Add code to estimate at what distance from a grounded target we should burn to come to a stop over the target.

lib/navigation.ks

@@ -1,5 +1,7 @@
 // functions for calculating steering values.
 
+runoncepath("/lib/node").
+
 function GetPitch {
   parameter v is SHIP:FACING:FOREVECTOR.
   return 90 - vectorangle(SHIP:UP:FOREVECTOR, v).
@@ -46,6 +48,41 @@ function CreateCircularizationNode {
   return n.
 }
 
+// The distance at which to start burning to reach a target on the ground.
+// REQUIRES a circular orbit.
+// TODO: it would be great to semi-automate this...
+function TargetBurnDistance {
+  return Sqrt(StoppingDistance()^2 + (SHIP:ORBIT:SEMIMAJORAXIS - SHIP:BODY:RADIUS - TARGET:ALTITUDE)^2).
+}
+
+// Stopping distance at current velocity. For a circular orbit this is valid at any point in the orbit.
+function StoppingDistance {
+  local dV is SHIP:VELOCITY:SURFACE:MAG.
+  return dV*BurnTime(dV)/2.
+}
+
+// Figure out our closest approach to our current target within the next
+// N seconds. (default orbital_period * 2)
+// This is a horrible, terrible, no good, very bad brute force hack.
+// function timeAtClosestApproach {
+//   parameter maxSeconds is SHIP:ORBIT:PERIOD * 2.
+
+//   // find the closest approach to pos
+//   local t is TIME.
+//   local minD is (PositionAt(TARGET, t) - PositionAt(SHIP, t)):MAG.
+//   local minT is t.
+//   until false {
+//     local d is (PositionAt(TARGET, t) - PositionAt(SHIP, t)):MAG.
+//     if d < minD {
+//       set minD to d.
+//       set minT to t.
+//     }
+//     set t to t+1.
+//   }
+
+//   return minT.
+// }
+
 // function PredictGeo {
 //   parameter t.
 

lib/ui.ks

@@ -49,5 +49,7 @@ function MakeMenu {
     stack:ShowOnly(top).
   }.
 
-  set top:AddButton(execLabel):onClick to callback:Bind(optionList).
+  if execLabel <> "" {
+    set top:AddButton(execLabel):onClick to callback:Bind(optionList).
+  }
 }