Reflection so it compiles when grav accel not exist.

src/kOS/Module/kOSProcessor.cs

@@ -15,6 +15,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using kOS.Safe.Execution;
 using UnityEngine;
 using kOS.Safe.Encapsulation;
@@ -510,7 +511,26 @@ public void InitObjects()
         // the KSP API.  It's in kOS.Safe.
         private void CalcConstsFromKSP()
         {
-            ConstantValue.G0 = PhysicsGlobals.GravitationalAcceleration;
+            // GravitationalAcceleration did not exist in PhysicsGlobals prior to KSP 1.6.x.
+            // This code has to use reflection to avoid calling it on older backports:
+            Type physGlobType = typeof(PhysicsGlobals);
+            if (physGlobType != null)
+            {
+                // KSP often changes its mind whether a member is a Field or Property, so let's write this
+                // to future-proof against them changing which it is by trying both ways:
+                FieldInfo asField = (physGlobType.GetField("GravitationalAcceleration", BindingFlags.Public | BindingFlags.Static));
+                if (asField != null)
+                    ConstantValue.G0 = (double) asField.GetValue(null);
+                else
+                {
+                    PropertyInfo asProperty = (physGlobType.GetProperty("GravitationalAcceleration", BindingFlags.Public | BindingFlags.Static));
+                    if (asProperty != null)
+                        ConstantValue.G0 = (double)asProperty.GetValue(null, null);
+                }
+            }
+            // Fallback: Note if none of the above work, G0 still does have a reasonable value because we
+            // hardcode it to a literal in ConstantValue before doing any of the above work.
+
 
             // Cannot find anything in KSP's API exposing their value of G, so this indirect means
             // of calculating it from an arbitrary body is used: