Lesson Notes By Weeks and Term v4 - SHS 2
Aircraft Structures and Control
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Aircraft Structures and Control
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Subject: Aviation And Aerospace Engineering
Class: SHS 2
Term: 2nd Term
Week: 4
Grade code: 3.1.3.LI.3
Strand code: 1
Sub-strand code: 3
Content standard code: 3.1.3.CS.2
Indicator code: 3.1.3.LI.3
Theme: Core Concepts in Aerospace Engineering
Subtheme: Aircraft Structures and Control
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Welcome, learners! In our previous lessons, we discussed how aeroplanes fly and control their movement using wings, ailerons, and rudders that push against the air. But what happens when we leave the atmosphere and go into space? In the vacuum of space, there is no air to push against. So, how does a satellite like GhanaSat-1 point its camera to take pictures of our homeland? How does a DStv satellite stay pointed at Ghana to give us our favourite TV channels? This lesson will explore the fascinating world of spacecraft control.
This lesson will be more engaging if you can show short video clips of reaction wheels spinning inside a satellite, or thrusters firing on the International Space Station (ISS). Many such videos are available on YouTube. A. The Two Types of Spacecraft Control
Unlike a car that just needs to be steered left or right, a spacecraft's movement must be controlled in two distinct ways: Attitude Control (Orientation): This is about the direction the spacecraft is *pointing*. Think of it like turning your head to look at something without moving your feet. For a satellite, its attitude determines where its solar panels are pointing (towards the sun), where its antennas are pointing (towards Earth), and where its cameras are pointed. The three axes of rotation are Pitch (nose up/down), Yaw (nose left/right), and Roll (tilting side to side), just like in an aircraft. *Analogy:* Imagine you are a photographer. Attitude control is like aiming your camera precisely at the subject before you take the picture. Orbit Control (Position/Trajectory): This is about the spacecraft's *path* through space. It involves changing the spacecraft's velocity to move it from one orbit to another, avoid space debris, or travel to another planet. This is also called manoeuvring. *Analogy:* For the photographer, orbit control is like walking from one location to another to get a different view of the subject. B. Mechanisms for Attitude Control (Pointing the Spacecraft)
In space, we cannot use rudders or ailerons. Instead, we use fundamental principles of physics, mainly Newton's Third Law and the Law of Conservation of Angular Momentum. Reaction Control System (RCS) Thrusters What they are: A network of small rocket engines (thrusters) placed around the spacecraft. They fire short, precise bursts of gas. How they work: This is a direct application of Newton's Third Law of Motion: For every action, there is an equal and opposite reaction. When a thruster on the left side of a satellite fires gas out to the left (action), the satellite is pushed to the right (reaction). By firing different thrusters in pairs, the satellite can be made to rotate around its pitch, yaw, or roll axis. Example: To make the satellite's nose turn to the right (yaw), you would fire a thruster on the left side pointing forward, and another on the right side pointing backward. Pros: Very powerful and fast. Can be used for both attitude and small orbit changes. Cons: They use fuel (propellant), which is limited. Once the fuel runs out, the thrusters are useless. The bursts can also cause slight vibrations. Reaction Wheels / Control Moment Gyroscopes (CMGs) What they are: Heavy, fast-spinning flywheels inside the spacecraft. Most satellites have three wheels, one for each axis (pitch, yaw, roll). CMGs are a more powerful version of the same concept. How they work: This uses the principle of Conservation of Angular Momentum. Angular momentum is the "amount of rotation" an object has. In a closed system (like a satellite in space), the total angular momentum must remain zero. Imagine sitting in a spinning office chair. If you hold a bicycle wheel and spin it clockwise, what happens to your body? You will start to rotate counter-clockwise. A reaction wheel does the same thing. The internal computer commands an electric motor to spin the wheel faster in one direction (e.g., clockwise). To conserve angular momentum, the entire spacecraft will rotate slowly in the opposite direction (counter-clockwise). To stop the rotation, the wheel is slowed down. To rotate the other way, the wheel is spun in the opposite direction. Example: To make the satellite's nose pitch up, the motor connected to the "pitch wheel" will be commanded to increase its spin speed in one direction. The satellite's body will react by pitching in the opposite direction. Pros: They don't use fuel, only electrical power (which can be generated by solar panels). They allow for very smooth and precise pointing. Cons: They can become "saturated" – meaning they are spinning at their maximum speed and can't spin any faster to create more torque. When this happens, the satellite must use its RCS thrusters to "dump" the momentum and slow the wheels down. They are also mechanical and can fail. Magnetic Torquers (Magnetorquers) What they are: Coils of wire inside the satellite. When an electric current is passed through them, they become electromagnets. How they work: These electromagnets interact with a planet's magnetic field (like the Earth's). Just like how two magnets can push or pull each other, the magnetorquer pushes against the Earth's magnetic field to create a gentle turning force (a torque) on the satellite. Example: Our own GhanaSat-1 used magnetorquers for its attitude control. By carefully controlling the electric current in its coils, it could orient itself relative to the Earth's magnetic field. Pros: Extremely simple, no moving parts, and use no fuel, only electricity. Cons: They are very weak. They only work in orbits where there is a significant magnetic field (e.g., around Earth or Jupiter), so they are useless for deep-space missions. C. Mechanisms for Orbit Control (Changing the Path)
To change a spacecraft's orbit, you must change its velocity (speed and/or direction). This change in velocity is called delta-v (Δv). This requires a much stronger force than attitude control. Main Rocket Engine(s) What they are: The primary, powerful engine(s) of the spacecraft. How they work: They operate on Newton's Third Law, just like RCS thrusters, but on a much larger scale. They burn a large amount of fuel to produce high thrust for a sustained period. When they are used: Orbit Insertion: Getting from a launch trajectory into a stable orbit. Orbit Circularisation: Changing an elliptical (oval) orbit into a circular one. Trans-planetary Injection: Firing the engine to leave Earth's orbit and travel to Mars or the Moon. De-orbit Burn: Firing the engine to slow down, causing the spacecraft to re-enter the atmosphere at the end of its life. Example: A communication satellite launched from French Guiana is first placed in a "transfer orbit." It then fires its main engine at the highest point of this orbit to raise the lowest point, making the orbit circular and geosynchronous (staying above one point on Earth).