Satellite Tidal Lock/Spin?

[Caelib]

One thing that has always puzzled me is how LEO or even GSO satellites maintain a tidal lock with the Earth? For example, suppose the satellite has an antennae that needs to be constantly pointed at the earth ... obviously this doesn't not happen without a great deal of effort, but does anyone know HOW they actually do this?

Is it just a really, really slow "spin" placed on the craft? If anyone can elaborate on this, please do or provide links to some articles that explain it because I cannot find much on the topic.

This is something that I would like to try to do in KSP, but I need a better understanding of how the problem is managed over Earth first

Thanks!

[peadar1987]

One thing that has always puzzled me is how LEO or even GSO satellites maintain a tidal lock with the Earth? For example, suppose the satellite has an antennae that needs to be constantly pointed at the earth ... obviously this doesn't not happen without a great deal of effort, but does anyone know HOW they actually do this?

Is it just a really, really slow "spin" placed on the craft? If anyone can elaborate on this, please do or provide links to some articles that explain it because I cannot find much on the topic.

This is something that I would like to try to do in KSP, but I need a better understanding of how the problem is managed over Earth first

Thanks!

I think (and I'm open to correction on this) that things in a gravity gradient will actually tend to orientate themselves naturally with their long axis pointing towards the earth, so no active control needed.

Unfortunately, KSP doesn't simulate this effect, so it's not much good to you.

Edit: After a quick trip to wikipedia, it doesn't look like this method is widely used. http://en.wikipedia.org/wiki/Gravity-gradient_stabilization

Edited July 10, 2014 by peadar1987

[Sky_walker]

Usually satellites rotate around one of its axes at a rate which matches the orbital period.

Yes, it requires high degree of precision and occasional corrections, however it's nothing like in Kerbal where you're forced to re-orient your satellites manually every few moments. Doing something like that would require huge amount of fuel (over a period of many orbits) and by this: significantly shorten a life time the satellite.

Gravity gradient is not feasible for vast majority of satellites and it can create additional dangers in a form of debris that are borderline impossible to detect.

Edited July 10, 2014 by Sky_walker

[K^2]

I think (and I'm open to correction on this) that things in a gravity gradient will actually tend to orientate themselves naturally with their long axis pointing towards the earth, so no active control needed.

It's a bit more complicated than that. For starters, it doesn't have to be the long axis. It depends on mass distribution and the field. But in sufficiently uniform gradient, it will be a line along the moment of distribution. But it's even worse. An object can't rotate around an arbitrary axis. In fact, there are only three mutually perpendicular axes that an object can rotate about without external torque being applied. These are principal axes. If you try set rotation around any other axis, the axis will tumble.

But these limitations are easy to design around if you specifically want a satellite that always points one end towards Earth. You need to design mass distribution so that tidal torque is minimized in that orientation and that one of principal axes, preferably the major one, is in the normal direction. You also want to balance light pressure and maybe add something to dampen oscillations. That way, if sat does end up getting knocked off the correct alignment, you can count on tidal torque to fix it, rather than just set the thing oscillating.

[peadar1987]

In case you're interested, the ISS uses reaction wheels, as per this video:

I assume they must desaturate them using some sort of reaction control system at some point.

@K^2, thanks for the info.

[Caelib]

Makes sense ... thanks for the info! I wonder how hard it would be for the RCS in KSP to be modified to use an "attitude lock" option on the orbital body.

[Camacha]

Unfortunately, KSP doesn't simulate this effect, so it's not much good to you.

Are you sure? Gravity is changing the higher you get and this is pretty much all you need. I meant to test this someday, but I never got around to it.

[peadar1987]

Are you sure? Gravity is changing the higher you get and this is pretty much all you need. I meant to test this someday, but I never got around to it.

Pretty sure. I think for orbital calculations, KSP treats each spacecraft as a lumped mass to speed things up.

[K^2]

Looking at the game's code, gravity and other inertial forces are applied at center of mass of the ship. In other words, no tidal effects are going to be present. On the other hand, drag and thrust are calculated per-part.

