X43a Scramjet Hypersonic Flight Testing
The X-43 is a deceptive aircraft at first glance: it looks like a gigantic and imposing, dark and futuristic space shuttle with a NASA logo emblazoned on its aft tail stabilizers. When shown with relative objects surrounding it such as the flight engineers below, however, the X-43 looks more like an expensive model than a real-life craft (the X-43 research vehicle measures 12 feet long). The X-43 is real, though; it has specific mission foci as an unmanned aircraft that manned testing vehicles cannot approximate. Below, it is shown attached to the appreciably larger Pegasus rocket launch vehicle, which is strapped in to its B-52 carrier craft (a common launch platform for X-program vehicles).
The X-43 was created under the Hyper-X program, which was a NASA Dryden / NASA Langley joint venture. X-43's purpose was to explore the limits of hypersonic air-breathing craft. The previous hypersonic experimental craft, the X-15, was a manned rocket that looked like a missile with a man strapped on it. X-15 flew in exactly the same manner as X-43 by being dropped from a B-52 with the notable exception that it was a self-contained rocket (with a live pilot) and needed no push from a booster rocket. Pete Knight flew the X-15 to Mach 6.7 in 1963. The fastest air-breathing craft, however, was the monstrous SR-71 Blackbird, which flew at a speed of Mach 3 on a regular basis. The X-43 was obviously never intended to carry men at this speed; it had a pre-programmed flight sequence which began by being dropped from a B-52 carrier aircraft and then pushed to cruising speed by a Pegasus rocket. A test launch demonstrating this sequence is visible below.
NASA has been experimenting with a scramjet design, because it offers an escape from the rocket paradigm, which is that the faster you want to go, the bigger the rocket needs to be. This is because the rocket needs an onboard source of oxygen to supplement its fuel source and produce combustion. Scramjets allow vehicle sizes to be smaller, which can increase the effectiveness and payload of various missions, and make projects cheaper. The website here: http://www.aviation-history.com/engines/ramjet.htm shows the two newer engine types designed to combat the oxygen storage problem and allow for smaller vehicles with larger payloads. Both have no moving parts and are relatively mechanically simple; both are also described as "aerodynamically complicated". The article here: http://www.grc.nasa.gov/WWW/K-12/airplane/ramth.html when translated describes an equation of thrust which explains how the engines work. Essentially, the yield (and resulting speed) of the thrust equation pictured in that article is a factor of gross thrust, ram drag, and pressure correction.
The article links posted throughout the text of this article are excellent resources for understanding the mathematics and science behind ramjet and scramjet engines, which are somewhat complicated in operation. The basic essence of difference between the two, however, is that ramjets slow down or "ram" the air to lower than supersonic speed before it enters the combustion chamber, while scramjets operate with the air moving at supersonic speed throughout the entire length of the engine and the entire propulsion process. Logically, this indicates that ramjets can operate at lower minimum speeds while scramjets have higher maximum speeds. Resources indicate that the shape of the exhaust nozzle is an important factor in determining normal operating ranges for the engines. What does not appear to be known yet, however, is how a ramjet/scramjet engine would operate in a conventional ground takeoff aircraft, because high speeds are required before this engine type can generate effective thrust.
NASA planned a number of vehicle variants to move progressively toward manned hypersonic scramjet flight, but the program has been suspended since 2004 (most likely due to lack of funding). The X-43A variant (successful flight test of this craft shown in the YouTube movie above) surpassed the minimum scramjet operation speed with the assistance of the Pegasus rocket. Its scramjet engine then propelled it for about 10 seconds and set a speed record of just under Mach 10 (about 7,000 miles per hour). It achieved this record in November of 2004, and was preceded by a failed test in June of 2001 and a success in March of 2004.
The intent behind testing of this engine platform was to develop a solid design scheme and eventually translate the technology to passenger airliners or passenger suborbital/orbital aircraft. One illustration noted that a craft with the hypothesized Mach 15 performance of the scramjet engine could make a Tokyo to New York flight that normally took 18 hours in 2 hours. Perhaps future develop and continuation of this research will yield developments that make the technology cost effective for commercial flight, but such testing is unlikely to be conducted under the NASA moniker. A program at the University of Queensland, Australia entitled HyShot has conducted similar testing, and may be the group that leads scramjets into the future.