# Study 02 — Stiffness Sweep (Method B)

**Type:** Parametric — CBUSH Spring-to-Ground Sweep
**Runs:** 22 (2 validation + 10 K_vert sweep + 10 K_lat sweep)
**Model:** CBUSH elements at 5 interface points, offset ground nodes (−10mm Z)

## What This Does

Replaces fixed BCs with CBUSH springs. Sweeps translational stiffness from 1e4 → 1e10 N/m on log scale. Two independent campaigns:
- Campaign 1: Sweep K_vert (laterals rigid)
- Campaign 2: Sweep K_lat (verticals rigid)

Finds the **knee point** where ΔWFE crosses the 1.0 nm budget.

## .sim Setup Required

1. Create 5 ground nodes (offset −10mm in −Z from each support node)
2. SPC 123456 on all ground nodes
3. Remove existing SPCs on 5 support nodes
4. Create 5 CBUSH elements (support → ground, global coords)
5. Create 2 PBUSH properties: 8001 (vertical, K=$K_vert) and 8002 (lateral, K=$K_lat)
6. Create 2 NX expressions: `K_vert`, `K_lat` (default = 1e12)

## Extractor

Same as Study 01: Zernike annular OPD, 50 modes, J5+ filtered, annular aperture.

## Output

ΔWFE vs K curves → knee points → recommended minimum stiffness for StarSpec.