NASA's Dragonfly Mission to Titan

This episode of Real Engineering is brought to you by Brilliant, a problem-solving website that teaches you how to think like an engineer. Over the past 100 years, advances in science and technology have allowed us to learn a lot about where we come from and how our planet has developed over time. Thanks to our discoveries of ancient fossils, we know that life on Earth has existed for at least 3.5 billion years. But since the Earth is thought to have formed about 4.5 billion years ago, we still have very limited knowledge about what caused life to form in the first place, and much of the evidence about how it may have actually developed has been destroyed by the Earth itself. life he created.

Unfortunately, we don't have a time machine, but we can look for places in our solar system that emulate early Earth. That's where Saturn's largest lunar “Titan” comes into the picture. Although it is about half the size of Earth, Titan has features that we believe are very similar to those of Earth in its early days. Titan has a thick atmosphere about 4.4 times denser than Earth's, and is the only Moon in the solar system to have any notable atmosphere. Titan also has large pools of liquid that follow a cycle similar to the rivers and seas we have here on Earth.

But, instead of water, Titan has seas of methane that evaporate into clouds, causing liquid methane to rain down. In terms of scientific information, Titan is a gold mine for scientists. But unfortunately, it is extremely difficult to get to, and because it is completely covered in methane clouds, it is almost impossible to study the surface from a distance. In 1997, a collaboration between NASA, the European Space Agency and the Italian Space Agency launched the Cassini space probe on a seven-year journey to reach Saturn. The probe was designed to study the entire Saturn system, including its rings and natural satellites.

But Cassini did not make the seven-year journey alone. Attached to the space probe was a small lander called Huygens that hoped to become the first spacecraft to land on Titan. Several months after entering Saturn's orbit, Huygens separated from Cassini and began its journey toward Titan. And he began sending back vital details about Titan's environment, such as the fluid properties of the atmosphere and the nature of the moon's surface. As it descended, Huygens recorded accelerometer data, which could be used to deduce properties such as fluid density, just as we knew about the probe's aerodynamic properties. Temperature and pressure readings were necessary to teach us about the thermodynamic properties of the atmosphere.

After two and a half hours of descent through the unknown, Huygens successfully landed on the surface, making it the farthest spacecraft landing from Earth ever completed. Although it was only designed to survive for about 90 minutes while on the surface, Huygens successfully recorded and sent back 350 images, revealing a world eerily similar to our own, with steep hills and valleys and rivers of methane making their way across the landscape. . . As our quest to learn more about the origins of life continues, NASA's new

mission

called 'Dragonfly' will begin its journey to Titan in 2026, and the work of the Cassini and Huygen

mission

will be vital to its success.

Dragonfly is a mobile lander equipped with 8 large rotors that will help it fly across the surface like a drone. An incredibly difficult engineering challenge, and the data obtained from Huygens will be incredibly valuable when designing the drone. Everything from sensor layout, battery capacity, power source, and propeller design will be dictated by what we learned, and those are exactly the engineering challenges we're going to investigate today. Dragonfly will have many of the same scientific instruments as the Curiosity rover. You will have a skid-mounted drill to take soil samples and run them through a mass spectrometer to learn more about the composition of the soil.

It will be able to rapidly analyze elemental compositions at landing sites before landing, using a neutron-activated gamma-ray spectrometer. This instrument normally requires cryogenic cooling, but thanks to Titan's sub-zero temperatures, this instrument can be passively cooled. However, it will need to generate its own neutrons rather than relying on cosmic rays to generate them, since the atmosphere blocks too much sunlight. When it lands, a seismometer will give us information about earthquakes and reveal the thickness and nature of Titan's icy crust lying over what is believed to be an ocean of liquid water. We think this because Cassini witnessed the surface change position 30 kilometers in just 2 years, indicating that the crust is floating on some type of liquid layer.

