Question

From the options given, select all that are true for turbofan engines with afterburners.

• A. Turning afterburner ON increases specific fuel consumption.
• B. Turbofan engines with afterburners have variable area nozzles.
• C. Turning afterburner ON decreases specific fuel consumption.
• D. Turning afterburner ON increases stagnation pressure across the engine.

Hint:

Afterburners = high thrust but poor efficiency. Always paired with variable–area nozzles; they increase stagnation temperature, not pressure.

Answer 1

The Correct Option is A, B

Step 1: Afterburner principle.
An afterburner injects fuel downstream of the turbine, using oxygen in the core stream to produce extra thrust by reheating. It is inefficient (high SFC), but gives large thrust augmentation for short duration (military).

Step 2: Evaluate each statement.
(A) True: Afterburner ON dramatically increases fuel consumption per thrust produced, so SFC rises.
(B) True: Variable area nozzle is essential. Afterburning raises jet temperature and volume flow; nozzle throat must enlarge to prevent choking and excessive back–pressure on turbine.
(C) False: Afterburning never reduces SFC; it always increases it.
(D) False: Afterburner increases stagnation temperature, not stagnation pressure. In fact, pressure drops across the burner due to frictional losses.

Final Answer:
\[ \boxed{(A),\ (B)} \]