Difference between revisions of "Axis"

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(*a few links; *SAS→reaction wheels; *all rotate around the COM; +control surfaces; +plane axes;)
(edit for better language and to correct inaccuracies)
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[[File:Rocket axes.svg|thumb|All axes of a rocket]]
 
[[File:Rocket axes.svg|thumb|All axes of a rocket]]
 
[[File:Plane axes.svg|thumb|All axes of a plane]]
 
[[File:Plane axes.svg|thumb|All axes of a plane]]
An '''axis''' is the imaginary line around an object can rotate. Each object has three axes around it can rotate. The force to rotate can be applied with [[reaction wheel]]s, [[control surface]]s, [[thrust vectoring]] (TVC) and [[RCS]]. While the stock rocket wheels support rotating around all axes, RCS only if there are thrusters at the side of the axis and TVC can only assists rotating orthogonally to the engines thrust.
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An '''axis''' is the imaginary line around which an object can rotate. A three-dimensional object such as a spacecraft has three noteworthy axes, corresponding to each dimension of space. Rotation is produced by torque. In reality, torque is the product of a linear force and its distance from the axis. In KSP, there are four ways to produce torque, each with its own limitations and side effects:
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* "Magic torque", produced by [[capsule]]s, [[probe]] bodies and [[reaction wheels]]. This is a twisting force - in any axis - applied at the location of whatever component provides it, with no other effects; it gets less effective further from the centre of mass.
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* [[Control surface]]s in atmosphere. Each control surface can only produce torque in one axis, but (by default) the game can figure out how to map control inputs to control surface actuations to produce the requested rotation.
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* [[Thrust vectoring]], produced by a gimbaled rocket engine. This can't produce rotation in the axis parallel to the engine's thrust (i.e. roll), and obviously only works while the engine is firing.
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* Any linear force applied at a distance from the center of mass, at an angle away from it. This is how [[RCS]] produces torque, but any net force can produce torque this way. In particular, unevenly placed boosters and drag-producing components can lead to a net force on one side, producing torque.
  
The size and mass of the craft affect the ability to rotate. In general, the further away of the axis a mass is the more force is needed to rotate it around the axis. So rolling is in most cases not a very big problem for a rocket while yawing and pitching without available TVC or RCS is relatively hard.
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The mass distribution of the craft affects how torque produces rotation. In general, the further away from the axis a mass is, the more torque is needed to move it around the axis. Most rockets are long and narrow; so rolling is easy, but yawing and pitching are hard. RCS enhances rotation by exploiting lever effects: the further the thrust is applied from the centre of mass, the larger the torque it produces.
  
By default the viewpoint of rotating is from the command module first added so the described movements here are only the default cases. It is possible to change the viewpoint from the rotation, by changing the part from which it is controlled. To do so right-click a part and select “Control from here”. Only command modules and docking ports are supporting this option and it will rotate always around the [[center of mass]] not around the selected part.
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By default the viewpoint of rotating is from the command module first added, so the described movements here are only the default cases. It is possible to change the viewpoint of rotation to another component. To do so right-click a part and select “Control from here”. Only command modules and docking ports support this option and it will rotate always around the [[center of mass]], not around the selected part.
 
 
== Yaw ==
 
A motion around the yaw axis results in moving the nose to the left or right and the tail to the other side. The axis is orthogonally to the display's plane by default. Wide or long crafts make rotation harder while the depth isn't very important.
 
  
 
== Pitch ==
 
== Pitch ==
Pitching will move the nose downwards and the tail upwards and the axis goes from one side to the other. Long or depth crafts will damp the rotation. A very wide craft won't impact the ability very much.
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Pitching will move the nose up or down and the tail in the other direction. The axis runs from port to starboard through the center of mass. More torque is needed to rotate long or deep craft in the pitch axis; width is irrelevant to pitch.
  
