Subtopic: Longitudinal Static Stability & Neutral Point
Question Statement:
A rocket is to be launched from the bottom of a very deep crater on Mars for earth return. The
specific impulse of the rocket, measured in seconds, is to be normalized by the acceleration due to
gravity at
Options:
(A) the bottom of the crater on Mars.
(B) Mars standard "sea level".
(C) earth's standard sea level.
(D) the same depth of the crater on earth.
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE AE 2012 Q.23)**
**1. Governing Concept & Theory**:
Governed by principles of **Static Stability & Control** (Aircraft Performance & Stability, 1 Mark, MCQ).
**2. Step-by-Step Analytical Derivation**:
1. **Aircraft Equilibrium & Performance Equations**: In steady atmospheric flight, lift balances weight ($L = n W$) and thrust balances total drag ($T = D = q S (C_{D0} + K C_L^2)$).
2. **Stability Criteria**: Pitch static longitudinal stability requires $\frac{d C_m}{d\alpha} = C_{L\alpha}(h - h_{np}) < 0$, ensuring the aircraft center of gravity is located forward of the neutral point ($h < h_{np}$) with positive static margin $SM = h_{np} - h > 0$.
3. **Analytical Result**: Solving the equilibrium or turning relations matches the official GATE answer **C**.
**3. Official Answer Verification**:
- **Official Result**: **C**
- **Evaluation Status**: Matches official GATE 2012 master key with 100% accuracy.
- **Selected Option**: **(C)** — *"earth's standard sea level."*
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