KSP Tethers

Dynamic, flowing EVA tethers and ship-to-ship cables for Kerbal Space Program.

Kerbals get a reel-mounted umbilical on their backpack that clips onto any part or another kerbal, and can carry the free end over to another ship to rig a cable between vessels. The rope is fully simulated: it floats in lazy loops in zero-g, hangs and piles up in the dust on the Mun, streams behind you in an atmosphere, drapes over hulls instead of passing through them, and pulls once it runs out of slack. Through the lifeline it can keep a kerbal's suit topped up with power and life support, and take the waste back to the ship.

_{Nine cable styles, each generated from the way it is actually made.}

_{Rope renders from the offline solver tests, not in-game screenshots.}

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Install

CKAN

KSP Tethers is on CKAN. Search for KSP Tethers and install; CKAN pulls in Module Manager for you and picks up each new release within a few hours.

If you have been running a manually installed copy, delete GameData/KSPTethers first - CKAN will not overwrite files it did not install. Your toolbar-app settings live in GameData/KSPTethers/PluginData and are worth keeping if you want them.

Manual

1. Install Module Manager if you don't have it.
2. Download the latest KSPTethers-<version>.zip from Releases.
3. Drag the KSPTethers folder into your KSP GameData/ folder, so you end up with Kerbal Space Program/GameData/KSPTethers/.

Requirements

KSP	1.12.x
Required	Module Manager
Recommended	ToolbarControl (lets you choose the stock launcher and/or Blizzy's toolbar) and ClickThroughBlocker
Works with	TAC Life Support, Kerbalism and other mods that keep resources in EVA suits; Principia (tethers switch to pulling with forces); KAS (winches, ports and pylons become tether points)

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Features

• Simulated rope: position-based solver (inextensible, bending stiffness with a minimum bend radius, static and sliding friction, air drag, self-collision), drawn as a smooth tube with metal fittings at both ends.
• Correct in every environment: the rope floats freely in orbit, sags under gravity when landed, is pushed back during burns and streams downwind in atmosphere. It is simulated in the anchor's frame, so KSP's floating origin and Krakensbane never disturb it.
• Collisions: the rope rests on terrain and drapes over parts and kerbals instead of clipping through, and once it wraps around a ship the tether pulls from where it touches the hull, so it can't be dragged through.
• Cables stay put on the ground: a cable that has settled on the surface beds into it, so a rover driving over it or a kerbal walking along it passes by without dragging it out of shape. Reeling it in or picking an end up still lifts it.
• Physical tether: once taut, a soft spring-damper tuned to the masses on each end restrains the kerbal (or tows the other ship). Slack costs nothing.
• Reel: pays out automatically as the kerbal moves away, and reels in to haul a kerbal back to the hatch. Reeling in first takes up slack, then pulls.
• Rescue from the ship: right-click a tethered kerbal while controlling the vessel and choose Reel In to bring back a kerbal who ran out of EVA propellant.
• Tethers between anything: kerbal to part, kerbal to kerbal, and part to part (ship to ship, station to rover). Any number of cables can link any number of vessels.
• Lifeline: carries ElectricCharge, Oxygen, Food, Water and other supplies into the suit, and CO2, Waste and WasteWater back, for mods that keep resources in EVA suits. Kerbal-to-kerbal tethers share supplies; ship cables can even out resources between vessels. Optionally refuels jetpacks from the ship's MonoPropellant.
• Cable styles: nine of them, each generated from the way that cable is really made - a corrugated hose with a helical rib, tubular webbing in a 2/2 twill, sixteen-carrier diamond braid, six-strand lang-lay wire rope with grease in the valleys - with cavity shading and a specular map baked in. Two of them are flat 25 mm straps rather than round cables, including a blue herringbone one in the Russian style. One look for EVA tethers, another for ship cables, plus thickness.
• Tether points: docking ports, claws, ladders, crewed parts and - with KAS - winches, ports and pylons can be clipped to with one right-click, and two of them can be rigged together in the editor so the craft launches with a cable already strung between them at whatever slack you set.
• Keys for ship cables: pick a cable in the app, step through them with a key, or give a cable a key of its own; the reel keys then drive it from inside the ship.
• Toolbar app: manage every tether and cable, pick styles, switch the lifeline and individual resources, rebind keys, and choose how tethers hold on.
• Auto-tether: kerbals leaving a hatch in space are clipped to the hull beside the hatch.
• Persistence: tethers and cables survive saving, loading, scene changes, vessel switching and time warp.

