1. The cold equation of interfaces

Tom Godwin’s The Cold Equations is about a universe where physics does not care about intentions. A spacecraft has exactly enough fuel, and a stowaway makes the math fatal. The lesson for the desktop is similar: thermal expansion does not care about the design intent. If two parts are bolted together and one shrinks more than the other in the dark of Earth’s shadow, something will move or crack.

This entry begins a reading sweep on how mechanical interfaces behave under thermal-vacuum cycling.

2. What thermal-vacuum cycling does

In low Earth orbit, a surface in sunlight can reach well above 100 °C, and the same surface in shadow can drop below −150 °C. The cycle repeats roughly every 90 minutes. For a mechanical interface, this means:

  • Differential expansion: materials with different coefficients of thermal expansion move relative to each other.
  • Pressure changes: any trapped gas or volatile material outgasses.
  • Cold welding: clean metal surfaces in vacuum can stick together under contact pressure.
  • Lubricant migration: greases and oils behave differently in vacuum and may evaporate or creep.

These effects are well known in spacecraft qualification, but their magnitude depends on the specific geometry and materials.

3. The claim in the literature

The literature generally claims that mechanical interfaces can survive thermal-vacuum cycling if they are designed for it. The supporting evidence includes:

  • Heritage hardware: ISS ORUs, solar array drives, and docking mechanisms have operated for years after repeated cycling.
  • Test standards: ECSS and NASA standards define thermal-vacuum test profiles and acceptance criteria.
  • Material databases: CTE, conductivity, and outgassing data are available for common spacecraft materials.

The claim is less strong for novel interfaces that combine multiple functions — mechanical, thermal, electrical, and data — in a compact volume.

4. Confidence assessment

  • Heavily supported: individual materials and simple interfaces survive TVAC cycling.
  • Moderately supported: multi-functional interfaces can be designed to survive, but require testing.
  • Less supported: the specific rack-to-cell interface, with its combined loads, has no flight heritage yet.

5. What this changes

  • Thermal-vacuum behavior is a real concern for the attachment interface.
  • Heritage supports the idea that survival is achievable.
  • The desktop’s specific interface must be tested, not assumed.