A pure ramjet engine cannot generate static thrust at zero flight velocity ($V_\infty = 0$, Mach 0) because:

(A) It lacks mechanical turbomachinery (compressor) to provide pre-combustion pressure rise without forward motion

(B) The fuel cannot vaporize at ambient sea-level temperatures

(C) The convergent-divergent nozzle chokes instantaneously at zero speed

(D) The flame speed exceeds the speed of sound inside the combustor

Answer Key:

A

### 5-Stage Derivation & Analytical Solution: 1. **Stage 1: Compression Mechanism:** Unlike turbojets, a ramjet has no rotating compressor blades. 2. **Stage 2: Stagnation Pressure Ratio:** Pre-combustion pressure ratio is given by $p_{0\infty}/p_\infty = (1 + \frac{\gamma-1}{2} M_\infty^2)^{\frac{\gamma}{\gamma-1}}$. 3. **Stage 3: Static State ($M_\infty = 0$):** At standstill, $p_{0\infty} / p_\infty = 1.0$. The cycle has zero pressure ratio and therefore produces zero net thrust. 4. **Stage 4: Selection:** Option A correctly describes the physical requirement for a booster (solid rocket / turbojet) to accelerate ramjets to operating speeds ($M > 2$). 5. **Stage 5: Reference:** Jack D. Mattingly, *Elements of Propulsion*, Chapter 4.

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