Decomposing Velocity to get velocity projected on another vector

[AaronLS]

I have two vessels nearby. Vessel A is moving towards Vessel B, but not directly at it. I want to decompose the velocity vector into two vectors, such that V = M + N where M is the vector pointing directly from A to B. I have a point on vessel B "magnetCenter" below and "attractedVessel" is the Vessel A.

I get a vector pointing from A to B by subtracting locations "B - A"

(magnetCenter - attractedVessel.findLocalCenterOfMass()).normalized

I then project the velocity onto that vector to get the component of the velocity pointing from A to B.

Vector3 velocityComponentTowardsMagnet = 
   Vector3.Project(attractedVessel.rb_velocity, (magnetCenter - attractedVessel.findLocalCenterOfMass()).normalized );


printIntermittantFormatted("rel V: {0:#.00}, rb_V: {1:#.00}", velocityComponentTowardsMagnet.magnitude, attractedVessel.rb_velocity.magnitude);

As expected the projected result's magnitude is smaller than original velocity, but when Vessel A is a good distance away and moving directly towards Vessel B, then I would expect the magnitude of the velocity and the magnitude of the projection to be very close. But I get a big difference such as "rel V: .74, rb_V: 4.36".

Very similar result from using vessel.rigidBody.velocity instead of vessel.rb_velocity. The two are very close but are not identical(when referenced in the same fixedUpdate)

Vector3 velocityComponentTowardsMagnet2 = 
   Vector3.Project(attractedVessel.rigidbody.velocity, (magnetCenter - attractedVessel.findLocalCenterOfMass()).normalized );

printIntermittantFormatted("rel V: {0:#.00}, V: {1:#.00}", velocityComponentTowardsMagnet2.magnitude, attractedVessel.rigidbody.velocity.magnitude);

I also tried this but it seemed to give me non-sense I think because it is not the relative velocity of the center of mass and it also incorporates angular velocity:

var relativeVelocity = attractedVessel.rigidbody.GetRelativePointVelocity(magnetCenter);

I have a feeling it's because I'm not accounting for the velocity of vessel B. I really need to get the true relative velocity and then do the projection on that. Such as the relative velocity when you've targeted one vessel.

[Crzyrndm]

Try using orbital velocity instead of the rigidbody velocities. May not help, but at the very least it'll be more consistent (KSP doesn't use a fixed physics reference frame position/velocity and using a velocity that isn't against a known reference frame sounds fishy to me). My suspicion is that the velocity of the active vessel is going to be non-zero and you aren't accounting for that with attractedVessel.rigidbody.velocity.magnitude

Target relative velocity

FlightGlobals.ActiveTarget.obt_velocity - FlightGlobals.ActiveVessel.obt_velocity

Edited June 29, 2015 by Crzyrndm

[AaronLS]

Thanks, helped me at least know that I was on the right track.

This was the culprit because somehow I accidentally called findLocalCenterOfMass instead of World:

magnetCenter - attractedVessel.findLocalCenterOfMass()