DRDO Scientist B (RAC) Interview

Maxwell's equations and what each one says physically; boundary conditions, plane-wave propagation, skin depth, and why a waveguide has a cut-off frequency · Transmission lines and antennas: characteristic impedance, reflection coefficient and VSWR, the Smith chart idea, dipole and patch antenna basics, gain, beamwidth and the Friis link budget · Analog circuits: BJT and MOSFET operating regions, biasing, small-signal gain, op-amp ideal assumptions, negative feedback and why it improves bandwidth and linearity, oscillators and the Barkhausen criterion · Signal processing: sampling theorem and aliasing, Fourier versus Laplace versus z-transform and when each is used, FIR versus IIR filters, convolution, the DFT and FFT, window functions · Communication systems: AM and FM bandwidth, digital modulation schemes such as BPSK, QPSK and QAM, bit error rate versus Eb/N0, Shannon capacity, channel coding and why satellite links use convolutional or LDPC codes · Digital electronics and microcontrollers: combinational versus sequential logic, flip-flop timing, setup and hold, FSM design, ADC and DAC types and resolution, SPI, I2C and UART, interrupts versus polling · Control systems as applied to electronics: transfer function, poles and stability, Bode and Nyquist plots, phase and gain margin, PID tuning, PLL operation · Space and defence electronics: radiation effects on semiconductors and hardening, telemetry and telecommand, radar range equation and Doppler, GNSS basics and why NavIC exists

26+ seeded questions

Data structures and algorithms: complexity analysis, hash tables versus balanced trees, graph traversals and shortest paths, dynamic programming, and choosing a structure for a real constraint such as memory on an embedded board · Operating systems: process versus thread, scheduling policies, deadlock conditions and avoidance, virtual memory and page replacement, synchronisation primitives, what a real-time OS guarantees that Linux does not · Databases: normalisation, ACID and isolation levels, indexing and the B+ tree, query optimisation basics, transactions and recovery, when to choose a NoSQL store · Computer networks: the TCP/IP layers, TCP congestion control and reliability, UDP use cases, IP addressing and routing, DNS and HTTP, latency versus bandwidth, and why a satellite link breaks standard TCP assumptions · Computer organisation: pipelining and hazards, cache hierarchy and locality, memory-mapped I/O, interrupts, instruction-set basics, and what makes an embedded processor different from a server CPU · Software engineering for critical systems: coding standards, static analysis, unit and integration testing, version control, configuration management, and why verification matters more than features in a launch-vehicle or weapon system · Theory and compilers: regular languages and finite automata, context-free grammars and parsing, P versus NP intuition, compilation phases, and what an optimising compiler can and cannot do · Applied topics boards ask: machine learning fundamentals such as overfitting and validation, image processing basics for remote-sensing or target data, cryptography basics, parallel programming with MPI or GPUs for simulation

26+ seeded questions

Thermodynamics: the four laws and what each forbids, entropy and irreversibility, Carnot, Rankine, Brayton and Otto cycles with their T-s diagrams, why reheat and regeneration raise efficiency, exergy · Fluid mechanics: continuity, Bernoulli and its assumptions, Navier-Stokes in words, Reynolds number and the laminar-turbulent transition, boundary layers, pipe losses, drag and lift, dimensional analysis · Heat transfer: conduction, convection and radiation with their governing relations, fin efficiency, Nusselt, Prandtl and Grashof numbers, LMTD and NTU for heat exchangers, radiation shields in a spacecraft · Strength of materials and machine design: stress-strain curve, Mohr's circle, bending and torsion formulas, Euler buckling, fatigue and S-N curves, factor of safety, failure theories, fits, bearings and gears · Finite element method and vibrations: discretisation, stiffness matrix assembly, element types, convergence and mesh refinement, natural frequencies and modes, damping, resonance and why a launch vehicle is modal-tested · Manufacturing and materials: casting, forging, welding and machining choices, tolerances and GD&T, additive manufacturing, heat treatment and the iron-carbon diagram, composites and why they are used in aerospace · Rocket propulsion and IC engines: thrust equation, specific impulse, nozzle expansion and why nozzles are bell-shaped, solid versus liquid versus cryogenic engines, combustion basics, turbocharging and emissions in IC engines · Measurement and control: strain gauges, thermocouples and RTDs, pressure transducers, uncertainty and calibration, open versus closed loop, PID in a mechanical system

