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What are the advantages and disadvantages of different spacecraft attitude control methods?

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Spacecraft attitude control is the process of orienting a spacecraft in a desired direction relative to the Earth, the Sun, the stars, or any other reference point. It is essential for communication, navigation, power generation, and scientific observation. However, different spacecraft attitude control methods have their own advantages and disadvantages, depending on the mission requirements, the spacecraft design, and the environmental factors. In this article, you will learn about four common spacecraft attitude control methods: passive, active, hybrid, and momentum.

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Devashish Bhalla

Scientist/Engineer at VSSC, ISRO | AIR 38 GATE-AE 2022 | Sat-LVI & QA for INSPIRESat-1 | Alumni of SSPACE, IIST | CSWP…
Juan Merino

Director, Head of Space Systems @ Startical

1 Passive methods

Passive methods rely on external forces or torques, such as gravity, magnetic fields, or solar radiation, to stabilize or align the spacecraft attitude. They do not require any power or actuation from the spacecraft, which makes them simple, cheap, and reliable. However, passive methods have limited accuracy and flexibility, as they depend on the availability and strength of the external forces or torques. They also may not be able to counteract the disturbances caused by other spacecraft components, such as reaction wheels or thrusters.

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Rishab Gangopadhyay

Flight Operations Engineer @ Blue Origin
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Passive methods of attitude control are actually quite common with deep space spacecraft, which experience different forces than LEO spacecraft that operate within Earth's magnetic field. Famously, the Kepler Space Telescope had to rely on solar radiation pressure to balance itself along its X axis (telescope bore sight). This was because the spacecraft had lost 2 of its 4 reaction wheels, meaning that it physically could not rotate along all three axes just using reaction wheels. Fortunately, the use of solar radiation pressure allowed Kepler to perform extremely fine pointing and continue its mission to find exoplanets.

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Nikhil Lakshman

Project Manager | Sansera Engineering Ltd - Aerospace Division | M.Tech | MIT | Manipal University |
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Spacecraft attitude control is crucial for maintaining the desired orientation or attitude in space. Various methods are employed to achieve this, each with its own advantages and disadvantages.

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2 Active methods

Active methods use onboard sensors, actuators, and controllers to measure and adjust the spacecraft attitude according to a desired reference. They can provide high accuracy and flexibility, as they can respond to various commands and scenarios. However, active methods require more power, mass, and complexity from the spacecraft, which increases the cost and risk of failure. They also may introduce unwanted vibrations or noise to the spacecraft, which can affect the performance of other systems or instruments.

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Rishab Gangopadhyay

Flight Operations Engineer @ Blue Origin
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Thrusters are one of the most commonly used forms of active reaction control. Despite the fact that reaction wheels are used for day-to-day pointing, the reaction wheels can become saturated, meaning that they have built up the maximum amount of angular momentum that they can handle. In order to reduce this momentum without also slewing the spacecraft, the momentum has to be unloaded while using thrusters to maintain the spacecraft's attitude.

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3 Hybrid methods

Hybrid methods combine passive and active methods to achieve a balance between simplicity and accuracy. They use passive methods to provide coarse attitude stabilization or alignment, and active methods to provide fine attitude correction or maneuvering. For example, a spacecraft can use a gravity gradient boom to point its nadir toward the Earth, and then use small thrusters to adjust its roll angle. Hybrid methods can reduce the power and mass requirements of the active methods, while improving the accuracy and flexibility of the passive methods. However, hybrid methods still have some of the disadvantages of both passive and active methods, such as limited availability of external forces or torques, and increased complexity and risk of failure.

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4 Momentum methods

Momentum methods use the conservation of angular momentum to control the spacecraft attitude. They store or exchange angular momentum between the spacecraft and a rotating device, such as a reaction wheel, a flywheel, or a control moment gyroscope. By varying the speed or orientation of the rotating device, they can create a torque that changes the spacecraft attitude. Momentum methods can provide precise and fast attitude control, without using any propellant or relying on external forces or torques. However, momentum methods have some drawbacks, such as saturation, desaturation, and gimbal lock. Saturation occurs when the rotating device reaches its maximum speed or momentum, and cannot provide any more torque. Desaturation is the process of dumping the excess momentum to an external source, such as a thruster or a magnetorquer, which can consume power or propellant. Gimbal lock is the loss of one degree of freedom when the rotating device is aligned with one of the spacecraft axes, which can limit the attitude control capability.

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Devashish Bhalla

Scientist/Engineer at VSSC, ISRO | AIR 38 GATE-AE 2022 | Sat-LVI & QA for INSPIRESat-1 | Alumni of SSPACE, IIST | CSWP | CSWA | HPAIR 2021 | Aerospace Engineering | Launch Vehicles | Small-Satellites
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Momentum methods are very effective for 3-axis stabilised spacecrafts. However, momentum dumping is a major drawback. While dumping the excess momentum, LEO small satellites can use thrusters and magnetorquers as the magnetic field intensity is high as we move close to earth surface. Whereas, GEO & bigger satellites can only rely on thrust based momentum dumping. Another drawback is related to high mass of reaction wheels corresponding to bigger satellites. Thus, there is a trade off needed between the size and saturation RPM, which can be used as a system design parameter.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Juan Merino

Director, Head of Space Systems @ Startical
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# Reaction Wheels (+) Precision No propellant consumption. (-) Limited by momentum accumulation Single-point failure # Magnetic Torquers (+) No need propellant. Simple and lightweight. (-) Need magnetic field. Limited torque generation. # Thrusters (+) High torque for quick attitude changes. Orbit adjustments. (-) Propellant limits S/C life. # Gyroscopes (+) Continuous torque w/o desaturation Precise rapid attitude control. (-) Complex, failures prone. Limited by angular velocities. # Gravity Gradient Stabilization (+) Passive. Reliable. (-) Only certain spacecraft shapes. Slow response to attitude changes. # Spin Stabilization: (+) Simple and energy efficient. Stability. (-) Limited control over S/C orientation. No for precise pointing.

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What are the advantages and disadvantages of different spacecraft attitude control methods?

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5

1 Passive methods

2 Active methods

3 Hybrid methods

4 Momentum methods

5 Here’s what else to consider

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What are the advantages and disadvantages of different spacecraft attitude control methods?

1

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1 Passive methods

2 Active methods

3 Hybrid methods

4 Momentum methods

5 Here’s what else to consider

Aerospace Engineering

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