Staying Cool in the Heat of the Moment
Sam Cassidy, Mechatronics Engineer
8 minute read
The History of Heat
Let’s start with the basics. Thermodynamics is the study and principles of how heat and other forms of energy interact with mechanical and environmental systems. The name is derived from the Greek word ‘Thermos’ meaning heat and ‘Dynamics’ meaning energy. “Heat energy” sounds much less official or cool to talk about so I’ll retain the former. The history of this engineering discipline dates back to nearly 3000 BC with the ancient Egyptians. At that time, it was theorized that energy was a fluid-like substance called phlogiston. The Egyptians were primarily focusing their research around fire as it was believed to be one of the natural forces of the world alongside earth, wind, and water. While they did not provide any dynamic principles, it’s assumed they were the first to recognize this as an area of study. Around 500 BC, 2,500 years later, it was the Greeks who made the next advancement when the concept of the vacuum was coined. This concept is fundamental to how many of the “modern” thermodynamic laws were discovered. Advancements in the fundamentals of thermodynamics then stalled until the 17th and 18th century when the study of gasses, primarily steam, helped drive us into the industrial revolution.
The Three Methods of Heat Transfer
Since the 17th century, three types of energy transfer have been established. The first is ‘Conduction,’ which is the process of energy transfer via physical contact between objects. A good example of this is how your pan heats up on an electric stove. While the coils of an electric stovetop generate heat, the direct physical contact of a pan conducts said heat and increases in temperature. At the molecular level, when an item heats up, the atoms vibrate rapidly, and this is what we feel as temperature. Conduction is the concept that the vibration of one atom directly hits a slower moving atom all while increasing the vibration of that adjacent atom. This process depends on several factors, such as temperature difference, contact surface area, and material itself.
‘Convection’ is a similarly simple concept wherein heat energy is transferred in a medium. Air is a great example of a medium as it’s the environment that an object exists in. Convection in its simplest form can be seen as steam from a pot of boiling water. As the water boils, it releases excess heat (energy) into the air around it. As the air around it heats up, it rises and likely comes into contact with your face as you hover over your macaroni and cheese. You think to yourself “One of these days I’ll learn that steam is hot”…from personal experience I can assure you that day will never come. As that painful steam rises, the air at the bottom is pulled in and introduced with steam where it too continues the cycle.
Our last method of heat transfer is what’s called ‘Radiation.’ This is the least visually represented principle, most people don’t think about it. Thermal radiation is when a material gives off heat energy in electromagnetic waves. This principle is notably different from nuclear radiation. While electromagnetic waves sound complex, a simple example of this is visible light. When you see videos of volcanoes erupting, the lava glowing is a perfect representation of radiation. The lava is at such a temperature that traditional conduction and convection heat transfer is not enough to regulate its thermal energy. The lava radiates light that releases more energy in its attempt to stay cool. It's important to note that radiation takes place with not only visible light but also infrared light (IR). Thermal imaging arrays work on the principle of interpreting these IR waves into a heat map. All materials at temperatures above absolute zero (-459.67°F or -273.15°C) exhibit some sort of radiation. Typically, the hotter an object is the more radiation it will release, or the “brighter” it will be.
The Sun is a great example of radiant energy. A good majority of the Sun’s energy is transmitted in a light frequency we cannot see. This is the same energy that gives you sunburn after you neglected to put on sunscreen. Below is an image of the Sun both in a visible light spectrum and an ultraviolet spectrum. You can see in the image that there is just as much if not more activity that we cannot see. At some point in all of our childhoods, we have harnessed the Sun’s radiation energy while using a magnifying glass to burn something. The principle of this works by taking the amount of sunlight hitting the area of the magnifying glass and condensing it down to a pinpoint. This energy is so great that it produces a heat cable of igniting combustible materials.
Visible Spectrum Image (left) and Ultraviolet Spectrum Image (right). Credit: University Corporation for Atmospheric Research.
