JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV LAUNCH VEHICLE AND MISSILE TECHNOLOGY Previous Question Paper (July/August - 2022 - R15)

Guidance and control systems are fundamental to the success of launch vehicles and missiles, ensuring they reach their intended target or orbit with precision. While guidance determines the required trajectory, control executes the necessary maneuvers to follow that path.

Guidance System: The guidance system acts as the 'brain' of the vehicle. It continuously calculates the vehicle's current position and velocity relative to the target or orbit using inertial measurement units (IMUs), GPS, or star trackers. It then determines the necessary steering commands to minimize the deviation from the pre-programmed trajectory. Common guidance laws include:

• Proportional Navigation (PN): Commonly used in missiles, where the rate of turn of the velocity vector is proportional to the line-of-sight rate.
• Inertial Guidance: Uses accelerometers and gyroscopes to track position through dead reckoning, independent of external signals.
• Terminal Guidance: Used in the final phase of flight to achieve high accuracy, often employing seekers (radar, infrared, or optical).

Control System: The control system acts as the 'muscles,' executing the commands generated by the guidance system. It stabilizes the vehicle against external disturbances like wind gusts or atmospheric turbulence. The primary methods for achieving control include:

• Thrust Vector Control (TVC): The most common method for launch vehicles, where the rocket engine nozzle is gimbaled (tilted) to change the direction of the thrust vector, creating a control moment.
• Aerodynamic Control Surfaces: Fins or canards that use aerodynamic forces to steer the vehicle, primarily effective within the atmosphere.
• Reaction Control Systems (RCS): Small thrusters located at the periphery of the vehicle used for attitude control, especially in the vacuum of space where aerodynamic surfaces are ineffective.

The integration of these systems forms a closed-loop feedback mechanism: the guidance system senses the state, computes the error, and sends a signal to the control system, which adjusts the vehicle's orientation or thrust direction. This continuous cycle ensures the vehicle maintains stability and achieves the mission objectives despite environmental variations and internal perturbations.

Launch vehicles require materials that offer high strength-to-weight ratios, excellent thermal stability, and resistance to extreme environmental conditions. The primary classes of materials used include:

1. Aluminum Alloys: Specifically 2000 and 7000 series, used for airframes and propellant tanks due to their low density and high strength.

2. Titanium Alloys: Used in high-temperature zones and pressure vessels due to superior corrosion resistance and strength at elevated temperatures.

3. Composite Materials: Carbon fiber reinforced polymers (CFRP) are extensively used for rocket motor cases, interstages, and fairings to significantly reduce structural mass.

4. Superalloys: Nickel-based alloys (e.g., Inconel) are critical for engine components, nozzles, and combustion chambers where extreme heat and pressure occur.

5. Thermal Protection Systems (TPS): Ablative materials like silica-phenolic composites or ceramic tiles are used to dissipate heat during atmospheric re-entry or high-speed flight.

The selection procedure involves a systematic evaluation based on the following criteria:

1. Structural Requirements: Determining the yield strength, ultimate tensile strength, and fatigue life required to withstand launch loads and vibration.
2. Thermal Performance: Evaluating the material's melting point, thermal conductivity, and coefficient of thermal expansion to ensure integrity during aerodynamic heating and engine firing.
3. Density and Mass Efficiency: Prioritizing materials with the lowest density that meet structural requirements to maximize payload capacity.
4. Environmental Compatibility: Assessing resistance to cryogenic temperatures (for liquid oxygen/hydrogen tanks) and chemical compatibility with propellants.
5. Fabrication and Cost: Considering the ease of manufacturing, weldability, and the overall cost-to-performance ratio, as complex aerospace-grade materials often require specialized machining or autoclave curing processes.
6. Reliability and Testing: Ensuring the material has a well-documented history of performance under flight conditions, often verified through non-destructive testing (NDT) and structural qualification tests.