SAS Extended - a KSP2 plugin

A precision flight-control mod for Kerbal Space Program 2 that turns the game's stock Stability Assist System into a full six-axis attitude autopilot — with fine-grained orientation offsets, an inertial attitude lock, and a self-tuning powered-hover controller.

Built on the KSP2 Redux / SpaceWarp2 modding stack (Unity 6, C# 9). Every mode below is implemented, in-game tested, and shipping. Showcase: https://www.youtube.com/watch?v=6EF_POq78kM

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Fine attitude control Stock SAS can only point one axis at a handful of preset directions and gives you no roll control at all. SAS Extended lets you pick a base direction — from Orbital, Surface, Target, or Special categories — and then dial in exact Heading / Pitch / Roll offsets relative to it. Select Surface Retrograde, pitch up 10°, and the vessel holds that attitude for you while the autopilot handles the torque. Each axis has a toggle: turn one off and that axis is left genuinely free to drift, exactly like stock SAS leaves roll uncontrolled — no silent snapping to zero.

Powered hover Hover mode does what no stock control can: it points the thrust axis up, leans into horizontal surface velocity to null out drift, and modulates the throttle to hold a commanded vertical speed. The throttle law is a gravity-compensated PID controller whose integral term self-tunes to each vessel's own hover-equilibrium throttle, so it needs no thrust/mass/TWR model. The same integral drives the tilt authority and the tilt-angle safety ceiling. It has been validated across extremes — from featherweight low-gravity landers to high-TWR craft under heavy gravity.

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Every mode

Group	Modes
Global	OFF · KILL ROT (capture & hold current attitude) · NODE (point at the maneuver burn vector)
Orbital	Prograde · Retrograde · Normal ± · Radial ±
Surface	Surface Velocity ± · Horizontal Velocity ± · Surface (horizon) · Up
Target	Target ± · Relative Velocity ± · Parallel ± (e.g. align to a docking port)
Special	Star ± (point at/away from the parent star) · HOLD (lock attitude in inertial space) · HOVER

Every pointing mode accepts the full Heading/Pitch/Roll offset trim, remembers its offsets as you switch between modes, and shows a live status readout (angle-to-target, angular velocity, or vertical/horizontal speed depending on mode). The window is draggable and its position, open state, and settings persist across sessions.

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Engineering highlights

This is a real-time control mod that has to cooperate with a physics engine it doesn't own. Some of the more interesting problems it solves:

• Quaternion attitude pipeline. All orientation is computed as full 3-axis rotations (LookRotation + Euler offset composition) and handed to the game's SAS via LockRotation, so heading, pitch, and roll are commanded together. Telemetry direction vectors are re-framed into a single coordinate system before use.

• Slew-rate limiting for ill-conditioned setpoints. A near-antipodal mode switch (Prograde → Retrograde) would otherwise hand the autopilot a 180°-flipped target in a single tick — the classic degenerate case for quaternion control, and a source of hard oscillation. The commanded setpoint is anchored to the vessel's actual current attitude each tick and slewed toward the target at a bounded rate.

• Throttle ownership via Harmony. The stock input handler re-pushes its own throttle every FixedUpdate, overwriting anything set from the update loop. A Harmony postfix injects the hover controller's commanded throttle after player input but before the push, so it sticks.

• A testable pure-math layer. The quaternion and hover-control math is extracted out of the MonoBehaviour into KSP-free pure functions.

• Custom UI controls. The UXML/USS interface is styled to match KSP2's own flight panels and registers custom UI Toolkit controls (LED toggle buttons, tab buttons) through the Redux control-factory registry.

Tech stack

Unity 6000.4.1f1 · C# 9 · ReduxLib → SpaceWarp2 modding stack · Harmony patching · UI Toolkit (UXML/USS) · ThunderKit build pipeline · Unity Test Framework

Building

SAS Extended builds through ThunderKit pipelines in the Unity Editor (Assets/SASExtended/Pipelines/ → Build for Editor / Build for Player / Deploy to Zip File), not a plain dotnet build. The compiled mod stages to Assets/Mods/__Testing/SASExtended/.

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_{Design north star: feature parity with KSP1 MechJeb2's "Smart A.S.S." module, rebuilt from the ground up for KSP2's Redux stack. Author: Falki.}