Scramjet Engine: Why in News Headlines Now?

The successful testing of the Scramjet engine designed by ISRO represents a major milestone in India’s future space programs. But what is a Scramjet Engine?

A scramjet engine, short for Supersonic Combustion Ramjet, is an advanced air-breathing propulsion system designed for high-speed flight in the hypersonic regime, typically defined as speeds greater than Mach 5.

Unlike traditional jet engines, scramjets do not use rotating compressors and turbines. Instead, they rely on the forward speed of the vehicle to compress incoming air before combustion.

What is a Scramjet Engine?

To make it simple, a scramjet is a supersonic combusting ramjet). This is a variant of a ramjet jet engine in which combustion takes place in supersonic airflow.

1. Air Inlet: Scramjets are most effective at high speeds where the incoming air can be compressed without the need for traditional compressors. The engine has an inlet that slows down and compresses incoming air.
2. Supersonic Combustion: As the air enters the combustion chamber, it is mixed with fuel (typically hydrogen) and ignited. Unlike traditional jet engines, where air enters the combustion chamber at subsonic speeds, scramjets rely on supersonic combustion, meaning the air is still moving at very high speeds when combustion occurs.
3. Expansion and Exhaust: The combustion process generates high-velocity exhaust gases, which are expelled from the engine to produce thrust. The high-speed exhaust gases are then expelled through a nozzle to further accelerate the exhaust flow.

As Scramjet is essentially a Ramjet with variations, it would be easy if we started our learning process from Ramjet. Let’s first start with the basics – the difference between a jet engine and a rocket engine.

What is the main difference between a Jet and a Rocket engine?

The main difference is that a rocket carries its supply of oxygen (Oxidizer Tank) for combustion while a jet engine utilizes oxygen from the atmosphere for combustion. This makes rocket engines less energy efficient when compared to Jet engines.

Jet Engine

A jet engine is a machine that converts energy-rich, liquid fuel into a powerful pushing force called thrust. The thrust from one or more engines pushes a plane forward, forcing air past its scientifically shaped wings to create an upward force called lift that powers it into the sky.

The first operational Jet Engine was developed by Hans von Ohain of Germany. But the credit for its discovery went to Great Britain’s Frank Whittle (1930).

The main processes in a Jet Engine

A Jet Engine has 3 main processes:

1. Compression

It increases the pressure of the air trapped inside the chamber.

2.  Combustion

It increases the temperature of the air-fuel mixture by releasing heat energy from the fuel.

3. Exhaust

It increases the velocity of the exhaust gases, thereby powering the vehicle. (Newton’s third law of motion)

Types of Jet Engines

All jet engines and gas turbines work in broadly the same way. Hence they all share five key components: an inlet, a compressor, a combustion chamber, and a turbine (arranged in exactly that sequence) with a driveshaft running through them. Even though they have similarities, jet engines differ in their extra components. The main types of Jet engines based on how air is compressed in the engine are as below:

1. Turbo Jet

The Turbojet engine is an air-breathing jet engine. This is one of the most common types of jet engines. It is still widely used in airplanes.

2. Turbofan Engines

Turbofans differ from turbojets in the way they have an additional component – a fan. The fan sucks in the air and then further gets compressed and combustion takes place in the burner.

3. Ramjet

High-speed forward motion is used to compress the air (no compressor). Fuel is injected into the combustion chamber where it mixes with the hot compressed air and ignites. The average speed of Ramjet is 3-6 Mach. But the ramjet efficiency starts to drop when the vehicle reaches hypersonic speeds.

4. Scramjet

Here also, the high-speed forward motion is used to compress the air (no compressor), but it is an improvement over the ramjet engine as it efficiently operates at hypersonic speeds and allows supersonic combustion. Speed is greater than 6 Mach (Six times the speed of Sound).

5. Dual mode Ramjet (DMRJ)

Type of jet engine where a ramjet transforms into scramjet over Mach 4-8 range, therefore, it can operate both in subsonic and supersonic combustion modes.

Scramjet (Supersonic Combustion Ramjet): Operation Details

To fire something we need air, that is oxygen. Rockets usually carry oxygen in an oxidizer tank even when it is traveling in Earth’s atmosphere. But, if we can use atmospheric oxygen during this time it would become energy efficient. Here comes the importance of the Scramjet engine.

