Truss S1 or P1 showing locations of Nitrogen Tank Assembly, Pump Module, and Thermal Radiator Joint
The following web sites provide excellent visuals of the Integrated Truss Assembly. You are able to locate the External Active Thermal Control Systems components on their pictures.
http://www.nasa.gov/externalflash/ISSRG/pdfs/integrated.pdf and http://www.nasa.gov/externalflash/ISSRG/pdfs/thermalcontrol.pdf
This site provides you with information on how the ISS works which includes the Thermal Control Systems. http://www.nasa.gov/externalflash/ISSRG/index.htm
Passive Thermal Control System
It is more difficult to maintain temperatures on the ISS than in your home, for two reasons. First, the outside temperatures on the ISS go from extreme hot to extreme cold multiple times each 24 hour period. The ISS rotates around the earth on average 15.729 times in 24 hours. That means there are 15-16 sunrises and sunsets per day. Second, the ISS is a closed system in which there is no air leaks as often found in homes. The ATCS does most of the work in maintaining the internal temperature of the ISS. However, there is also a Passive Thermal Control System (PTCS) at work helping to control temperatures. It is called passive because no fluid is flowing through it. The Passive Thermal Control System’s components are insulation, surface coatings, and electric heaters. These components are employed both inside and outside the ISS and work similar to the insulation and heaters in your homes. The outside surface of the ISS is a light color to help reflect some of the heat of the sun. The Passive Thermal Control System’s ability to provide heat and maintain temperatures plays a vital role in the TCS.
Orbital Replaceable Units (ORU) Telemetry In order to monitor and maintain the TCS and the many other systems onboard the ISS, a sophisticated technology that allows remote measurement, sensing, and control is employed. This technology is called telemetry and it means remote (tele) measuring (metry). The telemetry onboard the ISS sends data and messages back to earth from the sensors, computers, and crew members onboard the ISS. Over the years, problems with telemetry have been encountered due to the extreme temperatures, vibrations, accelerations, and signal travel time.
Some of the telemetry involves satellites that are in geosynchronous orbit with the ISS. This allows the ISS to send the message to a satellite, which then relays the message to earth. The system of satellites is called the Tracking and Data Relay Satellites (TDRS) and it uses S Band and Ku Band radios to send messages to earth.
The P1 Truss on the ISS has a UHF antenna that relays messages from the ISS crew to the astronauts involved in extravehicular activities and to shuttles when approaching the ISS. Messages can be sent directly from the ISS to earth by Ham Radio equipment onboard the ISS.
Exhibit 15. Ku Band Radio on ISS
Exhibit 16. UHF Antenna on P1 Truss
This site shows the location of the Tracking and Data Relay Satellites http://www.nasa.gov/externalflash/ISSRG/pdfs/communications.pdf
The telemetry aboard the ISS is not only used for voice communications, it also provides a two-way data exchange between the sensors and computers that monitor the many systems onboard the ISS and the Flight Controllers who regulate these systems. A special identification system uses Program Unique Identifiers (PUIs) to identify the sensor and computer that the data is coming from on the ISS. It is vital for Flight Controllers to understand the telemetry of the ISS, so that they can read the data generated by their system.
It is important for Flight Controllers to understand the pathways that telemetry data must follow to go from the ISS to Earth and vice versa. The many sensors and computers onboard the ISS send raw data to a Ground Processing System, which validates and processes the data. The Ground Processing System sends the data to the appropriate stations on earth as well as to an Archive System called Operational Data Reduction Complex (ODRC) were all data is stored for later retrieval if needed.
Exhibit 17. Telemetry Pathway Archive System
Ground Processing System also called Front-End Processors (FEPS)
Real Time Distribution
Users with Information Sharing Protocol Servers (ISP)
Since you are studying to be a Flight Controller, you will need to know the format for the Program Unique Identifiers (PUIs) for the ISS so that you can identify the data from the TCS sensors and computers.
This link is a paper from 2004 to update the telemetry system on the ISS http://www.tcl.tk/community/tcl2004/Papers/BrianOHagan/ohagan.pdf
This web site shows the computer locations within the ISS http://www.nasa.gov/externalflash/ISSRG/pdfs/computers_data.pdf
Space Flight Resource Management (SFRM) Significance: Team skills are essential if the ISS mission is to be successful. There are too many systems and procedures for one person to know it all. Flight Controllers must depend on members of their team to supply them with vital information that will lead to successful actions onboard the ISS. Flight Controllers must be able to function as a well organized team to make the correct decisions under some of the toughest circumstances.
