SDK Question How to calculate PMI and inertia tensor from real data ?

[Woodylepic]

I have some real moment of inertia data take from a nasa pdf
exp...

Ixx=28971 slug/feet
Iyy=828113 slug/feet
Izz=826797 slug/feet

I have converted all the data to kg/meter^2

I= Inertia x,y,z vector

Ixx=39279.402173991 kg/meter^2
Iyy=1122770.47987677 kg/meter^2
Izz=1120986.22343892 kg/meter^2


and I wont to incorporate this data in my vessel in Orbiter

pmi in Orbiter seem to be calculate by inertia tensor,

How to convert the real nasa data in to inertia tensor in Orbiter ?

 

[N_Molson]

This is extensively covered in Orbiter documentation.

Also, there is a small utility in the Orbiter package that can calculate an approximation for you (ShipEdit).

 

[Urwumpe]

That are moments of inertia, not principal moments of inertia - principal moments of inertia are moments of inertia divided by mass.

 

[martins]

"PMI" stands for "principal moments of inertia": it is a 3-vector containing the diagonal of the diagonalised inertia tensor. Note that orbiter uses a "mass-normalised" form of the inertia tensor (units 1/m^2). To calculate it from the values you have got, you need to divide by the vessel mass.

The PMI values in orbiter refer to the vessel centre of mass. This must coincide with the origin of the local vessel frame for angular momentum calculations to be performed correctly.

Orbiter assumes that the off-diagonal elements of the inertia tensor are zero. If this is not the case for your vessel, you need to rotate the local vessel frame to diagonalise the tensor. Small nonzero off-diagonal values are sometimes acceptable.

That are moments of inertia, not principal moments of inertia - principal moments of inertia are moments of inertia divided by mass.

I don't think the "principal" refers to the mass normalisation. That is just an Orbiter convention. Rather, "principal" refers to the principal frame in which the tensor is diagonal.

 

[Woodylepic]

So Martin I have just to divide the 3 inertial mass from my pdf to the total mass of my vessel in orbiter to get the pmi tensor in orbiter ? is that correct ?

exp:

I= Inertia x,y,z vector

I x=39279.402173991 kg/meter^2 / by mass of my vessel (26283.86347 kg) = 1.494
I y=1122770.47987677 kg/meter^2 / by mass of my vessel (26283.86347 kg) = 42.717
I z=1120986.22343892 kg/meter^2 / by mass of my vessel (26283.86347 kg) = 42.649

It is correct ?

 

[martins]

Yes, provided that all off-diagonal values of the tensor in your source are zero, i.e.

[math]
I_{ab} = 0 \; \mathrm{if}\; a \neq b\; \forall\; a,b=\{x,y,z\}
[/math]

I guess this is the case since you only listed the diagonal elements of the tensor.

 

[Woodylepic]

Thank

Thank for the answer Martin now I can calculate PMI tansor !:thumbup: