Start the Tunnel — AE 302 Module 5

The dirty secret of supersonic wind tunnels: turning them ON is the hard part. During startup a normal shock must sweep through the test section — and the second throat has to be big enough to swallow it (all its mass flow at reduced total pressure). Too small → the shock parks in your test section and the tunnel unstarts 💥. Too big → your diffuser recovers nothing. Design the second throat, press ▶ START, and find the knife's edge.

🎯 Test-section Mach 2.00

📏 Second throat A_t2/A_t1 1.20

Required to start: —

Status: idle

Efficiency score: —

The starting condition (Anderson §10.4):
While starting, a normal shock sits at the test-section Mach M. Total pressure drops across it: p₀₂/p₀₁ = f(M) < 1. Choked mass flow scales as ṁ ∝ p₀·A*, so to pass the SAME mass flow with LESS total pressure, the second throat must be larger:
A_t2/A_t1 ≥ p₀₁/p₀₂ = 1/(p₀₂/p₀₁) — at M = 2 that's ≥ 1.387, at M = 3 it's ≥ 3.045.
Once started (shock swallowed into the diffuser), you could in principle shrink the throat again — real tunnels use variable geometry for exactly this reason. Your efficiency score rewards the smallest throat that still starts.

Set your design, then hit ▶ START and watch the shock — will it swallow or stick?

Module 5 · Start the Tunnel