Mechanical Engineering for ISRO Scientist/Engineer 'SC'
What this domain tests
The ISRO Scientist/Engineer 'SC' (Mechanical) paper checks core mechanical engineering - thermodynamics, fluid mechanics, strength of materials, theory of machines, heat transfer, manufacturing and machine design - applied to launch-vehicle and satellite hardware: propulsion cycles, cryogenic turbopumps, lightweight stiffened structures, stage separations and vibration control. Expect numericals with space-relevant numbers (high Mach, cryogenic temperatures, thin-walled shells).
Core rules and concepts
- Steady-flow energy per unit mass: q − w = Δh + ΔKE + ΔPE. In a nozzle q = w = 0, so exit velocity follows from the enthalpy drop.
- Isentropic relations: T₂/T₁ = (p₂/p₁)^((γ−1)/γ). Choked mass flow through a throat: ṁ = A·p₀·√(γ/(R·T₀))·(2/(γ+1))^((γ+1)/(2(γ−1))).
- Beam bending: σ = M·y/I. Maximum transverse shear is 1.5·V/A for a rectangle and 4V/3A for a solid circle; thin-walled pressure vessels give σ_hoop = p·r/t.
- Rocket equation: Δv = Isp·g₀·ln(m₀/m_f); thrust F = ṁ·V_e + (p_e − p_a)·A_e.
- Use von Mises or Tresca yield criteria for ductile design, and Mohr's circle for principal stresses under combined loading.
Worked example
Combustion gas (γ = 1.2, R = 350 J/kg·K, T₀ = 3000 K) expands through a nozzle to Mach 3. Static temperature: T/T₀ = 1/(1 + 0.5(γ−1)M²) = 1/(1 + 0.1·9) = 1/1.9, so T ≈ 1579 K. Exit speed: V = M·√(γRT) = 3·√(1.2·350·1579) ≈ 3·814 ≈ 2440 m/s. The reasoning point is that the local speed of sound uses static temperature, not stagnation temperature.
Traps that cost marks
- Substituting T₀ into a = √(γRT) in high-speed nozzle problems.
- Mixing gauge and absolute pressures in NPSH and cavitation questions.
- Assuming choked flow depends on downstream pressure - it does not.
- Sign errors in shear-force and bending-moment diagrams, and in Mohr's circle rotations.
- Treating Isp (s) and effective exhaust velocity (m/s) as interchangeable without the g₀ factor.