27+ seeded questions

Circuit theory and network analysis: KVL and KCL, Thevenin and Norton, superposition, transient response of RL, RC and RLC circuits, phasors, resonance, two-port parameters · Electrical machines: transformer equivalent circuit and losses, DC machine EMF and torque, induction-motor slip and torque-slip curve, synchronous machines and power angle, starting methods, why BLDC and PMSM dominate spacecraft and drones · Power electronics and drives: diode and controlled rectifiers, buck, boost and buck-boost converters with their duty-cycle relations, PWM inverters, MOSFET versus IGBT selection, snubbers and switching losses, battery-charging topologies · Power systems: per-unit system, load flow intuition, fault analysis and symmetrical components, protection relays and circuit breakers, stability and the swing equation, renewable integration and grid frequency · Control systems: transfer functions, root locus, Bode and Nyquist stability, gain and phase margin, state-space representation, controllability and observability, PID tuning, digital control and sampling effects · Measurements and instrumentation: bridges, CTs and PTs, error and uncertainty, digital multimeters and oscilloscopes, sensors and signal conditioning, ADC resolution and noise · Electromagnetic fields: Gauss, Ampere and Faraday laws, inductance and capacitance calculations, eddy currents and skin effect, EMI and EMC in a satellite or defence system · Spacecraft and defence power: solar arrays and MPPT, Li-ion battery sizing and depth of discharge, power distribution and bus regulation, grounding and single-point failures

26+ seeded questions

Structural analysis: determinacy, shear force and bending moment diagrams, slope-deflection and moment distribution, influence lines, the stiffness method and how it connects to FEM · Reinforced concrete and steel design: limit-state philosophy and partial safety factors, singly and doubly reinforced beams, shear and development length, columns and slenderness, steel connections, plate buckling and IS 456 and IS 800 provisions · Geotechnical engineering: soil classification, effective stress, consolidation and settlement, shear strength and Mohr-Coulomb, bearing capacity, shallow versus pile foundations for heavy launch pads and reactor buildings · Fluid mechanics and hydraulics: Bernoulli and energy losses, open-channel flow and hydraulic jump, pumps and pipe networks, hydrology basics such as unit hydrograph and return period, drainage design for large campuses · Construction materials and technology: cement chemistry and hydration, concrete mix design, durability and cover, high-performance and heavy-density concrete, formwork and prestressing, quality control on site · Dynamics and special structures: natural frequency of a structure, response spectrum and IS 1893 seismic design, wind loads and IS 875, blast-resistant design, tall launch towers and large shells · Surveying, estimation and project management: levelling and total station, GPS and remote sensing in surveys, critical path method, cost estimation and rate analysis, contract types and safety on a construction site · Environmental engineering: water and wastewater treatment sequence, BOD and COD, effluent standards, and environmental clearance for a large facility

26+ seeded questions

Classical mechanics: Lagrangian and Hamiltonian formulations, conservation laws and symmetries, central-force motion and orbits, rigid-body rotation and gyroscopic effects, small oscillations and normal modes · Electromagnetism: Maxwell's equations in differential and integral form, electromagnetic waves and polarisation, radiation from an accelerated charge, waveguides, plasma frequency and why radio blacks out during re-entry · Quantum mechanics: the Schrodinger equation, particle in a box, harmonic oscillator and hydrogen atom, operators and commutators, the uncertainty principle, spin and the Stern-Gerlach experiment, tunnelling and its use in devices · Statistical and thermal physics: ensembles and partition function, Maxwell-Boltzmann, Bose-Einstein and Fermi-Dirac distributions, blackbody radiation, entropy and the third law, phase transitions · Nuclear and reactor physics basics: binding energy curve, fission and fusion, cross-sections, neutron moderation and the four-factor formula, criticality and control rods, radioactive decay and half-life, radiation detection and dosimetry · Solid-state physics: crystal structure and reciprocal lattice, band theory and the distinction between metals, semiconductors and insulators, p-n junctions, superconductivity basics, magnetic materials · Optics and lasers: interference and diffraction, resolution limits, Fourier optics basics, laser principles and population inversion, fibre optics, spectroscopy techniques and what they measure · Experimental physics: error analysis and significant figures, vacuum techniques, cryogenics, detectors, data fitting and what a chi-squared value means

