Columbus Cabin Heat Exchanger Dryout during ISS High Beta Angle Phase

Transcription

1 SpaceOps Conferences 5-9 May 2014, Pasadena, CA SpaceOps 2014 Conference / Columbus Cabin Heat Exchanger Dryout during ISS High Beta Angle Phase Laura Zanardini INSYEN, Wessling, Bavaria, Germany and Sinje Steffen DLR, Wessling, Bavaria, Germany This paper focuses on the details of a Cabin Heat exchanger (CHX) Dryout in Columbus, the European Laboratory part of the International Space Station (ISS), and the unique and challenging conditions, which occur during a high beta angle phase. A particular case happening between DOY154 and DOY163 of the year 2013 will be analyzed. Furthermore the paper illustrates how these difficult conditions can be overcome by the Flight Control Team at the Columbus Control Centre (COL-CC). A CHX Dryout and some of the challenges, which have been encountered during this specific high beta angle phase, are described below. The CHX Dryout is a regular maintenance activity, which is performed every 6 weeks (+/- 1 week) to change from one CHX core to the other. This activity is done after an extended period of time of the active CHX Core being exposed to water, in order to prevent microbial and fungal growth on the CHX hydrophilic coating. Such fungus growth may cause hardware damage and possible crew health issues, therefore is particularly important that a Dryout is regularly scheduled. The activity is nominally preceded by a Wet cycle phase lasting 7 days, in which the set point of the CHX control law is decreases to induce more condensation in the active core and to flush the core prior to its deactivation. In the particular case described here, the wet cycle has been skipped due to some anomalies in the TCS subsystem, which resulted in the impossibility of changing the CHX inlet temperature set point. At initiation of the CHX Dryout, the alternate Columbus Water Separator Assembly (CWSA) is activated, and the active CHX core, through which water is flowing, is swapped from one to the other. During the Dryout, both CWSA1 & CWSA2 are left active for a minimum of 16 hours. Having both CWSA on for that long amount of time ensures collection and separation of the condensate from the CHX to be dried out. At the end of the Dryout, the CWSA corresponding to the dried core is deactivated. An increase of humidity in the cabin is observed in the first few hours of the Dryout, which results in a higher dew point in the cabin. During high beta angle period, the Heater Control Unit (HCU) temperatures in certain areas, which are constantly shaded, are lower than usual. To avoid condensation of the Columbus external shell, the difference between the dew point and the lowest temperature on any of the heater strings shall be maintained at least at 4.4 C. While performing the Dryout during a high beta angle period, this requirement can be violated and corrective measures have to be taken to activate parts of the redundant HCU, supporting the active HCU in heating up the shell and avoid the risk of condensation. Another additional challenge in performing a Dryout during an high beta angle phase is the fact that, when the redundant HCU is turned on, all its 6 heater strings are enabled, so the initial power draw can be quite high and might exceed the power allocated to Columbus during such a critical power phase. Only certain strings need to be active, hence all should be disabled as soon as possible and, in case additional limitations are in place, different strings have to be activated in sequence to guarantee that the power limits are not violated and that there is no built up of condensation in Columbus. Columbus Flight Controller (STRATOS/SYSTEMS) COL-CC, INSYEN AG, Muenchener Str.20, Wessling, Germany/ laura.zanardini@dlr.de Columbus Flight Controller (SYSTEMS), COL-CC, DLR, Muenchener Str.20, Wessling, Germany/ sinje.steffen@dlr.de 1 of 12 Copyright 2014 by Laura Zanardini, Sinje Steffen, German Aerospace American Center (DLR). Institute of Aeronautics and Astronautics Published by the, Inc., with permission.

