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NASA technicians working on the X-43A prior to launch (March 27, 2004)
The
X-43 is an
experimental hypersonic aircraft design with multiple planned
scale variations meant to test different aspects of highly
supersonic flight. The initial version, the
X-43A, has been designed to operate at
speeds greater than seven times the
speed of sound (
Mach 7, about 5,000
mph or 8,050
km/h) and at
altitudes of 100,000
feet (30,000 m) or more. On
March 27,
2004, NASA succeeded in launching one of the aircraft to hypersonic speeds, three
years after a similar test failed. This test made the X-43A the fastest free flying
air-breathing aircraft in the world, though preceded as the first operating scramjet engine flight by an Australian
HyShot still attached to its launching missile, which flew in descending powered flight in
2002. (While
rockets can go faster, they carry an onboard load of
oxidizer rather than using
oxygen from the
atmosphere.)
The most recent success in the X-plane series of aircraft, the X-43 is part of NASA's Hyper-X program, involving the American space agency and contractors such as Boeing, Microcraft, and General Applied Science Laboratory (GASL). The budget for this ongoing program is approximately $250 million. A follow-on program to develop the X-43C has already begun.
The craft
The current X-43A aircraft is a small unpiloted test vehicle measuring just over 12 feet in length. The vehicle is a lifting body and waverider design, where the body of the aircraft provides a significant amount of lift for flight, rather than relying on wings. The aircraft weighs roughly 3,000 pounds or about 1,300 kilograms. Future X-43 versions are expected to have very similar body designs, though most of the following test vehicles are expected to be larger.
The X-43A is designed to be fully controllable in high-speed flight, even when gliding without propulsion. However, the aircraft is not designed to land and be recovered. Test vehicles are expected to crash into the Pacific Ocean when the test is over.
Traveling at Mach speeds produces a lot of heat due to friction from wind resistance. At high Mach speeds, heat can become so intense that metal portions of the airframe melt. The X-43A compensates for this by cycling water behind the leading edges of the aircraft, cooling those surfaces. In tests, the water circulation is activated at about Mach 3. In the future, fuel may be cycled through such areas instead, much like what is currently done in many liquid-fuel rocket nozzles and high speed planes such as the SR-71.
NASA's B-52B launch aircraft takes off carrying the X-43A hypersonic research vehicle (March 27, 2004)
The engine
The craft was created to develop and test an exotic type of engine called a supersonic-combustion ramjet, or "scramjet," an engine variation where internal combustion takes place within air that is flowing at supersonic speeds. The X-43A's developers designed the aircraft's airframe to positively affect propulsion, just as it affects aerodynamics: in this design, the forebody is a part of the intake airflow, while the aft section functions as a nozzle.
The engine of the X-43A is primarily fueled with hydrogen. In the successful test, about two pounds (or roughly one kilogram) of the fuel was used. However, because hydrogen poses certain difficulties in storage, transport, and even production, future X-43 versions may use more commonly available hydrocarbon fuels instead. Unlike rockets, scramjet-powered vehicles do not carry oxygen onboard for fueling the engine. Removing the need to carry oxygen significantly reduces the vehicle's size and weight. In the future, such lighter vehicles could bring heavier payloads into space or carry payloads of the same weight much more efficiently.
Scramjets only operate at hypersonic speeds in the range of Mach 6 or higher, so rockets or other jet engines are required to initially boost scramjet-powered aircraft to this base velocity. In the case of the X-43A, the aircraft was accelerated to high speed with a Pegasus rocket launched from a converted B-52 Stratofortress bomber. The combined X-43A/Pegasus vehicle is referred to as the "stack" by the program's team members.
The engines in the X-43A test vehicles are specifically designed for a certain speed range, only able to compress and ignite the fuel-air mixture because the incoming airflow is moving as expected. Future X-43 vehicles may incorporate moving panels within the engine to adjust for the movement of air at faster and slower speeds. The first two X-43A aircraft were intended for flight at approximately Mach 7, while the third is meant to fly at approximately Mach 10.
Tests
The Pegasus booster accelerating the X-43A, shortly after booster ignition (March 27, 2004)
NASA's first X-43A test on
June 2,
2001 failed because the Pegasus booster lost control about 13
seconds after it was released from the B-52 carrier. The rocket experienced a control oscillation as it went
transonic, eventually leading to the failure of the rocket's
starboard elevon. This caused the rocket to deviate significantly from the planned course, so the stack was destroyed by onboard
explosives as a safety precaution. An investigation into the incident stated that imprecise information about the capabilities of the rocket as well as its flight environment contributed to the accident, though no single factor could ultimately be blamed for the failure.
In the second test, the Pegasus fired successfully and released the test vehicle at an altitude of about 95,000 feet. After separation, the engine's air intake was opened, the engine ignited, and the aircraft then accelerated away from the rocket. Fuel was flowing to the engine for eleven seconds, a time in which the aircraft traveled more than 15 miles (24 km). After burnout, controllers were still able to maneuver the vehicle and manipulate the flight controls for several minutes as the aircraft was slowed down by wind resistance and took a long dive into the Pacific. Peak speed was at burnout of the Pegasus but the scramjet engine did accelerate the vehicle in climbing flight, after a small drop in speed following separation.
NASA plans to fly a third version of the X-43A in the fall of 2004. If successful, that test should take the vehicle to a speed of Mach 10 and further test the ability of the vehicle to withstand the heat loads involved.
Future development
Other X-43 vehicles are in the process of being designed. They are expected to have the same basic body design as the X-43A, though the aircraft are expected to be moderately to significantly larger in size.
X-43B
The next letter down the list, the X-43B, is expected to be a full-size vehicle, incorporating a turbine-based combined cycle (TBCC) engine or a rocket-based combined cycle (RBCC) ISTAR engine. Jet turbines or rockets would initially propel the vehicle to supersonic speed. A ramjet might take over starting at Mach 2.5, with the engine converting to a scramjet configuration at approximately Mach 5. If the vehicle is heading to space, it may activate rockets once the atmosphere becomes thin.
If the vehicle is ever demonstrated, it will most likely appear after the X-43C.
X-43C
The X-43C is another relatively small test vehicle, though somewhat larger than the X-43A. It is expected to test the feasibility of hydrocarbon fuel, potentially with the HyTech engine. While most scramjet designs to date have used hydrogen fuel, HyTech runs on conventional kerosene-type hydrocarbon fuels, which are much more practical for support of operational vehicles. A full-scale engine is now being built, which will use its own fuel for cooling. Using fuel for engine cooling is nothing new, but the cooling system will also act as a chemical reactor, breaking long-chain hydrocarbons down into short-chain hydrocarbons that burn more rapidly.
A test vehicle may be demonstrated by about 2008.
The X-43C may well be canceled as of mid-March, 2004.
- External link: www.aviationnow.com (http://www.aviationnow.com/avnow/news/channel_aerospacedaily_story.jsp?id=news/eng03194.xml)
The linked story reports the demise and quotes the appearance of Rear Adm. Stiedle appearing before a House Space and Aeronautics subcommittee hearing March 18 2004.
X-43D
The X-43D would expand the speed envelope to approximately Mach 15.
External links
References
de:Boeing X-43A
fr:X-43 Scramjet
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