Assessment of Technology Readiness Level of a Carbon Dioxide Reduction Assembly (CRA) for Use on International Space Station

Transcription

1 Assessment of Technology Readiness Level of a Carbon Dioxide Reduction Assembly (CRA) for Use on International Space Station Karen Murdoch Hamilton Sundstrand Space Systems International, Inc. Fred Smith NASA Johnson Space Center Jay Perry Marshall Space Flight Center Steve Green Southwest Research Institute Copyright 2004 Hamilton Sundstrand Space Systems International, Inc. Reprinted by SAE International with permission. ABSTRACT When technologies are traded for incorporation into vehicle systems to support a specific mission scenario, they are often assessed in terms of Technology Readiness Level (TRL). TRL is based on three major categories of Core Technology Components, Ancillary Hardware and System Maturity, and Control and Control Integration. This paper describes the Technology Readiness Level assessment of the Carbon Dioxide Reduction Assembly (CRA) for use on the International Space Station. A team comprising of the NASA Johnson Space Center, Marshall Space Flight Center, Southwest Research Institute and Hamilton Sundstrand Space Systems International have been working on various aspects of the CRA to bring its TRL from 4/5 up to 6. This paper describes the work currently being done in the three major categories. Specific details are given on technology development of the Core Technology Components including the reactor, phase separator and CO 2 compressor. INTRODUCTION The Carbon Dioxide Reduction Assembly (CRA) is being designed for implementation in the International Space Station as a Flight Experiment. In order to plan development tasks, the system must be assessed for technical and programmatic risk. A system is assigned a Technology Readiness Level (TRL) as a means to measure its overall technical maturity. A TRL rating of 6 is generally considered to be the jumping off point to move a technology into a flight program. TRL 6 is defined as Prototype. Components are flight configuration, the system has been demonstrated in a relevant environment, manufacturing has been demonstrated, and there is confidence for life. The definitions of technology readiness levels are given in Table 1. CARBON DIOXIDE REDUCTION ASSEMBLY (CRA) SYSTEM DESCRIPTION The block diagram in Figure 1 shows the major components and the system interfaces of the Carbon Dioxide Reduction Assembly (CRA). The CRA reacts waste hydrogen from the oxygen generator with waste carbon dioxide from the CO 2 Removal Assembly to form product water that can be used for crew consumption or oxygen generation. The heart of the system is a Sabatier methanation reactor, which reacts the hydrogen and CO 2 at high temperature and produces methane and water vapor. The gaseous product stream is cooled in a condensing heat exchanger where the water condenses to liquid. The liquid water is then separated from the gaseous methane in a rotary phase separator. The water product is delivered to the wastewater bus and the methane is vented overboard. Figure 2 shows the detailed system schematic. EVALUATION OF SABATIER TECHNOLOGY READINESS LEVEL (TRL) The following section details the assessment of Technology Readiness Level. For each component or category, an assessment is given of the current state of the technology and development required to reach TRL 6. TRL 6 is defined as flight configuration, demonstrated in a relevant environment. Manufacturing and system performance are demonstrated and confidence for life expectancy can be shown.

2 Table 1: Development Status Rating Criteria Development Status Core Technology Ancillary H/W Control & Integration Components 1 Basic Principle Observed and Reported N/A N/A 2 Conceptual Design Rough Sizing and System Schematic Feasibility Addressed Configuration 3 Feasibility Tested Experimentally Tested Analytically Tested Analytically Demonstrated 4 Breadboard ¼ to Full Size Tested; Validated in Laboratory; Life Not a Criteria Core of System Tested with Commercial Components System Tested with Data Acquisition Type Controller 5 Pre-Prototype Full Size, Zero-G Config; Validated in Relevant Environment; Materials for Expected Flight Requirements 6 Prototype Flight Config; Demonstrated in a Relevant Environment; Mfg. Demonstrated; Confidence for Life 7 DTO/Flight Prototype Fit, Form, & Function; Demonstrated in a Space Environment; Mfg. Process Developed 8 Qualification Fully Certified for Use; Life Verified (Test or Analysis) 9 Operational Performing Specified Mission; Life Demonstration System Tested with Commercial Components Grade Adequate to Demonstrate System Performance; Confidence for Life Fit, Form, & Function; Demonstrated in a Space Environment; Mfg. Process Developed Fully certified for Use; Life Verified (test or Analysis) Performing Specified Mission; Life Demonstration System Tested with Commercial Controller Control S/W Demonstrated (Comm Controller) Control S/W Validated on Representative Controller; Certified to Fly S/W Certified on Flight Controller Performing Specified Mission

