A mixed-compression supersonic intake operates at a design Mach number $M_0 = 3.0$. Which of the following statements regarding supersonic intake aerodynamics are **CORRECT**?

(A) Replacing a single normal shock with a series of multiple oblique shocks followed by a weak terminal normal shock significantly increases total pressure recovery

(B) During intake unstart, the terminal normal shock is abruptly expelled upstream of the cowl lip, resulting in a severe drop in mass flow rate and total pressure recovery

(C) In a scramjet engine, flow deceleration across the inlet is maintained entirely subsonic before entering the combustor

(D) Spillage drag occurs when an intake operates at sub-critical conditions where the shock structure sits ahead of the cowl lip

Answer Key:

Verified in official key

### 1. Strategy & Theoretical Principles 1. **Supersonic Inlets & Total Pressure Recovery:** A normal shock at $M_0 = 3.0$ causes an enormous loss of stagnation pressure ($p_{02} / p_{01} = 0.328$). By decelerating through multiple oblique shocks (each having $M_{1n} \ll M_0$), total pressure recovery is elevated to over $0.70-0.80$. 2. **Inlet Unstart:** In mixed-compression inlets, if the backpressure rises or internal boundary layer separation occurs, the internal terminal shock is violently disgorged out of the duct ("unstart"), accompanied by severe drag surge and buzz oscillations. 3. **Ramjet vs Scramjet:** In a **ramjet**, combustion is subsonic ($M_{\text{comb}} \approx 0.2 - 0.3$). In a **scramjet** (Supersonic Combustion Ramjet), the flow throughout the entire combustor remains **supersonic** ($M_{\text{comb}} > 1$) to avoid extreme dissociation and pressure losses. 4. **Spillage Drag:** Under sub-critical conditions, the pre-entry streamtube area is less than the capture area, forcing excess flow to spill around the external cowl lip, producing spillage drag. ### 2. Statement Evaluation - **Statement A:** **CORRECT.** Multiple oblique shocks maximize isentropic efficiency and total pressure recovery. - **Statement B:** **CORRECT.** Inlet unstart expels the shock system, devastating engine thrust. - **Statement C:** **INCORRECT.** Scramjet combustor operates with supersonic flow, not subsonic flow. - **Statement D:** **CORRECT.** Subcritical operation produces flow deflection and spillage drag. ### 3. Exam Trap & Standard Reference - **Trap Alert:** Remember the fundamental difference between ramjet (subsonic combustion) and scramjet (supersonic combustion). - **Standard Reference:** Heiser, W.H. & Pratt, D.T., *Hypersonic Airbreathing Propulsion*, AIAA Education Series.

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