ISS uses reaction wheels

ISS does a lot more attitude adjustment, however.

[deadshot462]

Another issue is solar wind. That could affect your rotation overtime even if you "synced" the rate with your orbit.

[Nibb31]

Satellites don't use "attitude or tidal lock". They simply spin at the same rate as they travel around the Earth so that they perform one full rotation per orbit. Once it's set up, this only requires minor corrections from time to time which is part of the routine station-keeping.

They are designed to carry enough fuel to maintain station-keeping for their lifetime. When propellant is nearly depleted, they use what's left to boost themselves to a graveyard orbit before being decommissioned.

[Crashy_McSplodey]

Satellites have, and continue to use a wide variety of attitude control systems. Mir and the ISS use the gyrodyne reaction wheel system, with rotating masses distributed throughout the structure. While spacecraft like Hubble, Chandra, Spitzer etc use the 3-axis reaction wheel system. Other satellites use electromagnetic torquers for fine adjustment and precision aiming of antennae. Of course ships such as the Apollo CSM, Space Shuttle, Dragon, and Orion systems use a propellant based attitude control as their reaction speeds generally need to be an order of magnitude quicker than those used in satellites, and satellites (usually) have longer operational lifetimes, and as such these propellantless systems offer greater longetivity.

Note that attitude control and station keeping systems are usually completely separate and different systems. The (usually) monopropellant systems used on satellites, such as for GEO satellites for example, are used to restore the orbital characteristics after various factors have cause the satellites orbit to change. These changes are caused by insolation, variations in the earths gravitational field throughout the orbit, atmospheric drag, and even the re-radiation of heat can, over time, alter the characteristics of a vessles orbit, all of which need to be compensated for.

So, statoin keeping is not attitude control, and there are more ways to control a spacecrafts attitude than there are to station keep.

[SciMan]

I've also heard of satellites that use magnetorquers, basically the satellite can control the power to an electromagnet, which acts on the Earth's magnetic field to change the orientation of the satellite. It's a small force, and it takes electrical power, but satellites are often designed for 10+ year operational lifespans, almost always have power readily available thanks to solar panels, and the perturbing forces that the magnetorquer is intended to counteract are quite small, so it's more effective than the disadvantages I just stated make it sound. They're usually not used for primary attitude control, instead they are used for things like de-saturating reaction wheels/control moment gyros, mid-precision attitude control, and can usually be used as a backup system if the primary attitude control systems fail partially. I'm pretty sure they can't be used to fully control the satellite's attitude, because there's only one axis they can apply torque on (parallel to the axis of the earth's magnetic field at the satellite's position)

Either way, they're useful, and hadn't been mentioned in this thread yet.

[Iansoreta]

The thread is still open right for new ideas, suggestions, etc. so please don't close this thread until a quick thread conclusion is made.

Edit: Remember to not close this thread without warning, or without making a conclusion to any topic as it make other people puzzled why this thread is closed without sharing new ideas, extras, and showing enlightenment. Did you heard that they haven't mentioned something new in this thread?

Edited July 12, 2014 by Iansoreta
additional comment

[kujuman]

Pretty sure. I think for orbital calculations, KSP treats each spacecraft as a lumped mass to speed things up.

I recall that at some point each part had its own orbital...stuff...calculated but that was removed. An interesting experiment would be to try and use non-craft merging physical connectors between vessels (like a rigid KAS) to see if you could see the effects in game.

[magnemoe]

I recall that at some point each part had its own orbital...stuff...calculated but that was removed. An interesting experiment would be to try and use non-craft merging physical connectors between vessels (like a rigid KAS) to see if you could see the effects in game.

Make sense they don't do orbital calculations on parts, more so as rotation stops if you warp.

Some did some test to find a gravity gradient, the effect should be where as two parts move away from each other in different rates depending on orbits.

Take two decent sized modules like orange tanks, connect with decopler and strut to kill decopler force.

now have axis point to kerbin and decople, the outermost will be in a higher orbit and fall behind. If axis point in the direction they are moving they will follow each other closely far longer.