In order to learn the team skills necessary for the ISS Flight Controllers, NASA has implemented a course of instruction called ISS Space Flight Resource Management (SFRM). This course integrates team building skills with technical learning (O’Keefe, 2008). Team training is seen as important as technical training, by senior Flight Controllers who believe that if an accident was ever to occur onboard the ISS, it would not be due to a lack of technical knowledge, but rather the lack of team skills (Baldwin, 2008).
Space Flight Resource Management: The SFRM course is modeled by a STAR. The center of the STAR contains the words Stop, Think, Act, and Review. This represents the approach each individual Flight Controller should take in every situation that requires decision making. The FC should Stop and focus on the situation. The Think allows the FC to collect his or her thoughts about the situation. The FC should focus on the crucial factors of the situation and how it is similar or different to other situations he or she has experienced. The Think involves the FC reviewing all the options available to remedy the situation and the risks involved with each option. In all situations a contingency plan should also be developed. This is where the Flight Controller must be knowledgeable of all the bits and pieces of information from the members of the Flight Controller Team. The Act comes only after the best option possible has been decided. Error-prevention techniques are employed to assure the actions are correctly followed. With every action taken, the FC must Review the process and its outcome. If an action does not lead to the appropriate outcome then the FC must start the STAR process all over again (Baldwin, 2008).
Exhibit 25. Space Flight Resource Management (SFRM) STAR
Each of the 8 points of the STAR represents inter-related team building skills. These skills are as follows:
It is important to understand the significance of each of these 8 team building skills. The success of the ISS mission is directly related to the degree that Flight Controllers incorporate these skills into their daily interactions.
Communication: Flight Controllers must have good verbal and written communication skills. However, it is just as important for them to have good listening skills. Communication is a two way road in which the ability to actively listen is just as important as the ability to communicate.
Cross-Culture: The people who contribute to the International Space Station are from 16 nations. Onboard the ISS are astronauts from several nations. Flight Controllers must be knowledgeable about the cultures that make up this diverse population. There is also diversity among the Flight Controllers at Mission Control Center. The Flight Controllers are from different cultures, social economic classes, genders, and races. It is vital to the success of the mission for all team members of the ISS team to be respectful of each other. Prejudice toward any member can lead to incorrect decision making and is detrimental to any team.
Team Care: It is important for each Flight Controller to be in the best physical and mental state. They have to take care of themselves by getting the proper rest and nutrition so that they can function at the peak of their abilities, especially if called upon in an emergency situation. The same is true for the collective nature of the Flight Control Team. It is important for each member to be aware of the needs of each other so that all can function at their best. A team is only as strong as its weakest link. Flight Controllers must help each other to build a team that operates at its fullest potential.
Decision Making: Because of the complexity of each decision that a Flight Controller has to make, she or he has a team of engineers and professionals to help them make their decisions. Decision making is a team effort which requires proper communications and “active listening.” Flight Controllers make decisions after following the STAR, Stop, Think, Act, and Review procedures.
Team Work: Every situation, which leads to a change in procedure or system, has implications throughout all the systems on the ISS. These situations require teamwork in order to get the task done successfully. Teamwork involves members being open, honest, and direct throughout discussions of the best possible solutions to accomplishing a task.
Leadership/Followership:Flight Controllers have dual roles. At times they are leaders and at other times they are followers. Both roles require good communication, active listening, and conflict management skills. The leadership role occurs when your system is having a problem and you are the most knowledgeable person. The followership role occurs when a system other than yours is having a problem and you are a member of the team working to solve the problem.
Conflict Management: As a Flight Controller, you have to be able to anticipate conflicts that may arise between members of your team and know how to effectively resolve these conflicts. Effective conflict management involves assuring that good communication and active listening does not deteriorate during conflicts. Rules for conflict resolution involve maintaining appropriate eye contact, voice tone, and body posture.
