Enforcement Officer / Accounts Officer (EO/AO) 2017 recruitment test. Transcribed from the Commission's own booklet (series A, GRS-A-TNS); answer as per the official UPSC final answer key. This question is in Part B. Answer last reviewed 24 August 2026.

ISRO in August 2016 successfully test launched Supersonic Combustion Ramjet (Scramjet) Engine. Which of the following statements with regard to Scramjet Engine is/are correct? 1. It can efficiently operate both in subsonic and supersonic combustor modes. 2. India is the first country to demonstrate the flight testing of a Scramjet Engine. Select the correct answer using the code given below.

(a)1 only
(b)2 only
(c)Both 1 and 2
(d)Neither 1 nor 2

Answer

Why

Correct — A, (a) 1 only. Taking the two numbered statements separately: Statement 1 — correct. The engine ISRO is developing under its air-breathing propulsion programme is a dual-mode ramjet. In that configuration a single duct works two ways. At lower flight speeds it behaves as a ramjet: the vehicle's own forward motion rams and compresses the incoming air, a diffuser slows that air to subsonic speed, and the fuel is burnt in a subsonic stream. As speed rises, slowing the air down that far becomes self-defeating — the losses and the temperatures grow unmanageable — so the engine transitions to scramjet operation, in which the fuel is injected and burnt while the air is still moving supersonically through the combustor. The changeover happens over roughly the Mach 4 to Mach 8 range, and the ability to work efficiently on both sides of it is exactly what statement 1 asserts. Note the careful wording: the statement speaks of what the engine can do, not of what the single test flight showed, and the August 2016 flight itself demonstrated the supersonic-combustion mode at Mach 6. Statement 2 — incorrect, and this is the statement the item was written for. India was the fourth country to demonstrate the flight testing of a scramjet engine, not the first; Russia, the United States and China had already flown such engines on hypersonic test flights. The claim of being first is the kind of exaggeration that spreads through headlines when a genuine achievement is reported, and a candidate who takes it at face value marks (c) instead of (a). So statement 1 alone is correct, which is option (a). The flight itself is worth holding as a set of facts. The Advanced Technology Vehicle, a two-stage sounding rocket, was launched on 28 August 2016 from the Satish Dhawan Space Centre at Sriharikota. About 55 seconds into the flight the two scramjet engines ignited at Mach 6 and burned for roughly five seconds. The engine uses hydrogen as fuel and takes oxygen from the atmosphere as its oxidiser. ISRO recorded that four critical technologies had been demonstrated: igniting an air-breathing engine at supersonic speed, holding the flame at supersonic speed, the air intake mechanism, and the fuel injection system.

Why the others are wrong

(b)2 only — This gets both statements the wrong way round. It rejects the dual-mode capability, which is a real property of the engine ISRO is building, and it accepts the claim that India was the first country to flight-test a scramjet, which is false — India was the fourth. A candidate arriving here has probably not read statement 1 carefully at all and has simply attached the answer to the more memorable-sounding of the two claims.
(c)Both 1 and 2 — This is the trap the item exists for. Statement 1 is correct, so a candidate who has verified it and then reads a flattering claim about India being first is strongly tempted to accept both. But the record is specific: India became the fourth country to demonstrate the flight testing of a scramjet engine, after Russia, the United States and China. The lesson generalises well beyond this item — on a two-statement question, a claim that a country is 'the first in the world' to do something should be treated as the statement most likely to be planted, and checked against a remembered ordinal rather than against a general impression of significance. Being fourth in a field of four is a genuine achievement; it is simply not the same claim.
(d)Neither 1 nor 2 — This is the over-correction. A candidate who knows that India was not the first to flight-test a scramjet may conclude that the entire item is a trap and reject both statements. That discards a statement that is true: the dual-mode ramjet does operate in both subsonic-combustion and supersonic-combustion modes, and that dual capability is central to the design. Suspicion is not a method; each statement has to be judged on its own, which is why the 'neither' branch of a two-statement code is chosen correctly far less often than it is chosen.

Concept

A rocket carries both its fuel and its oxidiser, and the oxidiser is much the heavier part of the load. An air-breathing engine takes its oxygen from the atmosphere instead, so for the portion of the flight that is inside the atmosphere the vehicle can carry far less mass — which is why air-breathing propulsion is treated as one of the routes to cheaper access to space and to a reusable launch vehicle. Three engine families matter. A turbojet uses a rotating compressor and works to moderate supersonic speeds. A ramjet has no moving compressor at all: the vehicle's forward speed rams air into an inlet, a diffuser decelerates it to subsonic speed and raises its pressure, and fuel is burnt in that subsonic stream; a ramjet cannot start from rest and loses efficiency beyond roughly Mach 5 to 6, because slowing hypersonic air to subsonic speed destroys energy and produces extreme temperatures. A scramjet — supersonic combustion ramjet — leaves the air supersonic through the combustor, which avoids those losses but makes the engineering brutal: the fuel has a few milliseconds to mix, ignite and burn while the air rushes past, so ignition and flame-holding are the hard problems. A dual-mode ramjet spans both regimes. ISRO's demonstration flew on 28 August 2016 aboard a two-stage Advanced Technology Vehicle sounding rocket from Sriharikota, ignited its engines at Mach 6, burned for about five seconds using hydrogen fuel and atmospheric oxygen, and made India the fourth country to flight-test a scramjet engine. It sat alongside the reusable launch vehicle technology demonstrator flown earlier in 2016, in the same programme of work on cheaper launch.