3 OXYGEN GENERATION ASSEMBLY CO2 COMPRESSOR CARBON DIOXIDE REMOVAL ASSEMBLY SABATIER REACTOR CO2 ACCUMULATOR CONDENSING HEAT EXCHANGER WATER PROCESSOR ASSEMBLY PHASE SEPARATOR OVERBOARD VENT Figure 1 Carbon Dioxide Reduction Assembly Block Diagram Carbon Dioxide Reactor Assembly (CRA) Schematic P Oxygen Generator Assembly (OGA) Avionics Air Assembly (AAA) Return SABATIER REACTOR T T TS T NO P PS T Rack Air P M NC P PS M 1152 N T TS COMPRESSOR ORU 9411 NC P P B A 1141 CRA CONTROLLER 8081 Carbon Dioxide Removal Assembly (CDRA) ITCS MTL Return ITCS MTL Supply 2512 Rack Air T TS M N T CONDENSING HEAT EXCHANGER 5461 TS L 7491 NO NO NO CO 2 ACCUMULATOR CH 4/CO 2 Vacuum Vent Waste Water Bus B P P ITCS MTL COOLANT A 4431 PUMP/SEPARATOR CRA ORU 9400 February 27, 2004 Figure 2 Carbon Dioxide Reduction Assembly Schematic

4 CORE TECHNOLOGY COMPONENTS Current TRL Level 5: Full size, zero-g configuration Desired TRL Level 6: Flight configuration, manufacturability demonstrated, performance in relevant environment. The core components of the CRA are the Sabatier reactor, condensing heat exchanger and phase separator, and CO 2 compressor. REACTOR The Sabatier reactor is the heart of the Carbon Dioxide Reduction Assembly. The reactor has been studied by Hamilton Sundstrand since the 1950 s with the development of a protoflight unit for the Air Force Manned Orbiting Laboratory in The reaction kinetics are well understood for a range of flow rates in excess of what is now planned for the International Space Station. The reactor diameter, length, heating, cooling and catalyst manufacturing process are unlikely to change from the current prototype that has been tested for many years. The major reactor development risk is the unknown impact of the catalyst fines. The most significant modification is likely inclusion of some type of filter and catalyst retention device in the bed to prevent the transplant of catalyst fines, which can foul downstream components. Catalyst fines generation is being evaluated this year under funding from JSC. The catalyst will be vibrated in a fixture to simulate launch vibration loads, as well as run cyclically to evaluate thermally generated fines. Fines will be collected and analyzed to provide data for sizing filters for the inlet and outlet of the reactor. Until the fines are quantified, it is difficult to predict the packaging impact of filters and/or bed retention devices. Catalyst poisoning is also being evaluated this year. Data collected from the four bed molecular sieve (4BMS) systems on ISS indicated a number of chemical contaminants in air that would be co-adsorbed with carbon dioxide and potentially delivered to the CRA. The chemicals have been grouped into classes to simplify testing. Four chemical contaminants, plus excess air will be injected into the carbon dioxide feed stream to test reactors. The performance of the reactors will be monitored to see if the reaction rate is degraded by the contaminants. Southern University, a research partner to Hamilton Sundstrand, will perform this work. evaluated as well as design for manufacturability in order to make the reactor a flight design. CONDENSING HEAT EXCHANGER The condenser has been demonstrated as a smooth bore ¼ tube with exterior fins to be used with air-cooling. The finned tube design was selected to minimize the volume of liquid slugs delivered to the phase separator. At ¼ nominal OD, the condensing liquid tends to form into chaplets that are separated by gas. The diameter is small enough that the gas pressure can easily move the water against gravity during 1-g testing. The condenser is air cooled because an interface with the liquid cooling loop on Space Station was undesirable due to failure modes that could cause localized boiling of the coolant. The shape and location of the condenser core are second order in impact to system performance. The materials used to manufacture the heat exchanger must be compatible with water, carbon dioxide and hydrogen. A condenser of the size and configuration base-lined for the CRA has been fabricated and is currently on test in the Sabatier Engineering Development Unit at JSC. Little development is required for the Heat Exchanger. The materials previously tested are acceptable for flight. The tube sections are welded in the Engineering Unit and this manufacturing approach will have to be evaluated for a flight design. PHASE SEPARATOR The product of the Sabatier reaction is methane and water. The water, once condensed, must be separated from the methane gas and delivered at pressure to the ISS wastewater bus. For the ISS CRA application, the water product must be pumped up in pressure from the sub-ambient pressure reactor system to the positive pressure waste storage tank. In micro-gravity, phase separation can be accomplished in a number of ways. A trade study conducted in 2001 selected a rotary disk type separator/accumulator as the appropriate technology for the operating conditions of the CRA. The rotary disk separator concept is well suited to the sub-ambient pressure requirement, and can be adapted to include redundant seals, a requirement due to the combustible gas product. There will probably be some changes to locations of fittings and mounting structure in order to meet the tight packaging requirements of the ISS application. The physical configuration will include vibration and thermal isolation. The materials of construction will need to be