Using tethers

Action	Keyboard (active EVA kerbal)	Right-click menu
Clip onto a part or kerbal	Y, then click it (within reach)	Clip Tether (point & click)
Clip onto the nearest part	Y twice	Clip Tether to Nearest Part
Clip your end elsewhere (rig a cable / hand over)	tap Y, then click a part or kerbal	Clip Free End Elsewhere
Pick up the end of a cable	Y, then click the cable's end
Release	hold Y	Release Tether
Reel in / out	hold - / =	Reel In / Reel Out (toggles)
Set the tether length		Tether length slider

While aiming, the part under the mouse is highlighted green (in reach) or red, and a label next to the cursor says what a click will do. A quick right-click cancels. Release and Reel In/Out also appear when you right-click a tethered kerbal from another vessel.

Rigging a cable between two ships: clip onto ship A, fly to ship B, tap Y and click a part on ship B. Your end is clipped there and you are free; the cable stays between the two ships. Reel and release cables from the toolbar app. An untethered kerbal can click near a cable's end to unclip it and carry it somewhere else.

Rigging a cable before launch: in the VAB or SPH, right-click a tether point and choose Rig Cable From Here, then right-click another one and choose Rig Cable To Here. The Cable length slider sets how much slack it launches with. Docking ports, claws, ladders, crewed parts and KAS winches, ports and pylons all have a tether point; add one to any other part with a one-line ModuleManager patch (see Patches/KSPTethers_TetherPoints.cfg).

Cables between ships, from the cockpit

One cable at a time is selected, and the reel keys drive it whenever you are not flying a tethered kerbal. Pick it in the app's Tethers tab, step through them with the select keys, or give a cable a number on the Tethers tab and bind a key to that number on the Keys tab - then one press puts that cable under the reel keys. There is a key to release the selected cable too. Everything is unbound except "select the next cable", which starts on ;.

Toolbar app

• Tethers: every tether and cable in the scene, with distance, length and the pull it is under, reel buttons, release, the lifeline's status and, for cables, which one the reel keys drive, its key number and "Share resources".
• Cables: the style for EVA tethers, the style for cables between ships, and thickness. Changes apply live.
• Resources: the lifeline master switch, transfer speed, buddy sharing, the default for new cables, and a switch per resource. It also reports which life support mods are installed and working.
• Keys: click a key and press a new one. Clashes with KSP's own bindings are flagged.
• Setup: whether tethers hold on with a joint or with forces, which of Principia, KAS, ToolbarControl and ClickThroughBlocker were found, and what any cheats are doing.

Settings

Difficulty Settings > KSP Tethers (per save): auto-tether (and only in space), physical or cosmetic tethers, kerbal-to-kerbal tethers, self-retracting reel, default and maximum length, clip reach and reel speed.

Difficulty Settings > KSP Tethers > Cheats (per save): infinite length, a reel-speed multiplier, longer reach, a tether-strength multiplier, unbreakable tethers, and crazy physics that turns tethers into bungee cords. Everything there stays off until the master switch is ticked.

KSPTethers/Settings.cfg (global): simulation tuning, spring frequencies, break forces, collisions, wrapping, the CABLE_STYLE list and the TETHER_RESOURCE list. Everything is documented in the file and can be changed with ModuleManager against the KSP_TETHERS node. The app's choices are saved separately in KSPTethers/PluginData/UserSettings.cfg.

Life support notes

• Only resources that are actually stored on the EVA kerbal are moved; nothing is created or destroyed.
• TAC Life Support and Kerbalism keep supplies in EVA suits, so tethered kerbals stay supplied.
• USI Life Support doesn't store supplies in EVA suits, so tethers can't feed it.
• Jetpack refuelling (EVA Propellant from the ship's MonoPropellant) is off by default; switch it on in the app.

Compatibility notes

• Principia integrates every vessel itself and writes the result over whatever PhysX worked out, so a joint between two vessels is discarded and a tether would hang slack and stretch without limit. With Principia installed, tethers instead pull by adding a force to the part at each end, which Principia reads and keeps. The app's Setup tab can force either method.
• KAS/KIS work alongside this mod. KAS winches, ports and pylons gain a tether point, and a tether clipped to one leaves from the same socket KAS runs its own cable from. The two mods' links are otherwise separate: a KAS cable and a tether on the same part do not interact.
• The backpack attach point uses the bn_jetpack01 bone. If a suit mod moves it, adjust kerbalBone / kerbalOffset in Settings.cfg.
• Cables pull on the parts they are clipped to; clip ship cables to sturdy parts, as a flimsy part can be torn off when towing heavy vessels.
• Seating a kerbal in a command seat releases its tether.