26+ seeded questions

Physical chemistry: thermodynamics and Gibbs free energy, chemical equilibrium and Le Chatelier, reaction kinetics and the Arrhenius equation, electrochemistry and the Nernst equation, phase diagrams · Quantum and spectroscopy: atomic structure, molecular orbital theory, IR, UV-visible, NMR and mass spectrometry and what each reveals about a molecule, selection rules, Raman spectroscopy · Inorganic chemistry: periodic trends, coordination chemistry and crystal field theory, organometallics, actinide and lanthanide chemistry, uranium and thorium compounds, and separation chemistry relevant to the nuclear fuel cycle · Organic chemistry: reaction mechanisms, stereochemistry, named reactions, aromaticity, polymer chemistry, and why certain propellants, binders and adhesives are chosen · Analytical chemistry: titrations and gravimetry, chromatography such as HPLC and GC, atomic absorption and ICP techniques, detection limits, calibration curves and uncertainty · Materials and solid-state chemistry: crystal structures, defects, ceramics and glasses, corrosion and its prevention, high-temperature materials for rockets and reactors, nanomaterials · Nuclear and radiochemistry basics: radioactive decay and half-life, isotopes and their separation, fission products, reprocessing chemistry such as PUREX, waste management and radiation safety · Energetic materials and propellants: oxidiser-fuel balance, specific impulse from a chemist's view, solid propellant composition, hypergolic liquids and cryogenic propellants, stability and safety testing

26+ seeded questions

Aerodynamics: lift generation and the Kutta-Joukowski theorem, boundary layers and separation, drag components, compressible flow and the Mach number regimes, shock waves and the area rule, hypersonic heating · Flight mechanics and orbital mechanics: equations of motion, static and dynamic stability, Kepler's laws, the vis-viva equation, Hohmann transfer, orbit types and why GTO is used, launch azimuth and the advantage of Sriharikota's latitude · Rocket propulsion: the ideal rocket equation and staging, specific impulse and thrust coefficient, nozzle design and expansion ratio, solid, liquid and cryogenic engines, combustion instability, thrust vector control · Aerospace structures: thin-walled structures and buckling, composites and laminate theory, fatigue and damage tolerance, aeroelasticity including flutter, modal testing of a launch vehicle · Control systems: transfer functions and state-space, stability via root locus, Bode and Nyquist, PID and LQR, Kalman filtering and sensor fusion for navigation, autopilot design and gain scheduling · Fluid dynamics and CFD: Navier-Stokes equations and their approximations, turbulence models, discretisation schemes, grid convergence and validation, when CFD can be trusted and when a wind tunnel is needed · Applied mathematics: ordinary and partial differential equations and their classification, numerical methods such as Runge-Kutta and finite differences, linear algebra and eigenvalues, probability and statistics for reliability, Fourier and Laplace transforms · Avionics and systems engineering: inertial sensors and their errors, GNSS and NavIC, telemetry, redundancy and fault tolerance, requirements and verification in a large programme

26+ seeded questions

Transport phenomena: momentum, heat and mass transfer analogies, Fick's law and diffusion, dimensionless numbers such as Reynolds, Schmidt and Sherwood, boundary layers and film theory · Thermodynamics and reaction engineering: phase equilibria and fugacity, reactor types and design equations, residence-time distribution, catalysis, exothermic reactor runaway and safety · Separation processes: distillation and the McCabe-Thiele method, absorption, extraction, membrane processes, ion exchange and solvent extraction as used in uranium purification and heavy-water production · Process control and safety: feedback control of a reactor or column, PID and cascade control, HAZOP and risk assessment, pressure relief, and why nuclear and defence plants use defence in depth · Physical metallurgy: phase diagrams including iron-carbon, diffusion and nucleation, heat treatment and TTT diagrams, dislocations and strengthening mechanisms, creep and fatigue, fracture mechanics · Extractive and process metallurgy: Ellingham diagram, iron and steel making, aluminium and titanium extraction, zirconium and uranium metal production, powder metallurgy, refining and recycling · Materials for extreme environments: zircaloy and stainless steels in reactors, radiation damage and embrittlement, superalloys and thermal barrier coatings in turbines, carbon-carbon and ceramic composites for re-entry, corrosion and hydrogen embrittlement · Characterisation and quality: microscopy, X-ray diffraction, hardness and tensile testing, non-destructive testing methods such as ultrasonic and radiography, welding metallurgy and heat-affected zones

26+ seeded questions