‘Like lighting a match in cyclone’ — Beauty of India’s hypersonic flight tech is in its engine

DRDO's successful test of a Hypersonic Technology Demonstrator Vehicle this month is a result of several indigenous tech achievements and know-how obtained over two decades.

The Hypersonic Technology Demonstrator Vehicle was launched from APJ Abdul Kalam testing range in Odisha’s Balasore | By special arrangement

Bengaluru: The successful test of the Hypersonic Technology Demonstrator Vehicle (HSTDV), conducted by the Defence Research and Development Organisation (DRDO) earlier this month, puts India in an elite group of nations that possess hypersonic cruise-vehicle technology. 

This is a result of several indigenous technological achievements and know-how obtained over the past two decades. This development assumes added significance in the backdrop of the ongoing tensions at the Line of Actual Control (LAC). 

Given that the Agni missiles and Indian Space Research Organisation (ISRO) launch vehicles already achieve hypersonic speeds (greater than Mach 5, or five times the speed of sound), one may wonder what makes HSTDV a big deal. 

To answer this, we take a closer look at the scramjet engine that powers HSTDV.

Ramjets and scramjets

Typical engines on airplanes, rockets, and missiles burn a mixture of fuel and oxidiser (combustion) to generate power (in a manner broadly similar to that observed in automobile engines), which is then used to create thrust forces to propel the vehicle. 

Missiles and other flight systems designed for supersonic speeds (above Mach 1 and below Mach 5) commonly use a ramjet engine. 

Unlike the gas turbine engines found on commercial transport airplanes, ramjets have no moving or rotating mechanical parts. At supersonic speeds, a stream of fast-moving air from the atmosphere, which contains oxygen, rushes into the engine. 

Fuel is injected into this airstream inside the engine, and a fast-moving mixture of fuel and oxygen is created. This mixture is ignited to initiate combustion and generate thrust. 

The ramjet engine is what powers the Brahmos cruise missile.

A scramjet — short for supersonic combustion ramjet — works on similar concepts as a ramjet, but is designed for operation at even higher flight speeds, going into the hypersonic territory. 

One of the key challenging aspects of a scramjet is that the air ingested by the engine flows through at very high speeds — this internal air-flow speed is itself supersonic. In this scenario, proper injection of fuel into the airstream, holding of a steady flame, and ensuring complete fuel combustion in the engine is an immensely challenging engineering task — one that demands a good scientific understanding of supersonic flow and combustion mechanics. 

It’s like lighting and holding a matchstick flame in the open during a cyclone with intense winds, as goes an analogy often used in aerospace engineering classrooms. 

The flame inside the engine, which initiates and sustains combustion, can very easily be extinguished by the high-speed airstream (known as flame blowoff), and this basically shuts down the engine, leading to immediate loss of thrust and thereby control of the vehicle.

HSTDV is powered by a scramjet, where the challenges mentioned above have been met through intricate design that performs careful conditioning of the internal airstream, and promotes a stable and continuous combustion process for steady engine operation. 

This was demonstrated during a recent test flight, where the vehicle cruised at Mach 6 (nearly 7,200 kmph) in free flight for more than 20 seconds powered solely by its scramjet.

In recent years, ISRO has also been engaged in independent efforts to develop scramjet technology. In August 2016, it completed a successful flight experiment designed to test scramjet engine technology developed in-house. 

Unlike the HSTDV test, where an entire flight vehicle was demonstrated, the ISRO tests were focused on the engine alone — two scramjet engines were bolted as add-ons to a rocket and operated in parallel to the rocket engine. These tests comprehensively demonstrated the workings of a scramjet over a range of Mach speed numbers and altitudes.

The scramjet advantage

The Agni missiles use a solid propellant that contains both fuel and the oxidiser needed for combustion. Similarly, ISRO space-launchers also carry both fuel and oxidiser onboard. 

In comparison, a scramjet engine draws oxygen from the atmosphere, resulting in significant savings in terms of the weight of oxidiser that does not have to be carried onboard. 

These savings directly translate to higher payload capacity, and/or extended flight range for the vehicle. 

Also, unlike solid-propellant engines, scramjets allow for a certain level of on-demand acceleration and deceleration by regulating the fuel burn rate, thereby enabling the vehicle to cruise in a controlled manner. 

The manoeuvrability of a scramjet-powered hypersonic vehicle adds a large degree of unpredictability to its flight path, making interception much harder than for a ballistic missile like Agni. 

This aspect naturally provides significant tactical advantages in certain operational scenarios.

What next?

While this milestone is certainly a boost to India’s drive towards greater self-reliance in meeting defence needs, there may be further technology gaps that need to be bridged in order to indigenously realise a field-ready hypersonic cruise vehicle platform. 

In addition to its own resources, the DRDO can leverage the leading scientific and academic research institutions in India to pursue an aggressive timeline for developing such a platform for strategic deterrence. 

Private industry can also play a role in this endeavour, not just as manufacturing partners but, potentially, as technology and product development entities. 

In the long term, creation of an ecosystem that enables synergy between industry and government-funded organisations/institutes for building advanced field-ready technologies augurs well for realising the vision of an Atmanirbhar Bharat.

Dr Duvvuri Subrahmanyam (@mangaloreman on Twitter) is an assistant professor of Aerospace Engineering at the Indian Institute of Science (IISc) in Bengaluru. His academic research interests lie broadly in the areas of experimental aerodynamics and fluid mechanics. Views expressed here are personal.