JNTU B.Tech III Year AEROSPACE PROPULSION- II Previous Question Paper (March/April - 2021 - R15)

High-speed propulsion systems, specifically the Ramjet and Scramjet, are designed for supersonic and hypersonic flight regimes where traditional turbojet engines become inefficient due to excessive inlet temperatures and mechanical complexity.

1. Ramjet Engine

A ramjet operates by utilizing the forward velocity of the vehicle to compress incoming air. It consists of a diffuser, a combustion chamber, and a nozzle. The diffuser slows the supersonic intake air to subsonic speeds, increasing its pressure through the ram effect. Fuel is injected into the subsonic stream in the combustion chamber, where it burns at constant pressure. The hot gases are then expanded through a convergent-divergent nozzle to produce thrust.

Schematic layout: [Inlet/Diffuser] -> [Fuel Injectors] -> [Combustor] -> [Nozzle].

2. Scramjet (Supersonic Combustion Ramjet) Engine

A scramjet is a variant of the ramjet where the airflow through the entire engine remains supersonic. Because the air is not slowed to subsonic speeds, it avoids the extreme stagnation temperatures associated with subsonic diffusion, making it suitable for hypersonic flight (Mach 5+). The engine features a highly integrated airframe-engine geometry where the forebody acts as the compression surface and the aft-body acts as the expansion nozzle.

Schematic layout: [Supersonic Inlet] -> [Supersonic Combustor] -> [Supersonic Nozzle].

Key differences include the combustion process: the ramjet relies on subsonic combustion, while the scramjet maintains supersonic flow throughout, necessitating specialized fuel injection systems to ensure efficient mixing and combustion within the extremely short residence time of the high-velocity air stream.

The Breguet range equation relates the range of an aircraft to its aerodynamic efficiency, engine performance, and weight fraction during cruise. For a jet-powered aircraft, the cruise is typically performed at constant altitude and constant velocity, assuming the lift equals weight (L = W) and thrust equals drag (T = D).

Starting from the definition of range, R = ∫ V dt, and using the fuel consumption rate, the change in weight is related to thrust by the specific fuel consumption (SFC):

Since T = D and L = W, we can write T = W / (L/D). Substituting this into the weight rate equation gives:

Rearranging for dt:

Integrating the range equation R = ∫ V dt from initial weight W_i to final weight W_f:

Assuming constant velocity (V), lift-to-drag ratio (L/D), and specific fuel consumption (SFC), the integral becomes:

The terms involved are defined as follows:

• R: The total range of the aircraft.
• V: The cruise velocity of the aircraft.
• SFC: The thrust specific fuel consumption, defined as the fuel mass flow rate per unit of thrust.
• L/D: The aerodynamic lift-to-drag ratio, representing the efficiency of the airframe.
• W_i: The initial weight of the aircraft at the start of the cruise.
• W_f: The final weight of the aircraft at the end of the cruise.