ISRO Achieves Breakthrough in Supersonic Combustion

As a part of the advanced technology initiative in the area of air-breathing propulsion, the Vikram Sarabhai Space Centre of ISRO at Thiruvananthapuram has successfully carried out the design, development, characterisation and realisation of the Supersonic Combustion RAMJET (SCRAMJET). Through a series of ground tests, a stable supersonic combustion has been demonstrated for nearly seven seconds with an inlet Mach number of six (which means six times the speed of sound).

Air Breathing Propulsion
Advanced reusable launch vehicles with air-breathing propulsion are expected to reduce substantially the cost of access to space which is presently about $12,000 per kg for launch into near earth orbit. The cost can be brought down by reducing the vehicle size and by recovering and reusing the hardware. More importantly, reducing propellant is imperative for low cost access to space as propellant forms about four fifth of launch vehicle mass at take-off (Space Shuttle: 78 percent and GSLV: 86 percent).

In the present day launch vehicles, the propellant constitutes more oxidiser than fuel burnt and oxidiser to fuel ratio varies from two to six (depending on whether it uses solid, liquid or cryogenic rocket propulsion). In air breathing propulsion, the need for carrying oxidiser is minimised (if not eliminated) by using oxygen from the air. But since dense air is available only up to about 50 km altitude, question arises regarding the extent of benefit of using air. However, three fourth of the propellant is consumed by the vehicle within this portion of the flight.

For example, taking off vertically, GSLV, which is used for launching satellites into 200 X 36,000 km
Geosynchronous Tranfer Orbit, consumes 75 percent of propellant before it reaches an altitude of 44 km. An air breathing launch vehicle can opt for off-verticle trajectory and benefit more by using oxygen from the air.

But there are challenges in the collection and use of air as the vehicle accelerates through the atmosphere. Space-faring nations are at present addressing the issues of air-breathing propulsion for a quantum jump in the performance compared to conventional rocket propulsion (solid, liquid or cryogenic). Also, air-breathing propulsion is the only feasible way for a powered return (cruise) flight necessary for reusable launch vehicle which should ‘fly back to base’.

Modern aircraft use turbojet air breathing engine (or its variant turbo-fan) for their propulsion. They
carry only fuel, and air pressurised by turbo-compressor to the required level, supplies the oxidiser. Fuel is injected and burned within the combustion chamber and expanding high pressure high temperature gases within the nozzle produce high velocity jet, generating thrust. Turbines are
used to drive the compressor. The compressor-turbine combination is the most complex and expensive element of the engine and also puts limits on its operability — turbojet engines do not operate at flight Mach numbers higher than three.

But at higher flight speeds, the ‘ram effect’ of air speed is capable of compressing air to required levels. This concept is used in a ramjet engine. The compressor-turbine combination is discarded and
the remaining ‘simple’ engine consists of only the air-intake, combustion chamber and nozzle.

In RAMJET, combustion chamber speeds are higher than that in turbojet but are still subsonic. As the flight speed increases, decelerating air from supersonic flight speeds to subsonic speeds becomes more and more inefficient, both in terms of pressure recovery and combustion processes. For flight Mach numbers of about six and above, there is a need to restrain the level of air deceleration and to retain its lower supersonic speed as it enters the combustion chamber. This necessitates Supersonic Combustion RAMJET or SCRAMJET.

In general, Specific Impulse, the performance parameter of a propulsion system which indicates how much thrust is generated for unit mass flow consumption of propellant, reduces with flight Mach number for air breathing propulsion. This, along with the problem of increasing air stagnation temperatures, generally limits the upper operability of SCRAMJET to flight Mach number of about ten. Beyond this, rocket engines will be called upon to propel the vehicle, though thrust levels and propellant requirement would be quite small — GSLV which uses only 15 percent of the total propellant for flight beyond Mach ten.

Air breathing propulsion for launch vehicles envisages a combination of propulsion cycles: turbojet engine (widely used in aircraft), ramjet engine (currently being used in missiles), SCRAMJET engine (being developed around the globe) and rocket engine (the workhorse of today’s launch vehicles). They could either function as separate propulsion systems co-located or as a “combined” propulsion cycle engine by synergistically integrating them. A variant, rocket based combined cycle engine, which avoids the complex (rotating) turbojet component where the rocket engine in its air augmented mode is used for the low speed flight, is also gaining popularity.

A less complex option is to combine the ramjet and the SCRAMJET (to have Dual Mode Ramjet for Mach 3 to 10) and have rocket engine separately.

Of the different propulsion cycles, SCRAMJET, with its supersonic combustion is the most critical. Fuel injection, mixing, ignition and flame holding as air travels at speeds greater than one kilometer per second within combustion chamber is often equated to lighting a candle in hurricane.

ISRO had earlier flown a rocket for testing a combined cycle system, for a low speed regime of up to Mach number 2.3. Thus, the present success of ISRO in demonstrating stable supersonic combustion through a series of ground tests for an equivalent flight Mach number of seven, assumes significance. Theoretical studies and extensive use of Computational Fluid Dynamics tools have helped in this achievement.

ISRO is planning to flight test an integrated SCRAMJET propulsion system (comprising air intake, combustor and nozzle) using a two-stage sounding rocket. Development of such a high technology system will be a major step towards the future space transportation systems.

(This article is contributed by Mr J D A Subramanyam, Project Director, Air Breathing Propulsion Project, Vikram Sarabhai Space Centre, Thiruvananthpuram)