Development of a Back Pressure Valve for the Spacesuit Water Membrane Evaporator (SWME)

Transcription

1 45th July 2015, Bellevue, Washington ICES Development of a Back Pressure Valve for the Spacesuit Water Membrane Evaporator (SWME) Brad Harris 1 and Thomas O. Leimkuehler 2 Paragon Space Development Corporation, Houston, Texas, John Fricker 3 Oceaneering Space Systems, Houston, Texas, A prototype Back Pressure Valve (BPV) for the Spacesuit Water Membrane Evaporator (SWME) has been designed, manufactured, and tested. The SWME provides heat rejection for the Portable Life Support System of the Exploration EVA Space Suit being designed for NASA by the CSAFE team. The BPV controls the heat rejection of the SWME by controlling the vapor pressure to which water in the SWME evaporates. The BPV is a custom manufactured sliding gate valve with an electromechanical actuator. An encoder is included for reporting valve position. A tapered orifice shape helps provide greater control of cooling at small valve openings. The prototype BPV has a very small leakage rate in the closed position (less than 10 W of cooling with water temperatures up to 40 C). This paper presents an overview of the BPV design and development test results. Nomenclature BPV = Back Pressure Valve COTS = Commercial Off The Shelf C-SAFE = Crew Spacesuit Accommodations for Exploration EHF = ECLSS Human-rating Facility EMU = Extravehicular Mobility Unit EVA = Extravehicular Activity NASA = National Aeronautics and Space Administration PLSS = Portable Life Support System RTD = Resistance Temperature Detector SWME = Spacesuit Water Membrane Evaporator I. Introduction HE Crew Spacesuit Accommodations for Exploration (C-SAFE) team has been tasked by NASA to develop, Tbuild, and certify NASA s next Exploration EVA Space Suit 1. The Spacesuit Water Membrane Evaporator (SWME) is a device that is being developed to provide heat rejection capability for such a spacesuit. Cooling is achieved by circulating the suit cooling water through microporous, hydrophobic tubes in the SWME. Liquid water remains within the tubes; however, a small amount of water vapor escapes through the tube pores and is exhausted to space vacuum, thereby removing heat through evaporation. The rate of heat rejection is controlled by regulating the vapor pressure on the shell side of the SWME with a Back Pressure Valve (BPV). The SWME may be more tolerant to contamination than the previously used sublimator on the Shuttle Extravehicular Mobility Unit (EMU) 2. Also, the SWME could potentially be used in a near-vacuum or micropressurized environment (such as the surface of Mars). Oceaneering Space Systems has chosen a Commercial Off The Shelf (COTS) device purchased from Liqui-Cel to serve as the SWME membrane. Paragon Space Development Corporation was tasked to develop a BPV to 1 Senior Aerospace Engineer, 1322 Space Park Drive, Suite C150, Houston, TX Senior Aerospace Engineer, 1322 Space Park Drive, Suite C150, Houston, TX CSAFE Life Support Subsystem Manager, Space Center Blvd., Houston, TX

2 regulate the amount of evaporative cooling from the SWME. After researching possible COTS options, it was decided a custom BPV would need to be developed to meet all the requirements. II. Requirements and Concept of Operations The Extravehicular Activity (EVA) would begin with the Portable Life Support System (PLSS) unpressurized volume venting air as the airlock de-pressurizes. Therefore, if the BPV is closed, it will need to withstand the pressure differential across the valve as one side transitions from nominal atmospheric pressure to vacuum. Once the external suit pressure reaches vacuum, the suit occupant could then adjust a control device (such as a dial) on the front of the suit which would send a signal to the BPV actuator and open the BPV as desired to achieve the proper amount of cooling for occupant comfort. Likewise, when the EVA is near termination, the suit occupant enters the airlock and represses. In the event the BPV is closed at that point, it should be able to withstand the resulting pressure differential across it as the airlock represses. The requirements for the First Generation BPV were as follows: The BPV orifice area shall be approximately 2.0 in 2. The BPV shall function with water vapor temperatures between 0 and 40 C. The BPV shall not leak more than 10W of heat with the valve in the closed position. The BPV shall consume 7W or less (while valve is moving). No power required to hold BPV in current position. The BPV should be less than 36 in 3 in volume. The BPV should be less than 2 lb in mass (dry). The BPV should have (at least) 10 set-point positions and report valve position. The actuator shall move the valve from any position to any other position in 2 minutes or less. III. First Generation Valve Paragon designed, manufactured and tested a First Generation BPV in Initial testing of Liqui-Cel membrane contactors (dating back to 2013) showed good performance. Testing looked at various orifice sizes, shapes, and valve types. Testing showed a large difference in cooling between orifice areas of 0 to 0.5 in 2. Testing also showed a small difference in cooling between 1.0 to 2.0 in 2. It was decided to employ a tapered valve orifice between 0 and 0.5 in 2 to provide better control. This all pointed the design toward a sliding gate valve. Also, a gate valve could easily be configured to have a low-profile as packaging within the PLSS volume becomes a challenge. Figure 1 shows the as-delivered First Generation BPV (attached to the prototype SWME). Figure 1. Paragon s First Gen SWME Back Pressure Valve (BPV) 2

