THE air-breathing propulsion experiment on August 28 was a success beyond expectations. A.S. Kiran Kumar, Chairman, ISRO, said India was only the fourth country to demonstrate the flight-testing of a scramjet engine. (The other three countries are the United States, Russia and Australia.) The two-stage vehicle used in the “experiment” was a modified sounding rocket called RH-560, developed by the Vikram Sarabhai Space Centre (VSSC) in Thiruvananthapuram. It weighed about three tonnes. After a flight lasting 300 seconds, the second stage of the RH-560 (RH stands for Rohini) fell into the Bay of Bengal, about 320 kilometres from Sriharikota. K. Sivan, Director, VSSC, called the “experiment” a fantastic success. Ground stations at Sriharikota tracked the flight of the vehicle from its lift-off at 6 a.m. to its splashdown in the Bay of Bengal, 300 seconds later. “The ignition of the scramjet engines in flight and holding the flame steadily for five seconds” when the air from the atmosphere is being rammed into the engines through an inlet at a supersonic speed of Mach 6 “was the objective of the experiment”, Sivan explained.
The experiment was the second step taken this year towards ISRO achieving a low-cost access to space. Reusable launch vehicles (RLVs), using scramjet engines, will lead to a drastic reduction in the cost per kg of payload delivered when a launch vehicle is used to put a satellite into orbit. ISRO took the first step, as part of its RLV programme, when its winged space plane called RLV-Technology Demonstrator lifted off on May 23 from Sriharikota, flew at a hypersonic velocity and made a hypersonic re-entry into the atmosphere. It then splashed down into the Bay of Bengal, about 412 km from Sriharikota. It survived the high temperatures during re-entry with the help of its thermal protection systems. The entire mission, from lift-off to touchdown, lasted about 770 seconds ( Frontline , June 14, 2016). The RLVs, using air-breathing propulsion, hold the key to low-cost access to space.
As the “Story of the Week” published on ISRO’s website entitled “ISRO’s Scramjet Engine Technology Demonstrator Successfully Flight Tested” explained: “Today, satellites are launched into orbit by multi-staged satellite launch vehicles that can be used only once (expendable). These launch vehicles carry oxidiser along with the fuel for combustion to produce thrust. Launch vehicles designed for one-time use are expensive and their efficiency is low because they can carry only two to four per cent of their lift-off mass into orbit. Thus, there is a world-wide effort to reduce the launch cost.”
Air-breathing engines, however, will bring down this cost drastically. For, air-breathing rocket systems use oxygen from the atmosphere and burn it with the stored on-board fuel to produce forward thrust, in contrast to the conventional chemical rocket systems which carry both the oxygen and the fuel on-board. So, air-breathing rocket systems are much lighter and more efficient, resulting in a reduction of overall costs. Besides, heavier satellites can be put into orbit. However, air-breathing engines can operate only when the launch vehicles fly in the atmosphere.
As the article on the ISRO website says: “A ramjet is a form of air-breathing jet engine that uses the vehicle’s forward motion to compress the incoming air for combustion without a rotating compressor. Fuel is injected into the combustion chamber where it mixes with the hot compressed air and ignites....
“Ramjets work most efficiently at supersonic speeds around Mach 3 [three times the speed of sound] and can operate up to speeds of Mach 6. However, the ramjet efficiency starts to drop when the vehicle reaches hypersonic speeds.
“A scramjet engine is an improvement over the ramjet engine as it efficiently operates at hypersonic speeds and allows supersonic combustion. Thus, it is known as supersonic combustion ramjet, or scramjet.
“A dual mode ramjet (DMRJ) is a type of jet engine where a ramjet transforms into scramjet over Mach 4-8 range, which means it can efficiently operate both in subsonic and supersonic combustor modes.
“An important development in ISRO’s Air Breathing Propulsion Project (ABPP) occurred on August 28, 2016, which was the successful flight testing of its scramjet.”
Developing a scramjet engine entailed mastering advanced technologies such as mixing the air coming in at a supersonic speed of Mach 6, igniting the engine and holding the flame within the length of the combustion chamber. “Igniting the engine when the air comes in at a supersonic speed and sustaining the flame” were the issues to tackle, Sivan said.
On August 28, the Advance Technology Vehicle (ATV), as the RH-560 is called, weighed 3,277 kg at lift-off and consisted of two stages: the first booster stage and the second sustainer stage. Two scramjet engines were strapped around the sustainer stage. After a smooth countdown of 12 hours, the RH-560 lifted off at 6 a.m. from the launch pad meant for sounding rockets at Sriharikota. The sequence of events was as follows: the booster stage burnt out at an altitude of 7 km and jettisoned. After a few seconds of coasting, the sustainer engine in the second stage ignited at an altitude of about 12 km. It took the second stage to a velocity of Mach 6. ISRO’s experiment began when the second stage reached an altitude of 20 km, about 55 seconds after lift-off. Atmospheric air was rammed into the scramjet engines through an inlet at six times the speed of sound, the air mixed with the hydrogen fuel in the combustion chamber, the scramjet engines ignited and the resultant flame held for five seconds. Then the burnout of the second stage took place. By then, the second stage had reached an altitude of more than 50 km. After a flight of about 300 seconds, the vehicle splashed down in the Bay of Bengal, about 320 km from Sriharikota.
T.S. Subramanian
