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35

If you watch these videos: ATV boost Zvezda boost ...you can see that the acceleration is quite gentle, but definite. The astronauts do need to hang on to something if they don't want to drift to the back of whatever room they're in. The first video was a reboost performed with the ATV service ship, as described in this article. Depending on what ...


22

The answer to the question, "Do the astronauts feel the station moving?" is yes, definitely, but sometimes in an "indirect" fashion. During Space Shuttle mission STS-109, when floating in my sleeping bag and waiting for slumber to come, I would notice that occasionally my body would softly brush up against one side or the other of said sleeping bag. A ...


21

Torque and/or mass. You can go with small reaction wheels which can only barely affect the attitude, or with massive ones, which... still barely affect the attitude. They are good for satellites that stay in the orbit for years, where you can afford hours per maneuver, but need years of operation. Meanwhile, RCS will run out of fuel quite fast, but it can ...


20

[Background: I'm writing this as a developer whose firmware's in flight on several substantial satellite missions. I've developed attitude control systems, working directly with RW hardware engineers.] As Hash says, there is a lubrication distribution issue with conventional mechanical-bearing reaction wheels. The result is increased wear, leading to wear ...


19

Background and Physics Note that there are actually two different but related types of actuators that use conservation of angular momentum1 to control a spacecraft's attitude (both of which may be lumped into "reaction wheel" by KSP): Reaction wheels (RWs, a.k.a momentum wheels) spin along a fixed axis at a variable speed. They change angular momentum by ...


17

For the record, there's actually a third possibility, Magnetorquers. Here's why you would use one vs the other. Magnetorquers - Inexpensive, low maintenance, but don't work in all situations. Used by LEO spacecraft typically, and small in size. They work by pushing off of a magnetic field. Reaction Wheels (Or Gyros) - The trick here is that you can't take ...


14

I found a great answer to your question from Robert Frost, Instructor and Flight Controller at NASA! It appears that the difference is one device has a dual purpose of stabilization as well as attitude control. The other has no active stabilization capability. Both are used for attitude control. Both are heavy flywheels. Both work by creating a ...


13

To answer your question "Is it true that..." then it is best to understand the context. The reaction wheel will be in a loop with a sensor that detects one or more dynamic properties of the satellite, such as an angle through a Earth or star sensor or a rate from a gyroscope. There are also likely to be some "external" actuators in a related control loop, ...


11

The reason for this is Newton's first law of motion: When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force (from here). This is the version for point-like objects. For extended objects, the law applies to the center of mass. When it activates its ...


11

It would be highly problematic for reaction wheels to serve dual purposes as reaction control devices and energy storage mechanisms. It might be possible, but the implementation would be extremely complicated. Starting with a simpler case of a three-wheel design, a desired spacecraft attitude and/or slew rate uniquely determines the wheel speed. That is, ...


11

The thruster configuration can be seen better in this image:(cropped from this document ). The attitude control thrusters are not in plane with the direction control thrusters. This Raytheon patent on MKVs states that "attitude control system includes multiple thrusters offset from the center of gravity that provide yaw, pitch and roll control." All four of ...


11

Yes. It is done by magnetic torquers. Magnetic torquer bars can provide control about the wheel-less axis, but they must also continue to dump momentum from the wheels to prevent wheel speed saturation. (emphasis mine) Source Source Another good reference on the attitude control system is here.


10

The rate of change is pretty low with pretty low acceleration. (Consider the mass of the ISS(370,000 kilos) and the output of the Progress booster engines at just under 3000N of thrust). One interesting experiment to consider is something you can try on a train that has open gangways. I.e. The cars are connected and it is one long tube. Stand at either ...


9

Reaction wheels consist of an electric motor attached to a flywheel. There are two causes of failure: mechanical electrical MECHANICAL FAILURES Both the flywheels and the motor can be damaged by the G-forces and the vibration caused during launch. Once in space, lubricating the bearing is almost impossible which leads to increase in friction and ...


8

I am just an enthusiast but I did work for a time as an intern at an aerospace company and had some exposure to satellite designs, mostly from the attitude control system programming viewpoint. That particular satellite had reaction wheels, magnetic bars and thrusters. The magnets were very useful to press against the Earth's magnetic field and allow the ...


8

It depends on the system architecture and the needs of the mission. If you're trying to momentum bias the spacecraft then it will be running at a nominal speed. If you really want to conserve energy then you might bring it down all the way. If you're running four reaction wheels arranged tetrahedrally (as is often done to provide failure tolerance) then you ...


8

There’s a very good answer to why all of those reaction wheels have been failing in a new video below. Basically, most of the spacecraft you’re referring to use the same kinds of reaction wheels from the same manufacturer. There’s some new studies that indicate solar activity has been causing damage to the wheels. Ok the plus side, more recent missions ...


7

Virtually all systems that do this use some sort of a continuous process. The typical process is something like this: A large change in momentum is required. The momentum change is handled by the reaction wheel. Reaction wheels basically work by changing the rotation of the spacecraft. You can think of it like standing in the center a roundabout and turning ...


5

They can definitely feel it when, like in January 2009, the Russians mess up and send crazy command data, resulting in too high of an acceleration and scary oscillations. Video of the ISS rock and roll dance here - the burn is in progress when the video starts, and cuts off about halfway through. What I wouldn't give to have an exterior camera view of the ...


5

A bit more detail on the causes of mechanical failures: Bearing cage instability (scroll to the sections on Momentum and reaction wheels) is a common phenomenon. It states: Bearing lubricant depletion between the ball race retainer causes cage instability and subsequent pointing errors, increased bearing torque, and wheel vibration. If this isn't ...


5

The short answer: yes, there are certain conditions that would allow desaturating reaction wheels without a secondary attitude control system (such as thrusters or magnetorquers), but those conditions are quite specific. Condition 1: You don't have to have the spacecraft at a certain attitude all the time. There might be times when you do want it at a ...


5

Can a reaction wheel shed weight to gain speed? What you need in a reaction wheel is not speed but torque. Less weight means less torque. The limit to a reaction wheel's speed is determined by motor design, friction etc. There's also a limit due to the centrifugal force in the wheel (when this force exceeds the material strength, the wheel will ...


5

It boils down to: how much spacecraft resource is required by the attitude control method you propose using? And sometimes the mission's pointing requirements play a significant role. Thrusters use propellant. Reaction wheels (and momentum wheels) use electric power. Spin stabilization uses neither, as long as you don't need to repoint the spacecraft — but ...


4

When you have two reaction-wheels on the same axis, you can accelerate one in one direction and the other in the opposite direction. The net torque would be 0, but you would have energy stored in both. When you then want to still use them as reaction-wheels, you just have to transfer momentum from one of the wheels to the other to get a net torque. However, ...


4

This short answer is meant to highlight the information in its more thorough form within this answer. If you find this interesting, read the full answer there, and if you are so inclined to up vote, consider doing it there rather than here. Since this and the other question are somewhat different, and each has good and non-identical answers, I don't see a ...


4

In principle, yes, you can use this technique to change your position in such a way that you return to your initial velocity at the end of a maneuver. In practice, it doesn't scale well. For an Earth-to-moon trip, you need a 770,000km rope if the brick is the same mass as your spaceship, and if you want to get there in a matter of days, you need to throw ...


4

Most reaction wheel assemblies use ball bearings between the rotor and housing, with some kind of lubricant, typically liquid, coating the ball bearings. There's a problem with liquid lubricants: They evaporate in vacuum. For this reason, most reaction wheel assemblies are hermitically sealed with a low pressure inert gas inside the container. Alternatives ...


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