Propylene Glycol Water Filter Sizing, Design, and Testing for Minimal Maintenance in Dream Chaser Cargo System Active Thermal Control

49th International Conference on Environmental Systems ICES-2019-213 7-11 July 2019, Boston, Massachusetts

C. Perich,1 and S. Cantrell-Avloes2 Sierra Nevada Corporation, Louisville, CO, 80027

M. Pinnola3 Sierra Nevada Corporation, Madison, WI, 53717

Sierra Nevada Corporation’s (SNC) Dream Chaser® spacecraft is under contract with NASA to resupply the International Space Station starting in 2021. The reusable vehicle design and build targets minimal maintenance and provides a quick return to flight. A primary subsystem of this spacecraft is the Active Thermal Control System (ATCS) which utilizes Propylene Glycol Water (PGW) 50/50 by volume coolant loops to maintain all spacecraft components and subsystems within their required temperature limits. The utilization of PGW coolant loops requires filtration of particulates, which is accomplished using carefully designed, sized and tested nonreplaceable filters. Current literature shows limited, if any, references regarding cleanliness standards, sizing guidance or testing filter capacity for this type of system. Most references focus on a filter's design in terms of monitoring pressure drop across the filter and its replacement upon reaching a target pressure. The SNC ATCS team has defined a process to size and test filter capacity for specific use with PGW for limited maintenance. This work also includes the development of a concept of operations that optimizes the use of ground equipment to reduce the amount of hardware launched. The approach combines testing, analysis and process control to size the hardware.

Nomenclature

Asurf = wetted surface area ATCS = Active Thermal Control System cm = centimeter CM = Cargo Module DI = Deionized dP = differential Pressure FOD = Foreign Object Debris KSC = Kennedy Space Center g = gram in = inches ICP-MS = Inductively coupled plasma mass spectrometry LEO = Lower Earth Orbit m = mass mm = millimeter PC = particle count PGW = Propylene Glycol Water psid = pounds per square inch differential ρ = density RI = Refractive Index

1 Systems Engineer III, Active Thermal Control System, 2000 Taylor Ave, Louisville, CO 80027. 2 Senior Systems Engineer, PE, Active Thermal Control Systems, 2000 Taylor Ave, Louisville, CO 80027. 3 Mechanical Engineer III, Active Thermal Control Systems, 1212 Fourier Dr, Madison WI 53717

I. Introduction NC’s Dream Chaser (UDC) spacecraft is a multi-mission space utility vehicle designed for transporting crew and S cargo to low-Earth orbit (LEO). It is comprised of two elements: the Uncrewed Dream Chaser reusable vehicle and the mission disposable Cargo Module (CM). A major subsystem of the Dream Chaser is the Active Thermal Control System, which serves the purpose of heat collection, rejection and transportation. The Active Thermal Control System utilizes a 50:50 by volume Propylene Glycol Water coolant and has two independent coolant loops. Both loops utilize pump packages that are comprised of a centrifugal pump, an inlet filter and an accumulator. Loop A has an additional pump and filter for redundancy. This paper discusses the sizing and testing of the pump filter intended for permanent use without required maintenance. Results presented here may be useful input for future spacecraft fluid filtration design.

II. Literature Review and Results Based on subsystem and vehicle requirements, the ATCS pump filter must be installed permanently and cannot require any maintenance. The literature review showed that no standard exists for assessing filter sizing methodology for non-replace filters; thus, the SNC thermal team has developed a new approach to size, test, and mitigate any risks to satisfy the unique requirements of this program. The literature, however, did provide general guidance with respect to particle generation sources, particle/filter interaction and general best practices. Particle/filter interactions have been well-established. For instance, filter openings have been known to first trap particles of sizes equal to or greater than the size of the openings. This interaction then creates smaller openings and passages which consequently trap smaller particles. These, in turn, create smaller openings and the cycle continues until the filter causes pressure drops to increase and stress the system1. The effects of cleaning and flushing the system have been assessed and leveraged in this work. In general, the literature and text books show there are a variety of filter types, filter purposes, and classes. The SNC filter uses a pleated wire mesh element. For the purposes of this research, four main sources of particulates from the fluid and hardware are assessed. The primary source is contribution from particulates located on the hardware upon delivery and is a function of the cleanliness of aforementioned hardware. The second potential source is the fluid itself, and the third is from wear and tear of moving parts of system components during cooling operations. The final potential source assessed is the corrosion of the material within the system due to the interaction of dissimilar materials. The Uncrewed Dream Chaser (UDC) is designed to last for the lesser of ten years or fifteen missions. PGW will not be drained from the UDC during the life of the vehicle unless sample data shows properties outside of specification or there is a larger system failure. New Cargo Module PGW will be loaded each mission. Therefore, this filter must be able to remove particulates from the four sources that accumulate over the life of the vehicle while still maintaining an acceptable pressure drop across the filter in the overall system. A comprehensive approach to account for all four sources of particulates over the life of the Dream Chaser ATCS was developed and leveraged to forge a method that would satisfy our design and operational requirements while taking into consideration the overall system design and concept of operations.