Situational Awareness: Situational Awareness requires Flight Controllers to determine the current status of their system’s condition. It requires Flight Controllers to understand how the status of their system affects the other systems onboard the ISS.
In order to maintain Situational Awareness of the Thermal Control System onboard the ISS, basic knowledge of thermodynamics is required.
The Laws of Thermodynamics
Significance: Thermo means heat and dynamics means the study of motion. Therefore, thermodynamics is a branch of physics, which involves the study of motion of heat energy in mechanical systems. The Thermo Control System onboard the ISS is governed by the laws of thermodynamics. These laws and the study of thermodynamics are essential knowledge, which allows Flight Controllers to understand the nature of the generation and flow of heat onboard the ISS. If an anomaly occurs onboard the ISS, the Flight Controller Team looks to THOR, ASTRO, and PRO to lead in fixing the problem.
Thermal Equilibrium: Thermal Equilibrium means that the amounts of heat in two objects are equal. For example, when loop A and loop B in the External Active Thermal Control System are both at the same temperature, then the loops are at thermal equilibrium. Most of thermodynamics can be modeled mathematically. In this situation, the reflexive property of equality of a = a models two objects at thermal equilibrium. The zeroth law of thermodynamics relates to the flow of heat at thermal equilibrium.
Zeroth Law: This law states that when two objects are at thermal equilibrium, the temperature of object A = temperature of object B, then the heat energy flow will be equal in both directions between the objects. This law can be modeled mathematically, as the transitive property of equality if A = B and B = C, then A = C. For example, when applied to the two loops A and B, if the temperature of loop A = 10 degrees Celsius and the temperature of loop B = 10 degrees Celsius, then the flow of heat from loop B to loop A is the same amount of heat flow from loop A to loop B. This law assures that when two objects are at thermal equilibrium they will remain at thermal equilibrium.
First Law of Thermodynamics:This law states that energy cannot be created or destroyed. While energy remains constant its form can change. This law can be modeled mathematically by the symmetric property of equality, which states if a = b, then b = a. For example, onboard the ISS electrical energy is inputted into running a computer. This electrical energy is used to operate the computer, but some of the electrical energy generates waste heat (the reason why computers get warm). According to the First Law of Thermodynamics the input of electrical energy = the work done by the computer + the waste heat. Because there are many computers and a multitude of equipment onboard the ISS there is a lot of waste heat generated. It is the job of the TCS to remove this generated waste heat from inside the ISS in order to maintain appropriate temperatures for the hardware and crew on the ISS.
Entropy: The Second Law of Thermodynamics involves the very interesting scientific concept of entropy. Entropy is the randomness or disorder in a system. The universe is always moving toward more entropy, which means that the entropy of any system (including the universe) only increases and never decreases. The hotter an object the more entropy it contains. This is because molecules that are moving rapidly in space (such as in a gaseous state) are more random than molecules moving slowly or in a solid state. For example, water when frozen has a lot less entropy than liquid water, which has a lot less entropy than water as steam. This example of water can be modeled mathematically by the transitive property of inequality, which states if a < b and b < c then a < c. This property models the entropy of water as it moves from ice to steam, where a = entropy of ice, b = entropy of liquid water, and c = entropy of steam. If the entropy of ice is less than the entropy of liquid water and the entropy of liquid water is less than the entropy of steam then the entropy of ice is less than steam.
Second Law of Thermodynamics:This law states that because a system is always moving toward greater entropy the energy flow of heat will always move in the direction from hot to cold or from higher temperatures to lower temperatures, which will increase the entropy of the cooler object by raising its temperatures. This law governs the flow of heat in the Internal and External Active Thermal Control Systems onboard the ISS. The heat generated onboard flows to the cooler water loops, which then flows through heat exchangers toward the still cooler ammonia loops. The ammonia loops then radiate heat to the still cooler atmosphere outside the ISS. This process of heat movement involves the following three processes: convection, conduction, and radiation of heat energy.
Conduction: Conduction is the transfer of heat of molecules in liquids and gases from higher temperature molecules to lower temperature molecules. This movement is governed by the Second Law of Thermodynamics, in which the molecules move from hotter to colder objects, thereby transferring heat until a thermal equilibrium is obtained. A noteworthy example is when a person touches a fire. The heat of the fire is transferred to your hand, which originally was at a lower temperature than the fire.
