diff --git a/core/src/net/sf/openrocket/rocketcomponent/FlightConfiguration.java b/core/src/net/sf/openrocket/rocketcomponent/FlightConfiguration.java
index f860d1d77..d8f21cbc9 100644
--- a/core/src/net/sf/openrocket/rocketcomponent/FlightConfiguration.java
+++ b/core/src/net/sf/openrocket/rocketcomponent/FlightConfiguration.java
@@ -6,6 +6,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -62,7 +63,7 @@ public class FlightConfiguration implements FlightConfigurableParameter<FlightCo
 	/* Cached data */
 	final protected Map<Integer, StageFlags> stages = new HashMap<Integer, StageFlags>();	// Map of stage number to StageFlags of the corresponding stage
 	final protected Map<MotorConfigurationId, MotorConfiguration> motors = new HashMap<MotorConfigurationId, MotorConfiguration>();
-	final private Collection<MotorConfiguration> activeMotors = new ArrayList<MotorConfiguration>();
+	final private Collection<MotorConfiguration> activeMotors = new ConcurrentLinkedQueue<MotorConfiguration>();
 	final private InstanceMap activeInstances = new InstanceMap();
 	
 	private int boundsModID = -1;
diff --git a/core/src/net/sf/openrocket/rocketcomponent/InstanceMap.java b/core/src/net/sf/openrocket/rocketcomponent/InstanceMap.java
index 1538e69aa..f40430d4c 100644
--- a/core/src/net/sf/openrocket/rocketcomponent/InstanceMap.java
+++ b/core/src/net/sf/openrocket/rocketcomponent/InstanceMap.java
@@ -1,9 +1,9 @@
 package net.sf.openrocket.rocketcomponent;
 
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import net.sf.openrocket.util.Transformation;
 
@@ -13,7 +13,7 @@ import net.sf.openrocket.util.Transformation;
  * @author teyrana (aka Daniel Williams) <equipoise@gmail.com>
  *
  */
-public class InstanceMap extends HashMap<RocketComponent, ArrayList<InstanceContext>> {
+public class InstanceMap extends ConcurrentHashMap<RocketComponent, ArrayList<InstanceContext>> {
 
 	// =========== Public Functions ========================