of 45 mA (1). The pressure transducer has been used on numerous missions such as Apollo and Viking (2).

1.1.2 Temperature Temperature is a measurement of the heat through the TPS material. This measurement gives the TPS engineers information on how the TPS material is responding in the entry environment. Multiple measurements at different depths in the same location will define how heat is conducted through the TPS material. The primary method of measuring temperature is with the thermocouple. Thermocouple sensors work by measuring the voltage between two dissimilar metals at a specific temperature. The voltage output of a thermocouple changes as a function of temperature, an effect known as the Seebeck Effect. The temperature can only be determined if the temperature of one of the metals is known. The measured voltage is then transformed to a digital signal by a specialized analog to digital converter called a Cold Junction Correction Analog to Digital Converter. The heat shield material is a good insulator and responds slowly to temperature changes. For this reason the temperature is only measured once every second. Typical thermocouple accuracy is 3% at 1000 degrees Celsius (3), and typical masses are on the order of 5 - 10 grams. The temperature range for a type K thermocouple is -200 degrees Celsius to 1,200 degrees Celsius. Page 2 of 11 Pages

One disadvantage to thermocouples is susceptibility to electromagnetic noise. Noise reduction can be made by converting the analog signal to a digital signal as soon as possible in the circuit. Thermocouples have been flown on numerous missions such as Pioneer Venus and Mars Pathfinder (2).

1.1.3 TPS Recession Sensor Recession is the amount of TPS material which is ablated during entry. The recession measurement shows how the TPS shield mass and shape changes during the entry. The amount of material which remains after the entry is also of interest to help optimize the sizing of future TPS systems. Recession is currently measured on probes by one of two sensor types. The Analog Resistance Ablation Detector (ARAD) or the Hollow aErothermal Ablation Temperature detector (HEAT) are the two most common types of sensors used to measure recession. Both sensors use two coils of resistance wire wound around a plug of ablative material. A hole is drilled into the TPS plug and the ARAD/HEAT sensor is glued into place with the RTV bonding agent. When the plug starts the ablation process a char layer is formed. The char layer is electrically conductive and completes the electrical circuit between the two coils of wound wire. The wire is provided with a constant current and the voltage is measured and provides an indication of the thickness of the TPS. The HEAT sensor was designed to have a higher signal to noise ratio to help alleviate the problems associated with the ARAD measurements on Galileo. The Galileo probe flew the ARAD sensor package. These sensors had some failures on the Galileo mission (4), but meaningful data was still collected. A high level of noise to signal ratio contributed to the failures. One possible reason for the failures is the electrostatic discharge interfering with the resistance values. The HEAT sensor was developed to overcome the shortcomings of the ARAD sensor package, and is expected to fly on the Mars Science Laboratory (MSL). The project is called the MSL Entry, Descent, and Landing Instrumentation or MEDLI.

1.2. Wired TPS Sensor Systems Currently the missions that do fly sensors to indicate atmospheric phenomena have them wired into the TPS of the spacecraft. Utilizing this architecture adds risk to the TPS system due to the process of routing wires in the shield and the difficulty of jettisoning the system after entry. Many current and past spacecraft engineers have decided not to fly sensors embedded within the TPS in an effort to mitigate the risk of spacecraft failure during entry. A wireless instrumentation system could collect measurements needed for scientists and engineers to improve future spacecraft design while lowering the overall risk of implementing sensors into the entry vehicles.

1.3. Wireless TPS Sensor Systems At the University of Idaho in 2006, research funding was provided by NASA Ames Research Center to design a wireless instrumentation system to be implemented within a spacecraft’s TPS. In the spring of 2007 an initial single node prototype system was developed to wirelessly transmit data from four thermocouples, embedded in a piece of LI-900 shuttle TPS, to a

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standalone data acquisition program. The system was successfully tested in the NASA Ames X-Jet facility to roughly simulate atmospheric entry (5). In the fall of 2007 the initial prototype system was improved to a multi nodal, multi sensor system to simulate a more complete model for TPS integration. The system consisted of three nodes, each node incorporated three thermocouples and a pressure sensor embedded in LI900 shuttle TPS. This system was again tested in the NASA Ames X-Jet facility. The test results indicated the multi nodal system prototype was a reliable and accurate method to obtain TPS sensor data through wireless transmission (6).

