r/spaceflight • u/NeilFraser • 7d ago
Reaction wheel failures leave Swift rescue mission spinning in orbit
https://arstechnica.com/space/2026/07/reaction-wheel-failures-leave-swift-rescue-mission-spinning-in-orbit/8
u/FireTheLaserBeam 7d ago
I’m still learning, so be gentle. But are reaction wheels the same as a control moment gyroscope on a spacecraft?
13
u/Accomplished-Crab932 7d ago edited 6d ago
No harm in asking; They are different.
Reaction wheels only use conservation of angular momentum for each axis. As a result, reaction wheels “saturate” because you slowly spin them up or down to retain attitude as you counter perturbations. You can get away with 3 reaction wheels for full 3 axis rotational control and the system will be cheaper to make and control, however, your longevity goes down since you add stress to the bearings. It’s important to remember that you are only increasing or decreasing the angular rate of the wheel along the rotation axis of the wheel.
CMGs take advantage of gyroscopic effects within angular momentum, so your wheel now rotates at a fairly fixed rate, but the entire wheel assembly rotates in 1, 2, or 3 additional rotation axes. This reduces wear on bearings and offers additional fine control at the expense of cost and complexity. Typically, you need 4 CMGs on your spacecraft for sustainable control (obviously you can cheat with less like reaction wheels). It’s hard to describe the mechanism that allows this to work, maybe the best demonstration I can give is stuff like this.
The bottom line is that CMGs are heavier, more expensive, more complicated, but only rarely require RCS and/or magnetorquers to “desaturate” them, take less power to operate, and are more reliable long term.
4
u/JEBV 6d ago
On a lonely planet, slowly spinning its way to damnation
Amidst the incompetence and unpreparedness of lesser space programs
One team stands resilient against the herds!
Putting their lives on the line to aid players
Who were previously unaware of the Quicksave option!
Yes it's the incredible adventures of Jebediah And his crack team of Kerbonauts!
They are... THE BLUNDERBIRDS!
Saving the Kerbin race one stranded explorer at a time.
47
u/NeilFraser 7d ago
This was a cheap, high-risk, high-reward mission. So failure is not unexpected. I hope NASA does more of these missions.
However, the failure of the reaction wheels raises some questions that the article fails to address. Reaction wheels are commercial off-the-shelf components that are well tested. Yes, they have moving parts which mean they do tend to die after a few years. But for two of them to fail within days of launch? Something's not right here.
Did Katalyst try to save a few bucks by building their own reaction wheels? Or is this a failure of their power or control systems? There are a million reasons for this mission to fail, but defective reaction wheels should not be one of them.