Ascíël coupler: the standard design, in modular habitat and starship architecture, for the coupler that binds adjacent modules into a single unit.
For such semi-permanent connections on a large scale, simple docking adapters are obviously unsuitable; tidal forces and other stresses common in large structures may cause a simple docking adapter to be stressed sufficiently to separate over time, and starship-level thrust applied to a modular design would cause near-immediate failure.
A variety of designs (often based on existing railroad couplers) were tried to prevent this while also avoiding the expense, wasted time, and potential damage involved in bolting or welding additional reinforcement onto the modules, with varying degrees of success, eventually converging on the modern Ascíël coupler.
The Ascíël coupler, as defined in IOSS 64212, makes use of the IUSI androgynous docking adapter (as defined in IOSS 52114) to achieve initial connection. (As such, it too comes in the three there-defined standard sizes.)
Once hard dock has been achieved, the surrounding coupler engages a nested pair of counterrotating helical screws, which intertwine from each side of the coupler to form a solid bond between the modules. Once the screws have advanced to the fully engaged position, twelve locking rods (six per screw, three being managed by each coupler) are electromagnetically released and are forced by springs into their extended position through holes in the screws, preventing them from rotating and thus from working loose over time.
When fully engaged, an Ascíël coupler has an effective strength equivalent to that of the surrounding module hull.
– A Star Traveler’s Dictionary