On Monday (Jul 22), the Indian Space Research Organisation (ISRO) will be test-flying a unique piece of technology known as an air-breathing propulsion system, WION has learnt. The test launch of this system is expected around 7:30 am IST, from India's spaceport Satish Dhawan Space Centre, Sriharikota. This flight test will be performed using a modified version of ISRO's 'Rohini-560' experimental rocket, dubbed as Advanced Technology Vehicle (ATV). The test articles would be mounted on the ATV rocket.

This test will be a follow-up to ISRO's maiden experimental flight of air-breathing propulsion technology that was carried out on August 28, 2016.


What Is Air-Breathing Propulsion?

Typically, a rocket consists of two tanks - a fuel tank and an oxidiser tank, both of which (known as propellants) comprise a significant mass of the rocket. The oxidiser is used to provide the necessary oxygen to enable the burning of the fuel, which together propel the rocket upwards.

In case of air-breathing propulsion systems, the rocket will carry its fuel, but will not carry an on-board oxidiser. Instead, this system will utilise atmospheric oxygen as an oxidiser to burn the fuel. This makes rockets significantly lighter and more efficient. However, such air-breathing technologies can be used only within the denser layers of the Earth's atmosphere, where there is an adequate supply of oxygen. Such technologies might be feasible in the denser layers of Earth's atmosphere (up to 70kilometres altitude), and thereafter the rocket must switch to another stage that has both fuel and an on-board oxidiser,

For context - India's largest rocket LVM-3 weighs around 640 tons at lift-off. Of the 640 tons mass, nearly 555 tons or 86 per cent is the propellant or the fuel and oxidiser combination. Of the 555 tons of propellant, nearly 70 per cent or 385 tons is oxidiser alone.

While the rocket weighs 640 tons, the cargo that it carries to space would be just about four to eight tons. Effectively, the cargo delivered to space by the LVM-3 rocket is anywhere between 0.6-1.25 per cent of the rocket's own lift-off mass. Almost all conventional rockets deliver only 2-4 per cent of their lift-off mass to orbit. However, when air-breathing technologies are incorporated, the avoidance of oxidiser in the atmospheric phase of flight can lead to significant improvement in rocket efficiency, bringing down rocket mass, and reducing rocket costs.

What Are The Types of Air-Breathing Propulsion?

Ramjet, Scramjet and Dual Mode Ramjet (DMRJ) are the three concepts of air-breathing engines which are being developed by various space agencies.

Ramjet

Ramjet engines have no moving parts and rely on the vehicle’s high speed to compress incoming air. The compressed air is mixed with fuel and ignited, producing thrust that propels it. Ramjet cannot fly on its own from the ground. A rocket will have to carry the ramjet engine to a supersonic speed and then the ramjet engine will take over and deliver speeds between Mach 3 and Mach 5. Mach 1 refers to the Speed of sound - 343m/sec or 1,234 kmph. The increase in Mach number denotes a corresponding increase in the speed.

For example, the Indo-Russian BrahMos missile uses a Ramjet engine. At launch, the BrahMos' first-stage booster engine would propel the missile upwards and then to supersonic speeds, after which the first-stage booster is separated. Then the Scramjet engine takes over and helps the missile fly at speeds close to Mach 3.

Scramjet

A Scramjet engine is an advanced version of the ramjet. Scramjet stands for Supersonic Combustion Ramjet. Scramjets operate at hypersonic speeds (Mach 5 and above). Scramjets do not have any rotating parts such as compressors or turbines. Instead, they rely on the high speed of the vehicle to compress incoming air, which is then mixed with fuel and burned to produce thrust.

While the vehicle is travelling at hypersonic speeds, air enters the scramjet engine through an inlet where it is compressed due to the vehicle's high velocity. This compression raises the air pressure and temperature. The compressed air is mixed with fuel (usually hydrogen) and ignited. The airflow through the engine remains supersonic throughout the combustion process, making it suitable for very high-speed flight. Scramjets are designed to operate efficiently at speeds typically between Mach 5 and Mach 10.

Dual Mode Ramjet

A Dual Mode ramjet (DMRJ) is a type of engine where a ramjet transforms into a scramjet over Mach 4-8 range, which means it can efficiently operate both in subsonic and supersonic combustor modes. Simply put, it combines the characteristics of a Ramjet and Scramjet.

ISRO's Scramjet Engine

The Scramjet engine designed and developed by ISRO uses Hydrogen as fuel and oxygen from the atmospheric air as the oxidiser. In August 2016, the maiden short-duration experimental test of ISRO’s Scramjet engine was accomplished with a hypersonic flight at Mach 6 and the Scramjet engines functioned for 5 seconds. ISRO’s Advanced Technology Vehicle (ATV), a modified RH-560, served as the solid rocket booster that carried the scramjet test articles to supersonic speeds. The then ATV carrying Scramjet engines weighed 3,277 kg at lift-off.

In the maiden test, ISRO successfully demonstrated critical technologies such as the ignition of air-breathing engines at supersonic speed, holding the flame at supersonic speed, air intake mechanism and requisite fuel injection systems.

Gradually, ISRO plans to develop an autonomous vehicle powered by Scramjet engine capable of accelerating for a period of 250 seconds. ISRO also intends to develop HAVA - Hypersonic Air Breathing Vehicle with Air Integration Systems. Such technologies are of strategic use and prove crucial in the development of advanced, high-speed aerospace vehicles. India is the fourth country to demonstrate the flight testing of a Scramjet engine.