 
== Roll ==
 
== Roll ==
Rolling allows the craft to rotate around its central axis. A very tall craft or aircraft won't change much, but a wide or depth craft will impact it. In most configurations rolling couldn't be supported with TVC. Placing an engine on the rockets side however allows to roll with TVC, but it is not very common. Because of this a rocket can easily start rolling if there is not enough SAS available to counteract and boosters on the side aren't placed at exactly the same height.
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Roll is a rotation around the axis from fore to aft. More torque is needed to rotate a wide or deep craft around the roll axis; length is irrelevant to roll. Most vehicles are larger in length than in either of the other dimensions, making roll the easiest rotation.
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== Yaw ==
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A motion around the yaw axis results in moving the nose to the port or starboard and the tail in the other direction. The axis runs from top to bottom through the center of mass. More yaw torque is needed to rotate wide or long craft in the yaw axis; depth (i.e. size from top to bottom) is irrelevant to yaw.
  
With rolling and pitching aircrafts can do more effective turns to the left or right than only with yawing. To do so roll to the desired side and then pitch upwards.
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Aircraft tend to be wider than they are tall and longer than they are wide. This makes rolling the easiest rotation, followed by pitching, while yaw is the hardest rotation for an aircraft. With this in mind, it is usually more effective to turn by rolling in the desired direction, then pitching up. With the right wing configuration, an aircraft in a rolled attitude is also subject to a yawing torque, which is especially efficient for slow turns.

Revision as of 02:44, 14 July 2014

All axes of a rocket
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All axes of a plane

An axis is the imaginary line around which an object can rotate. A three-dimensional object such as a spacecraft has three noteworthy axes, corresponding to each dimension of space. Rotation is produced by torque. In reality, torque is the product of a linear force and its distance from the axis. In KSP, there are four ways to produce torque, each with its own limitations and side effects:

  • "Magic torque", produced by capsules, probe bodies and reaction wheels. This is a twisting force - in any axis - applied at the location of whatever component provides it, with no other effects; it gets less effective further from the centre of mass.
  • Control surfaces in atmosphere. Each control surface can only produce torque in one axis, but (by default) the game can figure out how to map control inputs to control surface actuations to produce the requested rotation.
  • Thrust vectoring, produced by a gimbaled rocket engine. This can't produce rotation in the axis parallel to the engine's thrust (i.e. roll), and obviously only works while the engine is firing.
  • Any linear force applied at a distance from the center of mass, at an angle away from it. This is how RCS produces torque, but any net force can produce torque this way. In particular, unevenly placed boosters and drag-producing components can lead to a net force on one side, producing torque.

The mass distribution of the craft affects how torque produces rotation. In general, the further away from the axis a mass is, the more torque is needed to move it around the axis. Most rockets are long and narrow; so rolling is easy, but yawing and pitching are hard. RCS enhances rotation by exploiting lever effects: the further the thrust is applied from the centre of mass, the larger the torque it produces.

By default the viewpoint of rotating is from the command module first added, so the described movements here are only the default cases. It is possible to change the viewpoint of rotation to another component. To do so right-click a part and select “Control from here”. Only command modules and docking ports support this option and it will rotate always around the center of mass, not around the selected part.

Pitch

Pitching will move the nose up or down and the tail in the other direction. The axis runs from port to starboard through the center of mass. More torque is needed to rotate long or deep craft in the pitch axis; width is irrelevant to pitch.

Roll

Roll is a rotation around the axis from fore to aft. More torque is needed to rotate a wide or deep craft around the roll axis; length is irrelevant to roll. Most vehicles are larger in length than in either of the other dimensions, making roll the easiest rotation.

Yaw

A motion around the yaw axis results in moving the nose to the port or starboard and the tail in the other direction. The axis runs from top to bottom through the center of mass. More yaw torque is needed to rotate wide or long craft in the yaw axis; depth (i.e. size from top to bottom) is irrelevant to yaw.

Aircraft tend to be wider than they are tall and longer than they are wide. This makes rolling the easiest rotation, followed by pitching, while yaw is the hardest rotation for an aircraft. With this in mind, it is usually more effective to turn by rolling in the desired direction, then pitching up. With the right wing configuration, an aircraft in a rolled attitude is also subject to a yawing torque, which is especially efficient for slow turns.