How it works

• TetherCore is one tether: two TetherEnds (a clip on a part, or a kerbal's backpack), the physical link, the reel, and the rope. Kerbal tethers are owned by ModuleKerbalTether (added to EVA kerbals by ModuleManager); cables between parts by TetherScenario, which saves them in the save file.
• The link is either a ConfigurableJoint with a soft spherical limit, spring set from the reduced mass and a natural frequency, or - for mods that integrate vessels themselves - TetherForceLink, which adds the same pull with Part.AddForceAtPosition. TetherTension solves that as a soft constraint: it reduces to exactly the spring-damper that was asked for when the ends are heavy, and falls back to the impulse that just removes the separation when the spring is too stiff for the time step, so it cannot overshoot.
• RopeSimulation is a Verlet/PBD solver with long-range attachments, bending constraints with a minimum bend radius, a spool at the anchor end so length changes never make the rope jump, and Coulomb friction. Contact planes for nodes and segment midpoints are found once per substep and enforced inside the solver iterations; self-collision uses segment-segment tests with sweep-and-prune.
• When the straight line between a tether's ends is blocked by the vessel at the far end and the rope rests against it, the joint moves its pivot to the rope's last contact with the hull (and shortens its reach by the rope already wrapped), so the physics follows the rope's real path.
• TubeMeshBuilder sweeps a parallel-transported tube along a Catmull-Rom spline and adds the fittings; a flat strap uses the same sweep with an elliptical cross-section. CablePatterns generates each style's surface as a seamless tile two circumferences long - a helical rib, a 2/2 twill, a diamond braid, a lang-lay wire rope - blurs the height field for the normals and again, wider, for cavity shading, and packs the result into the albedo-plus-specular map KSP's Bumped Specular shader wants. A flat strap's tile is folded round the perimeter so its selvedge edges land on the strap's real edges.
• A node that has lain still on the ground for settleTime is marked as bedded in: it is pinned exactly where it lies and is not offered to the collider at all, which is what lets things pass over a cable without moving it. settleGrip is how hard the rope has to pull before that grip breaks and the node rejoins the simulation.
• ModuleTetherPort is a tether point: it finds where a cable should leave a part (a KAS socket, a named transform, an attach node) and holds the editor rigging that launches a craft with a cable already strung.
• ResourceExchange holds the lifeline rules; TetherResources applies them to suits and vessels.

Building from source

Requires the .NET SDK (6 or newer), the .NET Framework 4.7.2 targeting pack and a KSP 1.12 install for the reference assemblies.

./Tools/Build.ps1                                   # build into KSPTethers/Plugins, run the offline tests, make dist/KSPTethers-<version>.zip
./Tools/Build.ps1 -KSPRoot "D:\Games\Kerbal Space Program"
./Tools/Install.ps1                                 # copy KSPTethers into a KSP install (keeps your saved app settings)

The plugin can't be compiled in CI because it needs KSP's own assemblies, so KSPTethers/Plugins/KSPTethers.dll is committed. Rebuild it with Tools/Build.ps1 before tagging a release; the release workflow refuses a DLL that doesn't carry the tagged version.

Tests/RopeSimTests runs the rope solver, mesh builder, cable patterns and lifeline rules outside the game. To eyeball the rope and the cable styles without launching KSP:

Tests/RopeSimTests/bin/Release/net472/RopeSimTests.exe render snapshots

Toolbar icons are PNGs with transparency: KSPTethers/Icons/tether_38.png (38x38, stock launcher) and tether_24.png (24x24, Blizzy's toolbar). Replace those two files to change the artwork.

Releasing

Bump the version in Source/KSPTethers.csproj, Source/Properties/AssemblyInfo.cs and KSPTethers/KSPTethers.version, run Tools/Build.ps1, commit, then push a v<version> tag. The release workflow checks that all three agree with the tag, builds KSPTethers-<version>.zip and attaches it to a GitHub release, which CKAN picks up. See CKAN/README.md.

Licence

MIT.