2.0 Thermal Protection System Sensor Topologies 2.1. Wired TPS Sensor Systems The wired TPS sensor system, as seen in Figure 1, consists of multiple sensors placed and embedded strategically in the TPS of the spacecraft to gain information on re-entry performance. Each sensor is individually wired back to a data acquisition unit and the power bus within the spacecraft. These individual sensor wires are bundled together to reduce clutter and feed-through points.

Recession Sensor 4 Thermocouples

Large Power and Data Bus

Main Data Acquisition Unit

Power

1 Pressure Transducer 3 Strain Gauges

Figure 1. Wired Sensor System Block Diagram

2.1.1 Advantages The major advantage of using wired TPS sensors is that the system has flight heritage. A wired sensor system has flown on both Galileo and PAET (2). This system also minimizes the power needed to utilize TPS sensors because there is no need for support electronics such as the wireless system would need.

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2.1.2 Disadvantages Wired systems are heavy and cause unwanted clutter within the spacecraft. Feed through points needed to run wires to the data acquisition unit can impact the spacecrafts structural integrity. Manufacturing is complicated by a wired system. Since the TPS and the interior of the spacecraft are manufactured separately, the integration of the TPS sensors and the data acquisition system must be carefully considered. If a mission is designed to jettison the TPS, pyrotechnic wire cutters must be employed to detach, which introduces a single point mission failure potential.

2.2. Wireless Data Transmission with Battery Power The wireless TPS sensor system utilizing battery power is a fully wireless system, as seen in Figure 2. This system is similar to the wired TPS sensor system in that it consists of multiple sensors placed and embedded strategically in the TPS of the spacecraft to gain information on re-entry performance. Instead of routing wired data and power this system uses wireless communication and battery power to gather data. Sensors are directly connected to a battery powered transmitting node which digitizes the TPS sensor data and wirelessly sends it to receiving node connected to the data acquisition unit.

Figure 2. Wireless Battery Powered Sensor System Block Diagram

2.2.1 Advantages Implementing a fully wireless system can save a substantial amount of mass by removing wires normally used for data and power. The data and power wires account for a considerable amount of the total TPS sensor suite mass. Removing these wires also decreases feed through points to the inner spacecraft, increasing the structural integrity of these points. And without wire connections between the inner spacecraft and the TPS there is no need for pyrotechnic wire cutters during TPS jettison. The fully wireless TPS sensor data also becomes more robust since it is digitized before being sent to the data acquisition unit.

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2.2.2 Disadvantages The main disadvantage of a fully wireless system is that it does not have flight heritage. This system with the collaborated work of the University of Idaho and NASA Ames, as of summer 2008, is only at a TRL of 4. The wireless system electronics need to be test flown before this system can be considered as a viable solution. This system requires greater power due to the support electronics needed to process, digitize and transmit the TPS sensor data to the data acquisition unit. Although using battery power allows this system to be fully wireless, the weight of the actual batteries is greater than the small amount of wires need to run power. This suggests that a wireless data system with wired power could be more optimal in certain situations.

2.3. Wireless Data Transmission with Wired Power The wireless data transmission with wired power system is very similar to the fully wireless system. As seen from Figure 3, again we have TPS sensors wired to a wireless transmit node. This node then wirelessly sends the processes and digitizes data to the wireless receiving node, which is integrated into the data acquisition unit. The main difference between the two topologies is that the power needed by the wireless transmit nodes is still wired.

Figure 3. Wireless Sensor System with Wired Power Block Diagram

2.3.1 Advantages Again the main advantage of the wireless sensor with wired power topology is that it can save a substantial amount of mass by removing wires normally used for data. And compared to the fully wireless topology, using wired power actually has a greater mass savings over battery power. Removing the data wires also decreases the size of feed through points to the inner spacecraft, increasing the structural integrity of these points. And without data wire connections between the inner spacecraft and the TPS the pyrotechnic wire cutters have up to 85% less wires to cut through during TPS jettison. Again the wireless TPS sensor data also becomes more robust since it is digitized before being sent to the data acquisition unit.

