Through a series of ground tests, a stable supersonic combustion has been demonstrated for nearly 7 seconds with an inlet Mach number of 6 (i.e., six times the speed of sound).

As such technologies are in a very nascent stage of development the world over, ISRO considers this achievement as a major technology breakthrough in air- breathing propulsion. Other than USA, which has recently carried out in-flight demonstration of supersonic combustion for a short duration, work related to supersonic combustor designs in other countries like Japan, China, Russia, Australia, and Europe are either in their initial or ground testing phase.

Currently, the space transportation systems are expendable in nature and use the conventional chemical rocket systems for their propulsion. The cost per kg of payload of such expendable systems is quite high, being in the range of $12,000 to $15,000 per kg. If we have to make the access to space more affordable, this cost needs to be brought down by an order of magnitude to something like $500 - $1000 per kg. This will require a two pronged approach: (a) the systems are made recoverable and reusable (b) we adopt more efficient propulsion systems like air- breathing rockets.

Air-breathing rocket systems use atmospheric oxygen and burn it with the stored on- board fuel for producing the forward thrust - in contrast to the conventional chemical rocket systems which carry both the oxygen and the fuel on-board. As a result, the air-breathing systems become much lighter and more efficient leading to reduced overall costs. As the air- breathing systems have the capability to operate only during the atmospheric phase of flight, they always have to be mated with conventional chemical rockets, for meeting the final orbital velocity requirements.

A good example of air-breathing engines are the Turbojet engines used in aircrafts; however, they have limitations in operating only up to a maximum of Mach number 3. To travel beyond these Mach number regimes, Scramjet propulsion is the only viable option. The development of a Scramjet system is quite complex and involves a number of technological challenges, especially related to the mixing of very high speed air (velocity around 1.5 km/s) with fuel, achieving stable ignition and flame holding, in addition to ensuring efficient combustion, within the practical length of the combustor.

In the coming years, ISRO is planning to flight test an integrated Scramjet propulsion system comprising of air-intake, combustor and nozzle, by using a cost effective two stage RH-560 sounding rocket. Development of such a high technology system will go a long way towards meeting India's future space transportation needs.

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