5 The pressure rise requirement for the phase separator is 12 psid (10 psia reactor pressure to 8 psig bus pressure). The separator shown in figure 3, will separate gas and liquid at low speed, about 1000 rpm, until the separating chamber is full. The controller senses the differential pressure and when it reaches a high status, the motor is set to high speed. At about 2500 to 3500 rpm, the separator will pressurize the liquid enough to flow out of a check valve and to the waste tank. Figure 3 Sabatier Phase Separator CO 2 COMPRESSOR The Mechanical Compressor is being designed and fabricated by Southwest Research Institute (SWRI) in coordination with the hardware design efforts for the other core components. The CO 2 compressor is an oil-less, two stage piston compressor based on the legacy of oil-less technology that was developed for Space Station Freedom and demonstrated in a prototype (Figure 4) for ground tests. An Engineering Development Unit (EDU) is currently being manufactured for testing at MSFC in mid This EDU is designed to meet many of the flight hardware requirements including:? performance meets pressure and flow,? power projected to meet electrical power limit,? cooling meets cooling loop pressure drop, temperature rise and touch temperature limits. The flight requirements for packaging volume and weight have not yet been fully demonstrated in the EDU design. The EDU will be tested for acoustics, vibration and life with the results used to improve the flight unit. Current development activity includes particulate testing of the piston seals, as cylinder seal wear is considered to be the life limiting component of the compressor. A test has been ongoing since March 2003 in which molecular sieve dust (a potential discharge from the Four BMS CO 2 removal system) is injected into test cylinders with various cylinder/ring material combinations. The test cylinders are operated at the approximate speed and pressure ratio of the first stage of the flight compressor design. The flow is monitored as an indicator of seal wear. After 4000 hours of testing, the flow performance of all the test articles had degraded only slightly. Testing has also revealed that a solid piston seal ring will perform better than a gapped ring. The required in-flight operating life is 7000 hours; so it still remains to be seen whether the designs under test are adequate. These tests will be concluded in Figure 5 shows the results of 5500 hours of wear test data. The solid rings tested with two different cylinder wall materials show little evidence of performance degradation. The flow data in the chart is normalized against the initial flow rate of each cylinder at the beginning of life.

