Docking Adapter

Description

The Docking Adapter is a Physical Object that enables docking between two spacecraft during Rendezvous, Proximity Operations, and Docking (RPOD) activities. When attached to a spacecraft, the adapter monitors alignment with a target adapter and automatically initiates capture when the distance, axis angle, and roll angle are within configured tolerances. The adapter also models impact forces during capture and separation forces during undocking.

Example Use Cases

Automated Docking: Automatically capture a target spacecraft when the adapters are aligned within the specified tolerances.
Docking Telemetry: Monitor approach geometry including lateral offset, axial range, and alignment status for guidance algorithms.
Thermal Integration: Transfer heat between docked spacecraft through the docking interface.

Module Implementation

Capture Geometry

The docking adapter evaluates three alignment conditions to determine if capture can occur:

Condition	Parameter	Description
Distance	`CaptureDistance`	Separation between adapter origins must be less than $r_{c}$
Axis Angle	`CaptureAngle`	Misalignment between mating axes must be less than $θ_{c}$
Roll Angle	`CaptureRollAngle`	Clocking error about the mating axis must be less than $ϕ_{c}$

The axis angle $θ$ is computed as the angle between the adapter up vectors, which must be anti-aligned for proper mating:

θ = cos^{- 1} (- \hat{u}_{A} \cdot \hat{u}_{B})

where $\hat{u}_{A}$ and $\hat{u}_{B}$ are the up-axis unit vectors of each adapter in the inertial frame.

The roll angle $ϕ$ is the clocking error computed by projecting the right-axis vectors of each adapter onto the plane normal to the mating axis and measuring the angle between them. Setting CaptureRollAngle to 180 degrees disables the roll constraint.

Capture occurs when all three conditions are satisfied and CaptureEnabled is true (or the connected DockCaptureCommandMessage enables capture).

Port Metrics

The adapter computes and publishes alignment telemetry each update cycle:

Metric	Description
`TargetDistance`	Euclidean distance between adapter origins
`AxisAngle`	Misalignment angle between mating axes
`RollAngle`	Clocking error about the mating axis
`LateralOffset`	Perpendicular offset from the target adapter’s approach axis
`AxialRange`	Distance along the target adapter’s approach axis
`IsAligned`	True when axis and roll angles are within capture tolerances
`CanCapture`	True when aligned, within distance, and capture is enabled

Hub Selection

When docking completes, the heavier spacecraft becomes the hub and the lighter spacecraft becomes the child. The child’s dynamics are integrated through the hub while docked. Linear momentum is conserved by adjusting the hub velocity:

v_{h}^{'} = v_{h} + \frac{m _{c}}{m _{h} + m _{c}} (v_{c} - v_{h})

where $m_{c}$ and $m_{h}$ are the child and hub masses, and $v_{c}$ and $v_{h}$ are their pre-dock velocities.

Impact Force

When capture occurs with a closing velocity, the adapter applies an impact force computed from the impulse-momentum theorem:

F_{impact} = \frac{m _{r} \cdot v _{a}}{Δ t _{i}}

where $m_{r}$ is the reduced mass of the system, $v_{a}$ is the approach velocity component along the docking axis, and $Δ t_{i}$ is the ImpactAttenuationDuration. The reduced mass is:

m_{r} = \frac{m _{1} \cdot m _{2}}{m _{1} + m _{2}}

The force is applied equally and oppositely to both spacecraft for the attenuation duration. The MaxImpactForce parameter can limit the peak force magnitude. Setting ImpactAttenuationDuration to zero disables impact forces.

Separation Force

The UndockApplySeparation method releases the docked connection and applies a configurable separation force along the docking axis. The separation impulse is:

J_{sep} = F_{sep} \cdot Δ t_{sep}

where $F_{sep}$ is the SeparationForce and $Δ t_{sep}$ is the SeparationDuration. Both adapters apply their configured separation force simultaneously. The Undock method releases the connection without applying any force.

Thermal Connection

When both adapters have an attached Thermal Model, a conductive thermal connection is created upon docking. The effective contact area for heat transfer is:

A_{eff} = min (A_{1}, A_{2})

The connection is automatically removed when undocking occurs.

Assumptions/Limitations

Each adapter can have only one docking target at a time.
Capture requires a valid target adapter on another spacecraft; debris docking is not supported through this component.
Impact forces are only applied when the closing velocity along the docking axis is positive.
Thermal connections require both adapters to have an attached thermal model.
After undocking, the target reference on both adapters is cleared to prevent immediate re-capture.