• Scramjet engine designed by ISRO uses Hydrogen as fuel and Oxygen from the atmospheric air as the Oxidizer. This makes the system much lighter, more efficient, and cost-effective.
• The scramjet engine was test launched on board of Rohini 560 sounding rocket (Research Rocket). This RH 560 is a two-stage Advanced Technology Vehicle (ATV) and is designed & developed by ISRO to test the scramjet engine. ATV is the heaviest (3 tons) sounding rocket developed to date by the ISRO.

Key Features of Scramjet Engines

1. Air-Breathing: Scramjets are air-breathing engines, meaning they take in air from the atmosphere for combustion. This design eliminates the need to carry onboard oxidizers, making them more efficient for certain flight regimes.
2. Hypersonic Speeds: Scramjets are specifically designed for hypersonic flight, operating efficiently at speeds greater than Mach 5. They become more efficient as the speed of the vehicle increases.
3. No Moving Parts: Unlike traditional jet engines, scramjets have no rotating parts like compressors and turbines. Instead, they rely on the aerodynamic design of the engine to compress incoming air.
4. Limited Usefulness at Lower Speeds: Scramjets are most effective at high speeds, and they typically need to be accelerated to a certain speed before they become operational. Below a certain speed, other propulsion systems may be more practical.

India and Scramjet Technology

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India had started the efforts on Scramjet technology long back and we did ground testing in 2006 which validated the design of the engine. And now, India has become the fourth country to claim the successful testing of Scramjet Engine. Russia was the first country who claim scramjet flight test in 1991, and later US and European Space agency also joined the elite group.

Scramjet – Challenges              

• As mentioned earlier, Scramjet does not have a compressor to compress the air, instead, it uses its high-speed forward motion to compress the air. Therefore scramjet cannot produce efficient thrust unless boosted to high speed. From this, we understood that a scramjet engine cannot work until it attains enough speed. This is the main drawback of the Scramjet Engine.
• As we all know, Launch Vehicles have multiple stages with each having a different engine. After the fuel burns out in the first stage, the engine is jettisoned to reduce the weight of the vehicle. So when the second stage ignition begins, speed is enough for the scramjet to function. Therefore ISRO used a scramjet engine at the second stage during the flight test and using this technique it excelled in overcoming the drawback.
• Some of the other technological challenges handled by ISRO during the development of the Scramjet engine include the design and development of the Hypersonic engine air intake, the supersonic combustor, the development of materials withstanding very high temperatures, computational tools to simulate hypersonic flow, ensuring performance and operability of the engine across a wide range of flight speeds, proper thermal management and ground testing of the engines.

The importance of having this technology

• The efficiency of the launch vehicle depends on how much weight it can carry with a particular amount of fuel. Currently, Indian rockets carry a huge quantity of oxygen (approx.200 tons) that is nearly 85 percent of the rocket mass and it gets burnt during the atmospheric flight phase of the rocket. The use of Scramjet would help to reduce the weight of the launch vehicle and increase the speed. It would help India to do more with a limited space budget.
• The satellite launching business is gaining momentum and Antrix Corporation (the Commercial wing of ISRO) has been launching satellites at a lower cost when compared to other players. This can be further augmented by using Scramjet Engine which would help to minimize the cost.
• Scramjet engines can be used to augment ISRO’s reusable launch vehicle (RLV).

Avatar

Avatar (from “Aerobic Vehicle for Transatmospheric Hypersonic Aerospace TrAnspoRtation”) is DRDO’s reusable launch vehicle platform which is capable of carrying out satellite launches like normal aircraft, taking off vertically and landing back on a runway.

It is designed to use ramjets and scramjets for thrust. Each of these engines will be used in different stages of the flight wherein scramjets are used at hypersonic speeds.

Summary

In a nutshell, the development of Scramjet engines is an important milestone in ISRO’s endeavor towards its future space transportation systems. Air-breathing propulsion is a solution for a powered long return cruise flight necessary for reusable launch vehicles. In addition to this, DRDO has been working on developing the Brahmos II Cruise Missile based on the same scramjet technology. This calls for the faster development of ramjet or scramjet technologies which would help India execute advanced space missions in the future. Also, it will add colors to our much-hyped Make In India initiative.

Article contributed by Sree Resmi S