JNTU B.Tech IV ELEMENTS OF ROCKET PROPULSION Previous Question Paper (September - 2022 - R18)

The shape of a nozzle, specifically the contour of the divergent section, is critical in determining the efficiency of the conversion of thermal energy into kinetic energy. A conical nozzle is simple to manufacture but suffers from divergence losses because the exhaust gases are not directed purely axially, resulting in a momentum loss proportional to the cosine of the divergence half-angle. In contrast, a bell-shaped nozzle (or contoured nozzle) uses a parabolic or optimized curve to redirect the flow toward the axis, significantly reducing divergence losses and shortening the overall nozzle length for a given expansion ratio.

Thrust loss in an over-expanded nozzle occurs when the ambient pressure is higher than the nozzle exit pressure, leading to flow separation and shock waves within the nozzle that create asymmetric pressure distributions and potential structural damage. To overcome this, several methods are employed:

• Dual-Bell Nozzles: These feature a discrete step in the nozzle contour. At low altitudes, the flow separates at the step, effectively creating a smaller expansion ratio. At high altitudes, the flow attaches to the full wall, providing a larger expansion ratio.
• Altitude Compensating Nozzles (ACN): Designs like the plug nozzle or aerospike allow the ambient air to act as a flexible boundary, effectively adjusting the expansion ratio automatically as ambient pressure changes.
• Extendable Nozzle Extensions: These mechanical systems deploy an additional nozzle section after the vehicle has reached a certain altitude, transitioning from a low-expansion ratio configuration to a high-expansion ratio configuration to optimize performance across the flight trajectory.
• Gas Injection: Injecting secondary gas into the nozzle can force flow separation at a controlled location, preventing the formation of damaging internal shock structures.