Tether System

The space tether is modelled as a chain of rigid bodies connected by joints, simulated using Avian3D physics.

Configuration

Two constants in src/constants.rs control the tether geometry:

Constant	Value	Description
NUM_TETHER_JOINTS	15	Number of rigid body segments in the tether chain
DIST_BETWEEN_JOINTS	1.1	Distance between adjacent joints (metres, in simulation scale)

Entity structure

The tether is spawned by setup_tether (called in the Startup system chain in lib.rs). Each joint in the chain is an entity with:

• An Avian3D RigidBody, Position, and LinearVelocity component for physics simulation.
• A TetherNode { root: Entity } component that holds a reference back to the root joint entity. This allows any joint to quickly identify the tether it belongs to.

The root joint also carries an Orbital component so that the entire tether chain is propagated as a single orbital body and then the physics simulation handles relative motion within the chain.

Physics enable/disable radius

To manage performance, Avian3D physics is only active for entities within a certain distance of the camera target. This is controlled by PHYSICS_ENABLE_RADIUS and PHYSICS_DISABLE_RADIUS (defined in constants.rs):

• Entities within PHYSICS_ENABLE_RADIUS have their RigidBodyDisabled marker removed — physics is active.
• Entities outside PHYSICS_DISABLE_RADIUS have RigidBodyDisabled added — physics is suspended.

This is handled by the ssg_propagate_keplerian system, which checks distances each propagation step.

Floating origin interaction

Because orbital distances are enormous (hundreds to thousands of kilometres), the simulation uses a floating origin — the Bevy world origin shifts to follow the camera target. See Floating Origin for how this interacts with the tether physics.