2 Nomenclature CF A CHX COL CC CSA CW SA ESA HCU IP IM V ISF A ISS JAXA LEO M CC H M LU N ASA P RO T CV U SOS W OOV Cabin Fan Assembly Cabin Heat exchanger Columbus Control Center Canadian Space Agency Condensate Water Separator Assembly European Space Agency Heater Control Unit International Partner Inter Module Ventilation IMV Supply Fan Assembly International Space Station Japan Aerospace exploration Agency Low Earth Orbit Mission Control Center Houston Module Lighting Unit (in Columbus) National Aeronautics and Space Administration Power Resources Officer Temperature Control Valve United States Orbital Segment Water ON/OFF valve I. Introduction The International Space Station (ISS) is a collaboration of five different International Partners (IPs): National Aeronautics and Space Administration (NASA) from the USA, Roscosmos from Russia, Japan Aerospace Exploration Agency (JAXA) from Japan, European Space Agency (ESA) from Europe and Canadian Space Agency (CSA) from Canada, which all have their own control-centers. The assembly phase started in 1998 and was completed in 2011, with the delivery of its last pressurized module. The International Space Station has been the most advanced human outpost in space for over a decade and during its current exploitation phase is carrying on several experiments to investigate how the human body reacts to prolonged exposure to microgravity and radiation. It contains several laboratory modules, one of which is the Columbus module, launched in 2008 as the main contribution to the ISS from the European Space Agency. Since then it has been controlled by the Columbus Control Center in Oberpfaffenhofen near Munich, Germany. Hardware designed and manufactured for use in such an environment is exposed to the same conditions of microgravity and radiation. Some of the difficulties caused by this harsh environment for equipment are overcome in the design phase of the specific hardware, through the use of materials certified for the specific application field. Other problems are solved by adopting real time reactions while operating the equipment to guarantee its best performance with the evolution of the boundary conditions. The Columbus module is equipped with a series of subsystems to maintain the air temperature and humidity within an optimal range both for the crew and the electrical equipment. It is crucial that these tools are kept operational even with unfavorable environmental conditions, in order to guarantee that the critical functions and crew safety are guaranteed at all times. In addition, the formation of condensation on the shell of the module must always be avoided, since it can be the cause of hazard issues, if it gets in contact with electrical equipment. This happened already once in 2007 when a computer malfunction on the Russian segment left the ISS without thrusters, oxygen generation, carbon dioxide scrubber, and other environmental control systems. The root cause of the issue was condensation inside the electrical connectors, which led to a short-circuit that triggered the power off command to all three of the redundant processing units. In order to maintain the full functionality of the air conditioning subsystem in the Columbus module, one particular operation has to be performed regularly to avoid health problems for the astronauts and hardware damage: the Cabin Heat Exchanger Dryout process. It is critical for both the crew and the vehicle that this 2 of 12

3 operation is performed regularly and independently from the external environmental conditions. In the following chapters it will be analyzed in more detail how the modified boundary conditions can deeply influence the execution of the Columbus Cabin Heat Exchanger Dryout and which real time challenges have to be overcome to make sure that this activity is completed successfully. The first part covers a brief introduction of the Columbus subsystems involved in the temperature and humidity control and the Dryout process is explained with the support of telemetry analysis. In the second part a specific Dryout operation during a high beta angle period is taken as an example and compared with a nominal case; the effects of the modified conditions and the solutions adopted to overcome the challenges are analyzed in the last section. II. ISS Environment and High Beta Angle Overview Since 2000 the ISS has been permanently manned, at first with 3 crewmembers. After the addition of several more pressurized laboratories and modules 6 crewmembers are constantly on board since Each group of 3 astronauts who flies up and down together on a dedicated Soyuz vehicle, represents an Expedition and each group lives onboard the ISS for about 6 months. The ISS is a self-sustaining environment which produces, for example its own power, maintains a constant pressure and air flow and has a complex regenerative system to convert the carbon dioxide into oxygen and water. Nevertheless a lot of supplies, such as additional quantities of water, oxygen, nitrogen and propellant need to be delivered by the frequent visiting vehicles: the European Automated Transfer Vehicle (ATV), the Japanese H-II Transfer Vehicle (HTV), the Russian PROGRESS and the recent commercial US vehicles DRAGON and CYGNUS. 1 The Columbus module has no own resources: nitrogen, air, power and collection of condensate water are provided by the United States Orbital Segment (USOS). Nevertheless, like any other module, Columbus provides air circulation, temperature and humidity control, in order to guarantee the crew a comfortable environment while they are performing their daily activities, such as maintenance tasks, exercise and science experiments. While the internal environment can be actively controlled by the systems onboard, the external one is heavily influenced by the orbital parameters of the ISS. The International Space Station is located in Low Earth Orbit (LEO) at an altitude of about 400 km, its orbit around Earth takes around 90 minutes. The ISS is traveling with an average speed of km/h and every day the Station completes 15.7 orbits around the Earth. 2 One important parameter which describes the attitude of the Station with respect to the sun is the beta angle. This is the angle between the Sun vector, which is the vector between the earth and the sun and the ISS orbital plane. In Figure 1. the ISS beta angle and its variation during a one year period are represented. Figure 1. Beta Angle of the ISS and its periodical evolution over 1 year. As the beta angle increases, the ISS is exposed to more sunlight per orbit, and eventually it will be in 3 of 12

