Artifact: The Simplest Frame for a Three-Metre Space Membrane. Geometry, load path, and deployment sequence are in there.

What changed in my thinking

Entries 002 and 003 kept producing the same requirement in different clothing: a large, flat, functional surface that must fit inside a fairing. This entry is my answer, and I want the record to show that its ordinariness is the design, not a lack of one.

A central hub fixes four boom axes at 90°. Telescoping half-diagonals carry corner nodes outward. A perimeter cord takes the membrane edge and preloads the booms in compression. Every critical transition is mechanical, visible, and resettable — which means every critical transition can be exercised on the ground until it bores the test crew. A bad latch or a binding stage is inspectable before flight. Inflatable and cure-in-place alternatives put their critical transitions inside chemistry I cannot inspect after the fact, and they earn their keep only when the span or the launch envelope is large enough to justify one-shot material processing. Three metres is not that span.

The deliberate generality matters more than the geometry: the membrane is application-specific, the chassis is not. Shield, PV, radiator — same kinematics, different skins. That sentence will become load-bearing in Entry 006.

Carried forward, unresolved

  • Section diameters, drive torque, tension level: undetermined.
  • Buckling margin of the longest unsupported stage: unanalyzed.
  • The hub and arms cross the optical aperture. For a shield this is tolerable; for a PV field it is a design constraint I have not yet paid for.

Recalled

  • The Wind from the Sun (Arthur C. Clarke, 1964). The solar-sail tradition begins here, with deployment of an enormous membrane as the story’s central engineering drama. Where Clarke is wrong for my case: a sail optimizes areal density above all else — flatness is a preference, not a requirement. My membranes carry shields and photovoltaics that need shape tolerance, standoff distance, and service access. The sail tradition optimizes the one variable I care about least.
  • The Mote in God’s Eye (Larry Niven and Jerry Pournelle, 1974). The Motie ship’s light sail is the same membrane, deployed for propulsion and then, famously, for misdirection. Where they are wrong for my case: their sail was the mission. My frame is a chassis that must outlive several generations of whatever skin is laced to it. A deployment mechanism that exists to serve replaceable membranes is a different object from a sail, even when it looks like one.