Tutorial: Tidally Locked Bodies using Planety

Intro

Tidal locking is the process of one or two bodies in an orbital system gaining a set orientation relative to the other over time. The best example of this is our own moon; it's always facing us in the same direction along its orbit (hence the 'dark side of the moon'). In real life this phenomenon is caused by the rotation of the bodies. Let's take the moon as an example. The gravity of earth pulls harder on the side of the moon thats closest to us, causing it to 'bulge' outward towards the earth. Once upon a time, when the moon and the earth were not yet tidally locked, this bulge would rotate away from the earth as the moon rotated around its own axis. This would cause the bulge to be pulled back towards the earth-facing direction of the moon by earths gravity (tidal forces), against the direction of the moons rotation, thus slowing it down ever so much (this also causes a lot of internal friction. you may have heard of 'tidal heating'). Over millions of years, this eventually lead to the moon's rotation slowing down until it rotated exactly one time around its own axis for every rotation it made around earth; in other words, until the moon was always facing the earth in the same direction. This is what we call 'tidally locked'. This process happens in both directions; the earth is also being slowed down by the moon. A bulge forms on the earth where the moon is overhead, which is then pulled by the moons gravity as the earth rotates around its own axis (this is what causes the tides in our oceans). Because the gravitational pull of the moon on the earth is a lot smaller than that of the earth on the moon, earths rotation is slowing down at a much slower rate (just 2.3 milliseconds per century!)

Tidal locking doesn't just happen to planets or moons that are spinning faster than their companion. If a moon is spinning slower than the time it takes to complete its orbit, tidal forces will pull the bulge forward, in the direction of that moons rotation around its axis, speeding it up. Similarly, a planet that is spinning slower than the time it takes its moon to orbit it, will eventually speed up until it rotates exactly once around its own axis for every orbit its moon makes around it (but again, it will take much longer for it to become tidally locked than its moon).

The tidal locking effect occurs for every celestial body that orbits around a parent or is a parent itself: Moons, Planets, Binary systems and even Stars! Between a parent and a child, either the child is tidally locked or both child and parent are tidally locked. But the parent can never be tidally locked if the child isn't! For this reason, if you want to tidally lock a system in KSP2, always lock the least massive object first. You can then optionally lock the other object too should you so choose. Binary Systems consisting of objects with similar mass are quite prone to tidal locking in real life, so adding this to your Binary Systems in KSP2 can provide a nice touch of realism.

So, now that you know what tidal locking is, let's discuss how we can add it to a custom planet for KSP2.

Tidally Locking a Body in KSP2 with Planety

There is only one parameter that matters when it comes to tidal locking: rotation. This is how long a celestial body takes to spin once around its own axis, in seconds. To tidally lock an object orbiting a parent, all you have to do is set it to be equal to the period of its orbit in seconds. To tidally lock a parent being orbited by a child, you have to set its rotation to the orbital period of its child, again in seconds (remember that is is extremely rare in nature for a parent to be tidally locked while the child isnt).

To give an example, lets say we want to tidally lock a planet to Kerbol. First, you place the planet in an orbit of your choosing around Kerbol. Leave rotation at some arbitrary value for now (dont set it to 0; that will cause it to spin infinitely fast). Next, boot up a KSP2 campaign that has your planet pack active and open the tracking station. In the Information Panel of your new planet, find the value of its Sidereal Rotation under the Orbital Characteristics tab (ignore that it says m/s, this is a UI bug). Plug this value in your_planet.rotation in the .planety file of your planet pack, and if everything went right you should be left with a tidally locked planet in the game! If you're not happy with the side facing kerbol, you can manually rotate the planet by changing the third parameter (rotation around the Z-axis in degrees) in your_planet.axial_tilt = [0,0,0];

The process for tidally locking a planet relative to its moon is similar, except you have to use the Sidereal Rotation of the moon instead of that of the planet.