6 the required functions. What is not known is control limits and stability over time. A liquid sensor for the Sabatier was developed in This device shown in Figure 6, uses two conductive rings, along a length of pipe, that are electrically isolated from each other. A droplet of water bridges the two rings and gives a positive indication of water presence. This sensor was tested for short durations and will be incorporated into a long term Sabatier system test. Figure 4 SSF CO 2 Compressor Prototype Piston Ring Wear Test Data Solid/Al Solid/SS Split/Al Split/SS Test Time (hours) Figure 5 CO2 Compressor Piston Ring Wear Test Results ANCILLARY HARDWARE AND SYSTEM MATURITY Current TRL Level 5: System configuration tested with commercial components Desired TRL Level 6: Hardware grade adequate to demonstrate system performance VALVES AND SENSORS Most of the components of the CRA in the system can be demonstrated with commercial-off-the-shelf components, i.e. valves, pressure transducers, thermocouples, etc, where these types components have been demonstrated for flight. There are some components proposed in the current schematic that have not yet been proven to work for system control and performance. These include the modulating flow valve for CO 2, the CO 2 flow meter, the outlet regulator, and the liquid sensor. These four items require development and testing in a pre-prototype system to see if their functionality is adequate to perform Figure 6 Liquid Sensor The CO 2 modulating valve is being developed this year (2004) and the back pressure regulator will be developed next year. Carleton Technologies, Inc. is working with HS to develop these valves to meet the requirements of the Sabatier system. CO 2 is supplied to the Sabatier reactor from an accumulator that is constantly being filled by the compressor with CO 2 from the Carbon Dioxide Removal Assembly (CDRA) and then drained when the Sabatier is processing. The CO 2 accumulator pressure varies between 130 psia and 18 psia. The valve must be able to meter the CO 2 flow with accuracy of 0.1% of the desired flow. The CO 2 valve must have control capability of 0.1% of full range in order to maintain the CO 2 molar ratio between 3.3 and 3.7. Control of the feed gas mixture requires a flow measurement feedback to the flow control valve which would likely be a laminar flow element with a differential pressure transducer. The back pressure regulator must maintain the system pressure to psia with a gas flow rate that varies from full operation (0.29 lb/hr) to 25% flow (0.075 lb/hr) of a mixture of methane, CO 2 and hydrogen. The regulator must be able to handle system pressure transients without over-pressuring the rest of the system.

7 HAZARDOUS GAS CONTAINMENT Because methane is a product, leakage of gas in or out of the system creating the potential for a combustible mixture must be prevented. The system also operates at high temperature and these extreme temperatures must be contained. The seal approach used in the design of the components of the Sabatier ORU will affect both the safety of the system in the intrinsic leakage protection, as well as the volume required for each component. The CRA has been evaluated to determine the best approach to seal design. The subsystem schematic has been broken up into logical manifolds for the groups of components. The hot areas of the system, which include the reactor and heat exchanger, will be welded. All other areas of the system will incorporate two polymeric seals in a design that minimizes the seal surface area required. Figure 7 shows examples of some sealing designs considered for this application. Figure 8 shows the manifold designations for the CRA systems. Figure 7 Example Seal Strategies CRA Schematic Seal Zoning A COM R HX D B CON SEP L A A C Legend: A - Interface Manifold R / HX - Reactor / Heat Exchanger B - Motor Valve Manifold SEP - Separator C - Waste Water Manifold COM - Compressor D - Rack Air Manifold CON - Controller CONTROL AND CONTROL INTEGRATION Figure 8 CRA Schematic Assessed for Manifolds and Sealing Requirements CONTROL DEFINITION Current TRL Level 4: System Model correlated to key component performance, Hazard Analysis defines safety components Desired TRL Level 6: User Interface developed for flight system, system model augmented for FMEA demonstration The controls for the CRA have been developed to the point of defining all of the sensors and effectors required to operate the CRA safely. This includes sensors required for operation, fault detection, monitoring and isolation. Controller tests of the hardware have been defined which will allow the CRA to perform self-health checks periodically. The controls required to interact with other

