During the experiment in SDSC, the scramjet engine, affixed to a two-stage RH-560 sounding rocket developed in the 1970s, was ignited at an altitude of 20 km and allowed to burn fuel for five seconds before the rocket falls into the Bay of Bengal.

Unlike conventional rocket engines which carry both fuel and oxidiser, the scramjet engine with its air-breathing propulsion system technology will forcefully compress atmospheric oxygen when the rocket is in supersonic speed. The atmospheric oxygen will act as an oxidiser to burn the fuel (liquid hydrogen) being carried.

Isro scientists said that the scramjet engine would be tested about 55 seconds after take-off from the launchpad at SDSC. After the first stage of the rocket breaks off and falls into the Bay of Bengal, the rocket will continue moving up with available thrust. In the second stage, when the rocket is travelling at six times the speed of sound, the scramjet engine will be ignited at an altitude of 20 km and the fuel will burn for five seconds. The rocket will move further up for 40-70 km, before it coasts horizontally and begins its descent into the sea.

Isro scientists said that the engine, by using atmospheric oxygen, will eventually reduce the weight of the vehicle during lift-off by more than half, enabling it to carry heavier payloads into orbit. The scramjet engine is ideally suited for launch vehicles moving at hypersonic speed.

Director of Vikram Sarabhai Space Centre K Sivan said the experiment is important, as the engine will eventually be used in the RLV. "The ignition of the engine, the building up of pressure and the duration for which the engine can sustain the flame will be tested and monitored," he said. "The main concern is igniting the air-breathing engine in the air and then sustaining the flame at supersonic speed. If we can sustain it for five seconds, then it can last for even 1000 seconds."

Following the test on Sunday, the scramjet engine will be tested on a full-scale RLV, while the vehicle will be tested for its ability to land on a runway. While many countries like Japan, China, Russia and Europe are in the initial or testing phase of supersonic combustor technology, NASA demonstrated scramjet propulsion in 2004. Isro had previously carried out a ground test of a scramjet engine in 2006.