Scramjet and Air-Breathing Propulsion in India Explained

A scramjet is an air-breathing jet engine, short for supersonic-combustion ramjet, that burns fuel in air flowing through the combustor at supersonic speed, so a vehicle can fly at hypersonic speeds without carrying its own oxidiser. India has been working on scramjet propulsion through the Indian Space Research Organisation (ISRO) and the Defence Research and Development Organisation (DRDO), and the technology matters for both cheaper access to space and high-speed defence systems.

Unlike a conventional rocket, which must lift tonnes of liquid oxygen or solid oxidiser from the ground, a scramjet scoops oxygen from the atmosphere during flight. This explainer covers how scramjets work, what ISRO and DRDO have demonstrated, why the engines are so hard to build and what the technology could mean for India. It was last reviewed on 15 October 2026.

Quick Facts

Key Takeaways

• A scramjet is an air-breathing engine that sustains combustion in supersonic airflow and takes oxygen from the atmosphere instead of carrying oxidiser.
• ISRO flew a scramjet engine on a sounding rocket, the Advanced Technology Vehicle ATV-D01, on 28 August 2016 from Sriharikota.
• DRDO flew the Hypersonic Technology Demonstrator Vehicle (HSTDV) in September 2020, powered by a scramjet engine designed for hypersonic cruise.
• A scramjet cannot start from a standstill, so it needs a booster rocket or another engine to bring it to supersonic speed first.
• Scramjet combustion happens in milliseconds, and the extreme heating of the airframe and engine walls is the central engineering challenge.
• Successful scramjets could lower the cost of reaching orbit by reducing the oxidiser mass a launch vehicle carries from the ground.

What is a scramjet engine?

A scramjet engine is a jet engine that compresses incoming air purely through the shape of its intake and burns fuel in that air while it is still moving faster than sound. The name stands for supersonic-combustion ramjet, and the engine is often described as a ramjet whose airflow stays supersonic all the way through the combustor.

The engine has almost no moving parts. There is no turbine and no rotating compressor. The forward speed of the vehicle itself rams air into the inlet, where shock waves slow and compress it. Fuel, commonly hydrogen or a hydrocarbon, is injected into the compressed air, ignites, and the hot gases expand through a nozzle to produce thrust.

The engine only works inside the atmosphere, because it needs air. That limitation is also its advantage: the oxidiser, which makes up the largest share of the propellant mass of a conventional rocket, is simply collected from the sky.

How does a scramjet work?

A scramjet works by using high vehicle speed to compress air, mixing fuel into that supersonic stream, burning it and expelling the products to generate thrust. The flow path is usually divided into four sections.

• Inlet or intake: A sequence of ramps and shock waves compresses the air and slows it down, though not to subsonic speed.
• Isolator: A short duct that keeps the combustor’s pressure disturbances from travelling forward and disrupting the inlet.
• Combustor: Fuel is injected and burned in supersonic flow, often using flame holders or cavities to keep the flame stable.
• Nozzle: The hot, high-pressure gas expands rearwards, converting thermal energy into thrust.

Air takes only a few milliseconds to pass through the combustor. Fuel must mix, ignite and burn in that very short window. Engineers compare it to keeping a candle lit in a hurricane, and this is why flame stabilisation is one of the toughest problems in the field.

How is a scramjet different from a ramjet and a turbojet?

A scramjet differs from a ramjet mainly in the speed of airflow inside the combustor: a ramjet slows the air to subsonic speed before burning fuel, while a scramjet keeps it supersonic. A turbojet, in turn, uses rotating compressors and turbines, which limits its useful speed range to roughly Mach 3.

Because ramjets and scramjets cannot generate thrust at rest, practical vehicles combine them with a booster. India’s BrahMos cruise missile, for example, uses a solid-fuel booster to reach supersonic speed and then a liquid-fuelled ramjet for cruise. That is the ramjet stage of the same family of ideas that leads to the scramjet.

Why does India want scramjet technology?

India wants scramjet technology because it promises cheaper, more flexible access to space and gives the armed forces a route to hypersonic weapons and reconnaissance vehicles. The two goals share the same engine science, which is why both ISRO and DRDO have invested in it.

Space access

A typical launch vehicle carries a large mass of oxidiser. If an air-breathing stage could provide thrust in the lower atmosphere, the vehicle could leave much of that oxidiser behind, carry more payload for the same size, or become small enough to be reusable. ISRO has described air-breathing propulsion as a building block of future reusable launch vehicles.

