API Question ShiftCG

[gattispilot]

I am trying to get ShiftCG to function. I looked at the Atlantis code.

ShiftCG (OFS_LAUNCH_ORBITER-OFS_WITHTANK_ORBITER);

and here:

ShiftCG (OFS_WITHTANK_ORBITER);

OFS_WITHTANK_ORBITER and OFS_WITHTANK_ORBITER are both Vector3

const VECTOR3 OFS_ZERO             = { 0.0, 0.0,  0.0  };
const VECTOR3 OFS_LAUNCH_ORBITER   = { 0.0, 6.22,-7.795};
const VECTOR3 OFS_LAUNCH_TANK      = { 0.0,-1.91, 5.72 };
const VECTOR3 OFS_LAUNCH_RIGHTSRB  = { 6.2,-1.91,-5.68 };
const VECTOR3 OFS_LAUNCH_LEFTSRB   = {-6.2,-1.91,-5.68 };
const VECTOR3 OFS_WITHTANK_ORBITER = { 0.0, 4.79,-9.185};
const VECTOR3 OFS_WITHTANK_TANK    = { 0.0,-3.34, 4.33 };
const VECTOR3 ORBITER_DOCKPOS      = { 0.0, 2.44,10.44 };
const VECTOR3 OFS_MMU              = {0,2.44,10.44};

Ok. When I duplicate I get this error:
::ShiftCG' : function does not take 1 arguments

8.51.3.251 voidVESSEL::ShiftCG(constVECTOR3&shift) Shift the centre of gravity of a vessel.
Parameters: shift centre of gravity displacement vector [m]
Note: This function should be called after a vessel has undergone a structural change which resulted in a shift of the vessel’s centre of gravity (CG). Note that in Orbiter, a vessel’s CG coincides by definition always with the origin (0,0,0) of its local reference frame. Therefore, in order to achieve a shift of the CG by shift, this function performs the following actions: • Calls ShiftCentreOfMass (+shift) to align the vessel’s global position with the new CG position. • Calls ShiftMeshes (-shift) to compensate the mesh positions • Applies equivalent shift to all – thruster positions, – docking ports, – attachment points, – explicitly defined light source positions, – and to the cockpit camera position The net effect is a shift of the vessel frame of reference (and thus the CG by +shift, while the mesh positions remain in place in the global frame.

 

[martins]

Ok. When I duplicate I get this error:
::ShiftCG' : function does not take 1 arguments

I don't understand the error message. "::ShiftCG" refers to a function in the global namespace. Did you define a global ShiftCG function? I'm not even sure why the error message would print the two leading colons, rather than just "ShiftCG", but maybe that's to make it clear that it doesn't refer to the class member.

If you defined a global ShiftCG function, then a call to it from within a vessel class might be ambiguous. You can avoid this by explicitly calling this->ShiftCG instead.

 

[jedidia]

Are you calling it from within a vessel class?

 

[gattispilot]

Thanks. I got it.

if (key == OAPI_KEY_3)//
{
	
	ShiftCG(CGSHIFTOFFSET);
	

	return 1;
}

const VECTOR3 CGSHIFTOFFSET = { 0.0, 0, -4 };

I expected a tilt not a shift in direction. Trying to get a 16 degree up tilt.

 

[Urwumpe]

Trying to get a 16 degree up tilt.

You mean by aerodynamics?

 

[gattispilot]

You mean by aerodynamics?

Yes. This vessel had weights on the edge. Then when the weights were dropped the craft then tilted. So just has that shifted the COG.

I want to with a key stroke.

 

[Urwumpe]

Yes. This vessel had weights on the edge. Then when the weights were dropped the craft then tilted. So just has that shifted the COG.

I want to with a key stroke.

Yes. You can do that by moving the airfoil definition relative to the CoG as well.

 

[gattispilot]

Yes. You can do that by moving the airfoil definition relative to the CoG as well.

How?

Since in real life they shift the CoG. that is how it should be done here?

I Thought (dangerous I know) that by shifting the Cog in the z direction the craft would tilt back.

 

[Urwumpe]

Doesn't happen that way - conservation of impulse applies.

Usually, by shifting the CoG, the CoG shifts also relative to the aerodynamic centre of pressure. But in Orbiter, I am not sure if this is done automatically. I suspect not, since the API docs are not saying so.

 

[BrianJ]

Doesn't happen that way - conservation of impulse applies.

Usually, by shifting the CoG, the CoG shifts also relative to the aerodynamic centre of pressure. But in Orbiter, I am not sure if this is done automatically. I suspect not, since the API docs are not saying so.

I think ShiftCG() does shift aerofoils relative to CoG, but not sure either. So if you have aerofoils defined, centre of pressure should shift.

How?

Since in real life they shift the CoG. that is how it should be done here?

I Thought (dangerous I know) that by shifting the Cog in the z direction the craft would tilt back.

If your vessel is oriented as +Z = "forwards/velocity vector", +Y = "up", then you need to shift the CoG in the -Y direction to cause a "pitch up".

I have a hunch I know which vessel this relates to ;-)

 

[gattispilot]

Ok.. I have this:

const VECTOR3 CGSHIFTOFFSET = { 0.0, -4, 0 };

All that happens is the craft goes up. Still level though

 

[Urwumpe]

Ok.. I have this:

const VECTOR3 CGSHIFTOFFSET = { 0.0, -4, 0 };

All that happens is the craft goes up. Still level though

Yes - what do you expect on the ground? What matters is in aerodynamic flight!

See here:

Notice the offset CG by the heavy equipment?

That only has effect during aerodynamic flight. But since in Orbiter, you don't shift the CG because CG is origin of the coordinate system, you shift the rest of the spacecraft.

 

[gattispilot]

it is in space. It is orbiting Earth.

 

[Urwumpe]

it is in space. It is orbiting Earth.

Then dropping the ballast has no such effect by itself.

If the spacecraft is spinning, releasing the ballast could change the rotation impulse vector and make the spacecraft tumble around another axis, but it would not rotate and cease to rotate at 18° angle.

 

[gattispilot]

oh. Then how could I do it?

 

[Urwumpe]

oh. Then how could I do it?

Drop ballast, so the spacecraft is coarse stabilizing at the intended AOA during reentry.

Use RCS for fine tuning the attitude and turning to proper AOA before reentry.

 

[gattispilot]

But is there a way to check what the CofG shift should be.
This vessel has 2 sets of Ballast. 1 set makes it tilt 16 degrees and then the other level.

 

[Urwumpe]

But is there a way to check what the CofG shift should be.
This vessel has 2 sets of Ballast. 1 set makes it tilt 16 degrees and then the other level.

Yes - you keep track of it. Orbiter does not care how your vessel looks like inside, it only cares about how it interacts with the universe.

 

[gattispilot]

ok. But if i wanted a process.
Like at 1000KM weights drop so CofG shifts to allow a 16 degree tilt.

or should I at 1000KM set RCS to fire until at 16 AOA is set?

 

[gattispilot]

This is what I am trying to do. So press a key and the cruise mass drops thus changing the CoG. Then at another time press a key or automatic entry masses are dropped and now CoG is restored and capsule is level.