KSP-ReRoot — Set as root, in flight

A Kerbal Space Program mod that adds a "Set as root" action to the part context menu in flight. Its main purpose is EVA Construction: a Kerbal can never grab or detach the root part of a vessel, so this mod lets you move the root onto another part — freeing the old one for editing.

Unable to grab that damn docking port ?

Set the tank as a new root

This will save and reload the game

And now you can grab this nice little docking port...

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Why this mod exists

In the editor, re-rooting is trivial: the vessel tree is just data. In flight, KSP gives you no way to do it. That's a real problem, especially for bases built with EVA Construction:

• A Kerbal cannot detach the root part of a vessel (just like in the editor, the root can't be "grabbed").
• After a creative landing or an in-place build, the part you actually want to remove — an ugly/heavy docking port, a cupola on top, etc. — often is the root, so it's stuck.

Interestingly, Squad clearly intended to support this: the EVA-construction code even defines an unused ConstructionMode.Root mode — but it was never implemented. This mod fills that gap.

Re-rooting is not a physical operation. Physics relies on the PartJoints between adjacent parts, not on "who is the root". The root is only a reference point (transform, control, persistence). Re-rooting = rewriting pointers — so it is perfectly safe to do, KSP just never exposed it in flight.

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What it does / How to use

1. Right-click a compatible part → "Set as root". The action works on the vessel you're controlling and on any nearby loaded vessel (within ~200 m) — including debris you can't even switch to (handy in EVA Construction, where the thing you want to dismantle often isn't the vessel you control).
2. Confirm in the dialog. The game is saved and reloaded.
3. You're returned to the vessel you were controlling (not the one you re-rooted, unless they're the same) — and the target vessel is now rooted on the part you chose, correctly placed and physically intact.

The clickable button only appears on valid root candidates (the same parts the editor would accept — e.g. structural/stack parts, not radial batteries or antennas). On parts that can't be the root — the current root, or a physics-less part — you instead get a small read-only status line.

Tip: to free a stuck root part for EVA Construction, right-click an adjacent part of that vessel (it works even while you're on EVA next to it, since the vessel doesn't have to be the one you control) and "Set as root" — the old root is now a normal child you can detach.

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Requirements

• Kerbal Space Program 1.12.5, French or English
• ModuleManager (used to inject the action onto parts)

Installation

1. Build (see below) or download a release.
2. Copy the ReRootMod folder into your KSP GameData/ folder.
3. Make sure ModuleManager is also in GameData/.

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How it works (the interesting part)

Re-rooting a flying vessel turned out to be a minefield of KSP physics invariants. The approach that finally proved robust on everything from a tiny probe to a huge Eve base is a hybrid: do the tree/geometry work in memory with native KSP calls, then let KSP's own save/load rebuild the physics.

The pipeline (per "Set as root")

1. In-memory re-root of the live vessel:
   • Part.SetHierarchyRoot(newRoot) — inverts the parent chain (native KSP).
   • EditorReRootUtil.StripInvalidSymmetrySets(newRoot) — drops symmetry groups that are no longer valid after the root change.
   • vessel.rootPart = newRoot.
   • Reorder vessel.parts topologically (root first, every parent before its children) via a pre-order DFS.
   • part.UpdateOrgPosAndRot(newRoot) on every part — re-express each part's orgPos/orgRot relative to the new root.
2. Compute the new root's true surface placement from its live transform (lat/lon/alt, and srfRelRotation = Inverse(body.bodyTransform.rotation) * newRoot.rotation).
3. SaveGame to a dedicated file (ReRootReload, not persistent).
4. Patch that save's target VESSEL node: overwrite lat/lon/alt/hgt/rot + set skipGroundPositioning = True. The vessel is located by the new root's part uid (= flightID).
5. FlightDriver.StartAndFocusVessel("ReRootReload", idx) — reload, refocusing the same vessel.

The reload is what makes it robust: KSP rebuilds all joints, rigidbodies and physical-significance from scratch off the (now correct) tree, so we never have to get the live flight physics right ourselves.

Hard-won findings (why each step is there)

These are the non-obvious things that each cost a debugging round:

• Topological order is mandatory. KSP always writes root = 0 and resolves each part's parent by sequential index on load. If a child appears before its parent in the PART list, the load throws and drops the part (and the vessel deforms). Hence the in-memory reorder so the new root is parts[0] and parents precede children.
• Placement must be re-expressed for the new root. A landed vessel's lat/lon/alt/srfRelRotation describe the root. After re-rooting they still describe the old root, so on reload the (new) root is dropped at the old root's pose and the whole vessel ends up rotated/offset. We recompute and patch them.
• skipGroundPositioning = True on reload. KSP re-snaps a landed vessel to the terrain by raycasting from the root; with a new root that isn't the lowest point, that snap shoves parts underground. Skipping it makes KSP trust our exact coordinates (the vessel never actually moved). It's a one-load flag that flight logic resets after the vessel settles.
• Use a dedicated save file, not persistent. A scene-exit autosave to persistent clobbers the placement patch before the reload reads it. Saving and reloading our own ReRootReload file sidesteps that entirely.
• Only valid root candidates. A physics-less part (radial battery, antenna — physicalSignificance = NONE, no rigidbody) cannot be a flight root: children would joint to a mass-less anchor and the vessel drifts away. We reproduce the editor's isRootCandidate rules (flight-safe) and simply refuse the rest — which is exactly what the editor does.
• Don't fight the live joints. Doing the re-root purely in memory (no reload) scrambles the joint topology in flight — the same fragility that makes the stock Klaw buggy. Since the reload rebuilds joints from the tree, we just don't care about the transient live joint state.

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Limitations / notes

• Works on the active vessel and any nearby loaded vessel (right-click a part on it — within ~200 m of your active vessel). After the reload you're refocused on the vessel you were controlling, so the operation never steals control away from you.
• For the action to appear on a non-active vessel, your active vessel must be controllable (KSP's rule for unfocused part actions) — any controllable vessel works (a Kerbal on EVA, a rover, a probe…).
• The operation saves and reloads the game (brief loading screen). That's the price of letting KSP rebuild a clean physics state.
• Landed/splashed vessels are the primary target and are fully validated. Orbital vessels appear to work too.
• The action is independent of EVA Construction Mode — it's a standalone, technical vessel operation.

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Building

Set the KSPDIR environment variable to your KSP install, then:

• Windows: build.bat then install.bat
• Linux: ./build.sh then ./install.sh

build compiles ReRootMod.dll, bundles the ModuleManager patch and the localization files, and produces Release/ReRootMod.zip. install unpacks it into $KSPDIR/GameData/ReRootMod.

Localization

English (en-us) and French (fr-fr) are included, under GameData/ReRootMod/Localization/.

License

MIT (see LICENSE).