Ramjet and Scramjet Engines Questions and Answers

Answer: A

Forward speed alone supplies the compression that a turbojet obtains from its compressor. Removing the rotating machinery makes the engine mechanically simple and light, and allows flight speeds no turbojet could reach.

Answer: B

With no compressor there is nothing to raise the pressure when the aircraft is stationary. The engine must be accelerated to a useful speed by a rocket booster or a separate engine before it can begin to produce thrust of its own.

Answer: C

Below that speed the pressure ratio from ram effect alone is too small for a useful cycle. Efficiency climbs steadily above it, which is why ramjets are associated with the Mach 2 to 5 band rather than with any lower speed.

Answer: D

Correct answer: The temperature after full deceleration

A ramjet decelerates the captured air to subsonic speed before burning, so the combustor sees the stagnation temperature

At Mach 5 from 220 K that is already 1320 K before any fuel is added; by Mach 8 it is over 3000 K, at which the structure cannot survive, the air begins to dissociate, and adding heat yields little further thrust. The limit is thermal, not structural or a matter of fuel supply, and it is precisely the limit the scramjet exists to escape.

Answer: A

The inlet decelerates the flow through shocks to a comfortable subsonic Mach number before fuel is added. Burning in subsonic flow is far easier to achieve, which is why the ramjet preceded the scramjet by several decades.

Answer: B

Correct answer: In supersonic flow

The scramjet - supersonic combustion ramjet - decelerates the captured air only partially, so it enters the combustor still supersonic, perhaps at Mach 2 or 3 from a flight Mach number of 8. Burning in that stream avoids the enormous stagnation temperature that full deceleration would produce. The price is that the fuel has milliseconds in which to mix, ignite and burn, which is the central difficulty of the whole concept.

Answer: C

A bluff body or a step leaves a sheltered wake in which hot products linger and ignite the oncoming flow. Without such a region the flame would simply be swept downstream even at the reduced subsonic combustor velocity.

Answer: D

Correct answer: A few milliseconds

A combustor a metre or two long with flow passing at one to two kilometres per second gives a residence time of the order of a millisecond:

Everything - injection, penetration into the core flow, molecular mixing, ignition and heat release - must be complete within that window. It is why hydrogen, with its very fast chemistry, is the reference fuel, and why so much scramjet work is about injector geometry and mixing enhancement.

Answer: A

Rapid reaction chemistry suits the millisecond residence time, and the fuel can absorb a great deal of heat before combustion. Its low density and cryogenic storage are severe practical drawbacks, which is why hydrocarbon scramjets remain of interest.

Answer: B

Passing the fuel through structural passages absorbs heat that would otherwise destroy the airframe and preheats the fuel for combustion. It is one of the few thermal management options available when surface temperatures reach such levels.