Downside is that this effect is weak and the orbits are fast, It will not be enough to keep them pointed inward, perhaps if you set up an rotation first, but equal chance the tides would just generate oscillation.

[AngelLestat]

I guess most satelittes would use reactions wheels. Because gravity gradient it will be only enoght to produce this effect (in my opinion) with very long satellites.

[Camacha]

But it's even worse. An object can't rotate around an arbitrary axis. In fact, there are only three mutually perpendicular axes that an object can rotate about without external torque being applied. These are principal axes. If you try set rotation around any other axis, the axis will tumble.

Can you elaborate on this? I think I know what you are saying, but I am not sure I understand how that works.

Pretty sure. I think for orbital calculations, KSP treats each spacecraft as a lumped mass to speed things up.

Ah, sure, you are right there. I remember I intended to see whether I could make it work with KAS, as two things connected by a tether seem to have their own centre of gravity. That is actually what they tried the the Gemini program. Without much success though, as the tether was thought to be too short.

I might have to boot up KSP this weekend

[DerekL1963]

Satellites don't use "attitude or tidal lock". They simply spin at the same rate as they travel around the Earth so that they perform one full rotation per orbit. Once it's set up, this only requires minor corrections from time to time which is part of the routine station-keeping.

*Some* satellites most certainly do use "attitude or tidal lock". *Other* satellites simply spin at the same rate as they travel around the Earth. (Mostly because using gravity gradient stabilization requires a specific configuration that may or may not be feasible and compatible with the satellites mission.) Yet other satellites do neither.

They are designed to carry enough fuel to maintain station-keeping for their lifetime. When propellant is nearly depleted, they use what's left to boost themselves to a graveyard orbit before being decommissioned.

*Some* satellites are designed to carry enough fuel to maintain station keeping for their their life time. Others have no need of station keeping and carry no fuel at all. *Some* use their RCS (if they have it) to boost into a graveyard orbit. Others use their RCS (if they have it) to simply de-orbit themselves. Yet others are simply abandoned in place and if they're low enough then atmospheric drag eventually brings them down (though some are high enough that this make take tens, hundreds, or thousands of years).

Seriously, trying to lump all birds into one basket is a fool's errand. There are a handful of large families, but that's about the best you can do.

[K^2]

Can you elaborate on this? I think I know what you are saying, but I am not sure I understand how that works.

The angular momentum, L, is conserved. And for arbitrary rotation axis, L = Ỉۡ. However, the moment of inertia tensor, I, rotates along with the body. If ̉ۡ happens to lie along one of the principal axes, then Ỉۡ product remains constant despite the object'r rotation. But if not, Ỉۡ would change with object's rotation. So in order to keep L constant, ̉ۡ must change as well. That's axis tumbling.

[ecat]

I've also heard of satellites that use magnetorquers, basically the satellite can control the power to an electromagnet, which acts on the Earth's magnetic field to change the orientation of the satellite. It's a small force, and it takes electrical power, but satellites are often designed for 10+ year operational lifespans, almost always have power readily available thanks to solar panels, and the perturbing forces that the magnetorquer is intended to counteract are quite small, so it's more effective than the disadvantages I just stated make it sound. They're usually not used for primary attitude control, instead they are used for things like de-saturating reaction wheels/control moment gyros, mid-precision attitude control, and can usually be used as a backup system if the primary attitude control systems fail partially. I'm pretty sure they can't be used to fully control the satellite's attitude, because there's only one axis they can apply torque on (parallel to the axis of the earth's magnetic field at the satellite's position)

Either way, they're useful, and hadn't been mentioned in this thread yet.

Magnetorquers, can't say I've ever heard of them but the effect used to work in game with the KAS electromagnet. I wrote a brief note about the effect last year, see second craft in the post:

http://forum.kerbalspaceprogram.com/threads/37707-0-23-5-Magic-Smoke-Industries-Infernal-Robotics-0-16-5b?p=601202&highlight=magnetic#post601202

I've just tried a quick test and it no longer appears to work, I think the KAS code has been changed