Science behind the Scramjet Engine

Written by Anshika Yadav

What is the Scramjet Engine?
Scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion (hence ramjet), but whereas a ramjet decelerates the air to subsonic velocities before combustion, the airflow in a scramjet is supersonic throughout the entire engine allowing the scramjet to operate efficiently at extremely high speeds.

The Defense Research and Development Organization (DRDO) performed a major technological feat on September 7 when it flew a cruise vehicle at a hypersonic speed of Mach six for 20 seconds. The DRDO called the cruise vehicle Hypersonic Technology Demonstrator Vehicle (HSTDV). The centrepiece of the HSTDV was the developed air-breathing engine, which formed the HSTDV’s propulsion system. The mission’s aim was to prove this air-breathing scramjet engine in flight and it was achieved.

In an air-breathing scramjet engine, air from the atmosphere is rammed into the engine’s combustion chamber at a supersonic speed of more than Mach two.
In the chamber, the air mixes with the fuel to ignite a supersonic combustion but the cruiser’s flight will be at a hypersonic speed of Mach six to seven. So it is called supersonic combustion ramjet or Scramjet.

What’s the future?
• ISRO wishes to employ scramjets on future Single Stage To Orbit (SSTO) and Two Stage To Orbit (TSTO) missions. These missions would in the future employ rocket or turbine-based combined cycle engines. As mentioned above, ISRO is now working towards a bigger scramjet demonstrator in the near future.
• Overall, ISRO says that it has studied more than 100 different configurations to arrive at this particular outline.
• This new demonstrator is expected to be a 3.23-ton vehicle which will cruise at speeds of Mach 6 to Mach 7 and at altitudes between 24 km and 26 km
•. The scramjet that will power the flight vehicle is an evolution of the design tested in 2016 and will operate for 200-250 seconds while providing much greater thrust.
• Though Hydrogen is a cleaner-burning fuel resulting in only hydroxides and water emissions, Kerosene is being preferred since it is denser and does not pose handling difficulties unlike liquid Hydrogen
•. It also has a lower auto-ignition temperature of 220 °C as compared to 500 °C for Hydrogen. It is noteworthy, that extensive studies were conducted on Kerosene combustion in collaboration NIICHIMASH, Russia
• The new vehicle is expected to be statically and dynamically stable.

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