4 constant sunlight - in other words, there is no passing into the Earth s shadow for extended periods of time. While, on the contrary, when the beta angle is approximately zero, the amount of time which the ISS spends in eclipse is maximized. During the early stages of the ISS life, while the Space Shuttle was in service the beta angle played a very important role in the flight schedule. Every time the beta angle was higher than 60 degrees, the Shuttle could not be safely launched to the ISS due to the different thermal constraints of the two vehicles and the fact that their desired attitude to mitigate the effects of the beta angle were not compatible. 3 There are still certain effects coming from high beta angles on the ISS, which need to be worked from the operation teams, with planned actions to minimize its effects on the equipment on board, especially in terms of power and thermal requirements: Power limitations might become necessary depending on the angle between the sun and the solar arrays, since the shadowing on the ISS solar panels reduces the amount of power generated. Those limits are usually identified in advance by the Power Resources Officer (PRO), a group of flight controllers of the Mission Control Center in Houston (MCC-H). These limitations are then forwarded to all positions in Houston and the IPs and each position is responsible for taking the necessary actions on their hardware in order to meet the power restrictions. The temperature of the modules is impacted as well by the amount and angle of the sunlight it absorbs. In the case of Columbus, based on its position on the ISS, this means a decrease of the shell temperature in the port area towards Node 2. There can be communication outages (Ku-Band), because the Ku-Band antenna has to be parked to avoid breaching the Antenna low temperature limits. While most of the system telemetry is transmitted via the S-Band antenna, which is not impacted by this, the science data and the onboard video are acquired through the Ku-Band antenna, therefore there is a high impact on experiments in such cases. III. Dryout Process in Columbus Figure 2. Schematic of Columbus Air Conditioning System (source: ESA) The Columbus air condition system maintains the temperature and humidity levels in Columbus within 4 of 12

5 the required range and filters the air. Fresh air is provided by the station: oxygen is nominally generated by the Russian Elektron, nitrogen is introduced by Russian assets and carbon dioxide is removed by the Russian Vozdukh. This fresh air is drawn into Columbus by the IMV Supply Fan Assembly (ISFA), supported by one Cabin Fan Assembly (CFA). It is combined with 50% of used air from the Columbus cabin and then passes into a filter to remove particles out of the air. A schematic of the Columbus air conditioning system can be found in Figure 2. The Temperature Control Valve (TCV) (belonging to the Cabin Heat exchanger) separates the filtered air stream into one stream cooled in the active Cabin Heat exchanger (CHX) core and dehumidified in the Condensate Water Separator Assembly (CWSA). The other air stream flows through the core which is not cooled. The ratio of the two air streams is based on the difference between the temperature set point and the actual cabin temperature measured by the Cabin Temperature Sensors. The cool, dry air and the bypassed air are combined again and diffused into the cabin. The condensate is forwarded to the USOS via the Condensate Return Line. After being coated with water for an extended period of time, microbial and fungal growth can occur on the CHX hydrophilic coating, causing damage and possible crew health problems. A periodic Dryout of the CHX is needed to prevent this fungus growth on either core, therefore this activity is scheduled with a periodic interval of 42 days (+/- 1 week). During the Dryout operation, which last about 17 hours, the following actions are performed in sequence: During initiation of COL CHX Dryout, the redundant (alternate) CWSA is activated, the cold CHX core is swapped by closing the related Water On/Off Valve (WOOV), which determine the cold and the warm core, and the so called TCV sweep is performed to invert the movement of the Temperature Control Valve. For the duration of the Dryout, both Condensate Water Separator Assemblies (CWSA1&2) are left active for a minimum of 16 hours. Having the CWSA of the CHX that is being dried out active for at least 16 hours ensures that the residual amount of condensate is pushed out of the dried out CHX. The termination phase of the Dryout deactivates the CWSA corresponding to the dried core. During the activity several parameters deviate from their nominal values; the ones that are meaningful in this specific case are the cabin temperature, the cabin air humidity and, as a consequence, the dew point. A. Cabin Temperature Figure 3. Columbus Cabin Temperature during a Dryout A cabin temperature drop is expected to occur at the very beginning of a Dryout activity. This is due to the fact that for about 7 minutes 100% of the air flow is cooled (both CHX cores are active) and for a further 12 minutes 50% of the air flow is cooled (one active core with TCV set at 50%). The original value of the cabin air temperature is typically recovered after 2-4 hours, depending on the Columbus thermal conditions (both internal loads and external environment). A typical temperature curve during a Dryout is shown in Figure 3. 5 of 12

6 B. Cabin Air Humidity During the TCV sweep, a high increase in cabin air humidity is expected. This is due to the fact that a humid CHX core is no longer cooled by the water loop and is flushed with a large amount of air. Some of the water condensed and collected in the CHX core is sucked by the CWSA to be separated, but a certain amount of the water re-evaporates and is transported into the cabin via the air flow, hence increasing the cabin humidity. The evolution of air humidity (in percentage) during a Dryout is illustrated in Figure 4. The original value of the cabin humidity is typically recovered within 8-12 hours. C. Dew Point Figure 4. Columbus Cabin Air Humidity during a Dryout As the dew point is highly dependent upon temperature and relative humidity, it is also affected by the ongoing Dryout operations, and its behavior can be traced as shown in Figure 5. The Dew Point decreases in the first minutes of the dryout, while the air is cooled during the sweep of the TCV; then, as soon as the cabin temperature is back to its nominal range, it follows the trend of the humidity, slowly recovering its nominal values within a few hours. Figure 5. Columbus Dewpoint during a Dryout The Dew Point γ is obtained through the Magnus formula: ( ( )) RH γ = (T, RH) - ln 100 exp bt c+t T dp = ln ( ) RH bt c+t ; (1) cγ(t, RH) b γ(t, RH) ; (2) 6 of 12

7 Where: T is the Temperature in C RH is the relative humidity in percentage b and c are two constants 4 equal to respectively and T dp is the temperature of the dew point in degrees Celsius. IV. Columbus Dryout Challenges during High Beta Angle phases A high beta angle changes the environmental conditions of the ISS, as already mentioned in Chapter II. Especially during an operation such as the Columbus Dryout, which already heavily influences the Columbus environmental parameters (Chapter III), the contribution of the high beta angle leads to a particularly challenging situation. The major items of concern are the possible condensation on the Columbus shell and the limited power availability. A. Possible Condensation due to Low Shell Temperature During the high beta angle phase, due to its position with respect to the ISS structure, the Columbus module is shadowed by the ISS structure. This configuration impacts the temperature of the shell, especially on the port cone side (the side which is attached to Node 2). The influence of the Beta Angle is so significant that the Shell Heater temperature cannot be maintained within the set limits. In fact, during this phase the temperature on the port cone regularly decreases below the nominal low limit of 20 C. To avoid condensation and freezing on the module shell, the Columbus module is equipped with a network of heaters, a total of 156 heaters (78 primary and 78 secondary) grouped in 12 circuits, also called strings (6 primary and 6 secondary). Figure 6 shows an overview of the distributions of the heater elements and the HCUs on Columbus. The small, black triangles indicate the heaters. Figure 6. Columbus Shell Heater positions (source: ESA) The shell heaters are controlled by the Heater Control Unit (HCU), which switches the circuits on and off, based on the shell temperature upper and lower set-points that are monitored by temperature sensors (Thermistors). In order to guarantee that the Columbus shell is not subject to condensation, the readings of each thermistor have to be above the dew point plus a temperature error of 4.4 C at any time. The 7 of 12

