Initial orbiting craft

src/Craft/Krakensbane.cs

@@ -2,6 +2,7 @@ using System.Diagnostics;
 using Godot;
 using ImGuiNET;
 using Quadratic.Carto.MathExt;
+using Vim.Math3d;
 
 namespace Quadratic.Carto.Craft;
 
@@ -20,8 +21,24 @@ public sealed partial class Krakensbane : Node3D
     public Node3D? FocusedVessel { get; set; }
 
     const float maxDistance = 500.0f;
+    const float planetRadius = 6372.0f;
 
-    DVector3 originPosition = DVector3.Zero;
+    DVector3 originPosition = DVector3.UnitY * planetRadius;
+
+    /// <summary>
+    /// Something to manage the non-inertial frame of reference.
+    /// </summary>
+    /// <para>
+    /// It's good to have an inertial frame of reference when you're close to
+    /// a lot of static stuff, like on the ground. However, a non-inertial frame
+    /// of reference might be more appropriate when you're in deep space.
+    /// </para>
+    /// <para>
+    /// This provider field is null if we're using an inertial frame of reference.
+    /// When it's not null, it should be responsible for managing the acceleration,
+    /// angular acceleration, and even positions and/or rotations of children
+    /// objects.
+    INonInertialFrameProvider? nonInertialFrameProvider;
 
     public override void _PhysicsProcess(double delta)
     {
@@ -52,6 +69,27 @@ public sealed partial class Krakensbane : Node3D
 
             originPosition += vesselPosition.AsVim().AsDouble();
         }
+
+        // Apply gravity
+        foreach (var child in GetChildren())
+        {
+            if (child is RigidBody3D rb)
+            {
+                var localPosition = rb.Transform.Origin.AsVim().AsDouble();
+                var globalPosition = originPosition + localPosition;
+                var gravityVector = -globalPosition.Normalize();
+                var floatGravityVector = gravityVector.AsSingle().AsGodot();
+                var distance = globalPosition.Length();
+                var gravity = 9.81f * planetRadius * planetRadius / (distance * distance);
+                rb.ApplyCentralForce(floatGravityVector * (float)gravity);
+            }
+        }
+    }
+
+    public DVector3 GetPositionOf(Node3D node)
+    {
+        var localPosition = node.Transform.Origin.AsVim().AsDouble();
+        return originPosition + localPosition;
     }
 
     public override void _Process(double delta)
@@ -61,3 +99,9 @@ public sealed partial class Krakensbane : Node3D
         ImGui.End();
     }
 }
+
+public interface INonInertialFrameProvider
+{
+    public DVector3 GetCurrentAcceleration();
+    public DVector3 GetCurrentAngularAcceleration();
+}

src/Craft/Vessel.cs

@@ -0,0 +1,14 @@
+using Godot;
+
+namespace Quadratic.Carto.Craft;
+
+/// <summary>
+/// A Vessel is the smallest unit of object in the game that is considered a
+/// whole on its own, such as a space station or an astronaut. Connections can
+/// be made between vessels, but each vessel is still a separate entity.
+/// </summary>
+/// <remarks>
+/// Vessel is an abstract class, and is meant to be extended by concrete 
+/// implementations.
+/// </remarks>
+public abstract class Vessel { }

src/Craft/VesselNode.cs

@@ -0,0 +1,9 @@
+using Godot;
+
+namespace Quadratic.Carto.Craft;
+
+/// <summary>
+/// A VesselNode is the node that represents a <see cref="Vessel"/> currently
+/// loaded into the game world.
+/// </summary>
+public abstract partial class VesselNode : Node3D { }