# Where is the center of mass of the ISS relative to it's internal coordinates?

I'd like to find a fairly precise location of the center of mass of the ISS. I understand it moves for several reasons, including shifting of loads, flexing, and thermal expansion, but to say 1 meter precision.

I'm guessing it falls within the main structural truss, within Truss Segment Zeros S0, near where the "crew axis" crosses, but I haven't found anything definitive.

I'd like to plot the microgravity field in 3D using Blender. It is calculated referenced to the ISS orbit, but it is pinned to the ISS at the stations center of mass.

above: International Space Station, from here.

above: Handy little 3ds model of ISS imported into Blender, from here.

above: 2D Vector plot of acceleration relative to the orbit in a given plane.

The information you need can be found in On-Orbit Assembly, Modeling, and Mass Properties Data Book, Volume I & II, International Space Station Program, January 2008 (JSC 26557) (you need an updated version for more current data, here it's Revision AB).

First, the Space Station Analysis Coordinate System (SSACS) is defined as follows (see page 26):

The origin is located at the geometric center of the mid-ship Integrated Truss Segment (ITS) S0. The longitudinal x-axis of multiple core modules, including the Zarya Functionalni Gruzvoi Blok (FGB) and Unity Node 1, is parallel with the analysis coordinate system axis XA, positive in the direction of the velocity vector. Positive YA axis runs parallel with the starboard truss from the center point at S0. Axis ZA completes the triad, pointing to the nadir.

The center of mass positions are then given in (see page 82)

Distance along X/Y/Z-axis from origin in SSACS

The largest part of this document is then devoted to giving you the information about the different configurations of the ISS. For example, for the configuration after the separation the shuttle during STS-132 (see page 542), you get:

Center of mass:

X:
-4.37 m (-14.34 ft)

Y:
-.81 m (-2.66 ft)

Z:
3.43 m (11.25 ft)

Note that the document I linked above ends after STS-133 (early March 2011). That's after most of the assembly was finished, but about 8 t of equipment was since installed, most of it during STS-134, so you may have to look for a more current version of the document if you need high precision information on the current state of the station.

• Wow great! It will take me a bit of time to look through this, thank you very much!! Briefly and roughly speaking, the CG is trailing a few meters "behind and below" the spacecraft origin, to using completely imprecise, non-engineering language. I promise to do my due diligence and read through. Thank you very much for such a thorough answer!!
– uhoh
Feb 25, 2017 at 13:11
• It's great to see a Blender user here by the way, especially one who likes spacecraft!
– uhoh
Feb 25, 2017 at 13:37