Convection: Convection is the passage of energy or heat from one substance to another. On Earth this occurs naturally because of gravity. For example, the air above a home radiator heats up and rises. It is less dense than the cooler air so gravity pulls it up. A current develops because as the air rises and cools its density increases so gravity pulls it down again. This process produces a free convection current. In a microgravity environment, as found on the ISS where gravity is not available to create currents, a fan or pump most be used to create a convection current. This is exactly what happens in the Internal Thermal Control System of the ISS. Water flows over heated experiments or equipment and collects its waste heat. A Pump Package Assembly is used to force convection for heat transfer. This heat transfer obeys the Laws of Thermodynamics. To regulate this passage of heat on the ISS thermal and radiant insulation is employed. Thermal insulation is material that reduces the conduction of heat by covering pipes to prevent the passage of heat transfer between the pipes and its surroundings. Radiant insulations are materials that prevent the passage of heat from radiation sources. Metals and particularly gold (Au) are good radiant insulations. Onboard the ISS, thin films of gold is sometimes used to reflect the radiant heat energy of the sun.
Radiation: Radiation is the transfer of heat energy through empty spaces in the form of rays or electromagnetic waves. Radiation unlike convection and conduction does not require a medium for its transfer of energy. Therefore, radiation through the radiators on the outside of the ISS is able to reject their heat to outer space. Radiant energy from the sun is a major source of power on the ISS. Solar power is the use of the sun’s radiant energy to generate electricity.
The Third Law of Thermodynamics: The Third Law of Thermodynamics deals with the absolute minimum temperature that a substance can reach. Thermodynamics uses a Kelvin scale of temperatures to represent this minimum temperature of zero Kelvin. It is not possible for a substance to reach zero Kelvin, because it means that the substance would have no heat or entropy. The Third Law of Thermodynamics states that it is impossible for an object to reach zero Kelvin. Scientists often use the Kelvin scale, while for non-scientific purposes most countries use the Celsius scale. However, the United States uses the Fahrenheit scale. Because of the different countries involved in the ISS, it is important to know how to convert between all three scales and to always specify temperature with the proper scale.
Exhibit 26. Temperature Conversion Formulae
C = K – 273.15
K = C + 273.15
F = (9/5) K - 459.67
K = (5/9)(F + 459.67)
F = (9/5)C + 32
C = (5/9)(F – 32)
Newton’s Law of Cooling: An examination of the rate of heat loss is necessary to understanding the Thermal Control System. Newton’s Law of Cooling states that the rate of heat loss of an object is proportional to the difference in temperatures between the object and its surrounding. The mathematical model for Newton’s Law of Cooling involves a differential equation that is found in calculus. For example, the ammonia in Loop A radiator is 13 degrees Celsius, due to waste heat collected and needs to be cooled to 3 degrees Celsius by radiating heat to space during the night. How long will this take if the night air outside the station is -6 degrees Celsius? We know that according to the Laws of Thermodynamics heat will flow from the radiator to the cooler space air. We know that the initial temperature of the radiator is 13 degrees C, and the initial temperature of the air is -6 degrees C. The difference between these two temperatures is 19 degrees C, which can be considered the change in y for this equation where y is the dependent variable of temperature of the objects (which simply means that the change in temperature is dependent on the amount of time). The change in x, the independent variable of this equation is the time change that is needed to go from 13 to -7 degrees C. In algebra, the change in y divided by the change in x is the rate of change or the slope of a line. In calculus this slope is the first derivative of an equation written d (Temp)/d (time). Since Newton’s Law of Cooling states that the rate of heat loss d (Temp)/d (time) is proportional to the difference in temperatures the mathematical model would be d (Temp)/d (time) = - k (Temp of radiator – Temp of air), with k representing the proportionality constant for this equation. If k is known then the time it takes to change the temperature on the ISS can be calculated. The negative sign in front of the k signifies that the ISS is cooling down so that the slope of the line will be negative. Don’t worry if you do not understand the algebra yet, you will when you study Algebra 1 and 2.
The integration of mathematics and science with the technology onboard the ISS is exactly what keeps the ISS functioning at optimal conditions. Now that we have explored the basics of the ISS and its TCS, we are ready to launch into becoming a certified student Flight Controller.