Scramjet
A scramjet (supersonic combustion ramjet) is a type of jet engine. Scramjet engines are intended to propel aircraft at hypersonic speeds. It consists of a constricted tube through which air is compressed, fuel is combusted, and the exhaust is vented at higher speed than the intake air. However, like a ramjet, there are ether few or no moving parts, the compression being provided by the fact that the engine is moving through the air, and air is being rammed into the air intake (and thus compressed).Like a ramjet, the scramjet must already be moving extremely fast before it will start working but, theoretically, speeds in excess of Mach 20 are possible.
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2 Applications 3 Simulations and testing of models 4 Scramjet programmes 5 References |
Since the engine itself is moving through the air, the engine walls (and any other parts) generate multiple shock waves that inhibit the predictable and proper mixing of fuel and air in order for the combustion to occur in a well-defined manner. And, in layman's terms, the last thing you want when you're going mach 5 is to have lots of fuel in or near your engine that may or may NOT be combusting!
Scramjets, like all jets, are undergoing continuing development and refinement. But, it would be best to say they are now in a primitive stage of development, and have been for the last 30 years. Early tests are promising, having demonstrated speeds far in excess of what is possible with previous jet engines.
Scramjet technology has significant potential, so governments and organizations around the world are researching it for use in hypersonic vehicles. An aircraft using this type of jet engine could dramatically reduce the time it takes to travel from one place to another, potentially putting any place on Earth within a 90-minute flight.
Other uses of such an engine is on a (presumably) reusable space delivery vehicle. One of the main limitations of rockets is that they must carry oxygen with them; a scramjet would allow atmospheric oxygen to be used instead. This would provide a 700% (7x) efficiency boost to a space vehicle that used it.
A limitation of previous jet engine design has been the fact that when the speed of the air flowing through the engine reaches the speed of sound (when the aircraft itself is moving at around Mach 5), the fuel fails to burn properly. Scramjets attempt to overcome this effect in a number of ways. Usually, the inlet is much wider (typically the entire underside of the craft) so the air remains at supersonic speeds. Some designs use reactive chemicals or gases other than standard jet fuel. Combustion occurs similarly to inside a diesel engine - the combination of heat and pressure is high enough to ignite the fuel-air mixture without the need for a flame holder.
This type of engine is very complex to design due to limitations on both simulating its operation and testing those simulations with actual working engines.
Subsonic gas behaviors are governed by several relatively simple rules: the ideal gas law, the Bernoulli's principle, the laws of thermodynamics, and others. However, at supersonic speeds, one of the most important, the bernouli principle, no longer applies, and the concept of Laminar flow of air over a surface is also entirely different.
There are wind tunnels that can provide airflows above mach 1, and rumors of (military) ones that can produce even mach 3 airflows. However, these wind tunnels lack several features:
However, hypersonic simulation computer code is NOT well verified. These simulations are very mathematically intensive (see computational fluid dynamics for why). So even running a simulation can take a long time. This added to the severe lack of ability to verify any given simulation means it is very difficult to design a working engine.
The team took a unique approach to the problem of accelerating the engine to the necessary speed by using an Orion-Terrier rocket to take the aircraft up on a parabolic trajectory to an altitude of 314 km. As the craft re-entered the atmosphere, it dropped to a speed of Mach 7.6. The scramjet engine then started, and it flew at about Mach 7.6 for 6 seconds. [1]. This was achieved on a lean budget of just A$11.5 million (US $1.1 million), a tiny fraction of NASA's $US 250 million to develop the X-43A.
NASA has partially explained the tremendous difference in cost between the two projects by pointing out that the American vehicle has an engine fully incorporated into an airframe with a full complement of flight control surfaces available.
Theory
Applications
Simulations and testing of models
Computer software and configuration information that simulates the 'fluid dynamics' of the air going past an object at a variety of speeds is by now quite accurate for the subsonic range of speeds, object shapes, object sizes, and atmospheric variations (temperature, density/altitude, humidity). This software is also pretty accurate for airflows that are in the mach 1 to 2 range. Most large university aerospace engineering departments own a wind tunnel that can generate these speeds, so many variations are (as engineers say) 'well characterized'.Scramjet programmes
HyShot
On July 30, 2002, the University of Queensland's HyShot team successfully conducted the first ever test flight of a scramjet. X-43A
NASA first successfully flew its X-43A scramjet test vehicle on March 27, 2004 (an earlier test, on June 2, 2001 went out of control and had to be destroyed). Unlike the University of Queensland's vehicle, it took a horizontal trajectory. After it separated from its mother craft and booster, it briefly achieved a speed of 5,000 miles per hour (8,000 km/h), the equivalent of Mach 7, easily breaking the previous speed record for level flight of an air-breathing vehicle. Its engines ran for eleven seconds, and in that time it covered a distance of 15 miles (24 km).Russia and France (and NASA)
On November 17, 1992, Russian scientists with some additional French support successfully launched a scramjet engine in Kazakhstan. In 1998, NASA worked with Russian scientists to test a ramjet-scramjet engine. Data regarding whether the internal combustion took place in supersonic air streams was inconclusive, according to NASA. Scramjet testing is believed to be continuing in Russia.GASL projectile
At a test facility at Arnold Air Force Base in the U.S. state of Tennessee, GASL fired a projectile equipped with a hydrocarbon-powered scramjet engine from a large gun. On July 26, 2001, the four-inch (10-centimeter) wide projectile covered a distance of 260 feet in 30 milliseconds (roughly 5,900 mph or 9,500 km/h). The projectile is supposedly a model for a missile design. Many do not consider this to be a scramjet "flight," as the test took place near ground level. However, the test environment was described as being very realistic.References