3 ms·

I always assumed satellites and probes used small jets of hydrazine to orient themselves. I know fule capacity is limited but I just figured they used it sparin

I always assumed satellites and probes used small jets of hydrazine to orient themselves. I know fule capacity is limited but I just figured they used it sparingly. It interesting to see how they actually do it. There is a video of a walking cube on the Web I suppose it's the same principal, it seems like magic. I wonder if the magnets will interfere with sensors?

planteen 10y ago
Typically, a spacecraft will have reaction wheels or CMGs to generate internal torques which let you change attitude by the conservation of angular momentum. However, there are also external torques in the system. In low Earth orbit, you get atmospheric drag. Solar radiation pressure also affects spacecraft. To overcome this saturation or buildup of momentum in wheels, you need to generate an external torque. This can be done with torque rods that work against Earth's magnetic field. But such a strategy does not work in deep space with no magnetic field, so you would need to use thrusters instead. You typically try to keep your nagnetometer far from torque rods and wheels. Voyager had a huge boom to keep the magnetometer far from the spacecraft I believe. Not sure how you get good separation at femtosat distances.
zackelan 10y ago
> I always assumed satellites and probes used small jets of hydrazine to orient themselves. It's important to distinguish between a change in orbital velocity (how high the orbit is, how fast it is, and the plane it's orbiting in) and a change in orientation/attitude (where the satellite is pointing). For changes in orbital velocity, some form of thrust is required. This can be as simple as pressurized gas being ejected from a nozzle without being burned. The thrust vector of this propulsion needs to pass through the center of mass of the spacecraft to avoid making it tumble. These propulsion events are quite rare, and many smaller satellites don't even carry the hardware necessary for them. They're launched into one orbit and stay there, with the exception of the tiny amount of atmospheric drag eventually causing the orbit to decay. Changes in attitude are much more frequent. If the spacecraft is otherwise unoccupied, it might want to orient itself so its solar panels get maximum sun exposure and the batteries stay charged. If it's over a ground station for a communication pass, it will want to orient its antenna for maximum efficiency. And if it has cameras or other sensor packages, it might need to point them in a particular direction to achieve a mission objective. Since those attitude changes are so frequent (several of them might take place in a single ~90 minute orbit), using up propellant to achieve them isn't really an option. Also, because the goal is rotation around the center of mass, it wouldn't be feasible to use the same thruster you use for orbit changes. You'd need 3 thrusters, orthogonal to each other and offset from the center of mass, plus all the accompanying plumbing. So, it's much more common to have reaction wheels [0] and/or torque rods [1], which are electrically powered instead of consuming fuel. You typically need 3 of them, oriented in the X, Y, and Z axes, for full control. As spacecraft get smaller, this consumes a larger portion of its mass budget, which is why the small size of the reaction wheel described in the article is so important. 0: https://en.wikipedia.org/wiki/Reaction_wheel https://en.wikipedia.org/wiki/Reaction_wheel 1: https://en.wikipedia.org/wiki/Magnetorquer https://en.wikipedia.org/wiki/Magnetorquer

planteen 10y ago

Typically, a spacecraft will have reaction wheels or CMGs to generate internal torques which let you change attitude by the conservation of angular momentum. However, there are also external torques in the system. In low Earth orbit, you get atmospheric drag. Solar radiation pressure also affects spacecraft. To overcome this saturation or buildup of momentum in wheels, you need to generate an external torque. This can be done with torque rods that work against Earth's magnetic field. But such a strategy does not work in deep space with no magnetic field, so you would need to use thrusters instead. You typically try to keep your nagnetometer far from torque rods and wheels. Voyager had a huge boom to keep the magnetometer far from the spacecraft I believe. Not sure how you get good separation at femtosat distances.

zackelan 10y ago

> I always assumed satellites and probes used small jets of hydrazine to orient themselves. It's important to distinguish between a change in orbital velocity (how high the orbit is, how fast it is, and the plane it's orbiting in) and a change in orientation/attitude (where the satellite is pointing). For changes in orbital velocity, some form of thrust is required. This can be as simple as pressurized gas being ejected from a nozzle without being burned. The thrust vector of this propulsion needs to pass through the center of mass of the spacecraft to avoid making it tumble. These propulsion events are quite rare, and many smaller satellites don't even carry the hardware necessary for them. They're launched into one orbit and stay there, with the exception of the tiny amount of atmospheric drag eventually causing the orbit to decay. Changes in attitude are much more frequent. If the spacecraft is otherwise unoccupied, it might want to orient itself so its solar panels get maximum sun exposure and the batteries stay charged. If it's over a ground station for a communication pass, it will want to orient its antenna for maximum efficiency. And if it has cameras or other sensor packages, it might need to point them in a particular direction to achieve a mission objective. Since those attitude changes are so frequent (several of them might take place in a single ~90 minute orbit), using up propellant to achieve them isn't really an option. Also, because the goal is rotation around the center of mass, it wouldn't be feasible to use the same thruster you use for orbit changes. You'd need 3 thrusters, orthogonal to each other and offset from the center of mass, plus all the accompanying plumbing. So, it's much more common to have reaction wheels [0] and/or torque rods [1], which are electrically powered instead of consuming fuel. You typically need 3 of them, oriented in the X, Y, and Z axes, for full control. As spacecraft get smaller, this consumes a larger portion of its mass budget, which is why the small size of the reaction wheel described in the article is so important. 0: https://en.wikipedia.org/wiki/Reaction_wheel https://en.wikipedia.org/wiki/Reaction_wheel 1: https://en.wikipedia.org/wiki/Magnetorquer https://en.wikipedia.org/wiki/Magnetorquer