Propulsion System Classification Questions and Answers

Answer: C

Mass flow follows density almost directly, but the colder air raises the achievable pressure ratio and cycle efficiency. The two effects oppose each other, which makes a simple density scaling conservative rather than wrong.

Answer: D

The turboprop has the poorer core yet matches the turbofan overall, because it accelerates far more air far more gently. Since the two efficiencies multiply, an advantage in one can genuinely compensate for a deficit in the other.

Answer: A

Correct answer: More than one type is needed

Read the table down the columns. Only the rocket works at rest, at Mach 5 and outside the atmosphere, but a rocket must carry its own oxidiser for the whole ascent, which is what makes single-stage runway-to-orbit so hard. The air breathers each cover part of the range and none covers it all, so a runway-launched orbital vehicle needs a combined-cycle or staged arrangement - turbine to accelerate from rest, ram or scramjet through the atmosphere, and rocket beyond it.

Answer: B

Both statements are true and the second is the reason. The turbojet's large velocity difference wastes energy in the jet at those speeds, which is why the turbofan displaced it on essentially every transport aircraft built since the nineteen sixties.

Answer: C

Both hold and the second is the reason it produces no thrust at rest. Having no compressor makes it mechanically simple and capable of very high speeds, but it cannot start itself from a standstill under any circumstances.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the whole argument. Because \( \eta_o = \eta_{th}\,\eta_p \), a gain in one factor can be cancelled by a loss in the other. Raising the turbine entry temperature improves the thermal efficiency but also raises the jet velocity, which lowers the propulsive efficiency - so the two must be pursued together, and the bypass ratio is the lever that restores the second.

Answer: A

Both statements are true and the second is the reason. With no intake there is no density term and no flight speed term in the thrust equation, which is exactly what makes the rocket usable outside the atmosphere altogether.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the cause. Specific thrust is thrust per unit mass flow, so lowering it means more air is needed for the same thrust, and more air means a larger fan and a larger nacelle. That is the price of the high bypass ratio - more weight, more drag and more ground clearance trouble, all accepted for the fuel saving.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The tip sees the vector sum of the rotational and forward velocities, so it reaches sonic conditions well before the aircraft does. Once it does, compressibility losses and noise rise sharply and the propeller’s efficiency collapses, which is what sets the practical ceiling of around four hundred knots for a conventional propeller.

Answer: D

The reason is true but it undermines the assertion rather than supporting it. At that condition the velocity increment and therefore the thrust are zero, so a practical cycle deliberately accepts some inefficiency to produce useful thrust.