Space and defence technology items reach EPFO papers as two-statement questions far more often than as single-fact questions, because the format lets the examiner pair one technical claim with one claim about precedence or priority. The precedence claim is where candidates come unstuck. National achievements are routinely reported as firsts, and candidates absorb the headline rather than the ordinal; the discipline is to hold the specific number — fourth country, third country, first in Asia — for each achievement, because that number is exactly what the examiner will test. The technical claim, by contrast, is usually a fair description of how the device works and can be reasoned about.

Key facts

ISRO flight-tested its scramjet engine on 28 August 2016 from the Satish Dhawan Space Centre, Sriharikota.
The engines were carried by a two-stage Advanced Technology Vehicle sounding rocket.
They ignited at Mach 6 about 55 seconds into the flight and burned for roughly five seconds.
The engine uses hydrogen as fuel and takes oxygen from the atmosphere as the oxidiser.
Technologies demonstrated: ignition of an air-breathing engine at supersonic speed, flame-holding at supersonic speed, the air intake mechanism and the fuel injection system.
India was the fourth country to demonstrate the flight testing of a scramjet engine; Russia, the United States and China had done so earlier.
A ramjet decelerates incoming air to subsonic speed before combustion; a scramjet burns fuel in air that remains supersonic.
A dual-mode ramjet operates in both modes, transitioning over roughly the Mach 4 to Mach 8 range.
Air-breathing propulsion matters because a rocket's oxidiser is the greater part of its propellant mass.
'Scramjet' is short for supersonic combustion ramjet, which the paper's stem spells out in full before glossing it.

One technical claim and one claim about precedence — the second is where the item is planted

1. Dual-mode operation — correct→The engine is a dual-mode ramjet, a single duct working two ways. At lower flight speeds it behaves as a ramjet: the vehicle's own forward motion rams and compresses the incoming air, a diffuser slows that air to subsonic speed, and the fuel is burnt in a subsonic stream.

Why it has to change over→As speed rises, slowing the air down that far becomes self-defeating — the losses and the temperatures grow unmanageable — so the engine transitions to scramjet operation, burning fuel while the air is still moving supersonically through the combustor. The changeover happens over roughly the Mach 4 to Mach 8 range.

2. 'First country' — incorrect→India was the fourth country to demonstrate the flight testing of a scramjet engine; Russia, the United States and China had already flown such engines on hypersonic test flights. Being fourth in a field of four is a genuine achievement — it is simply not the same claim.

The rule this generalises into→On a two-statement question, a claim that a country is the first in the world to do something is the statement most likely to have been planted, and it should be checked against a remembered ordinal rather than against a general impression of significance.

(d) Neither 1 nor 2 — the over-correction→A candidate who knows India was not first may conclude the whole item is a trap and reject both statements, discarding one that is true. Suspicion is not a method; each statement has to be judged on its own, which is why the 'neither' branch is chosen correctly far less often than it is chosen.

The flight itself→28 August 2016 from the Satish Dhawan Space Centre, Sriharikota, on a two-stage Advanced Technology Vehicle sounding rocket. The engines ignited at Mach 6 about 55 seconds into the flight and burned for roughly five seconds, on hydrogen fuel with atmospheric oxygen as oxidiser. Four technologies were demonstrated: ignition at supersonic speed, flame-holding at supersonic speed, the air intake mechanism and the fuel injection system.

Study next

Ramjet, scramjet and turbojet compared — how each compresses air and over what speed range each worksISRO's Reusable Launch Vehicle technology demonstrator and the stages of that programmeSounding rockets in the Indian programme and what they are used forCryogenic and semi-cryogenic engines, and why the cryogenic upper stage took so longMach number, and why hypersonic flight begins around Mach 5

Common traps

Accepting a 'first in the world' claim about an Indian achievement without checking the ordinal; here the correct number is fourth.
Rejecting both statements because one of them is false — the codes are there to be judged statement by statement.
Confusing what the engine is capable of with what a single test flight demonstrated.
Mixing up ramjet and scramjet — the difference is whether the air is still supersonic when the fuel burns.
Assuming an air-breathing engine can launch a vehicle from rest; a ramjet or scramjet needs to be accelerated to speed first.

Science and technology current affairs in EPFO papers usually pair a mechanism with a milestone. Prepare each major test or mission as four lines — what it was, when and from where, what was demonstrated, and where India stands in the ordering of countries that have done it. That last line is the one the two-statement format keeps returning to.

Related PYQs

No directly related past PYQ was found.

Practice

practice — not a real PYQ
In a scramjet engine, unlike in a ramjet, the air entering the combustion chamber is
- (a)decelerated to subsonic speed before the fuel is burnt
- (b)kept at supersonic speed while the fuel is burnt
- (c)compressed by a rotating compressor driven by a turbine
- (d)mixed with an oxidiser carried on board the vehicle
Answer(b) kept at supersonic speed while the fuel is burnt
practice — not a real PYQ
In ISRO's scramjet engine flight test of August 2016, the oxidiser used was
- (a)liquid oxygen carried on board
- (b)oxygen taken from the atmosphere
- (c)nitrogen tetroxide
- (d)hydrogen peroxide
Answer(b) oxygen taken from the atmosphere

Why