# ISS Structural Integrity

The ISS occasionally fires thrusters to correct orbit and uses reaction control wheels to adjust attitude. It also experiences various external forces - tidal, aerodynamic, etc.

From the outside, it seems very flimsy - modules connected to each other just by the docking rings. Is that really all that holds it together? The docking rings? Or is there some sort of additional structure to give the whole thing rigidity?

• The ISS experiences aerodynamic forces? Only the drag lowering its orbit height, but aerodynamic requires air.
– Uwe
Nov 18, 2017 at 10:39
• @Uwe There is a little bit of air up there (at approx. 400 km altitude): braeunig.us/space/atmos.htm Looks like something like 10^-12 as dense as at the surface. I personally have no idea how significant that is, but it's not zero. At the very least, an answer over at Aviation indicates that drag is a significant force at that altitude: aviation.stackexchange.com/questions/34150/… Nov 18, 2017 at 12:40
• That "drag lowering its orbit height" is an aerodynamic force. Nov 18, 2017 at 12:52
• Aerodynamics uses the continuum assumption. The assumption is true for the air pressure where planes fly but not for the extreamly low pressure at a satellites orbit. At 400 km height you can't ignore the discrete molecular nature of gases. For a LEO, statistical mechanics is a more accurate method than is continuum aerodynamics. See wikipedia for aerodynamics and the Karman line at 100 km. Wings produce a lift force below the Karman line, but not at the height of the ISS
– Uwe
Nov 18, 2017 at 22:03
• Another thing to remember is that the forces applied to the station are relatively small. Even more powerful reboosts result in only fractions of a g, and drag (however you describe the mechanics) is a small force distributed across the whole frontal area of the station. Nov 20, 2017 at 18:15

The ISS is pressurized to nominally Earth sea-level atmospheric pressure - about 1013hPa (14.7 pounds per square inch). Docking rings look to be about 2.2 meters in diameter, so appear to have a cross-sectional area of about 3.8m² (5880 square inches). That means each docking ring (assuming they are all that size) is constantly under about 385 kN (86,500 lbs) of force as the internal atmosphere tries to push the mated sections apart.

To put that in perspective, the total mass of the station is about 420 metric tons (924,740 lb), (equivalent to ~4120 kN on the earth surface).

Structures needed to contain a human-friendly atmosphere are by no means flimsy, especially those built large - the forces involved can be quite substantial.

• The force on one side of the docking ring is counteracted by the force on the other side, for a net zero force. In the same way, you don't feel atmospheric pressure on your hand when you hold it out as the pressure is the same each side. Unless you mean external docking rings? Nov 21, 2017 at 8:09
• The net zero force you @SirAdelaide describe only means there's no resulting acceleration. It doesn't mean there aren't massive internal forces putting all of the ISS skin under tension. Would you say that the skin of an inflated balloon, at rest, is not under tension? I'd also suggest that you do feel atmospheric pressure on your hand, but you know no different so don't notice it. Nov 21, 2017 at 8:36
• @SirAdelaide It's not about air pressure acting on the surface of the ring itself, but over the entire surface of the pressure vessel, and the otherwise unbalanced force over the open area of the hatchway. Nov 22, 2017 at 2:13

"Is that really all that holds it together?"

Well, no.

There are several different attach mechanisms used to hold the various parts of the ISS together. The mechanism used depends on what kind of component is being used (pressurized, unpressurized) and who built it (US, Russia, Japan).

• The US pressurized modules are held to each other by Common Berthing Mechanisms (CBMs).

• The interface between the US pressurized modules and the truss is at the lab and the S0 truss segment and is called the Lab Cradle Assembly aka the Module to Truss Segment Attach System (MTSAS).

• The truss segments are held to each other by the Segment-to-Segment Attach System (SSAS) or the Rocketdyne Truss Attachment System (RTAS).

• The US side is connected to the Russian side by an Androgynous Peripheral Attachment System (APAS).

• The Russian side uses Probe & Drogue and Hybrid interface mechanisms.

• The "porch" aka the JEM-EF is attached to the JEM by the unique Exposed Facility Berthing Mechanism (EFBM).

• Excellent collection of documentation! Thank you!! Nov 18, 2017 at 2:57
• So does it work like the lens on a SLR camera? Nov 19, 2017 at 19:49
• @JDługosz Which mechanism are you asking about? Nov 19, 2017 at 21:30
• @OrganicMarble the robust physical coupling of the bayonet mount. The “alignment guide” and “latch” look familiar. None of the pictures show how they bind together when docked. Nov 19, 2017 at 22:16
• You might enjoy reading the document linked in the Segment-to-Segment Attach System part of my answer. It's where all the pictures came from, and has much more detail on each mechanism. Nov 19, 2017 at 22:23