3 Figure 2 shows the tapered orifice. The orifice is divided into two symmetrical halves. The shaft from the motor is located directly above the space between the two halves. When the valve is closed, the gate completely covers the orifice openings. When the valve is opened, it slides to expose the tapered ends of the orifice first to provide better control at smaller heat loads. Comparing to Figure 1, the tapered ends of the orifice (not visible under the gate) point toward the right, and the valve opens by moving the gate to the left. Figure 2. Tapered Orifice Allows Better Control This BPV is primarily made of aluminum. It weighs 1.45 lb and its volume is 11.8 in 3. The sliding gate is actuated by a COTS stepper motor that includes an encoder for reporting valve position. The BPV seal is simply metal to metal. Both the valve body and the sliding gate are coated in Tufram to reduce the coefficient of friction. Four spring plungers are used (as shown in Figure 3) to provide down force on the gate. The gate has ramps so that the spring plungers are only engaged when the gate is in the closed position. Figure 3. Spring Plungers Provide Down-Force In Closed-Position 3

4 IV. Test Setup and Approach The BPV was assembled with the prototype SWME and tested in the ECLSS Human-Rated Facility (EHF) at Paragon in Tucson, AZ. The EHF is an 8 ft diameter vacuum chamber with a cold trap. The cold trap was used to collect all the water evaporated by the SWME. This assembly was tested at pressures down to 0.5 torr. A chiller cart provided water at 200 lb/hr and controlled SWME inlet temperatures (5-40ºC). A photo of the test article is shown in Figure 4. The BPV was tested at various SWME inlet temperatures and valve positions. The test matrix is shown in Table 1. Figure 4. SWME/BPV in the EHF Table 1. First Generation BPV Test Matrix Test Case # Configuration 2 5 C SWME Inlet (vary valve position) 2A 10 C SWME Inlet (vary valve position) 3 20 C SWME Inlet (vary valve position) 3A 30 C SWME Inlet (vary valve position) 4 40 C SWME Inlet (vary valve position) 5 Max Leak Rate (with valve closed) 4

5 V. Test Results Testing varied and measured the BPV position for various SWME inlet coolant temperatures. A preliminary check (at ambient conditions with water flowing through the SWME) was conducted to make sure the inlet and outlet RTDs were measuring approximately the same temperature. Testing also characterized the performance of the SWME prototype. The coolant inlet and outlet temperatures, flow, pressure, and differential pressure were measured. In addition, the back-pressure inside the SWME shell was measured (using a Baratron device) for various SWME inlet temperatures and valve positions. The BPV performed well, meeting most of the requirements. As shown in Figure 5, the BPV met its 10W leakage requirement for SWME inlet temperatures less than 30ºC; however, it did not meet the 10W leakage requirement for 40ºC (17W). Stronger spring plungers will be considered for the Second Generation Valve. Data shown in Figure 6 illustrates the SWME performance. Again, the data shows that there is very little difference in heat rejection between a 1.0 in 2 and 2.0 in 2 orifice. Figure 5. Closed-Valve Heat Leak Data for First Generation Valve 5

6 Figure 6. SWME Performance Data vs BPV Orifice Size VI. Second Generation Valve Paragon was tasked to develop a Second Generation BPV improving upon the previous design, particularly from a mass, volume, and power standpoint. Paragon was also tasked to provide Oceaneering with a backup or emergency BPV. After careful consideration, it was decided to combine the primary and backup valve features into the same valve body to save mass and volume. Instead of a single gate, it was decided to employ a dual-gate valve with separate actuators as shown in Figure 8. The two actuators are controlled by separate controllers and powered by separate power supplies within the PLSS to yield a single fault tolerant system. Even if one of the gates failed closed, the other could provide all the cooling needed for a 1 hour emergency scenario. A viton seal was placed between the SWME and BPV as shown in Figure 8. Figure 7. Second Generation BPV 6

7 Figure 8. Second Generation BPV and SWME When the BPV gates open (simultaneously), the gates slide under the two actuators, providing a more compact design than the first generation valve. Key characteristics of the Second Generation BPV include: Weight of 0.37 lb (less than 0.7 lb requirement) Volume of 3.24 in 3 (less than the 11 in 3 requirement) Full valve travel time of 24 seconds (less than 30 second requirement) Maximum valve driving power of 3.16W (less than 4W requirement). The Second Generation BPV was manufactured and tested at Paragon. The SWME and Second Generation BPV are shown in Figure 9 (installed in the vacuum chamber during testing). Testing determined that the Second Generation BPV met all of its requirements including the 10W heat leakage requirement for SWME inlet temperatures between 5 and 40ºC as shown in Figure 10. Figure 9. SWME and Second Generation BPV Installed in Vacuum Chamber 7

8 Figure 10. Closed-Valve Heat Leak Data for Second Generation Valve VII. Forward Work Future design work will include employing radiation hardened actuators and considering all flight like requirements (such as launch, shock and vibration loading, lunar dust, etc.) that were not considered in this developmental stage. VIII. Summary A custom SWME BPV prototype has been developed by Paragon for the Oceaneering-led C-SAFE team for NASA s Exploration EVA Spacesuit. The BPV is a compact unique design (weighs 0.37 lb, volume of 3.24 in 3 ) that meets the intent of both the primary and emergency BPV functions. Further development will bring this design to a flight certified system. References 1 Stein, M., Williams, Jr., A. L., Radford, T., Splawn, K., Dougherty, M., Oelke, M. L., Battisti, B., Harris, B., and Schuck, D., Technology Development Efforts for an Exploration Spacesuit, AIAA , 43rd International Conference on Environmental Systems (ICES), Vail, CO, July Bue, G. C., Makinen, J., Cox, M., Watts, C., Campbell, C., Vogel, M., Colunga, A., and Conger, B., Long-Duration Testing of a Spacesuit Water Membrane Evaporator Prototype, AIAA , 42nd International Conference on Environmental Systems (ICES), San Diego, CA, July