Ramjet and Scramjet Engines Questions and Answers

Answer: B

Correct answer: 5 m/s squared

The net force is thrust minus drag, and Newton’s second law gives

Using the thrust alone gives 15 m/s squared, which is the usual error: at high speed the drag is a large fraction of the thrust, and the acceleration available is the small difference between two large numbers.

Answer: C

Correct answer: 6.7

The ratio is

Specific impulse measures thrust per unit weight flow of propellant, and the air breather scores so much better simply because the oxidiser is not counted - it is taken from the atmosphere. Since oxygen makes up most of the mass of a rocket’s propellant load, leaving it behind transforms the figure. That single advantage is what justifies the formidable difficulty of the scramjet.

Answer: D

Correct answer: More heat to be added before thermal choking occurs

Adding heat to a supersonic flow drives its Mach number down towards unity; in a constant area duct there is a definite amount of heat beyond which the flow chokes and the whole flow path is disrupted, often unstarting the inlet. Diverging the duct works in the opposite sense, accelerating supersonic flow, so the two effects can be balanced and much more heat released before sonic conditions are reached. This is why scramjet combustors are given a diverging area.

Answer: A

Correct answer: 37.5 per cent

The ramjet band spans three Mach numbers of the eight the vehicle must cover:

The remaining 62.5 per cent must come from something else - a turbine engine or booster below Mach 2, a scramjet above Mach 5 - which is exactly the argument for combined cycle propulsion. No single engine type covers the whole range.

Answer: B

Dividing gives \( 36.7/1.89 \approx 19.4 \). That steep growth with Mach number is why ram compression is negligible at low speed and dominant at high speed, and why the ramjet has such a narrow useful band at its lower end.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. A ramjet has no compressor; all of its pressure rise comes from decelerating the oncoming air, so at rest there is no compression, no pressure ratio and no thrust. Useful operation begins around Mach 2 and is good from there to about Mach 5. A rocket booster or a carrier aircraft must therefore bring the vehicle up to speed before the ramjet can take over.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the reason for the first. At Mach 8, decelerating the flow to subsonic speed would raise the stagnation temperature above 3000 K from a 220 K ambient - beyond any material, and high enough for the air to dissociate and absorb the heat that should have made thrust. Burning in the still-supersonic stream keeps the static temperature far lower, at the cost of a millisecond in which to mix and react.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and connected. Hydrogen has about 120 MJ per kilogram against kerosene’s 43, it is a superb heat sink for regenerative cooling of a structure under severe aerodynamic heating, and above all its reaction chemistry is fast enough to complete within the millisecond available. Its drawback is not technical performance but density: liquid hydrogen demands a tank several times the volume of a kerosene tank of the same energy, which is why the fuel question remains open.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second states why. On a hypersonic vehicle the forebody generates the shock system that performs much of the inlet compression, and the afterbody forms the expansion surface of a nozzle far too large to be enclosed. Engine and airframe are therefore one aerodynamic object: a change to the forebody angle alters the inlet capture and the combustor entry conditions, and a change in engine operation alters the vehicle’s pitching moment.

Answer: C

Correct answer: Statement I is incorrect but Statement II is correct

Statement II is correct and is precisely why Statement I is not. Turbine engines run out at about Mach 3 as inlet temperatures overwhelm the compressor, ramjets need about Mach 2 before ram compression is adequate and fade by Mach 5 or 6, and scramjets take over from there. No single type spans the range, so a vehicle flying from rest to hypersonic speed must hand over between engine types - the combined cycle argument.