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2.3.2 Disadvantages Again the main disadvantage of the wireless data and wired power system is that it does not have flight ght heritage. This system is only at a TRL of 4. The wireless system electronics need to be test flown before this system can be considered as a viable solution. This topology also requires greater power due to the support electronics needed to process, digitize and transmit the TPS sensor data to the data acquisition unit. Although using the wireless data with wired power topology is the greatest in mass saving, it doesn’t implement a fully wireless system, there are still a small number of wires needed to provide power, which may require wire cutters.

3.0 Case Study 3.1. Case Study Overview The main configuration for the case study was based on the L-EX TPS sensor layout scaled to the dimensions of the MSL TPS. L-EX is a proposed mission designed to imitate Earth atmospheric re-entry from a moon mission to give further insight to developing the space shuttle’s successor, Orion. MSL is a robotic rover that will assess whether Mars was, was or still is, an environment able to support life life. It is one element of NASA's Mars Exploration Program, Program a long-term endeavor of robotic exploration of our neighbor planet planet. MSL is scheduled for launch in 2009. The amount of TPS instrumentation for this L L-EX TPS sensor sor suite is unparalleled. The premise of this case study was to implement a well instrumented heat shield such as the proposed L-EX L sensor suite on a large heat shield such as the MSL TPS. Below, Figure 4 shows the sensor suite layout of the L-EX (7).

Figure 4: L L-EX Fore-body TPS Sensor Layout Page 7 of 11 Pages

3.2. Design Considerations The three TPS sensor topologies were investigated in a comparative study. The fore-body TPS and aft-body TPS of the proposed L-EX mission were inventoried for the number of sensor nodes and the number of supporting wires needed. The implementation of the L-EX sensor suite was then transposed on to the MSL sized TPS using the three sensor topologies. Each TPS sensor node remained consistent and only the wires, batteries, and wireless topologies were changed to demonstrate the advantages of using a wireless or reduced wired system.

3.3. Design Specifications The size of the average wire run was estimated to be 10 m with an average wire mass of 0.00145 kg/m (for 28 gauge wire). Each wireless node, created by the University of Idaho Thermal Exposure senior design team, was weighed and found to be 55 grams. The mass of the current battery system was found to be 70 grams. Each TPS sensor node was then investigated to determine the number of wires required to make the node functional.

3.4. Design Calculations The mass of the wires, wireless nodes, and other supporting equipment were tallied to give final mass totals. The mass for each wire was calculated using equation 1. Each wire was then multiplied by the number of wires going to each TPS sensor node to give the total mass of the wires.

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The wireless system with wired power calculations were derived by reducing the lengths of the data lines from 10m, needed for the average wire run to the systems data acquisition, to 1m, needed to connect sensors to nodes. The two power wires for each node were still estimated at 10m. This represented the replacement of wired data transmission with a wireless sensor topology using wired power. The transmission node mass total was calculated using equation 2.

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The third topology, the battery operated system, eliminated all the 10m wire runs for data and power, and replaced them with a completely wireless topology. Data lines again were estimated at 1m to transmit sensor data to the transmission node. The transmission nodes, receiving nodes and batteries were then weighed and implemented into the final calculations.

3.5. Results Table 1 shows the results for L-EX sensor layout on the MSL scaled fore-body TPS utilizing the fully wired topology. The mass of the fully wired system was calculated to be 7.511 kg. This mass is strictly based on the mass of the wires and does not include wire harnesses and other hardware to secure the system to the spacecraft. Including these parameters would only increase the fully wired system mass.

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Pressure Heat Plugs Strain Gauge

Quantity 13 23 32

Wires 6 8 8

Fore Shield Wired System Length Per Unit [m] Total Length [m] 10 780 10 1840 10 2560 Total Mass [kg]

Mass [kg] 1.131 2.668 3.712 7.511

Table 1. Fore-body Wired TPS Sensor System Mass Table 2 shows the results for L-EX sensor layout on the MSL scaled fore-body TPS utilizing the wireless data, wired power topology. The mass of the fully wired system was calculated to be 4.6694 kg. This is approximately a 38 percent reduction in mass from the fully wired system.

