Journal of Aeronautics & Aerospace Engineering
Open Access

Supersonic Combustion Ramjet: An Overview

Sushree Sunena^{* *}Correspondence: Sushree Sunena, Department of Biotechnology, Utkal University, Bhubaneshwar, Odisha, India, Email:

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A scramjet (supersonic combustion ramjet) is a type of ramjet air breathing jet engine that uses supersonic airflow for combustion. A scramjet, like a ramjet, relies on high vehicle speed to compress incoming air violently before combustion (thus the name), but unlike a ramjet, where the air is decelerated to subsonic velocities before combustion, the airflow in a scramjet is supersonic throughout the engine. This enables the scramjet to run at exceptionally high speeds while being efficient. Scramjet engines are a type of jet engine that generates thrust by the burning of fuel and an oxidizer. Scramjet-powered aircraft, like conventional jet engines, carry the fuel on board and receive the oxidizer by inhaling ambient oxygen (as compared to rockets, which carry both fuel and an oxidizing agent). This constraint confines scramjets to suborbital atmospheric propulsion, where the oxygen content of the air is adequate to keep combustion going.

The scramjet is made up of three fundamental parts: a converging inlet, which compresses incoming air; a combustor, which produces heat by burning gaseous fuel with atmospheric oxygen; and a divergent nozzle. The scramjet engine is the way of the future for aerospace. Scramjet engines bridge the gap between extremely efficient turbojets and rocket engines’ high speeds. To completely comprehend what a SCRAMJET, or Supersonic Combustion Ramjet, is, first understand what a ramjet is. A ramjet is a jet engine that runs on air. It does not require a compressor because it compresses the incoming air using the engine’s forward motion. The independence of ramjet and scramjet engines from the need to carry liquid oxygen on board is an intriguing advance. Scramjets can only travel at near-hypersonic speeds due to the nature of their design.

Scramjets rely on the high kinetic energy of a hypersonic flow to compress incoming air to operational conditions because they lack mechanical compressors. As a result, a scramjet-powered vehicle must be propelled to the requisite speed by another means, such as a turbojet, railgun, or rocket engine. Scramjets, like ramjets, are meant to operate in the hypersonic flight regime, beyond the reach of turbojet engines, and bridge the gap between turbojets’ high efficiency and rocket engines’ high speed. While turbomachinery- based engines are very efficient at subsonic speeds, they become increasingly inefficient at transonic speeds because turbojetcompressor rotors require subsonic speeds to operate. While the flow can be decelerated to these conditions from transonic to low supersonic speeds, doing so at supersonic speeds results in a massive temperature gain. Scramjets, like ramjets, are meant to function in the hypersonic flight regime, which is beyond the range of turbojet engines. They bridge the gap between turbojet efficiency and rocket speed.

While turbomachinery-based engines are very efficient at subsonic speeds, they become increasingly inefficient at transonic speeds because turbojet engines’ compressor rotors require subsonic speeds to operate. While the flow can be decelerated to these conditions from transonic to low supersonic speeds, doing so at supersonic speeds causes a massive temperature increase. Ramjets exploit the high-speed properties of air to physically ‘ram’ air into the combustor through an inlet diffuser. The air upstream of the inlet is unable to move out of the way rapidly enough at transonic and supersonic flight speeds, and is compressed within the diffuser before being diffused into the combustor.

In a ramjet, combustion occurs at subsonic speeds, similar to turbojets, but the combustion products are then propelled to supersonic speeds using a convergent-divergent nozzle. Ramjets cannot start from a stop because they lack mechanical compression, and they rarely reach adequate compression until supersonic flight. The speed of the airflow at combustion is the difference between a ramjet and a scramjet. While a ramjet engine reduces airflow to subsonic rates before combustion, a scramjet engine maintains supersonic airflow throughout the engine, allowing for supersonic combustion. Because supersonic flow generates more response, the scramjet can function at hypersonic speeds more efficiently.

Scramjets have numerous benefits as well as drawbacks. As previously stated, one of the engine’s advantages is that it does not require an on-board supply of oxidizer. Another significant benefit is that the engine contains no moving parts, making it easier to produce and maintain. Although scramjets appear to be simple in theory and design, putting them into practice poses numerous obstacles. The temperature on the plane is significantly higher than the air around it, necessitating the use of novel materials that can withstand these temperatures. Furthermore, scramjets are incapable of producing thrust at zero velocity, necessitating the useof a secondary propulsion system to achieve a sufficient operating flight Mach number. Despite these obstacles, scientists believe it is necessary to continue creating and testing new products. The reason for this is that scramjet engines allow for flight at fifteen times the speed of sound, which is desirable for a variety of purposes, including air travel and missiles.