III. Technical Approach The team evaluated and mitigated each of the four sources of particulates. Analyses were used where justified and a test or maintenance program was defined as further mitigation.

A. Particulates as a Function of Hardware Cleanliness The cleanliness level of the ATCS systems is defined by an SNC internal standard as an allowable number of particles per size as a function of wetted surface area. For the purposes of determining particulate build up and the effect on filter performance, each particle left on ATCS hardware after precision cleaning is assumed to be a sphere with constant volume and mass. This assumption was used to obtain a worst case density of the filter cake. A systems

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Copyright © 2019 Sierra Nevada Corporation material analysis of the wetted surfaces was completed and results determined a system materials composition percentage of stainless steel and aluminum. From there, the distance of the ATCS flow path was determined and, as a result, dictated particulate accumulation quantity between the fill port and the filter. Per current ATCS architecture, the pump packages are located in the nose of the vehicle, while the fill port is located on the aft end. In order to provide conservative margin, this flow distance was increased to half of the total length of the ATCS. The wetted surface area of all plumbing and cooling loop components was calculated based on that assumption. Combining these factors, the team defined the estimated mass accumulation as a function of cleanliness reflected by Equation 1:

푚 = 휌 ∗ 1000 ∗ ∗3∗( 푃퐼 ∗ ∗ 푟 ∗ 푃퐶)∗ 퐴 (1) where m is particle mass, ρ is density, PC is the particle count, and Asurf is the wetted surface area. This analysis provided a total particulate mass expected to be seen by the system during the original fill operation. Experience indicates that most filter designs assume an operational margin of least 300% following a sizing calculation. This heuristic method was incorporated in the pump filter sizing and margins. Based on system and pump performance a filter pressure loss of no more than 2.0 psid (13.8 kPa) was allotted. As long as the pump filter has a pressure loss less than this, the pump is able to overcome the system pressure loss and provide adequate coolant flow rate to meet heat rejection requirements. In terms of system assembly and integration, environmental and component cleanliness levels are either equal to or more stringent than those imparted on the pump package, eliminating further external particle contribution. Literature review also provided guidance that welds themselves do not produce many particles on average, providing further cleanliness confidence during the assembly and integration phases2.

B. Particulates Originating from the Fluid Itself Another mitigation strategy in place to limit particulate generation and accumulation is the cleanliness levels of the PGW. The PGW is a mixture of AMSOIL ANT Propylene Glycol and Type III Deionized (DI) Water, prepared locally at 200 cleanliness level per DC Program reguirements, which encompass IEST-STD-CC1246E, Product Cleanliness Levels and Contamination Control Program as shown in Table 1. One important note is that actual cleanliness far exceeds specification cleanliness in many instances2; this affect, however, has not been incorporated in this assessment to maintain conservatism. The fluid is processed through a five micron (absolute) filter prior to loading onto the ground coolant system. The ground coolant system will then circulate the fluid through this filter and an air trap to eliminate all gas bubbles and particulates from the coolant. These ground system filters and air traps are replaceable and their associated delta pressures will be monitored for performance degradation. This specific five micron (absolute) filter size was selected to be more stringent than the system cleanliness levels of 200. Also taken into consideration was the pump minimum clearances, which would allow for particles up to 22 microns to pass through without concern of immediate build-up. This concept of operations ensures that fluid particulates introduced into the ATCS are less than five microns, which is substantially less than critical tolerances within component packages.

