Ramjet vs Scramjet
Supersonic and Hypersonic flight are dreams of aeronautical engineers, where flying as many times the speed of sound is a technically difficult endeavor. Even though supersonic is a realized dream the cost makes it difficult to be used financially.
Ramjet and Scramjets are engines using its own speed to compress air and propel the engine. The ramjet engine technology practically applies in many cases ranging from missiles, supersonic aircraft to artillery rounds, while the scramjet engines are still highly experimental.
More about Ramjet
The ramjet, sometimes called a stovepipe jet or athodyd, is an air breathing jet engine, which uses the engine’s forward motion to compress incoming air, without a rotary compressor present in jet engines.
By design, ramjets cannot produce thrust at zero velocity, initially when they are still. Therefore, the aircrafts require a propulsion system to initiate the movement for compression in the ramjet to undertake. For optimal operation ramjets need velocities around Mach 3 and can operate up to speeds of Mach 6. The operation of the Ramjet is based on the Brayton cycle.
The intaken air is compressed using nozzles created in the compression areas, and the flow speed is reduced to subsonic speeds to allow better combustion. A flame holder ignites the mixture to produce high pressure high velocity gas stream that exits the engine at supersonic speeds.
Ramjets can be used for applications requiring a small and simple engine for high speed use, such as Russian Indian stealth BrahMos missiles, and Indian Akash missile uses ramjet technology. They have also been used successfully, though not efficient, as tip jets on helicopter rotors. The jet engines of legendary Lockheed SR 71 act as ramjets as the aircraft speeds more than the speed of sound.
Advantages of ramjets are independent of oxygen supply and include no rotating parts which makes it easier to manufacture and maintain.
More about Scramjet
A scramjet (Supersonic Combustion RAMjet) is a variant of a ramjet in which combustion takes place when the airflow is supersonic. As in ramjets, scramjets also compress the incoming air before combustion using the speed of the vehicle. However, ramjets decelerate the airflow to subsonic speeds within the engine prior to combustion, though airflow in a scramjet is supersonic throughout the entire engine. Supersonic flow creates more reaction giving the capability for scramjets to operate efficiently at Hypersonic speeds; theoretical top speed of a scramjet lies between Mach 12 (15,000 km/h) and Mach 24 (29,000 km/h), and the fastest air-breathing aircraft has Scramjet engines; the NASA X-43A reached Mach 9.6.
Like ramjet, scramjets have no moving parts inside the engines and inherit all the fundamental requirements like primary propulsion system to accelerate them to supersonic speeds. While scramjets are conceptually simple in design and construction, actual implementation is limited by extreme engineering challenges. The aerodynamic drag at Hypersonic flight within the atmosphere is immense, and temperatures found on the aircraft and the engine is much greater than the temperature surrounding the air; hence requiring new materials to endure the temperatures. Maintaining combustion in the supersonic flow poses additional challenges because, within milliseconds of time, the fuel must be injected, mixed, ignited, and burned.
In addition to the advantages of common ramjets, scramjets have a higher specific impulse (change in momentum per unit of propellant) than conventional jet engines. Ramjets produce specific impulse between 1000 and 4000 seconds, while a rocket only provides 600 seconds or less.
Because scramjets have the high theoretical performance, they have been suggested as the power plant for next generation orbital vehicles and the NASA conducts extensive research in the scramjet technology arena.
What is the difference between Ramjet and Scramjet?
• Flow inside the ramjet is subsonic while, in the Scramjet, it is supersonic.
• Scramjets produce higher specific impulse.
• Theoretically Ramjets have a velocity range of 1 to 6 Mach while, in Scramjets, the range is 12 to 24 Mach. However, the fastest practically obtained speed is 9.6 mach obtained by the X-43A.
Diagram Sources:
http://en.wikipedia.org/wiki/File:Ramjet_operation.svg
http://en.wikipedia.org/wiki/File:Scramjet_operation_en.svg