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X-31 Enhanced Fighter Maneuverability Demonstrator
The X-31 Enhanced Fighter Maneuverability (EFM) demonstrator, currently being flown at NASA's Dryden Flight Research Center, Edwards, Calif., providing information which is invaluable for proceeding with the designs of the next generation highly maneuverable fighters.
The X-31 program is showing the value of using thrust vectoring (directing engine exhaust flow) coupled with advanced flight control systems, to provide controlled flight to very high angles of attack. The result is a significant advantage over conventional fighters in a close-in-combat situation.
Background
"Angle-of-attack" (alpha) is an engineering term to describe the angle of an aircraft's body and wings relative to its actual flight path. During maneuvers, pilots would like to fly at extreme angles of attack to facilitate rapid turning and pointing against an adversary. With older aircraft designs, entering this flight regime often led to loss of control, resulting in loss of the aircraft, pilot or both.
Three thrust vectoring paddles made of graphite epoxy and mounted on the X-31's aft fuselage are directed into the engine exhaust plume to provide control in pitch (up and down) and yaw (right and left) to improve maneuverability. The paddles can sustain temperatures of up to 1,500 degrees centigrade for extended periods of time. In addition, the X-31s is configured with movable forward canards, wing control surfaces, and fixed aft strakes. The canards are small wing-like structures located just aft of the nose, set on a line parallel to the wing between the nose and the leading edge of the wing. Normally "weathervaned" with the prevailing airflow, these devices are programmed to be used for aerodynamic recovery from high angles of attack in event of thrust vectoring system failure. The strakes are set along the same line between the trailing edge of the wing and the engine exhaust. The strakes supply additional nose down pitch control authority from very high angles of attack. Small fixed nose strakes are also employed to help control sideslip.
The X-31 flight demonstration program is focused on agile flight within the post-stall regime, producing technical data to give aircraft designers a better understanding of aerodynamics, effectiveness of flight controls and thrust vectoring, and airflow phenomena at high angles of attack. This is expected to lead to design methods providing better maneuverability in future high performance aircraft and make them safer to fly.
F-18 and the X-31.
X-33 Advanced Technology Demonstrator
The X-33 is being developed under a joint agreement between NASA and Lockheed Martin's Skunk Works as a technology demonstrator of a future single-stage-to-orbit Reusable Launch Vehicle (RLV).
Two significant goals of NASA's Reusable Launch Vehicle development program are lowering the cost of putting a pound of payload into space from $10,000 to about $1,000, and dramatically increasing the reliability of space flight. By reducing the cost of placing payloads into low earth orbit, commercial RLVs would create new opportunities for space access and significantly improve U.S. economic competitiveness in the worldwide launch marketplace.
The first test flight of the X-33, designed to fly as high as 55 miles and reach speeds of 12,000 mph (Mach 15), is expected during the summer of 2000. A series of up to 15 test flights is planned, sending the X-33 to the edge of space, followed by its atmospheric reentry and aircraft-like landing. The test program, along with associated ground-based research and development work, is expected to provide Lockheed Martin the information and technology to proceed with development of a commercial RLV called VentureStar. When operational, VentureStar is expected to eventually replace the Space Shuttles as NASA's next-generation Space Transportation System. NASA would then be a customer, not the operator, of the commercial RLV.
The X-33, 63 feet long and 68 feet wide, is a 53 percent scaled prototype of the proposed VentureStar. The design of both vehicles is based on the wingless lifting body concept pioneered at the NASA Dryden Flight Research Center and tested in six unique aerodynamic configurations between 1966 and 1975. Data from the lifting body program contributed to the design and operational profile of the Space Shuttles, and is being used again in the X-33 and the proposed VentureStar.
Each of the 15 suborbital missions for the uncrewed, autonomously flown X-33 will begin with a vertical launch from Edwards AFB and end with a runway landing at one of two sites, Michael Army Airfield at Dugway Proving Grounds, Utah, or Malmstrom AFB, Great Falls, Mont. The first five flights are currently scheduled to land in Utah, followed by two at Malmstrom AFB. Up to eight more test flights may be flown based on the results of the initial series and additional test objectives that must be met.
On the 950-mile flights to Malmstrom AFB, the X-33 will be airborne about 20 minutes, reach an altitude of about 55 miles, and achieve speeds of about 12,000 mph. The flights to Dugway, 450 miles from Edwards AFB, will take about 14 minutes, have a top speed of 8,300 mph and a peak altitude of about 30 miles.
The Vehicle
The wedge-shaped X-33 features an airframe built of titanium and composite materials. Small aft-mounted elevons and two small vertical rudders will provide pitch, yaw, and roll control in the atmosphere. Eight reaction control motors - gaseous jets - will be used for control at very high altitudes where the atmosphere is too thin for aerodynamic control surfaces.
Nestled tightly inside the rear half of the airframe are two liquid hydrogen tanks made of graphite epoxy. The forward half of the airframe is filled with an aluminum liquid oxygen tank. Together, they will fuel the two J-2S Linear Aerospike engines that will power the X-33 to speeds of Mach 15 (15 times the speed of sound) and altitudes of more than 260,000 feet during the test flights. Engine thrust at launch will be 410,000 lbs.
Linear aerospike engines were first developed more than 30 years ago, but were not considered mature enough for space flight until recent advances in materials and manufacturing. They use the same type of fuel as most standard rocket engines but do not have the familiar bell-shaped nozzle. The engines use the atmosphere as part of its nozzle. The airflow surrounding the rocket's exhaust plume keeps it contained so the engine is working at peak efficiency through its entire burn cycle, unlike traditional rocket engines which cannot compensate for variations in atmospheric pressure.