Mitigation was then Table 1: Cleanliness levels as dictated by the SNC Dream Chaser Program considered in terms of concepts Particle size (microns) Particles per 0.1 square of operations in filling the meter of surface area ATCS coolant loops in both the 15-25 2949 UDC and CM. The ATCS Cleanliness level: 200 25-50 1069 spans both elements of the 50-100 154 Dream Chaser Cargo System, 100-200 15 meaning that part of the loop 200+ 0 exists on the reusable UDC, while the rest exists on the Cargo Module, which will be jettisoned and disposed of prior to re-entry with each mission. In assembly, coolant is filled during the UDC initial build and topped off to account for specification leakage throughout the life of the vehicle. Each CM will be filled prior to delivery to Kennedy Space Center (KSC) for processing and integration into the UDC. The only source of particulates at the initial fill would be the result of the cleanliness level of the internal system. This concept of operations means that each new Cargo Module will have

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Copyright © 2019 Sierra Nevada Corporation proper ground system filtering to ensure that the CM leg of the ATCS coolant loop will not contribute to the accumulation of the reusable UDC particulate count. The ground cooling system filter will be able to affectively remove any foreign object debris (FOD) or particulate generation due to initial fill of the CM, removing its contribution to the ATCS total system particulate count. This in effect helps to reduce the particulates seen by the UDC pump filters.

C. Particulates Originating from Wear and Tear of Hardware The third source of particulates is due to the wear and tear of hardware and is mitigated using three methods. First PGW serves as the lubricant to all of the moving parts, and substantial evidences from PGW use in the automotive industry supports this assertion. Leveraging this lubricant aspect of the coolant, particulate generation due to friction is greatly reduced. The PGW serves to lubricate all moving parts within the system: the pump package shaft and a temperature control valve. The stainless steel pump package impellor rotates on a carbon composite hydrodynamic journal bearing. When operating at design conditions, the hydrodynamic bearing supports the full load of the shaft on a thin fluid film; therefore there is no contact between the shaft and bearing3. Introduction of particles to the fluid film may cause shaft contact with the bearing, which over time may lead to degradation of the bearing surface and reduce pump life. Since PGW already contains carbon, further carbon particulate generation due to rotational operations is not a significant concern in terms of material compatibility. The only risk associated with this operation is the possibility of substantial accumulation and formation of carbon masses within the coolant, which would be isolated via aforementioned filtration processes. The second mitigation strategy is a direct result of the design and conops of all ATCS moving parts. Both valves in the system have external hard stops preventing them from generating particulates internal to the system. The last method is an extensive wear-in program as part of acceptance testing of the hardware followed by precision cleaning. The accumulation from wear and tear post assembly was assumed to be negligible. Finally, the fluid rate itself being in the laminar flow regime leveraged literature conclusions that the ratio of contaminated resistance to clean resistance was independent of the Reynolds number, meaning that our flow rate would not be a significant contributing factor to particle generation via fluid-mechanical interactions1.

D. Particulates Resulting from Corrosion Most aerospace components, such as propellant tanks and valves, require design consideration of large temperature fluctuations which drive exponential decrease in particulate generation as the hardware ages2. The Dream Chaser ATCS corrosion sensitive materials, however, remain within temperature ranges that are not particularly sensitive to corrosion contribution. Therefore, this factor has been disregarded as a substantial contributor to fluid line particulate generation. This assumption has not been verified to vet for unexpected nuances of material capability, and thus requires risk mitigation via compatibility testing. This test will determine the rate of particulate release as a function of time and temperature. This test is to be conducted by SNC using ASTM G71, Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes. If the test results show that this assumption is incorrect, the team is protecting a pump performance margin that will allow us to install a filter into the Cargo Module ATCS for each mission without impacting the UDC design and turn around processing. Furthermore, planned life testing as detailed in Section F will provide additional insight into the long term generation of corroded particulates thoughout the vehicle life.

IV. Testing To support the analyses and further mitigate risk, the team defined three test programs for the filter. The first test was designed to characterize the capacity loading of the design and verify filter capacity in support of overall vehicle life requirements. This test uses a standard test rig and fine dust per industry standards. The second test will be used to determine the health of the fluid before and after each mission. The third test will be used to verify the team’s assumptions regarding galvanic corrosion for the life of the vehicle.

E. Filter Testing To validate the filter design based on the previous analysis, the pump filter vendor performed a contaminant tolerance test. The filter contaminant test schematic is shown in Figure 1, and the experimental test stand is shown in Figure 2 and Figure 3.