8 temperature error considers the uncertainty on the thermistor measurement, the dew point calculation, the gradient between the coldest spot on the shell and the sensor location. During the high beta angle phase, the temperature of the shell is lower than usual and during the Dryout phase the dew point is higher than normal (as pictured in Figure 5). This combination of events causes the reduction of the delta between the two values and it can happen that the delta reaches values lower than the margin of 4.4 C. This can be seen in Figure 7 as Shell to DewPoint DV graph, which represents the difference between the lowest shell heater temperature and the dew point plus the margin. If this value goes below zero, as in the left example during a high beta angle period, the shell is at risk of condensation. Figure 7. Lowest HCU string temperature in comparison to the dew point and Shell to dew point values during a high beta angle period - left - and a nominal one - right - B. Limited Power Availability As mentioned above, another impact of the high beta angle on the entire ISS power distribution system is the fact that, due to the unfavorable position of the Solar Arrays and the additional shadowing caused by the forced parking of some structural elements (for example the Ku-Band antenna), some restrictions on the power availability have to be applied to the downstream users. As already mentioned, these limitations are calculated and provided to the users by the responsible Power Resources Officer in MCC-H, who is in charge of analyzing in detail the attitude of the ISS, the power generated by the Solar Arrays and the loads requested by each module, equipment or payload, per power channel at any moment during a critical timeframe. The configuration of the power channels and the amount of power which can be granted to each downstream user are then reported to each responsible position in the different Flight Control Teams; for Columbus, COL-SYSTEMS and STRATOS are responsible for analyzing if the expected power consumption of the module is below the requirements for the whole duration of the power-down period on both power channels feeding the module. In case the power consumption is higher than the limit, non-critical equipment or science payloads have to be deactivated, or loads have to be balanced between the two power channels. Usual actions in case a power-down that have to be performed are: The deactivation of one string of the Columbus Module Lighting Unit (MLU). These are the Columbus lights, where one string consist of 4 MLUs The disabling of the Heater Control Unit control law or The possible re-planning of payload science runs to a timeframe outside the power-down. In addition, reconfiguration from nominal to redundant units (usually powered by different power channels) can also be performed; but, in this case, it has to be verified that the change in the load distribution is not violating the available power on the other channel. This is important as it might have impacts on the entire power chain and would ultimately impact several other equipment/user power allocation. 8 of 12

9 V. Real-Time Solutions As explained above, the risk of condensation on the Columbus shell is increased during a high beta angle phase of the ISS. To prevent this hazardous situation, different approaches can be taken, depending on the power configuration. A. No Power Limitations in place during a Dryout In case there is a high beta angle, but no power limitations have been deemed necessary for the Columbus module, there is the possibility of additionally heating the shell. As most of the equipment in Columbus with a critical function, the shell heater hardware is one failure tolerant, meaning that, in case of a failure in the Heater Control Unit number 1, there is a redundant unit which can be activated with a dedicated set of heaters, as already mentioned in chapter IV. In nominal conditions it is sufficient that only one HCU controls the temperature of the shell. If during a Dryout the temperature of one or more strings decreases to a point, where it is close to the limit of condensation (dew point C as previously mentioned), it is possible to turn on the second HCU and enable the temperature control of the critical strings to additionally heat the coldest spots on the shell. This action is usually very effective and, within minutes it is possible to see the increase of the temperature readings of the thermistors. A real time example for using the second HCU for heating the shell is shown below in Figure 8, where is possible to notice that the parameter Shell to DewPoint DV raises immediately above zero, as soon as HCU number 2 is switched ON: Figure 8. Shell temperature during Dryout with additional HCU string heating B. Power Limitations in place during a Dryout In the event that power limitations are in place for Columbus, power-down steps need to be taken. In order to protect ongoing science experiments, it is beneficial to first try to power-down or shift power loads from the system, before turning off payloads. The most effective reaction is usually to disable the HCU temperature control as this terminates one of the highest power draws of the entire Columbus system. This stops the HCU control law, which regulates the heating of the thermistors to keep their readings within the set limits. The power draw necessary to keep the shell temperature between the nominal range of 20 C to 23 C can 9 of 12

