diff --git a/boot/rocket.ks b/boot/rocket.ks
index db61768..c80efca 100644
--- a/boot/rocket.ks
+++ b/boot/rocket.ks
@@ -8,6 +8,7 @@ deletepath("/boot/rocket").
 compile "0:/ui/rocket" to "1:/init".
 compile "0:/lib/navigation" to "1:/lib/navigation".
 compile "0:/lib/throttle" to "1:/lib/throttle".
+copypath("0:/lib/math", "1:/lib/math"). // larger when compiled
 compile "0:/launch" to "1:/launch".
 copypath("0:/execnode", "1:/execnode"). // larger when compiled
 
diff --git a/launch.ks b/launch.ks
index 8f238e7..694b43e 100644
--- a/launch.ks
+++ b/launch.ks
@@ -1,5 +1,5 @@
-run once "lib/throttle".
-run once "lib/navigation".
+runoncepath("lib/throttle").
+runoncepath("lib/navigation").
 
 parameter APOAPSIS_TARGET is 80000.
 parameter GRAVITY_TURN_START is 8000.
@@ -20,16 +20,10 @@ from { local x is 5. } until x = 0 step { set x to x - 1. } do {
   wait 0.5.
 }
 
-// throttle controls
-when TWR() > 1.5 then {
-  if SHIP:ALTITUDE > 32000 {
-    lock THROTTLE to 1.0.
-    return false.
-  }
-
-  lock THROTTLE to ThrottleToTWR(1.5).
-  return true.
-}
+// Hold throttle to maintain 1.5 TWR.
+// We do *not* use a PID Loop here because we can
+// calculate the correct value discretely.
+lock THROTTLE to ThrottleToTWR(1.5).
 
 // Main ascent control.
 lock THROTTLE to 1.0.
diff --git a/lib/throttle.ks b/lib/throttle.ks
index abe939a..ce34adb 100644
--- a/lib/throttle.ks
+++ b/lib/throttle.ks
@@ -1,29 +1,30 @@
 // Functions for calculating thrust values.
 @lazyglobal off.
 
+local G is 9.81.
+lock G to GetGravAcc().
+
+function GetGravAcc {
+  if HAS_GRAV_SENSOR = true {
+    return SHIP:SENSORS:GRAV:MAG.
+  }
+  return SHIP:BODY:MU / ((SHIP:BODY:RADIUS+SHIP:ALTITUDE)^2).
+}
+
 // Returns the throttle value you should use to achieve the
 // target TWR. If TWR can't be achieved, returns 1.0. (full throttle)
 function ThrottleToTWR {
   parameter targetTWR is 1.5.
 
   local m is SHIP:MASS.
-  local g is 9.81.
-  if HAS_GRAV_SENSOR = true {
-    set g to SHIP:SENSORS:GRAV:MAG.
-  }
-  return min((targetTWR*m*g)/SHIP:AVAILABLETHRUST, 1.0).
+  return min((targetTWR*m*G)/SHIP:AVAILABLETHRUST, 1.0).
 }
 
 // Calculates the ship's current TWR.
 function TWR {
   local m is ship:mass.
-  local g is 9.81. // fallback to this approximation that doesn't deal with
-                   // altitude change.
-  if HAS_GRAV_SENSOR = true {
-    set g to SHIP:SENSORS:GRAV:MAG.
-  }
   local t is THROTTLE * SHIP:AVAILABLETHRUST.
-  return t/(m*g).
+  return t/(m*G).
 }
 
 // Check for various sensors and set appropriate global constants.
@@ -51,7 +52,6 @@ function BurnTime {
   local m is SHIP:MASS * 1000.        // Starting mass (kg)
   local e is CONSTANT():E.            // Base of natural log
   local p is en[0]:ISP.               // Engine ISP (s)
-  local g is 9.80665.                 // Gravitational acceleration constant (m/s²)
 
-  return g * m * p * (1 - e^(-dV/(g*p))) / f.
+  return G * m * p * (1 - e^(-dV/(G*p))) / f.
 }