Conduction in Space
KMI’s spacecraft will generate heat in various ways. To start, all powered components will generate heat as they carry out their specific functions. This heat needs to be managed to move it away from the generating component. Space is a tricky environment as there is no medium (no air) to help manage heat through convection. On Earth, our spacecraft would cool itself down just by the air touching it. In space, that heat has nowhere to go. Let’s use the example of an electric motor. An electric motor will generate heat in space, which warms the motor material. This heat conducts into the mounting bracket as it makes physical contact. This mounting bracket will conduct further, moving that heat energy into the body of the spacecraft (again, physical contact). The principle of conduction works just the same in space as it does on Earth. Unregulated, this will continue until the material heats up so much that it begins to glow to the point where it then radiates energy (through visible light). Our spacecraft serving as its own nightlight in the darkness of space sounds neat but is also very problematic. Components will fail and our mission will perish if we get anywhere near that level of heat. If you're wondering how hot something needs to be to glow, I'll refer you to what’s called a Draper Point. The Draper Point is the point at which almost all solid material begins to glow from heat. This is approximately 977°F (525°C).
What's the environment like?
In order to cool our heat generating systems, we first need to control the base temperature of our components. The temperature of space is quite volatile, for items in Low Earth Orbit variations from -65ºC to +125ºC are expected. These temperature swings are primarily due to the shadow of the Earth blocking the Sun’s warmth. It’s anticipated that a satellite will experience 9,000 temperature cycles per year. Our spacecraft needs to shield itself from this excessive temperature in order to better control its own thermal generation. So, mylar coatings are used to reflect the visible and infrared rays produced by the Sun. Reflecting this energy allows for us to lower our baseline temperatures, thus making cooling easier. The most simple example of mylar is the emergency blankets that look like tin foil, which work to retain body heat by keeping radiation cooling energy away all while keeping your personal radiation energy close to you. The space version of this is the same principle but with additional layers. We plan to have a mylar coating wrapped around the thermally sensitive portions of our design similar to the image below. Color choice is also important due to the reflection of solar energy. If you have had a black car, you likely know that the heat it takes on in the summer Sun is quite intense. Black as a color absorbs most of the Sun's visible and infrared light thus making it heat up. Compared to white, which reflects the majority of the Sun’s rays, it makes for quite the temperature difference between colors. If we were to make the majority of our craft dark colors, it would make it more inclined to take on solar energy. On the other hand, if we made our craft light colors it would reflect the most solar energy. There are some additional trivial aspects to color choice but let's leave it at this for now.
NASA Engineer applying a mylar coating to the Cassini spacecraft.
Expelling Heat
Now that we have limited incoming thermal energy, how do we expel the heat we make with our powered components? The answer is simple, …magic! I wish. That would make my job much simpler as an engineer for KMI. Unfortunately, magic isn’t in the cards yet so let’s resort to some engineered options. Our first option is Heat Tubes. Heat tubes work on the concept of balancing the hot parts of the craft with the cold parts. These are hollow tubes that run throughout the spacecraft and are filled with ammonia in a liquid and vapor form. The hot parts heat up this fluid and transform it to a vapor. The vapor travels to colder parts of the craft and is condensed back into a liquid. The liquid moves back to the hot parts and the cycle continues. A large benefit to heat tubes is the lack of mechanical components, as outside of the tube there are no other components involved. This technology works for balancing our heat displacement but has a minimal impact on the overall capacity for us to cool down. One example of heat management would be keeping heat away from our propellent (fuel). Heat tubes simply take the heat energy from the propellant area and push it somewhere not as sensitive. Each portion of the spacecraft has heat requirements and using heat tubes will help manage each component individually.
In order for our spacecraft to cool down, we need to wick away the heat. The principle of radiation is used when no medium is available since energy can be transferred through electromagnetic waves. NASA and its partners paved the way for cooling via radiation with mechanisms called radiators. Radiators are large panels that have cooling loops embedded in them. These cooling loops bring in fluid heated by waste heat and spread that energy over a large surface area. This large surface area dissipates the energy along the IR spectrum. In the design of radiators, size matters. The cooling capacity of a radiator is directly proportional to the size of it.
What's Next?
Thermodynamic management is one of the larger hurdles to cross before KMI launches our mission. With the help of research done by NASA and others, we can move forward with a multitude of options while we attempt to keep our spacecraft cool in space. The ‘Law of Conservation of Energy’ in physics states that energy cannot be created or destroyed. Since laws in space are the same as that of Earth (well, laws of physics are at least), any and all energy we have on our craft (batteries, propellant, etc.) does not simply disappear when we are done using it. Rather, we need to push it outside of ourselves and into the abyss of space. Just as heat is a liability, it can also be an asset when used in the right ways. KMI is taking the same approach in turning the liability of orbital debris into an asset for in-space manufacturing, as a step along the path of #KeepingSpaceClearForAll.
Recommended column to read next: Xenon: Propellant of the Cosmos