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Comment by ericpauley | Hacker News Reader

The article briefly mentions dumping energy at regular periods without using thrusters. Does anyone know what mechanism they're using to do this? At first glance I'd assume it would look like rotating/configuring the satellite so the solar wind imparts angular momentum.

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Excellent question and it turns out there's a whole article about the solution! https://www.esa.int/Enabling_Support/Operations/Rescuing_Int...

> By using a specially designed sequence of manoeuvres, the control team realised they could redistribute the angular momentum stored onboard the satellite using two different reaction wheels spinning in opposing directions, causing the spacecraft to flip.

> "So at this point we knew we could control the build up of energy absorbed from the Sun, and christened this new manoeuvre the z-flip".

There’s actually several small to medium effects on any spacecraft in orbit around earth. There will be tiny perturbations from the interaction of earths electromagnetic field as it moves through it slowly, there’s atmospheric drag on solar panels and the spacecraft body that can create “aerodynamic” torques related to what the normal orientation is relative to the earth. There’s even smaller perturbation from the solar wind… some spacecraft have a thing (or can fudge one using some part of the electrical subsystems) called a “magneto-torquer” which is basically a solenoid/electromagnet designed to tug on the earth’s magnetic field so they can trade electrical power for physical force.

It’s a pretty cool thing that seems to be happening more and more frequently with spacecraft flight dynamics teams developing such a detailed model of all these forces that they can invent new “stable modes” like the one they used in this article, or the Kepler missions K2 mode where they found another useful mode instead of just ending the mission. It’s not as dramatic as something like a solar sailing mission, but I think it’s a pretty awesome technical achievement, like watching a trained stuntman drive a car on two wheels through a slalom course, like a form of highly skilled, kinetic, robot based gymnastics.

I think what they are referring to is "angular momentum dumps". The reaction wheels have a fixed maximum amount of angular momentum. This is a function of the speed of the wheels themselves since the mass/moment of inertia is fixed.

Over time external forces continue to add momentum to the system (from drag/solar radiation pressure). In order to maintain a desired pointing direction this means the wheels must increase in velocity to "absorb" this momentum.

Eventually you need to "dump" the momentum using another external force, such as thrusters, or in this case the same solar radiation pressure.

Goal is to orient spacecraft such that external forces can be used to spin down the various reaction wheels, thereby "dumping" momentum out of the wheels.

This is pretty common on all spacecraft, but gets complicated/innovative when you have to do it in an emergency or if you don't have a full set of reaction wheels.

Just adding it because I think it's a cool use of the word. When reaction wheels are operating at maximum speed and can no longer exchange momentum with the spacecraft in both directions, they are considered 'saturated'.

'Saturated' is used in control systems engineering for any value (sensor input, drive output etc.) that has reached maximum or minimum value and can't be further increased/decreased. It's the same sense of the word as used in the HSV colour space in computer graphics.

https://en.wikipedia.org/wiki/Tennis_racket_theorem

Just turn it 180 every few days this way, so the torque from external influences is cancelled out.

The NASA video on that page is mesmerizing.

https://en.m.wikipedia.org/wiki/Control_moment_gyroscope

Perhaps? The ISS uses these.

"using its highly sensitive reaction wheels"

Yes, the article uses those words. But an understanding of elementary physics tells us that momentum is conserved in an isolated system. Spending the reaction wheels can cause the outer frame of the satellite to rotate in a certain fashion but it cannot possibly alter the overall momentum or angular momentum of the entire satellite unless it somehow interacts with an external force.

I guess the motors are lossy and some of it gets converted to heat if they speed the wheels up and down

That won't solve the conservation of momentum problems though, it'll just use more energy to get to the same state. If you could demonstrate controlling the rotation of a spacecraft just by using more inefficient motors you would upend a large portion of modern physics and could probably go and collect your Nobel prize within the week.

Ok, read TFA now. It sounds like they need to dump energy from the reaction wheels. Maybe they can do this by inducing a spin, rotating 180 on a different axis then cancelling the spin?

You can't cheat conservation of momentum, no matter how cleverly you move your spacecraft. That's why all the solutions involve expelling reaction mass. (Or, in the case of solar wind, interacting with an external force.)

Right, the reaction wheels help if you want to maintain orientation of the satellite under external forces that want to (gently!) rotate it. And eventually your reaction wheel saturates and you can't get more useful rotation out of it.

This is where thrusters are useful, because you can use the thrusters to give you a "source of force" while you spin the reaction wheels back down. I believe this is where the original parent's question is coming from - if you have no thrusters, how do you avoid saturation?

Presumably, they've worked out a way to accurately predict the external forces and gently operate against them while spinning the wheels down in a way that doesn't make the satellite spin faster.

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