Difference between revisions of "Center of thrust"

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[[File:Cot icon.png|framed|left|Icon]]
 
[[File:Cot icon.png|framed|left|Icon]]
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[[File:Com and cot.png|thumb|The thrust points away from the center of mass]]
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The '''center of thrust''' (abbreviated '''COT''' or '''CoT''') is the midpoint where [[w:Thrust|thrust]] from all a [[craft]]'s [[reaction engine]]s balances and the direction in which a craft's thrust is acting. It is often called the '''thrust vector'''<!-- at least in real life -->, though this may emphasize the direction at the expense of the position.
  
[[File:Com and cot.png|thumb|The thrust points away from the center of mass]]
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Along with the [[center of mass]] (COM), and the [[center of lift]] (COL) when inside an [[atmosphere]], it is one of three key physics properties determining a craft's behavior while in motion. Display of the COT can be toggled in the [[editor]] where it is seen as a magenta and black checkered sphere with a magenta arrow extending out of it.
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The '''center of thrust''' (abbreviated '''COT''') is the direction in which the thrust is acting and on which point the thrust is acting on. Because it has also a direction component it's usually called '''thrust vector'''<!-- at least in real life -->. The COT and center of mass should be in one line to minimize steering losses. It is important to note that the COT is calculated based on all running [[engine]]s on the [[craft]] and not just the engines in any individual stage. The center of thrust will change when engines get activated, deactivated or run out of fuel. Also when the craft's configuration changes, due to docking with other craft or jettisoning parts, the thrust vector will change.
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== Usage ==
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Like [[w:Lift (force)|lift]], thrust is a force acting on a craft and the center of mass acts as the fulcrum. The center of thrust has a vector arrow pointing the direction that thrust will push out of the COT, causing the craft to accelerate per Newton's Third Law. The interaction between the COT and COM is charted along the axis running through the exact center of each of them. For the craft to travel straight and true along that line, the thrust vector points the exact opposite direction along the axis from where the craft is going (equal and opposite reaction).  
  
Is one of the three “centers” displayed in the [[VAB]] and [[Spaceplane Hangar]] along with the [[center of lift]] and [[center of mass]] abbreviated COL and COM respectively.
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If the vector of thrust tilts at an angle to that line, the result will be ''asymmetric thrust'' which generates turning torque around the COM. This is how [[thrust vectoring]] works, but torque steering can be excessive due to design. For example, if the engines on a plane are above the COM, they may point straight back but the COM-COT line doesn't; thus the thrust vector is tilted relative to it and the plane will nosedive. Such torque can sometimes be balanced out by [[control surface]]s, [[RCS]], or [[SAS]], but it is more fuel efficient to make a balanced craft with its COM in-line with its COT. And the more thrust the engines generate, the stronger any torque steering becomes.
  
== Reading the Center of Thrust ==
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=== Changes in-flight ===
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It is important to note that the center of thrust depends on all ''running'' [[reaction engine]]s on the [[craft]]. Unfortunately, the editor calculates the COT based on the thrust of all engines present, regardless of staging. In use, the COT will change when any single engine's thrust changes due to activation, throttling, shutdown, or any other change. This includes when a craft's configuration changes due to jettisoning an engine or docking with another craft that has an active engine.
  
The center of thrust is used mainly to determine a rocket's behavior in flight by comparing its position to the center of mass. It's basically the same as the center of lift but this is generated by engines rather than wings. If the COT is left of the COM then the rocket tilts right, too far right, and it tilts left. It behaves similarly if it is too far forward or backwards.
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A highly unbalanced rocket (e.g. a rocket that loses one of multiple engines due to rapid unplanned disassembly) might produce so much torque at full throttle that its other controls are too weak to prevent it from rotating. This means it will be mostly uncontrollable. It might be possible, however, to control such a craft by throttling the engines down to a point where the torque they produce is no greater than the torque from SAS.
  
