Propulsion System Classification Questions and Answers

Answer: C

Thrust for an air breathing engine follows the mass flow it can process, which follows density. A rocket carries both propellants and takes in nothing, so neither density nor flight speed appears in its thrust equation at all.

Answer: D

Specific thrust falls rather than rises, because more air is being accelerated by less. That is exactly why a high bypass engine must process far more air and therefore be physically much larger for the same total thrust.

Answer: A

Propulsive efficiency approaches one as jet velocity approaches flight velocity, at which point the velocity increment and the thrust both vanish. A practical cycle therefore accepts some loss deliberately in exchange for thrust the aircraft can use.

Answer: B

Correct answer: Reducing the specific thrust alone

Raising the pressure ratio and the turbine entry temperature improve the thermal efficiency, and matching the jet velocity more closely to the flight velocity improves the propulsive efficiency - all three raise the product. Reducing the specific thrust on its own merely means the same thrust is produced by more air, which enlarges the engine; the gain comes only through the velocity matching that usually accompanies it, and it is that matching, not the specific thrust figure, that does the work.

Answer: C

With no compressor there is nothing to raise the pressure when the aircraft is stationary, so the engine produces no thrust whatever. It must be boosted to operating speed by a rocket or another engine before it can take over.

Answer: D

Correct answer: They achieve higher cruise speeds than turbofans

That is the one thing a turboprop cannot do. The propeller tip speed limit caps it at around four hundred knots, well below a turbofan’s cruise. The genuine reasons are the other three: better efficiency at low and moderate speeds, excellent field performance from the large slipstream over the wing, and a good match to short sectors where little time is spent at cruise anyway.

Answer: A

Because they multiply, values of 0.45 and 0.70 give 0.315 rather than 1.15, which would exceed one and be impossible. The multiplicative relation is why a weakness in either component caps the result rather than merely reducing it.

Answer: B

Decelerating hypersonic flow to subsonic speed would raise the temperature beyond any material's limit, which is precisely the difficulty the design avoids. The flow stays supersonic through the combustor, leaving milliseconds for mixing and burning.

Answer: C

A turbojet has high specific thrust and poor subsonic fuel consumption, while a high bypass turbofan has the reverse. The figure describes engine size for a given thrust rather than the economy with which that thrust is produced.

Answer: D

Correct answer: An increase in propulsive efficiency

Everything about a tip approaching sonic speed is unwelcome. Compressibility losses rise, the noise rises sharply, and the combination sets the practical cruise speed limit - those three are all consequences. Propulsive efficiency falls rather than rises, because the blade sections are no longer working efficiently and the energy goes into shock losses and noise instead of into thrust.