Maarg Manthan · Topic 12.23

Air-Breathing Propulsion System

Science & Technology › Space Technology and ISRO · Topic 12.23

An air-breathing propulsion system uses oxygen from the atmosphere to burn its fuel, instead of carrying its own oxidiser as a rocket does. Jet engines, ramjets and scramjets are air-breathing. A ramjet has no moving parts, compresses incoming air by the speed of the vehicle, and works at about Mach 2 to 5. A scramjet, or supersonic combustion ramjet, burns fuel in air that stays supersonic inside the engine and works above Mach 5, up to about Mach 10 or more. Because the vehicle does not have to carry oxygen, an air-breathing launch vehicle could be lighter and cheaper. ISRO tested a scramjet engine on a rocket on 28 August 2016 and the Defence Research and Development Organisation flew a hypersonic technology demonstrator in September 2020.

5 min read · UPSC Prelims · Science & Technology · Space Technology and ISRO

At a Glance

Ramjet Mach 2 to 5
Scramjet Mach 5 and above
Fuel advantage No onboard oxidiser
ISRO test 2016 scramjet
DRDO test HSTDV, 2020
Exam link UPSC Prelims, Science & Technology: Space Technology and ISRO

Where Does It Fit?

This topic follows the account of Astrosat. It returns to propulsion technology and presents a possible next generation of launch and missile systems, which use air as the oxidiser. Previous topic: Astrosat, Astrosat-Chandra-Hubble Joint Detection of Cosmic Explosions. Next topic: NASA’s Asteroid Impact and Deflection Assessment Mission.

How Do Air-Breathing Engines Differ from Rockets?

Engine	Features
Rocket	Carries both fuel and oxidiser. Works in vacuum. High thrust, but heavy because of the oxidiser
Turbojet and turbofan	Compressor and turbine move air. Used in aircraft up to about Mach 3
Ramjet	No compressor. Air is slowed and compressed by the forward speed in an inlet, then burned with fuel. Needs a boost to start. Works at about Mach 2 to 5
Scramjet	Air flow stays supersonic in the combustor. Works above Mach 5 and has been demonstrated up to about Mach 9.6 in tests
Dual mode ramjet-scramjet	Switches mode as speed rises
Rocket based combined cycle	Uses a rocket at low speed and air-breathing mode at high speed
Limitation	Cannot start from rest, needs air, and works only in the atmosphere

What Have India and Others Achieved?

ISRO scramjet test: ISRO’s Advanced Technology Vehicle (ATV-D02) launched on 28 August 2016 from Sriharikota carried two scramjet engines, which burned for about five seconds at Mach 6 at an altitude of about 20 km
Hypersonic Technology Demonstrator Vehicle: DRDO tested the HSTDV, a scramjet powered cruise vehicle, on 7 September 2020 at Mach 6 for about 20 seconds
Ground test: DRDO conducted a long duration ground test of an active cooled scramjet combustor in 2025. Check the details
BrahMos: the supersonic cruise missile of India and Russia uses a liquid fuelled ramjet in its second stage to reach about Mach 2.8
BrahMos-II: a planned hypersonic version of about Mach 7
X-43A of NASA: reached about Mach 9.6 in 2004
Hypersonic weapons: China, Russia and the US have developed hypersonic glide vehicles and missiles
Reusable launch vehicle: ISRO is developing a winged reusable vehicle and an air-breathing second stage in the long term

What Else Should You Remember?

Mach number: the ratio of speed to the speed of sound; hypersonic means Mach 5 and above
Heating: at hypersonic speed, air friction creates temperatures of thousands of degrees, so special materials and cooling are needed
Inlet and combustor: the shape of the inlet slows air with shock waves, and the combustor must burn fuel in air moving at supersonic speed within milliseconds
Fuel: hydrogen is common for tests, and kerosene is used for missiles
Advantages for launch: an air-breathing first stage could cut lift-off weight and cost
ISRO design: the ATV engine used hydrogen and was a step to an air-breathing launch vehicle
Wind tunnels: hypersonic test facilities simulate high speed airflow, and India has built hypersonic wind tunnels
Strategic use: hypersonic missiles are hard to intercept because they are fast and manoeuvre, a factor in defence planning, as in Space Weaponry: Anti-Satellite (A-SAT) Technology
RLV-TD: the reusable launch vehicle technology demonstrator flew in May 2016 and autonomous landing experiments (LEX) were done in 2023 and 2024
Space plane: a concept of a plane that takes off from a runway to reach orbit
Challenges: ignition at high speed, structural heat and long duration operation
Link: rocket basics are in Categories of Launch Vehicles and Their Uses

Why Does It Matter for the Exam?