The COT should also be pointing at the COM as well, if it is pointing away from the COM then the rocket will be top-heavy and be prone to tilting. This is usually called the pendulum rocket fallacy. In Kerbal Spaaace Program, such crafts will be less efficient, as [[thrust vectoring]] needs to be applied to keep the craft pointing straight, and the thrust spent doing that is no longer available to push the rocket up like it normally would be. Steering problems can be fixed by adding engines with [[thrust vectoring]], adding an [[RCS]], or adding [[control surface]]s.
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=== Pendulum rocket fallacy ===
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It is tempting to think about placing the engines of a rocket at the top, so that the thrust vector points down toward the center of mass. This way, it seems that the engines will pull the rocket up, and that the rocket's weight will keep it hanging directly under the engines, so that it stays upright. Unfortunately, this is not true. Since the rocket's weight acts as a force exactly through its center of mass, gravity doesn't provide any torque to oppose the rotation of the rocket. Any imperfection in the alignment of the thrust vector, or any outside torques, will still cause rotation. This is equally problematic regardless of the location of the engines. The stability of a rocket ultimately depends on its ability to actively counteract imperfections. In KSP, this is most often accomplished by using engines with thrust vectoring. When such an engine gimbals, it slightly modifies the thrust vector. [[SAS]] uses this as needed to correct for imbalances, along with [[RCS]], [[reaction wheel]]s, and [[control surface]]s.
  
 
== See also ==
 
== See also ==
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* [[Center of mass]]
 
* [[Center of mass]]
 
* {{Wikipedia|Pendulum rocket fallacy}}
 
* {{Wikipedia|Pendulum rocket fallacy}}
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[[Category:Physics]]

Latest revision as of 22:05, 14 March 2015

Icon
The thrust points away from the center of mass

The center of thrust (abbreviated COT or CoT) is the midpoint where thrust from all a craft's reaction engines balances and the direction in which a craft's thrust is acting. It is often called the thrust vector, though this may emphasize the direction at the expense of the position.

Along with the center of mass (COM), and the center of lift (COL) when inside an atmosphere, it is one of three key physics properties determining a craft's behavior while in motion. Display of the COT can be toggled in the editor where it is seen as a magenta and black checkered sphere with a magenta arrow extending out of it.

Usage

Like lift, thrust is a force acting on a craft and the center of mass acts as the fulcrum. The center of thrust has a vector arrow pointing the direction that thrust will push out of the COT, causing the craft to accelerate per Newton's Third Law. The interaction between the COT and COM is charted along the axis running through the exact center of each of them. For the craft to travel straight and true along that line, the thrust vector points the exact opposite direction along the axis from where the craft is going (equal and opposite reaction).

If the vector of thrust tilts at an angle to that line, the result will be asymmetric thrust which generates turning torque around the COM. This is how thrust vectoring works, but torque steering can be excessive due to design. For example, if the engines on a plane are above the COM, they may point straight back but the COM-COT line doesn't; thus the thrust vector is tilted relative to it and the plane will nosedive. Such torque can sometimes be balanced out by control surfaces, RCS, or SAS, but it is more fuel efficient to make a balanced craft with its COM in-line with its COT. And the more thrust the engines generate, the stronger any torque steering becomes.

Changes in-flight

It is important to note that the center of thrust depends on all running reaction engines on the craft. Unfortunately, the editor calculates the COT based on the thrust of all engines present, regardless of staging. In use, the COT will change when any single engine's thrust changes due to activation, throttling, shutdown, or any other change. This includes when a craft's configuration changes due to jettisoning an engine or docking with another craft that has an active engine.

A highly unbalanced rocket (e.g. a rocket that loses one of multiple engines due to rapid unplanned disassembly) might produce so much torque at full throttle that its other controls are too weak to prevent it from rotating. This means it will be mostly uncontrollable. It might be possible, however, to control such a craft by throttling the engines down to a point where the torque they produce is no greater than the torque from SAS.

Pendulum rocket fallacy

It is tempting to think about placing the engines of a rocket at the top, so that the thrust vector points down toward the center of mass. This way, it seems that the engines will pull the rocket up, and that the rocket's weight will keep it hanging directly under the engines, so that it stays upright. Unfortunately, this is not true. Since the rocket's weight acts as a force exactly through its center of mass, gravity doesn't provide any torque to oppose the rotation of the rocket. Any imperfection in the alignment of the thrust vector, or any outside torques, will still cause rotation. This is equally problematic regardless of the location of the engines. The stability of a rocket ultimately depends on its ability to actively counteract imperfections. In KSP, this is most often accomplished by using engines with thrust vectoring. When such an engine gimbals, it slightly modifies the thrust vector. SAS uses this as needed to correct for imbalances, along with RCS, reaction wheels, and control surfaces.

See also