Which of the following statements regarding ramjet propulsion systems are **CORRECT**?

(A) A ramjet has zero static thrust at launch ($V_0 = 0$) because it lacks rotating turbomachinery to produce compression

(B) The combustion process in a conventional ramjet takes place at subsonic flow speeds ($M \approx 0.2$ to $0.3$)

(C) Thermal choking in a constant-area ramjet combustor occurs when heat addition drives the exit Mach number to $M = 1$

(D) A ramjet can operate efficiently at hypersonic Mach numbers ($M_0 > 8$) without dissociation losses

Answer Key:

Verified in official key

### 1. Strategy & Theoretical Principles 1. **Zero Static Thrust:** A ramjet relies entirely on the forward momentum of the aircraft (ram pressure recovery) to compress incoming air. At $V_0 = 0$, ram compression is zero, resulting in zero static thrust. Hence, ramjets require an auxiliary booster (e.g. solid rocket) to accelerate to operating speed ($M_0 > 2$). 2. **Subsonic Combustion:** Incoming supersonic air is decelerated through the intake diffusers to low subsonic Mach numbers ($M \approx 0.2 - 0.3$) before entering the combustor to ensure flame stabilization and minimize stagnation pressure loss. 3. **Thermal Choking:** By Rayleigh flow theory, heat addition to a subsonic stream increases its Mach number until $M = 1$ (thermal choking), which chokes the engine intake. 4. **Hypersonic Limit ($M_0 > 8$):** Decelerating flow from $M_0 > 8$ to subsonic speeds produces extreme static temperatures ($T > 3000\text{ K}$), causing massive molecular dissociation ($O_2 \to 2O$, $N_2 \to 2N$), which absorbs combustion energy and severely degrades thrust. This necessitates supersonic combustion (Scramjet). ### 2. Statement Evaluation - **Statements A, B, C:** **CORRECT.** - **Statement D:** **INCORRECT.** Severe dissociation occurs; scramjets must be used instead. ### 3. Exam Trap & Standard Reference - **Trap Alert:** Ramjets cannot generate static thrust at rest ($F_0 = 0$). - **Standard Reference:** Heiser, W.H. & Pratt, D.T., *Hypersonic Airbreathing Propulsion*, Chapter 1.

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