Ranges: ramjet and scramjet Mach numbers
Indian tests: ISRO 2016 and DRDO 2020
Missile link: BrahMos ramjet
Current affairs link: hypersonic missile tests
Common errors: saying that scramjet works from rest or that it carries oxidiser

As an analytical point, air-breathing engines could lower launch costs and give new missile capability, but need long years of testing. India’s tests have shown basic competence, and sustained effort is needed to reach practical systems.

Why Is This Topic Important?

Types Ramjet and scramjet
Tests ISRO and DRDO
Use Launch and defence

What Came Next?

The next topic covers NASA’s asteroid impact and deflection assessment mission. Next topic: NASA’s Asteroid Impact and Deflection Assessment Mission.

Key People and Terms

Mach Multiple of sound speed
Hypersonic Above Mach 5
Ramjet Engine with no compressor
Scramjet Supersonic combustion engine
Oxidiser Substance that supports burning
HSTDV Hypersonic Technology Demonstrator Vehicle

Exam Corner

Points to Remember

Ramjet: Mach 2 to 5; scramjet: above Mach 5.
ISRO scramjet test: 28 August 2016.
DRDO HSTDV: 7 September 2020.
Air-breathing engines do not carry oxidiser.
BrahMos second stage uses a ramjet.

Do Not Confuse With

Ramjet and scramjet Subsonic combustion versus supersonic combustion
Rocket and scramjet Carries oxidiser versus uses air
Supersonic and hypersonic Above Mach 1 versus above Mach 5

Memory Hook

Ramjet to five, scramjet beyond; no oxygen tank to carry, but start from a booster.

Mains Angle

Questions ask for engine types and Indian tests. These points help.

Advantage Lighter and cheaper
Status Tests
Challenges Heat and ignition

A Question You May Face

An original practice question, not a past paper question.

“Explain the working of air-breathing engines such as ramjet and scramjet and discuss India’s progress and their strategic importance.”

How to Answer

Working Air as oxidiser.
Types Ramjet, scramjet.
India ISRO, DRDO tests.
Way Sustained testing.

GS Relevance

Prelims: ramjet versus scramjet, Mach ranges, Indian tests. GS Paper 3: science and technology, defence and space.

Frequently Asked Questions

What is an air-breathing engine?

An air-breathing engine takes oxygen from the surrounding air to burn its fuel, unlike a rocket, which carries its own oxidiser. Jet engines, ramjets and scramjets are air-breathing engines. They are lighter for the same energy but can work only within the atmosphere, so they cannot reach orbit alone.

What is the difference between a ramjet and a scramjet?

A ramjet slows the incoming air to subsonic speed before burning fuel and works at about Mach 2 to 5, while a scramjet keeps the airflow supersonic inside the combustor and works above Mach 5. Neither has a compressor, and both need a booster to reach the speed at which they can start.

What did ISRO achieve in its 2016 scramjet test?

On 28 August 2016, ISRO's Advanced Technology Vehicle (ATV-D02), a two stage solid rocket, was launched from Sriharikota and carried scramjet engines which operated for about five seconds at around Mach 6 and 20 kilometres altitude. It was the first flight test of ISRO's scramjet and a step toward an air-breathing launch vehicle.

What is the Hypersonic Technology Demonstrator Vehicle?

The Hypersonic Technology Demonstrator Vehicle, HSTDV, is an unmanned scramjet powered vehicle developed by DRDO. In a test on 7 September 2020 it reached Mach 6 for about 20 seconds, which made India one of a few countries to demonstrate hypersonic air-breathing flight, and it is meant for future hypersonic cruise missiles.

Why are air-breathing engines useful for launch vehicles?

A rocket carries a large mass of oxidiser, which is most of its lift-off weight. An air-breathing first stage would take oxygen from the air instead, so the vehicle could be lighter and the cost per kilogram to orbit lower. This is why agencies study scramjets and combined cycle engines, although they are not yet operational.

PYQ Practice — Statement Analysis

1 A scramjet burns fuel in a supersonic airflow.

True

It works above Mach 5.

2 A ramjet can produce thrust when the vehicle is at rest.

False

It needs forward speed.

3 The BrahMos missile uses a ramjet in its second stage.