10 be up to 960W (160W per string). The amount of power needed is in direct correlation to the beta angle (higher beta angles correspond to higher power draw to keep the shell temperature constant) and to the ISS attitude, as it can be seen in Table 1. ISS Attitude +XVV Beta Angle ( ) Power (W) -75 to to to to to to to to to to to to to to to to Table 1. ISS Attitude -XVV Beta Angle ( ) Power (W) -75 to -70 N/A -70 to -60 N/A -60 to -50 N/A -50 to to to to to to to to to to to to 70 N/A 70 to 75 N/A Heater power consumption in different ISS attitudes and beta angles In this case, the following actions can be taken: 1. Enabling Temperature Control Law When the HCU temperature control is disabled the temperature starts to decrease immediately. In case any of the thermistors shows a temperature close to the dew point plus the 4.4 C error during a Dryout the corresponding string would need to be activated by enabling the temperature control law for that specific string. As this will draw about 160W and could breach the given power limits, the possible use of an additional 160W need to be pre-coordinated and agreed upon with the Power Resources Officer in MCC-H before the power-down would start. 2. Performing a Pre-Heat before the Power-Down A second possibility to prevent the situation that any temperature on the shell might get too low is to perform a pre-heat before the power-down. This means that the shell temperature set points are increased to a range between 25 C and 28 C for six hours before the power-down. This ensures that the shell temperatures are above thermal limits for at least 7 hours. With a pre-heat the shell temperature is higher than usual and when the heaters are turned off for the power-down, a longer time is needed for the temperature to decrease to the critical temperature, when condensation could occur. In Figure 9 an example of a pre-heat can be found. The thermistors shown are the ones around the port cone, which is usually the coldest part of the Columbus shell. In case of a CHX Dryout, it has been seen that the critical phase in terms of temperature and dew point values, lasts only a few hours, therefore, pre-heating the Columbus shell is an effective real time solution to avoid condensation during a Dryout in high beta angle periods. 10 of 12

11 Figure 9. Pre-Heat of 6 hours and subsequent disabling of the HCU temperature control VI. Conclusion A Dryout is an activity which is important to perform regularly (42 days with a margin of one week) to avoid microbiological and fungal growth contamination of the wet CHX core. The operational solutions described in this paper represent a series of actions which can be taken to avoid delaying this important maintenance activity due to environmental conditions. As specified in the previous chapters, the hazard of condensation during a Dryout in Columbus is a serious issue, especially when this activity is performed during particularly critical timeframes, such as a high beta angle periods. Nevertheless, Columbus Flight Controllers have developed several operational workarounds, which allow this activity to be performed without any condensation risk to the Columbus module: the Heater Control Unit is the key to prevent possible condensation on the shell during a Dryout. Especially during the initial phase of the Dryout, when the risk of condensing on the shell is higher, the redundant control unit can be activated to assist heating up low temperature areas of the shell. If the shell temperature decreases towards the critical limit in cases where power limitations are in place and the temperature control law of the active HCU has been disabled, additional power can be allocated after coordination with MCC-H counterparts to be able to activate the individual heater strings. Another possibility is to perform a pre-heat of the shell in advance to allow a straightforward execution of the Dryout with a higher initial temperature of the shell. To be able to prevent the condensation issue in real time the Flight Controllers constantly monitor the dew point and the temperature of the shell and can intervene to recover the situation when needed with the solutions described above. Acknowledgments The authors would like to thank the Columbus Project Managers, Gerd Soellner and Dieter Sabath, for the support in making the realization of this paper possible and the Columbus STRATOS Flight Control Team for the technical background and the inspiration in finding real time solutions to overcome the daily challenges in the operation of Columbus. L.Z. author thanks INSYEN AG in the persons of Dave McMahon 11 of 12

12 and Austin Gosling and S.St author thanks DLR in the person of Thomas Kuch for the opportunity to present this paper. They also thank Andrew Caldwell for proofreading. References 1 Uhlig, T., N. A. and Kehr, J., How Columbus Learnt To Fly, Hanser Fachbuch, NASA, Reference Guide to the International SpaceStation, pages/station/news/iss Reference Guide.html. 3 NASA, Space Shuttle Mission STS-133 Mission Press Kit, STS- 133.pdf. 4 Barenbrug, A. W. T., 1974 Psychrometry and Psychrometric Charts, 3d ed., Physical Description, Published [Johannesburg]: Chamber of Mines of South Africa, of 12