Entering the Hypersonic Era

A bullet train or a passenger flight are fast moving vehicles that you have ever experienced .Basically a passenger flight moves in a speed range of 500-600 mph . Now imagine how it will be moving at a speed of 7000 mph i.e. more than 9 times the speed of sound. On 7th September 2020, DRDO conducted a successful HTDV (Hypersonic Technology Demonstrator Vehicle) test, comprising a scramjet engine in the wheeler island, ODISHA. In this experiment the HTDV reached the speed of Mach 6 just in 6 seconds, after which India got into a hypersonic missile club. Where Mach number is defined as the speed of an object with respect to speed of sound in air.

Earlier the jet engines are supposed to have fuel propellant consisting of oxidizer. which are supposed to be less efficient, and more costly. Therefore, we need a propulsion system which can use atmospheric oxygen for the combustion process. RAMJET (Range: Mach1-Mach5) is an engine system which compresses atmospheric air for the combustion process. But the problem arises after reaching supersonic speed. Specifically, in RAMJET air enters at supersonic speed, but later slows down to subsonic speed in the combustion chamber. As the Mach number increases to 5 the engine efficiency decreases significantly due to the shock wave experienced by the inlet. Later this issue was solved by an advanced version of the RAMJET engine, called SCRAMJET (supersonic combustion RAMJET) (Range: Mach 6 - March 15).

Now the question is what is SCRAMJET and how it works? SCRAMJET is a simple air breathing engine model which is envisioned at speeds up to at

least Mach 15. Which operates on Mach number >6. In scramjet air enters through the inlet, where it is compressed by supersonic compressor. After the ignition process the hypersonic exhaust comes through the nozzle. As the air has speed in the range of supersonic then the combustion is completed in the time span of less than 1x10-3 seconds. Where the fuel is injected in the perpendicular cross-section of airstream for getting maximum fuel efficiency and minimum fuel loss. Then thrust equation for this system will be

F=meVe - moVo+(Pe – PO)Ae

Where F = thrust, moVo = free stream mass flow, meVe = exit mass flow, (Pe– PO) = pressure difference in between inlet and nozzle, Ae = nozzle cross section.When an engine operates at hypersonic speed then the selection of fuel is the most important task. The ideal fuel should have high calorific value, low ignition energy, wide flammability limits, high density. Currently effervescent kerosene and liquid hydrogen are the most used fuel in the SCRAMJET system as these fuels are satisfying most of the above conditions.There are many challenges in operating a SCRAMJET engine. First of all, it cannot start from rest. That’s why a booster is used to reach supersonic speed. the engine needs a constant air flow, so that the vehicle should be in the 0-50 km atmospheric height. In a hypersonic motion the temperature on the surface of the vehicle reaches around 30000c which is enough to melt metals like iron, copper and aluminium . For that reason, a mixture of titanium carbide and boron nitride nanotubes are used on the surface of the moving body. Also, the changing of direction is much more difficult in such high moving vehicles.

NASA had done SCRAMJET flight testing of X-43A aircraft. Also, China and Russia claimed to have hypersonic missile systems. Now SCRAMJETs are being used in missile and unmanned jets. The day we use SCRAMJET in the commercial passenger flights, we can travel from Chennai to Delhi in just 10 minutes.

REFERENCES

https://www.nasa.gov/missions/research/f_scramjets.html

https://www.isro.gov.in/launchers/isro%E2%80%99s-scramjet-engine-technology-demonstrator-successfully-flight-tested