We can also expect incredible photos of Titan's surface, just like the photos we're currently getting from Mars. Since the air is denser on Titan and gravity is one-seventh that of Earth, the

dragonfly

will be able to achieve more thrust on a planet that needs less lift. Drastically reduce energy consumption compared to land. However, finding that energy to fly over Titan's surface is not easy. Due to Titan's distance from the Sun and its thick atmosphere, sunlight on Titan's surface is about 100 times weaker than on Earth, making solar panels impractical. Fortunately, we have plenty of practice on a different type of power source through missions like the Curiosity Rover, which was powered by a radioisotope thermoelectric generator.

The RTG works by converting the heat from the natural decay of a radioisotope into electricity. Now this is not traditional nuclear power as I have mistakenly stated in the past. The RTG does use radioactive materials to generate electricity, but not through nuclear fission. It uses a simple principle called the Seebeck Effect to generate electricity. Basically, the Seebeck effect allows us to generate an electric current through a heat differential, since the charge carriers will go from hot to cold. So if we have a heat source and a way to cool we can generate a sustained electric current.

Fortunately, radioactive substances generate heat as they decay. Choosing a suitable radioactive material is our first challenge. In any spacecraft, a compact and lightweight design is paramount, but we also need the material to have a long half-life to ensure a long-lasting power source. We also need it to primarily produce alpha waves, since this form of radiation is more easily converted to heat in a compact space. As a result of these requirements, plutonium-238 (Pu-238), strontium-90 (Sr-90) and curium-244 are the most widely used fuels. Next, we need a material that is both a thermal insulator to maximize our temperature differential and an electrical conductor to maximize our current.

These two material properties are usually linked. Materials like copper are good thermal and electrical conductors, and a material like iron is a poor thermal and electrical conductor. Using these materials together can create a crude thermoelectric generator, but the efficiency is very low. If we can create a material with the best of both properties, then we can achieve greater efficiency. Leading to the use of materials such as lead telluride and labels, which is an alloy of tellurium (Te), silver (Ag), germanium (Ge) and antimony (Sb). The thermoelectric generator used by the Curiosity Rover could generate 110 watts of electrical power.

But we will lose some power generation capacity during Dragonfly's eight-year journey to Titan, since we can't turn a radioactive element on and off as needed to conserve energy. In fact, Dragonfly's cruise vehicle will need to be equipped with radiators to bleed that thermal energy into space and prevent overheating, just as the Curiosity rover did. We will also lose power to keep the spacecraft at operating temperature, as Titan's surface can reach temperatures as low as -180°C, and to keep some vital systems and scientific experiments running, leaving us with about 75 watts to charge. while we are underway. the terrain at best.

All of our activities will occur during Titan's daylight hours, so we will try to charge our batteries during Titan's nights, which last 192 hours, the same as its daylight hours. So, it makes sense to fully charge our battery in those 192 hours. Giving us a 14 kWh battery. For comparison, a typical Tesla battery is about 75 kWh. With a specific energy of 100 Wh/kg, that will make our battery weigh 140 kilograms. In practice, a smaller battery will probably be used, and even a 30 kilogram, 3 kWh battery would provide up to 2 hours of flight at 10 m/s, providing a huge range of 72 kilometers.

Even more incredible when you consider that the Curiosity Rover has only traveled 21 kilometers during the last 7 years of its stay on Mars. Of course, Dragonfly won't fly its maximum range in a single hop and will likely make shorter, safer hops between interesting spots during the day on Titan, and one of the most impressive things about this mission is how the spacecraft will navigate the surface. Since Titan is so far from Earth, it will be difficult to send basic information to and from Earth. The power requirements for transmitting data increase dramatically as a result of the inverse square law.

The Huygen probe had the advantage of being able to transmit information to the Cassini space probe, which had a larger antenna and greater power. Unfortunately for the Dragonfly mission, Cassini is no longer in orbit around Saturn and will fall into Saturn's atmosphere in 2017. Therefore, Dragonfly will need to devote both power and weight to a large, high-gain antenna in order to communicate with the Earth's Deep Space. In addition to the additional power and weight requirements, Titan B-Roll (E3)'s average round-trip communication time is about 2 and a half hours, making it impossible to fly the spacecraft in real time.

