API Question How do I convert 1 vessel's thrust vector to another vessel's local vessel coordinates?

[nbcfrosty]

Hello. I want to transmit the child vessel's thrust forces to the parent. I am trying to do this:

VECTOR3 childThrustVector, globalThrust, parentLocalThrust;
        auto child = oapiGetVesselInterface(attach.child);
        child->GetThrustVector(childThrustVector);
        child->GlobalRot(childThrustVector, globalThrust);

        v->Global2Local(globalThrust, parentLocalThrust);
        
        v->AddForce(parentLocalThrust, attach.pos);

But in practice the parent vessel goes flying out of the atmosphere. Any help with this would be greatly appreciated.

 

[Urwumpe]

You must use a matrix there, there is no inverse function for GlobalRot. I thought about pairing Local2Global with Global2Local, but thats for positions, not forces.

If I am not wrong, its just GetRotationMatrix of your vessel transposed.

 

[nbcfrosty]

I haven't checked the numbers but testing with an SRB on DG this seems to work?

DLLCLBK void opcPreStep(double simt, double simdt, double mjd) {
    VESSEL* v = oapiGetFocusInterface();
    auto handle = v->GetHandle();
    if (attachments.find(handle) == attachments.end()) {
        return;
    }
    auto info = attachments[handle];
    for (int i = 0; i < info.attachments.size(); i++) {
        auto attach = info.attachments[i];
        if (!oapiIsVessel(attach.child)) {
            continue;
        }
        VECTOR3 childThrustVector, globalThrust, parentLocalThrust;
        auto child = oapiGetVesselInterface(attach.child);
        child->GetThrustVector(childThrustVector);
        child->Local2Global(childThrustVector, globalThrust);

        v->Global2Local(globalThrust, parentLocalThrust);
      
        v->AddForce(parentLocalThrust * simdt, _V(0, 0, 0));
    }
}

And this also seems to work:

VECTOR3 childThrustVector, globalThrust, parentLocalThrust;
        MATRIX3 R;
        auto child = oapiGetVesselInterface(attach.child);
        child->GetThrustVector(childThrustVector);
        child->GlobalRot(childThrustVector, globalThrust);
        v->GetRotationMatrix(R);
        parentLocalThrust = tmul(R, globalThrust);
        v->AddForce(parentLocalThrust * simdt, _V(0, 0, 0));

 

[nbcfrosty]

I also wanted to check if that the multiplication with simdt is right.

 

[n72.75]

I also wanted to check if that the multiplication with simdt is right.

I don't think you want to to multiply by simdt. AddForce takes two arguments, a force vector, and a vector pointing to the point at which the force acts in local vessel coordinates relative to the center of mass.

 

[n72.75]

I should also mention that if you're trying to make a vessel with multiple stages, it is probably far easier to do this with docking ports.

 

[nbcfrosty]

Trying to give my take on Kulch's UCGO Cargo Deck.

auto attach = info.attachments[i];
        if (!oapiIsVessel(attach.child)) {
            continue;
        }

        VECTOR3 childThrustVector, globalThrust, parentLocalThrust;
        MATRIX3 R;
        auto child = oapiGetVesselInterface(attach.child);
        child->GetThrustVector(childThrustVector);
        child->GlobalRot(childThrustVector, globalThrust);
        v->GetRotationMatrix(R);
        parentLocalThrust = tmul(R, globalThrust);
        v->AddForce(parentLocalThrust, attach.pos);

I think this is the final solution, I wasn't sure if I needed to multiply the force vector with simdt but looks like I don't need to.