diff --git a/core/src/net/sf/openrocket/simulation/RK4SimulationStatus.java b/core/src/net/sf/openrocket/simulation/RK4SimulationStatus.java
index 67465e16d..d42d951fa 100644
--- a/core/src/net/sf/openrocket/simulation/RK4SimulationStatus.java
+++ b/core/src/net/sf/openrocket/simulation/RK4SimulationStatus.java
@@ -8,7 +8,6 @@ public class RK4SimulationStatus extends SimulationStatus implements Cloneable {
 	private Coordinate launchRodDirection;
 	
 	private double previousAcceleration = 0;
-	private AtmosphericConditions previousAtmosphericConditions;
 	
 	// Used for determining when to store aerodynamic computation warnings:
 	private double maxZVelocity = 0;
@@ -26,7 +25,6 @@ public class RK4SimulationStatus extends SimulationStatus implements Cloneable {
 			this.previousAcceleration = ((RK4SimulationStatus) other).previousAcceleration;
 			this.maxZVelocity = ((RK4SimulationStatus) other).maxZVelocity;
 			this.startWarningTime = ((RK4SimulationStatus) other).startWarningTime;
-			this.previousAtmosphericConditions = ((RK4SimulationStatus) other).previousAtmosphericConditions;
 		}
 	}
 	public void setLaunchRodDirection(Coordinate launchRodDirection) {
@@ -49,18 +47,6 @@ public class RK4SimulationStatus extends SimulationStatus implements Cloneable {
 		this.previousAcceleration = previousAcceleration;
 	}
 	
-	
-	public AtmosphericConditions getPreviousAtmosphericConditions() {
-		return previousAtmosphericConditions;
-	}
-	
-	
-	public void setPreviousAtmosphericConditions(
-			AtmosphericConditions previousAtmosphericConditions) {
-		this.previousAtmosphericConditions = previousAtmosphericConditions;
-	}
-	
-	
 	public double getMaxZVelocity() {
 		return maxZVelocity;
 	}
diff --git a/core/src/net/sf/openrocket/simulation/RK4SimulationStepper.java b/core/src/net/sf/openrocket/simulation/RK4SimulationStepper.java
index 2801fbc6a..9c1b825af 100644
--- a/core/src/net/sf/openrocket/simulation/RK4SimulationStepper.java
+++ b/core/src/net/sf/openrocket/simulation/RK4SimulationStepper.java
@@ -102,11 +102,17 @@ public class RK4SimulationStepper extends AbstractSimulationStepper {
 		store.timestep = MathUtil.max(MathUtil.min(store.timestep, maxTimeStep), MIN_TIME_STEP);
 		checkNaN(store.timestep);
 
+		/*
+		 * Get the current atmospheric conditions
+		 */
+		calculateFlightConditions(status, store);
+		store.atmosphericConditions = store.flightConditions.getAtmosphericConditions();
+
 		/*
 		 * Compute the initial thrust estimate.  This is used for the first time step computation.
 		 */
-		store.thrustForce = calculateAverageThrust(status, store.timestep, store.timestep,
-				status.getPreviousAtmosphericConditions(), false);
+		store.thrustForce = calculateAverageThrust(status, store.timestep, store.longitudinalAcceleration,
+												   store.atmosphericConditions, false);
 		
 		/*
 		 * Perform RK4 integration.  Decide the time step length after the first step.