The Indian Space Research Organisation joined an elite club when, on Sunday (August 28, 2016), it successfully launched a rocket using a scramjet engine that was developed indigenously. This is ISRO’s first major step towards developing an air breathing propulsion system. The scramjet engine functioned for around six seconds. There are many reasons why the use of a scramjet engine is so attractive. A scramjet engine uses oxygen present in the atmospheric air to burn the hydrogen fuel. As a result, the amount of oxygen required to be carried on board would be reduced considerably as atmospheric oxygen is utilised to burn the fuel in the first stage. In general, propellant accounts for nearly 85 per cent of the weight of a rocket, and oxygen accounts for nearly 60 per cent of the weight of the propellant. Scramjet-powered rockets also have several times greater thrust compared with rockets powered by liquid fuel or even cryogenic fuel. Since about half of the propellant is required for the first stage to achieve the required velocity, a rocket using a scramjet engine would be significantly lighter and smaller and, therefore, cheaper. Alternatively, rockets fired by scramjet engines will be able to carry more payload.
Sunday’s test flight, which attained six times the speed of sound (Mach 6) and was able to achieve ignition and maintain stable combustion even at such high velocity for about six seconds, is a big technological achievement. This is akin to “lighting a matchstick in a hurricane condition and sustaining the flame” for six seconds. The air intake mechanism and fuel injection systems were also successfully demonstrated during the maiden test flight. Since it relies on oxygen present in the atmosphere, the trajectories of scramjet engine-powered rockets are vastly different from conventional ones — rockets with scramjet engines should remain in the atmosphere for a longer period than normal rockets. Typically, scramjet rockets climb to a certain altitude and remain in the atmosphere for as long as possible to achieve the required velocity. It will take many years before a commercial rocket powered by a scramjet engine takes to the sky as there are several challenges to be overcome. One challenge will be to test the engine at higher Mach speeds and prolong the period of combustion. Since the scramjet comes into play only when the rocket goes beyond Mach 5, an engine that initially works at subsonic speed (as a ramjet) and later as a scramjet has to be developed. But as in the case of the successful test flight of a reusable vehicle, the first experimental flight using a scramjet engine is a technological demonstration of ISRO’s capability and will go a long way in redefining its position as one of the leading space agencies in the world.