The Keystone 25 clamshell layout was not the first option. It was the third. This note documents the two alternatives we tested and why they were abandoned.
The first layout used a rigid rear panel with four captive fasteners providing access to the primary volume. The logic was straightforward: keep the structural shell intact, contain the servicing to a single panel. This approach is common in production systems for good reason - it's structurally efficient and manufacturing-friendly.
The failure mode emerged during simulated field service under time pressure. With gloves, four fasteners in the corners of a rear panel are manageable. Under stress, in reduced light, after the third service event, the corner fastener nearest the right hip attachment point became a consistent problem. Not because it was designed badly, but because of geometry - it was in shadow, at an awkward angle, and required a specific tool orientation that conflicted with natural grip under pressure.
This is not a fatal flaw. Many production systems live with worse. But the brief for Keystone 25 was repair-forward, and "manageable" isn't the same as "designed for service."
The second variant used a hinged side panel, replacing two of the four fasteners with a single retention latch. This reduced service time substantially and resolved the corner fastener problem. In dry conditions, it was faster to open than Variant A and easier to close securely.
The problem was environmental. The hinge mechanism accumulated debris in the pivot. This was manageable with maintenance, but required a specific cleaning step that wasn't intuitive. More significantly, the retention latch introduced a single-point-of-failure characteristic - the panel could not be secured without the latch functioning correctly.
The clamshell layout splits the chassis along the primary load axis, opening like a book. This means:
The clamshell creates its own constraints - it adds weight to the spine hardware and requires careful geometry on the load-path across the split line. Both are solved problems in the A03 revision. The spine hardware weight penalty is 47g. The load-path is managed through a bridging extrusion that transfers shear across the joint.
Whether that trade is correct depends on the use case. For the Keystone 25 brief - sustained use, field service, repair-forward - it is.
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Keystone 25 →