Defence applications

Hypersonic cruise missiles fly at very high speeds within the atmosphere, which shortens reaction time for the target and makes interception difficult. A scramjet gives a missile sustained powered cruise rather than a brief boost. DRDO’s demonstrator work is aimed at this class of systems, along with possible reconnaissance uses.

What did ISRO achieve with its 2016 scramjet engine test?

ISRO successfully flew a scramjet engine on 28 August 2016, when an Advanced Technology Vehicle (ATV-D01) lifted off from the First Launch Pad at Sriharikota, in Andhra Pradesh. The flight demonstrated that a scramjet could ignite and sustain combustion in real flight conditions on an Indian-built engine.

The vehicle was a two-stage solid rocket based on ISRO’s RH-560 sounding rocket. A solid first stage carried the second stage up through the atmosphere. The scramjet engines were mounted on the second stage and operated during the coast phase after the booster had accelerated the vehicle to supersonic speed at high altitude.

The main points of the test were:

• The engines used hydrogen as fuel and ambient atmospheric oxygen as oxidiser.
• The engines operated for roughly five seconds at a hypersonic speed of around Mach 6.
• Ignition and sustained combustion in supersonic flow were demonstrated, along with passive thermal protection of the engine and air intake.
• The vehicle was a technology demonstrator and did not carry any satellite.

The short burn time was typical of an early demonstration. Sustained operation for minutes rather than seconds is the next step, and ISRO has since pursued ground tests of longer-duration combustors.

What other air-breathing propulsion tests has ISRO carried out?

ISRO has run several air-breathing and hypersonic experiments beyond the 2016 scramjet flight, with the RLV-TD Hypersonic Flight Experiment (HEX) of May 2016 being the most visible. The Reusable Launch Vehicle Technology Demonstrator (RLV-TD) is a winged vehicle designed to test hypersonic re-entry, autonomous landing and thermal protection.

The programme sits within a broader roadmap for reusable launchers. ISRO’s work on air-breathing propulsion includes:

• Scramjet flight experiment: the 2016 ATV-D01 mission described above.
• RLV-TD HEX-01: a winged vehicle flown in May 2016 to study hypersonic aerodynamics and heat protection at around Mach 5.
• Ground testing of combustors: longer test runs of scramjet combustors in laboratory facilities to understand durability.
• Dual-mode concepts: studies of engines that operate as a ramjet at lower speeds and a scramjet at higher speeds.

The sounding rocket platform is attractive because it offers a cheap way to reach the required speed and altitude, and it allows engineers to gather real flight data before attempting a full air-breathing launch stage.

What is DRDO’s Hypersonic Technology Demonstrator Vehicle (HSTDV)?

The Hypersonic Technology Demonstrator Vehicle (HSTDV) is an unmanned scramjet-powered vehicle developed by the Defence Research and Development Organisation (DRDO) to demonstrate hypersonic cruise. DRDO conducted the first flight test of the HSTDV in September 2020 from the Abdul Kalam Island range off the Odisha coast.

The demonstrator was launched on top of a solid rocket booster derived from the Agni family. After the booster took the vehicle to a high altitude, the cruise vehicle separated and its scramjet ignited. DRDO reported that the vehicle reached hypersonic speeds of around Mach 6 and sustained flight for about twenty seconds, in tests that validated several scramjet technologies, including ignition, combustion and flight control at hypersonic speed.

The HSTDV is a technology bed rather than a missile in service. Its purpose is to prove key technologies that could later be used in hypersonic cruise missiles and other platforms. DRDO laboratories such as the Defence Research and Development Laboratory (DRDL) in Hyderabad have also run long-duration ground tests of actively cooled scramjet combustors, which are essential to extend flight time from seconds to minutes.

What other Indian programmes use ramjet and hypersonic technology?

Several Indian programmes use ramjet propulsion or hypersonic concepts, and each contributes experience to the scramjet effort. These include the BrahMos missile, the Solid Fuel Ducted Ramjet (SFDR) and hypersonic cruise missile development.

DRDO has also reported tests of long-range hypersonic missile systems in recent years. As details of such programmes are often released only in part, this article avoids precise performance figures for them.

What are the main challenges in building a scramjet?

The main challenges in building a scramjet are extreme heating, very short combustion time, difficulty starting the engine, and the lack of realistic ground-test facilities. These problems make scramjet development slower and costlier than that of almost any other propulsion system.