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Figure 1: Filter Test Schematic

Pressure drop through the test loop was measured using a tare unit, shown in Figure 2. A pretest pressure drop across the filter unit was determined using flight representative flow rates and temperatures. With the isolation valve closed, measured test dust quantities were incrementally added via the funnel in the particle injection loop, with the directional valve open. Test condition PGW was used to rinse the funnel, helping to deposit all contaminant into the loop. The contaminant affects from a dry vs. slurry form loading have shown no negligible difference in previous studies4. The directional valve was then closed, isolation valve opened, and mainline valve closed to permit PGW flow through the injection loop. The isolation valve was closed, mainline valve opened and flow was allowed to stabilize for one minute minimum prior to recording the pressure drop across the filter. The test was repeated until the desired pressure drop across the filter was reached.

Test dust per ISO 12103-1, A4 Coarse Test Dust was selected as the contaminant for the test, as the particle size distribution, shown in Table 2, more closely matches the expected particles in the ATCS loop based on a system cleanliness level of 200 and shown previously in Table 1. The capacity test was terminated after loading the system with 0.88 grams of dust, resulting in an increase of 0.4 psid (2.8 kPa) measured across the system. This was the expected amount of contaminant for a 300 level cleanliness. This test results show a linear relationship between particulate added and increase in pressure loss, as can be seen in Figure 4. Figure 2: Filter Test Setup

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Table 2: Particle size and distribution by volume per ISO 12103-1 A4, Coarse Test Dust Particle size (microns) Distribution by volume ISO 12103-1, A4 % less than 0.97 .74-.83 1.38 1.8-2.1 2.75 5.5-6.3 5.50 11.5-12.5 11.00 21.0-23.0 22.00 36.0-38.5 44.00 58.0-60.0 88.00 85.0-86.5 124.50 93.0-94.0 176.00 97.2-98.2 248.90 99.0-100.0 352.00 100.0

Figure 3: Particle Injection Loop

Figure 4: Pressure Differential vs. Total Dust Particulate Added

These test results were compared with vendor historic data, which showed a filter capacity of 0.0505 gram/in2 (0.00783 gram/cm2). Our design has a filtration surface of 10.5 in2 (67.7 cm2). resulting in a filtration capacity of 0.53 grams. The historic data set up used hydraulic fluid as the operating fluid, with different operating and acceptance 6 International Conference on Environmental Systems

Copyright © 2019 Sierra Nevada Corporation parameters from our intended use; however, the results are still valuable in that the same type of filtration medium, filtration rating, and element configuration were used. Additionally, literature shows that fluids with higher density and viscosity will have a smaller contaminant tolerance4; therefore this estimate is a conservative estimate of our filter capability. Based on these results, it is conservatively estimated that the filter has a margin of at least 2000% based on a 40 micron absolute filter size and the aforementioned assumptions detailing concept of operations.

F. Fluid Testing To further mitigate risk and fill the gaps in analysis, future testing will be conducted based on the following guidelines listed in this section. The fluid characteristics will be tested at least three times per mission. The first test occurs after initial mix of Propylene Glycol with DI water. The second sample will be taken before encapsulation just before launch. The last sample will be taken on the runway post landing before connecting the ground cooling cart up to the flight system. This testing will be used to determine the life and health of the PGW. Industry standards will be used to accomplish these objectives. In addition, these samples will be tested to determine the content of added particulates due to galvanic corrosion by comparing pre and post flight samples. Each batch of new PGW mixture will be tested for a baseline profile before being used on the vehicle. The specific tests and their respective purposes can be found in Table 3.

Table 3: Future Tests and Descriptions Test Description Electrical Determine life of fluid. As the PG quality degrades over time its electrical conductivity Conductivity changes and as such is a good indicator of how much degradation of the PG has occurred in PGW solution. Mass Spectrometry Provide information regarding metal or organic material content added to the fluid. Identify (ICP-MS) source of particles and at-risk hardware. Mass/Volume Determine how much particle mass has been added to the fluid. Provides correlation to vehicle performance and determination of future maintenance needs. Organic Material PG itself has substantial amount of organic material to start with. Recording the baseline and comparing future test results will tell the team potential source and rate of degradation to determine the rate of maintenance if it all. At the time of this paper, SNC is still researching what method is best used to test organic material content. pH Utilize ASTM D1287, Standard Test Method for pH of Engine Coolants and Antirusts to determine pH and RI. pH values will change based on what constituents are in the fluid mix and is an early indicator of PG breakdown within the mixture. SNC will use both a handheld pH meter and lab testing. The handheld meter will be calibrated against the lab testing and used in the mixing process of PG with water. Refractive Index Utilize ASTM D1287, Standard Test Method for pH of Engine Coolants and Antirusts to (RI) determine pH and RI. RI will determine ratio of PG to DI water. SNC will use both a handheld RI meter and lab testing to determine these values. The handheld meter will be used in the mixing of PG with water at the SNC facilities.

