Which of the following process(es) is/are involved in the compression of air in an
ideal ramjet engine?
Options:
(A) oblique shock
(B) mechanical compression
(C) normal shock
(D) subsonic diffusion
Official Answer Key:
A, C, D
Step-by-Step Solution & Explanation
### **Detailed Step-by-Step Solution (GATE AE 2026 Q.44)**
**1. Intuitive Physical Setup & Given Parameters**:
An ideal ramjet engine operates at supersonic flight speeds without any rotating turbomachinery (no compressor or turbine).
We need to identify all aerodynamic compression processes involved in decelerating freestream supersonic air to the subsonic combustor inlet (MSQ).
**2. Step-by-Step Aerodynamic & Propulsion Derivation**:
1. **Supersonic Diffuser / Inlet Architecture**:
- In a supersonic ramjet intake, compression occurs purely through aerothermodynamic deceleration:
1. **Oblique Shocks**: Formed on the external centerbody spike or ramps to decelerate the flow efficiently from high supersonic Mach number with minimal stagnation pressure loss. Statement A is **TRUE**.
2. **Normal Shock**: Occurs at or near the throat to transition the flow from supersonic to subsonic regime. Statement C is **TRUE**.
3. **Subsonic Diffusion**: A diverging subsonic diffuser downstream of the terminal shock further decelerates the subsonic flow, converting dynamic pressure into static pressure prior to entering the combustion chamber. Statement D is **TRUE**.
2. **Mechanical Compression**:
- A ramjet engine has **no rotating parts** or mechanical compressors (unlike turbojets or turbofans).
- All compression is achieved aerodynamically through ram effect and diffusion.
- Therefore, mechanical compression is **NOT involved**. Statement B is **FALSE**.
**3. Comprehensive Options & Distractor Analysis**:
- **(A) oblique shock**: ✅ **CORRECT**. Used for efficient external supersonic compression.
- **(B) mechanical compression**: ❌ **INCORRECT**. Ramjets have no rotating compressor blades.
- **(C) normal shock**: ✅ **CORRECT**. Terminal shock that brings flow to subsonic speeds.
- **(D) subsonic diffusion**: ✅ **CORRECT**. Divergent duct that lowers velocity to burner inlet speeds (~Mach 0.2).
**4. Official Master Key & Exam Takeaway**:
- **Official Answer Key**: **A, C, D**
- **Exam Takeaway**: Ramjet compression is purely aerodynamic: Oblique Shocks $\to$ Terminal Normal Shock $\to$ Subsonic Divergent Diffusion.