28 votes
Accepted

How does desaturation of the reaction wheels work?

"[T]here seems to be no mention for the need to manually slow down the reaction wheel speed and instead merely applying an external torque has the effect of reducing the wheel speed. This confuses me ...
Ilmari Karonen's user avatar
23 votes
Accepted

How does “space weather” cause reaction wheel failure?

Ceramic bearings are a relatively new concept. They were a hot research topic in the late 1970s and didn't become commercially available until the mid 1990s. The Far Ultraviolet Spectroscopic ...
David Hammen's user avatar
  • 73.8k
22 votes
Accepted

Why did SpaceX choose gas thrusters over reaction wheels/CMGs?

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 ...
SF.'s user avatar
  • 54.9k
18 votes

What does "reaction" in "reaction control system" and "reaction wheel" refer to?

I think there is little doubt that "reaction" refers to the "equal and opposite reaction" of Newton's third law. First consider "reaction control system". In principle, ...
Mark Foskey's user avatar
  • 10.5k
15 votes

Reaction Wheels vs. Momentum wheels

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 ...
fred_dot_u's user avatar
  • 1,870
14 votes
Accepted

Could Stirling Engines work on sunlight alone?

Could Stirling Engines work on sunlight alone? With the heated side facing the sun and the rest in its own shade, could two contra-rotating Vacuum Stirling Engines and flywheels, in tandem, accumulate ...
uhoh's user avatar
  • 149k
12 votes
Accepted

Does this kill vehicle contain momentum wheels? Watch how it moves!

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 ...
Andreas's user avatar
  • 1,385
12 votes
Accepted

Does Hubble need to dump momentum of its reaction wheels?

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 ...
Organic Marble's user avatar
11 votes

Why does the torque due to a reaction wheel shift to the center of mass of the spacecraft?

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 ...
gosnold's user avatar
  • 1,298
10 votes

Why aren't magnetic bearings used more frequently in reaction wheels?

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 ...
Geuis's user avatar
  • 201
10 votes

To what extent could JWST continue to be useful after the propellant runs out?

Could JWST be moved to an Earth-trailing orbit (or other useful orbit) without sacrificing too much precious remaining propellent? The JWST will be moved to an Earth-trailing heliocentric orbit as an ...
David Hammen's user avatar
  • 73.8k
9 votes

How does “space weather” cause reaction wheel failure?

The study was specifically of ITHACO reaction-wheel assemblies that proved to be highly prone to failure in the space environment. These assemblies used steel bearings. Interference with electronics ...
Christopher James Huff's user avatar
8 votes
Accepted

At what RPM does a desaturated reaction wheel run?

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 ...
Schlusstein's user avatar
  • 2,147
6 votes
Accepted

Attitude control systems with only reaction wheels

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 ...
Tom Spilker's user avatar
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6 votes
Accepted

Why did Cassini have reaction wheels but Galileo, Juno and New Horizons don't?

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 ...
Tom Spilker's user avatar
  • 18.3k
6 votes

How does desaturation of the reaction wheels work?

Reaction wheels are used to control attitude of the spacecraft and they work due to conservation of momentum. Let's say we have a reaction wheel on the x-axis of the space craft, aligned with the ...
Ludo's user avatar
  • 14.2k
6 votes

What does "reaction" in "reaction control system" and "reaction wheel" refer to?

As Organic Marble mentioned in a comment, "reaction" is used in the same sense as Newton used it: "equal and opposite reaction". A reaction wheel is designed to produce a reaction, ...
Woody's user avatar
  • 21.3k
5 votes

Why do reaction wheels fail so "frequently"?

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, ...
uhoh's user avatar
  • 149k
5 votes

Why do reaction wheels fail so "frequently"?

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 ...
Puffin's user avatar
  • 9,504
5 votes

Voyager 1's tape recorder and other angular momentum management issues

There are no reaction wheels on Voyager. They were new technology at the time (the first use I can find is the OGO series of satellites, first launched in 1964), that may have been an argument against ...
Hobbes's user avatar
  • 127k
5 votes
Accepted

Can a reaction wheel decelerate to add momentum in the same direction?

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 ...
Hobbes's user avatar
  • 127k
4 votes

Why are some reaction wheels sealed with low pressure gas, others with vacuum?

I think you are right, the pressure of the gas does not inhibit the evaporation of lubricants. The same is true for air and water vapour. If we compress air from outdoors to 200 bar, a lot of water ...
Uwe's user avatar
  • 1,714
4 votes

Do reaction wheels generally run in vacuum, or are they pressurized? Sealed or vented?

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 ...
David Hammen's user avatar
  • 73.8k
4 votes

Reaction Wheels vs. Momentum wheels

I'm getting a bit confused on the difference between reaction wheels and momentum wheels. Reaction wheel and momentum wheel are near synonyms. Manufacturers of reaction wheels / momentum wheels (e.g.,...
David Hammen's user avatar
  • 73.8k
4 votes

How often do reaction wheels require desaturation, normally?

For some small sats, the unloading is done automatically by the attitude control system when the wheel gets to a particular speed. So that would depend on how much wheel momentum you accumulate over ...
jimlux's user avatar
  • 321
4 votes
Accepted

Reactionless Electrodynamic Propulsion in LEO

This answer responds to the torque part of the question. "Magnetorquers" have been used on satellites for decades, mainly comprising a coil of wire for each axis of interest. They have the benefits ...
Puffin's user avatar
  • 9,504
4 votes
Accepted

How does the Bias Momentum Wheel and the Reactive Momentum Wheel differ?

It might be that the problem arises from terminology: there is no term "Reactive Momentum Wheel" in common use. The following terms are actually used: Reaction wheel This has an average speed of ...
Puffin's user avatar
  • 9,504
4 votes

Reaction Wheels may be Worse than they Appear?

Magnetorquers are the most similar thing. They only work if one is around a strong magnetic field, usually low Earth orbit. They work by orienting the spacecraft to the magnetic field lines of the ...
PearsonArtPhoto's user avatar
  • 121k
4 votes
Accepted

Does the ISS keep pointing prograde for most of the time?

"the station spends most of the time travelling in the direction of movement" Indeed. Orientation wise, the ISS mostly flies in a "+XVV" attitude - it's positive X axis (the end ...
Organic Marble's user avatar
3 votes

DIY Reaction wheel and actuation accuracy

You can either use a high-speed stepper motor, the kind used in hard disks and CD-ROM drives, or even a brushless DC motor. While you may order or otherwise improvise the flywheel itself, you won't ...
SF.'s user avatar
  • 54.9k

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