To further mitigate risk, a materials and fluid compatibility test is in development to investigate the long term particulate generation of PGW in contact with stainless steel and with aluminum, specifically with respect to galvanic corrosion. This test will be conducted using an adaption of ASTM G71, Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes. This test will be used to better scope concern regarding long term corrosion of the vehicle, and in turn the amount of particulates expected to reach the filter. If the test shows a risk of increased corrosion that exceeds the filter capacity, there exists two options to mitigate this risk: drain the system of PGW and refill with a noncorrosive fluid between missions or install an additional filter on the CM that will be disposed of with CM on reentry.

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V. Further Mitigation Strategies

G. Real Time Performance Monitoring In order to ensure the health of the ATCS pumps, pressure sensors are located at the inlet and outlet of each pump package in order to provide real time data of pump pressure head. These pressure sensors provide insight into pump and pump filter health. A low inlet pressure indicates potential leakage and a high delta pressure indicates a potential system blockage outside the pump package. A blockage inside the pump package such as the filter being undersized will result in both a low delta pressure and low flow rate measured downstream of the pump package. These pressure sensors allows for system controls to flag substantial deviations and to switch over to either the redundant pump assembly or the redundant coolant loop. In addition, the team has insight into pump current which will provide indication of blockage. Combining pump pressure, flow rate, and current data will develop a health status relationship that will be used to assess overall pump package and system health.

H. Post Landing Maintenance Post landing maintenance of the pump packages includes limited life tracking and utilization of post-landing cooling cart. The pump package service life requirements are dependent upon the ATCS concept of operations over the course of the vehicle life. Since a redundant pump is located on Loop A, and an entire redundant cooling loop is integrated into the ATCS, the total pump life can be allocated equally amongst all three pumps. These pumps will be tracked throughout their life as limited life hardware to ensure that distribution of use does not exceed individual allocations. In order to minimize pump wear and to prolong life, a coolant cart will be utilized to maintain active cooling of the Uncrewed Dream Chaser post landing and during all ground operations. After landing, a cooling cart will be attached to the UDC via quick disconnects at the aft end of the vehicle. Samples of the flight PGW will be taken for testing mentioned in Section F. Cart pumps will then push fresh fluid through the system in lieu of ATCS pump usage, as done prior to vehicle power on. As mentioned, this cart pump has filters sized to fluid through a five micron (absolute) filter and air trap to eliminate all gas bubbles and particulates from the coolant. This will help minimize flight pump usage, help remove any particulates in the loop generated during flight cooling, and to maintain proper component lubrication for optimal functionality. Current baseline is to store the vehicle with PGW in the ATCS loop in order to minimize introduction of air and particulates into the system and to streamline maintenance operations.

VI. Conclusion SNC’s ATCS teams intends to publish papers resulting from this effort documenting results of life testing , material compatibility, and operational performance results at a later date. Based on our assessments and analysis, and with significant conservatism added to protect the longevity of our system, our customer agrees that this new approach meets the intent of the requirement dictated and as such satisfies the design. Furthermore, changes in the system plumbing material from Stainless Steel to Titanium will require further assessment.

Acknowledgments The authors would like to thank all of our team members at Sierra Nevada Corporation, NASA, Barber-Nichols Inc, and Norman Filter Company.

References 1Jan, D., Guernsey, C., and Callas, J., “Propulsion System Filter Sizing Considerations for the Galileo Spacecraft,” AIAA/SAE/ASME/ASEE 26th Joint Propulsion Conference,90-1941, AIAA, Washington, DC, 1990. 2Jan, D. and Guernsey, C., “A Procedure for Sizing Propulsion System Filter Capacity,”AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference and Exhibit, 92-3535, AIAA, Washington, DC, 1992. 3Vance, J., Zeidan, F., and Murphy, B., Machinery Vibration and Rotordynamics, John Wiley & Sons, Hoboken, New Jersey, 2010, pp. 172. 4“Shuttle Filter Study – Volume I Characterization and Optimization of Filtration Devices,” Wintec Corp, NASA-CR-140386, Los Angeles, 1974.

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