8 subsystems have been defined, including the sensors and effectors that are needed. Some of the control logic for optimized operation of the system still need to be developed. The control of the CO 2 flow valve with feed back from the CO 2 flow meter is one example. Also, control of the cooling air through the reactor is not yet resolved. The cooling air valve can be operated based on commanded flow rate of the reactants, or by a temperature schedule. MODELS Mathematical models of the Sabatier reactor, CO 2 compressor and carbon dioxide removal assembly (CDRA) have been separately developed and verified against test data. The CDRA and compressor models have been effectively exercised to support development of the control algorithm for operating the compressor. The system is now optimized to minimize the compressor operating time, number of starts and total power without starving the Sabatier reactor. Further analysis is in process to evaluate the control logic under various crew loading scenarios. The Sabatier model has been verified against test data. It is currently being integrated with the Oxygen Generation Assembly (OGA) model that was created for flight software development. The Sabatier model will be integrated with the CDRA model to further define the controls for the system. The system controls of the CRA must be integrated into the models and then optimized. The system model must be augmented to demonstrate failure modes and hazard controls. There is no user interface currently developed. SABATIER SCAR The Carbon Dioxide Reduction Assembly will be installed in orbit, after the launch of its resident rack. Design features have been incorporated in the OGA rack to facilitate this operation. The CO 2 accumulator has been defined and is currently being manufactured by MSFC. The accumulator will be installed into the rack as rack resident hardware prior to the launch of the Oxygen Generation System (OGS) rack. The hydrogen line has been designed and a keep-out zone has been defined to preserve the scar. The keying needed for the quick disconnects have been defined which will prevent mis-mating of connectors on the panel. Ground integration of a prototype CRA with the flight OGA and CDRA is the ultimate risk mitigation tool to ensure that the systems will work together when installed on orbit. Table 2: Evaluation of Sabatier Technology Readiness Level (TRL) Summary Table Core Technology Components Ancillary H/W & System Maturity Control & Control Integration Present TRL Development Required to Reach TRL 6 Modulating valves: Control range stability over time Complete a system model including interfacing systems Reactor: Particle filtration Catalyst Poisoning Materials Manufacturing Vibration Isolation Heat transfer isolation Condenser: Materials Fabrication Separator: Demonstrate Pumping Pressure Rise CO 2 Compressor: Materials Acoustics Performance Manufacturing Latching valve: demonstrate dependability Outlet Regulator Liquid Sensor Hazardous Gas Containment: Address leakage Manufacturing Demonstrate failure modes and controls Develop user interface Demonstrate control schemes (i.e. measuring inlet and outlet pressure to set flow control valve position) Dissolved Gas in Product Water Test & Analysis Evaluate concepts

9 SUMMARY/CONCLUSIONS: Technology Readiness Level assessment is a tool that is useful for determining the maturity of a system and what further development is required. The Carbon Dioxide Reduction Assembly has been assessed as a Level 4/5 (Breadboard/Pre-prototype). Current development efforts are focused on bringing the technology to Level 6 (Prototype). The assessment summarized in Table 2 shows the areas of the system that require development. JSC and MSFC have planned to continue developing the CRA to Level 6 and to build a prototype that can be integrated on the ground with the flight OGA and CDRA prior to their launch. This ground integration is a key component to mitigating the risks associated with installation and operation of the CRA on orbit. ACKNOWLEDGMENTS The authors would like to acknowledge the support of the Advanced Life Support division of the Johnson Space Center for their continued interest and support of the development of the Carbon Dioxide Reduction Assembly. CONTACT Karen Murdoch Hamilton Sundstrand Space Systems International, Inc. 1 Hamilton Road MS 1A-2-W66 Windsor Locks, CT (860) (860) fax karen.murdoch@hs.utc.com Jay Perry NASA Marshall Space Flight Center Mail Code FD21 Marshall Space Flight Center, Alabama (256) (256) fax Jay.L.Perry@nasa.gov Fred Smith NASA Johnson Space Center 2101 NASA Road 1 Mail Code EC3 Houston, TX (281) (281) fax frederick.d.smith1@jsc.nasa.gov Steve Green Southwest Research Institute 6220 Culebra Road San Antonio, TX (210) (210) fax sgreen@swri.edu