Hypersonic Aircraft: X-51 Waverider
In April the US Air Force, Boeing and United Technologies will launch a hypersonic jet called the X-51 Waverider that will fly at 4,000 miles per hour -- six times the speed of sound -- or Mach 6. The small jet is shaped like a missile. It will be attached to the wing of a B-52 bomber and flown to an altitude of 50,000 feet. During its maiden flight, the rocket-shaped aircraft will be dropped from its carrier over the Pacific Ocean, where it will fall freely for 4 seconds. Then, a tactical missile rocket will fire, boosting the jet to faster than four times the speed of sound. Its massive motor will spring to life, and quickly accelerate it to a speed of about 5,800 km/hr, near the Mach 5 horizon. Once this speed is reached, its one-of-a-kind scramjet engine will roar to life, and will accelerate the craft even further, to Mach 6, or about 7,400 km/hr or 4,000 mph at that altitude. From New York's JFK to London's Heathrow at that speed? 52 minutes! By comparison, the supersonic Concorde had an average cruise speed of Mach 2 or about 2,140 km/h or 1,330 mph.
The US Air Force has over the past few years been developing a new type of airplane, a scramjet-powered hypersonic craft, capable of reaching as much as Mach 7 in speed. Experts in charge of the project have already flown the X-51 Waverider while attached to a B-52 bomber, and they recently announced that they were about to begin the first of four test flights to see the X-51 actually separating from its host, and going hypersonic. The goal engineers tried to achieve was to create a plane that would travel faster and further than all of its predecessors. St Louis-based Boeing Defense, Space & Security Systems has been working on the airplane since 2003, and officials highlight that the new airplane is the first hypersonic vehicle to fly in the United States for the past six years. Behind this project are the Air Force Research Laboratory and also the Defense Advanced Research Projects Agency (DARPA). DARPA had a lot to do with many of the objects in the modern world that we see and touch every day, such as the Internet and the computer mouse, but its name is barely ever mentioned in association with these accomplishments. It is heavily involved with classified research projects.
The fastest aircraft ever, an unmanned aircraft predecessor called the X-43, hit 9.8 times the speed of sound or Mach 9.8 in 2004. It was powered by hydrogen and flew for just 10 seconds. The second fastest, the X-15, was powered by rockets and was basically a spaceship -- many of its flights, mainly in the 1960s, qualified its pilots as astronauts!
The X-51 is a US Air Force program designed to push the limits of high-speed flight. The airplane, built by Boeing and powered by the United Technologies subsidiary Pratt & Whitney Rocketdyne, would be the third-fastest aircraft in history and the fastest powered by jet fuel. If the X-51 works as planned -- not a sure thing given that the first X-43 failed -- it would be only the third flight to reach hypersonic speeds.
What is Hypersonic Flight?
Hypersonic flight is defined as aircraft speed faster than five times the speed of sound or Mach 5, as opposed to the start of supersonic flight at Mach 1. Grand plans for hypersonic vehicles have been around for decades, but their goals were often unrealistic and not matched by budgets, resulting in failure. The approach on X-51 has been to demonstrate the technologies that could one day enable craft like single-stage-to-orbit vehicles. Potential applications for hypersonic technology are superfast airplanes, missiles and reusable space launch vehicles. The reason why Mach 5 is considered to be the hypersonic flight threshold by many is the fact that a combination of phenomena, including the molecular dissociation and ionisation of the airflow around the plane, starts to play a very significant part in the overall forces that particular craft is subjected to. Coming up with a more rigid definition for when hypersonic flight begins is rather difficult, because many characteristics change at different speeds, not all at once.
The programme is more than a speed contest. It's part of an effort to go fast and travel further without having to carry lots of rocket fuel. For 300 seconds the X-51 will be powered by a hydrocarbon-based jet fuel called JP7 -- 30 times longer than the X-43. As it stands now, the Space Shuttle and other space-bound vehicles have to carry huge tanks of oxygen along board to keep the rocket fuel burning. If engineers could build a craft that could use the oxygen in the air for at least part of its journey, the craft could carry more useful supplies and equipment into space. NASA began looking at the technology (it ran the X-43 program) to develop a less bulky and expensive way of getting to space.
The Unique Scramjet
The engine on the X-51, called a supersonic-combustion ramjet, or "scramjet," achieves a number of tricky tasks simultaneously:
. It uses air not just to burn the fuel, but also to provide the compression for the combustion that in most motors is provided by a fan or a piston;
. The plane's air-intake chamber is shaped in such a way that inrushing air is slowed to about half the speed of the plane, ie, Mach 3;
. The slowed air is directed to the combustion chamber to increase the pressure there to the optimal level for burning the fuel;
. The fuel is then pressed into double service: it is used for combustion and its way the fuel sucks heat from the engine; and
. This cools the engine and heats the fuel to its optimum temperature for combustion.
The whole process has been compared to keeping a match lit in a hurricane. But that understates the problem: hurricane winds are nowhere near Mach 3!
Test Flight Schedule
Boeing says it expects to fly the X-51 in mid-April. The plane is ready to go, and the maiden flight was tentatively scheduled for March 17, but it has been put off until the US Air Force can carve out some time on the equipment needed for launch and line up the necessary air space clearance. This spring's first flight will be one of four scheduled for the programme. After that, researchers would like use their findings to design a system that could both go faster, which could get them closer to space, and slower, which would allow them to depart from the ground. And maybe, someday, bound from New York's JFK for London's Heathrow!
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