Instead of Dragonfly flying using its own vision, just like the autonomous drones we have here on Earth, Dragonfly will use its cameras along with built-in gyroscopes and accelerometers to travel from point to point. Dragonfly will be trained to identify suitable landing sites that are flat and free of obstacles such as large rocks and rough terrain. Dragonfly was originally intended to fly with a single rotor, but as helicopters are mechanically complex in the way they vary rotor pitch to vary lift, the idea was never developed. But with the rise of multirotor drone technology in the last decade, the idea of ​​a quadcopter became much more feasible.

Dragonfly will feature a total of 8 rotors mounted in pairs in a quadcopter design. Unlike a helicopter rotor, which is designed to rotate at a constant rate, the speed of each rotor can be regulated electrically to vary the amount of lift generated. Although it is less efficient to have rotors in this upper configuration compared to a normal quadcopter, it provides additional lift while also offering some redundancy, as the aircraft will be able to achieve stable flight even with the loss of an engine or rotor. . Since Titan's atmosphere is composed primarily of nitrogen and is much colder than Earth's atmosphere, the viscosity is also much lower.

Along with the higher density, this means Dragonfly's rotors will operate in a fluid with a much higher Reynolds number than if they were operating on Earth. The Reynolds number is essentially a quantity that tells engineers whether laminar or turbulent flow will develop. It's a bit confusing since the number is not constant for all situations and depends largely on the application, but in general it can be described by this equation for flow in a pipe. Where the inertial forces, which try to keep the fluid flowing, are the numerators, and the viscous forces, which try to slow the fluid, are the denominator.

Here a higher density will increase the Reynolds number and therefore increase the probability of turbulent flow, and a lower viscosity will also increase it. Exactly the scenario we found on Titan. As a result, the propellers had to be designed differently to work as efficiently as possible on Titan. In fact, they have a design much more similar to wind turbine blades than normal propellers, with a large twist in the aerodynamic profile. These propellers are one meter in diameter, much smaller than the blades of any wind turbine here on Earth, but as a result of that higher Reynolds number, the same principles ofdesign.

Wind turbines are also designed to resist surface dirt buildup, which will be valuable property on another planet with maintenance crews billions of miles away. We must also take into account the lower speed of sound on Titan, which is about 194 m/s compared to 340 m/s, so the formation of shock waves can occur much earlier at the tips of our propellers. This means that we must take into account the tip speeds which are determined by the diameter of the propeller and the speed of rotation. Even with these unique environmental factors, Dragonfly will have a mass of around 450kg with fantastic range and top speed.

Allowing Dragonfly to first land on the sand dunes of Titan's equatorial region before finally arriving at Selk Crater, an impact crater believed to contain all the building blocks of life we ​​are familiar with here on Earth. Earth, and that can give us some clues. on the fundamental question of the human being. How we got here. You may have noticed that I mentioned the inverse square law, but I didn't fully explain it. This is a law that applies to a large number of physical properties and simply states that the intensity of a point source of energy will decrease with the square of the distance you move away from it.

This applies to gravity, electric fields, and radiation, like the

dragonfly

trying to send radio waves to Earth. To learn more about it, you can take this gravitational physics course to learn how Newton deduced that the force of gravity obeys the inverse square law and help you understand some of the terms I use in my videos. This is just a small chapter of a course on Brilliant, and you can choose from many other courses to test your brain and learn more about our universe. Or you could complete one of Brilliant's daily challenges. Every day, Brilliant presents you with interesting scientific and mathematical problems to test your brain.

Each daily challenge provides the context and framework you need to approach it, so you learn the concepts by applying them. If you like the problem and want to learn more, there is a course quiz that explores the same concept in greater detail. If you are confused and need more guidance, there is a community of thousands of students discussing problems and writing solutions. Daily Challenges are thought-provoking challenges that will take you from curiosity to mastery one day at a time. If I've inspired you and you want to find out, go to shiny.org/RealEngineering and sign up for free.

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