There are numerous U.S. hypersonic aircraft designs utilized to various degrees over the years. The only vehicle successfully used is the Space Shuttle, although this ceases to be an aircraft while in orbit. The newest major design for a hypersonic aircraft comes in the form of the Boeing X-51.
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How a Scramjet Works
To achieve hypersonic speeds, generally considered to be Mach 5 or above, all U.S. hypersonic aircraft must utilize a type of engine known as a scramjet. This uses a a small tube, which allows air to enter and be compressed by the speed of the vehicle itself. The jet fuel is sprayed into the air, ignited and forced out at a higher rate than the intake, creating a strong thrust. Unlike other jet engines, scramjets have no turbine, making the design less likely to experience failure. However, one major problem with scramjets is the fact that they can only be activated when the aircraft has already achieved hypersonic flight, meaning another means of propulsion needs to be used to get the vehicle to Mach 5 or above. The scramjet will maintain and increase the speed.
Above right: Scramjet operation. (Supplied by Emoscopes at Wikimedia Commons; GNU Free Documentation License; http://upload.wikimedia.org/wikipedia/commons/e/ed/Scramjet_operation.png)
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Designing a Waverider
One of the major challenges with hypersonic vehicles is the shock wave effect created by the high speeds. To combat this, researchers developed a design for U.S. hypersonic aircraft that uses these shock waves as a lifting surface to improve the lift-to-drag ratio. Generally, the designs utilize a very small and flat nose cone, slowly expanding in size to the rear, where either carat or delta wings are placed. Carat wings look like upside down “V" shapes, while delta wings are shaped more like a triangle. Waverider vehicles use the design to keep the shock wave under control, usually around the wings, which increases the air volume underneath the craft, providing the necessary additional lift.
Above left: NASA scramjet. (Supplied by NASA; Public Domain; http://upload.wikimedia.org/wikipedia/commons/4/4a/X43a2_nasa_scramjet.jpg)
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The Space Shuttle
Developed during the 1970s, the Space Shuttle orbiter is the only U.S. hypersonic aircraft used on a regular basis, although it is scheduled to end in 2010. Essentially a “space glider," the Space Shuttle uses rocket technology, spacecraft designs and traditional aviation elements to complete missions from the surface of the Earth into orbit. It is capable of carrying crews and payloads into space and returning. Five usable orbiters were constructed, along with a prototype.
The way the Space Shuttle orbiter achieves hypersonic flight is through a series of propulsion systems. It utilizes two solid rocket boosters that launch on the ground, providing the first two minutes of ascension. After the boosters are ejected from the vehicle, the main engines fire, fueled by a large external tank. This helps the Space Shuttle obtain hypersonic speeds of roughly 26,000 feet per second (8,000 m/s).
Above right: Discovery. (Supplied by NASA; Public Domain; http://upload.wikimedia.org/wikipedia/commons/4/4c/STS-121-DiscoveryEnhanced.jpg)
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The only current U.S. hypersonic aircraft outside of the Space Shuttle is the Boeing X-51. Using scramjet technology, the Boeing X-51 is a waverider plane similar in design to many previous hypersonic vehicles. Designed under the management of the U.S. Air Force (USAF) Research Laboratory's Propulsion Directorate, additional program influence comes from NASA, DARPA and Pratt & Whitney Rocketdyne.
In ideal conditions, the 26 foot-long (8 m) Boeing X-51 is capable of achieving flight speeds of Mach 7, approximately 5,000 miles per hour (8,050 km/h). It is flown into a high altitude of 50,000 feet (15.24 km) aboard a B-52 bomber and launched. An ATA CMS solid rocket booster is fired, helping the vehicle obtain Mach 4.5, at which time the scramjet takes over and brings the vehicle to at least Mach 6.
Above left: X-51 Waverider. (Supplied by U.S. Air Force; Public Domain; http://upload.wikimedia.org/wikipedia/commons/9/97/X51waverider.jpg)
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Previous Hypersonic Aircraft
The XB-70 Valkyrie was designed during the 1950s by the USAF Strategic Air Command with the purpose of performing deep penetration nuclear bombing runs. It had six engines that allowed it to fly to altitudes of 70,000 feet (21,000 m), providing a defense against other aircraft. Two prototypes were built, but the program was canceled in 1961.
NASA developed an experimental hypersonic aircraft during the late 1990s known as the X-43. The design was similar to the Boeing X-51, using a B-52 Stratofortress to achieve altitude before firing a rocket and using its own scramjet power to maintain flight. Despite setting the world speed record of Mach 9.8 in 2004, the program was canceled.
The Blackswift program, under the auspices of the DARPA Falcon Project, attempted to develop a reusable U.S. hypersonic aircraft between 2003 and 2008. It was designed as a fighter-sized unmanned vehicle capable of committing to rapid airstrikes against enemy forces.
Above left: XB-70 in flight. (Supplied by NASA; Public Domain; http://upload.wikimedia.org/wikipedia/commons/7/7c/North_American_XB-70_in_Flight_EC68-2131.jpg)
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Problems with Hypersonic Technology
There are many problems associated with obtaining and maintaining hypersonic speeds. Wing design is an important element, as higher speeds require special wing designs. However, these same wings are ineffective at slower speeds. Another challenge is the fact that hypersonic craft generate high levels of heat, which can impact the integrity of the vehicle itself. To combat this, designers implement a variety of different elements such as carbon composite heat shielding and coolant pumped onto the skin of the aircraft.
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The Future of Hypersonic Designs
Despite the future success or failure of the Boeing X-51, it is guaranteed that the U.S. will continue its work with hypersonic aircraft. The possibilities for vehicles that can travel at these speeds in both space exploration and military applications is seemingly endless. The challenges that the speeds present have slowly been overcome through the years, meaning new problems will also be researched and most likely solved. The future for U.S. hypersonic aircraft is surely to be a costly, but adventurous endeavor.
Above right: Waverider at Mach 7. (Supplied by NASA; Public Domain; http://upload.wikimedia.org/wikipedia/commons/5/55/X-43A_%28Hyper_-_X%29_Mach_7_computational_fluid_dynamic_%28CFD%29.jpg)
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Boeing X-51 Waverider: http://www.boeing.com/defense-space/military/waverider/index.html
NASA X-43: http://www.nasa.gov/missions/research/x43-main.html
Wings for All Speeds: http://research.lifeboat.com/surf.htm