• Thermal loads: At hypersonic speed, air friction and shock compression raise surface temperatures to levels that can melt ordinary metals. Engine walls need regenerative cooling, using the fuel itself as coolant, or heat-resistant materials.
• Ignition and flame stability: Fuel has only milliseconds to mix and burn. Designers use cavities, struts and carefully shaped injectors to hold the flame.
• Inlet unstart: If combustor pressure disturbs the inlet, shock waves can be expelled forward and engine thrust can collapse abruptly.
• Integration with the airframe: The whole underside of the vehicle often acts as part of the inlet and nozzle, which couples aerodynamics, propulsion and structure into one problem.
• Testing limits: Wind tunnels can simulate hypersonic flow only for fractions of a second or at reduced scale, so flight tests remain essential.
• Low thrust at lower speeds: A scramjet is inefficient below hypersonic speeds, so the vehicle needs a booster or a combined-cycle engine.

Which other countries have flown scramjets?

The United States, Russia, China and Australia are among the countries that have flown scramjet-powered test vehicles, so India is part of a small group. The best known early success was NASA’s X-43A, which in 2004 reached nearly Mach 10 in a short unmanned flight powered by a scramjet.

Other notable efforts include Australia’s HyShot experiments, which launched scramjets on sounding rockets in the early 2000s, and US programmes such as the X-51A Waverider. Russia carried out some of the earliest scramjet flight experiments in the 1990s using test beds based on missile systems. India’s approach, using sounding rockets as flight platforms, resembles the low-cost route that Australia followed.

No country has yet put a fully operational scramjet-powered launcher into regular service. Most work remains at technology demonstration level, which means India’s progress is competitive with the global state of the art.

What is the future of scramjet propulsion in India?

The future of scramjet propulsion in India lies in longer-duration engines, air-breathing stages for reusable launchers and hypersonic cruise systems for defence. The most likely sequence runs from long ground tests of cooled combustors, to extended-duration flights, to integrated vehicles.

For space launch, ISRO’s long-term idea is a two-stage-to-orbit reusable system in which an air-breathing first stage accelerates through the atmosphere and a rocket upper stage finishes the job in space. This concept needs an engine that works over a wide range of speeds, which is why dual-mode combustors and combined-cycle designs matter.

Progress also depends on materials, simulation tools and test infrastructure. Computational fluid dynamics, high-enthalpy test facilities and advanced ceramics and composites will shape the pace. Given the complexity, any timeline should be treated as uncertain.

Bottom Line

Scramjet propulsion is a demanding but promising technology, and India has moved from theory to flight demonstrations through ISRO’s 2016 ATV-D01 test and DRDO’s HSTDV flight in 2020. The step from seconds of burn time to sustained, controllable hypersonic flight is the real test, and it will determine whether India’s air-breathing engines can power reusable launchers and hypersonic systems.

Frequently Asked Questions

What is a scramjet in simple words?

A scramjet is a jet engine that burns fuel in air flowing faster than sound, using the atmosphere’s oxygen instead of carried oxidiser. It has no turbine or compressor blades because the vehicle’s speed squeezes the air. It works only at hypersonic speeds, generally above Mach 5.

When did ISRO test a scramjet engine?

ISRO tested a scramjet engine on 28 August 2016 using the Advanced Technology Vehicle (ATV-D01) launched from Sriharikota. The hydrogen-fuelled engines operated for about five seconds at around Mach 6. The flight was a technology demonstration and carried no satellite.

What is the difference between a ramjet and a scramjet?

A ramjet slows incoming air to subsonic speed before combustion, while a scramjet burns fuel in air that stays supersonic. Ramjets work from about Mach 2 to Mach 5, and scramjets take over at higher hypersonic speeds. Both need a booster because neither can produce thrust from rest.

What is DRDO’s HSTDV?

The HSTDV, or Hypersonic Technology Demonstrator Vehicle, is a scramjet-powered unmanned vehicle developed by DRDO. It was flown in September 2020 from the Odisha coast and reached hypersonic speed of around Mach 6 for about twenty seconds. It is a test bed for technologies that could later power hypersonic cruise missiles.

How could scramjets reduce the cost of space launch?

Scramjets could lower launch cost because an air-breathing stage collects oxygen from the atmosphere rather than carrying it from the ground. This reduces the mass of the vehicle and may allow reusable designs. The benefit is still a long-term goal, since sustained scramjet flight has only been shown in short tests.

Why are scramjets so difficult to develop?

Scramjets are difficult because air passes through the combustor in milliseconds, surface temperatures are extreme, and ground facilities cannot fully reproduce hypersonic flight. Engineers must keep the flame stable, cool the engine and avoid inlet unstart. These problems make flight tests essential and development slow.

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