A linear aerospike engine is about 75 percent smaller than a standard rocket of comparable thrust, which translates to a lighter spacecraft and lower operating costs.
NASA Dryden contributed to the X-33/VentureStar design process by testing a one-tenth scale, half-span model of the X-33 at speeds of about 750 mph with an SR-71 Blackbird. The model, with a linear aerospike engine, was mounted on a test fixture attached to the upper fuselage of the SR-71. During flight the model validated Lockheed's computational predictive tools about the aerodynamic performance of the spacecraft design, and showed how the engine plume would interact with the aerodynamics of the vehicle. The engine was not hot fired during the flights, although gaseous helium and liquid nitrogen were cycled through the engine during the tests.
The X-33's exterior is covered with several types of Thermal Protection System (TPS) materials. These heat resistant materials will shed searing temperatures generated by high speeds through the atmosphere much like the tiles used on the Space Shuttles. A carbon-carbon cap covers the nose and can withstand temperatures of 2,000 degrees (F). Metallic Inconel honeycomb tiles, used for temperatures between 1,300 to 2,000 degrees (F), cover the entire bottom of the vehicle plus leading edges of the rudders and wings, and the rear portions of the fuselage. Flexible Nomex insulation cover the upper surfaces of the vehicle where temperatures are not expected to exceed 900 degrees (F).
A Global Positioning System in the X-33 will be coupled to the vehicle's flight control and inertial navigation systems to keep the craft on a precise flight path from launch to landing. The vehicle will fly autonomously with each test flight individually programmed into the flight control and navigation systems. Test conductors at the X-33 Flight Operations Center at Edwards AFB will also have the ability to control the vehicle during flight.
Thousands of sensors on the vehicle will collect performance and status data throughout each flight. On-board data transmitters will send the information to the ground where test personnel at the X-33 Flight Operations Center and NASA and Air Force Mission Control Centers will monitor the status of all systems, the overall operation of the vehicle, and flight safety.
Gross vehicle weight at launch will be 273,000 lbs., with 210,000 lbs of that amount represented by the combined weight of the liquid oxygen and liquid hydrogen fuel.
X-34: Demonstrating Reusable Launch Vehicle Technologies
NASA’s X-34 technology demonstrator is a flying laboratory for technologies and operations applicable to future low-cost, reusable launch vehicles. It is one of a family of technology demonstrators aimed at lowering launch costs from $10,000 to $1,000 per pound.
On Aug. 28, 1996, NASA signed a contract now worth $85.7 million with Orbital Sciences Corp., of Dulles, Va., for X-34 design, development and test flights. The 50-month contract includes three flight test vehicles. NASA and other government agencies are spending an additional $16 million for wind tunnel testing,
thermal protection systems,vehicle health monitoring, ground support, engine testing and flight support. Orbital has invested $10 million in corporate funds for modifications to its L-1011 carrier aircraft to accommodate the X-34. NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the program.
The suborbital technology demonstrator is 58.3 feet (17.77 meters) long. It has a 27.7-foot (8.44 meter) wingspan and stands 11.5 feet (3.5 meters) tall. It is designed to be air-launched from Orbital Sciences Corp.’s L-1011, a commercial jetliner originally modified to carry the company’s expendable Pegasus launch vehicle.
The X-34 will be dropped from the L-1011, ignite its engine, and fly a preprogrammed flight profile before making an automated approach and landing on a conventional runway – a first for an American launch system.
The X-34 is capable of flying up to eight times the speed of sound and reaching altitudes of approximately 50 miles. It will be a workhorse for testing high reliability, low-cost technologies and operations needed to develop and operate the next generation of space vehicles.
The X-34 is scheduled to make a total of 27 unpowered and powered flights during its test program, beginning in early 2000. Captive-carry flights to prove the airworthiness of the combined L-1011 and X-34 vehicles began in June 1999. The first X-34 vehicle, designated A-1A, will be used for a series of tow tests on the ground at NASA’s Dryden Flight Research Center, Edwards, Calif. The A-1A vehicle will then be used for a series of unpowered test flights. It will be released from Orbital Sciences Corp.’s L-1011 carrier aircraft over the Army’s White Sands Missile Range, N.M., and land on the Army runway.
At the same time, Orbital will complete assembly of the second X-34 flight vehicle, designated A-2. Its Fastrac rocket engine will be installed on the vehicle and test fired on the ground at Holloman Air Force Base, N.M., test facilities. After these ground test firings, the first series of powered flight tests of the X-34 will be conducted from, and land at, Dryden.
The first series of powered flights at Dryden will gradually expand the X-34’s flight envelope. Initial powered flights will be at speeds of about Mach 2.2–2.2 times the speed of sound – and then gradually increased to approximately Mach 5 over eight powered flights.
The A-2 vehicle then will be shipped to NASA’s Kennedy Space Center, Fla., for a second series of flight tests. These flights, which will reach speeds of up to approximately Mach 4.6, will demonstrate rapid turnaround flight operations required for reusable launch vehicles of the future. This series of seven flights will be conducted by flying an average of once every 14 days. One of these flights will demonstrate a 24-hour turnaround capability for the X-34.
The remainder of the test program, which involves the third X-34 vehicle, designated A-3, will be completed at Dryden. These test flights will expand the rocket plane’s performance to its maximum speed of up to Mach eight and altitudes up to 250,000 feet (76.2 kilometers), while also testing additional reusable launch vehicle technologies as carry-on experiments. The program plans to demonstrate a cost goal of $500,000 per flight. |
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