Pressure Power Pressure Data to Node Heat Plug Data to Node Strain Gauge Data to Node Node Power Wireless Node Hardware Mass Receive Node Mass

Fore Shield Wireless System with Wired Power Quantity Wires Length Per Unit [m] 13 2 10 13 4 1 23 8 1 32 8 1 36 2 10 37 na na 1 na na

Total Length [m] 260 52 184 256 720 na na Total Mass [kg]

Mass [kg] 0.377 0.0754 0.2668 0.3712 1.044 2.035 0.5 4.6694

Table 2. Fore-body Wireless Data, Wired Power, TPS Sensor System Mass Table 3 shows the results for L-EX sensor layout on the MSL scaled fore-body TPS utilizing the wireless data, battery powered topology. The mass of the fully wired system was calculated to be 5.8061 kg. This is approximately a 23 percent reduction in mass from the fully wired system. This is less than the wireless data, wired power topology, but has the benefit of implementing a fully wireless system.

Pressure to Node Heat Plugs Strain Gauge Node Hardware Mass Node Battery Mass

Fore Shield Wireless System with Batteries Quantity wires length per unit [m] 13 6 1 23 8 1 32 8 1 37 na na 36 na na

Total Length [m] 78 184 256 na na

Mass [kg] 0.1131 0.2668 0.3712 2.035 2.52

Total Mass [kg]

0.5 5.8061

Receive Node Mass

Table 3. Fore-body Wireless Data, Battery Power, TPS Sensor System Mass From the calculations used to find the three different sensor topology masses, equations for the mass of each topology with respect to the number of nodes were able to be derived. Figure 5 shows the linear approximation of each topology’s mass due to the number of sensors to be implemented. These equations are based off a MSL sized fore-body TPS. Realistically the fully Page 9 of 11 Pages

wired plot would be the only topology close to a linear realization as depicted below. Both the wireless topologies would be more of a staircase function, increasing to the next level each time another node is introduced.

Figure 5. Sensor System Mass Due to the Number of Sensors

4.0 Conclusion The wireless data transmission of sensor data on future missions will reduce the mass and the complexity associated with the instrumentation of the TPS. The wireless system reduces the number of wires and helps in the integration of the sensor network by allowing sensors senso to be placed in a myriad of topologies without significant changes. Reducing ing wire runs reduces the risk for jettisoning TPS during EDL. For the case study of the L-EX EX sensor layout scaled to a MSL sized TPS, it revealed that integrating a wireless sensor topology could create mass savings of approximately 38 percent or 2.85 kg. This impressive mass savings will create a much lower impact to the entire mission when implementing a TPS sensor suite, enabling the research needed to successfully design future missions.

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5.0 References 1. Mensor Corporation. 6100 Pressure Transducer Data Sheet . www.mensor.comf. [Online] Mesnsor Corporation. [Cited: June 15, 2007.] http://www.mensor.com/pdf/cds6000f.pdf. 2. C. Davies, M. Arcadi. Planetary Mission Entry Vehicles Quick Reference Guide. Version 3. Hanover : NASA Center for Aerospace Information, 2003. 3. Omega Corporation. Thermocouple Resource Page. www.omega.com. [Online] http://www.omega.com/prodinfo/thermocouples.html. 4. Thermal Protection System and Facility Needs for Demanding Future Planetary Missions. B. Laub, E. Venkatapathy. Lisbon, Portugal : 2003. Proceedings of the International Workshop on Planetary Entry and Descent Trajectory Reconstruction and Science. 5. Wireless Sensors in Thermal Protection Systems. G. Swanson, T. Reid, D. Berk, J. Wagoner, D. Atkinson. International Planetary Probe Workshop 5 Proceedings, 2007. 6. L. Wells, J. Sochacki, J. Pentzer, B. Holmes, C. Johnson. Wireless Thermal Protection Sensor System. s.l. : Senior Design Report, 2008. 7. M. Munk, J. Fu, etc. LE-X Instrumentation: TIM. 2007.

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