Top 15 Technologies
9.
X-33 Advanced Technology Demonstrator scheduled to fly in 1999
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The X-33 Advanced Technology Demonstrator is scheduled to begin test flights in 1999 at Edwards Air Force Base.
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The X-33, which is the largest and most advanced reusable launch vehicle (RLV) technology demonstrator, is the flagship in NASA's (www.nasa.gov) fleet of RLVs, which includes the vertical takeoff and landing Clipper Graham, and the air-launched X-34.
Under a cooperative agreement between NASA and Lockheed Martin (www.lockheedmartin.com) Skunk Works, the X-33 will be developed to increase reliablility and lower the cost of putting a pound of payload into space from $10,000 to $1000. This cost reduction may create new opportunities for a commercial RLV, which could improve U.S. economic competitiveness in the worldwide launch marketplace. NASA has committed to funding $941 million for the X-33 program through 1999, while Lockheed Martin will invest $212 million.
The X-33 represents a half-scale prototype of Lockheed Martin's RLV labeled "VentureStar," which the company plans to develop in the next century. From the X-33's ground research and test flights, Lockheed Martin will decide whether it should proceed with the VentureStar's development.
X-33
Length: 63 ft
Width: 68 ft
Takeoff weight: 273,000 lb
Fuel: LH2/LO2
Fuel weight: 210,000 lb
Main propulsion: 2J-2S Linear Aerospikes
Takeoff thrust: 410,000 lb
Maximum speed: Mach 15+
Payload to low-Earth orbit: N/A
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VentureStar
Length: 127 ft
Width: 128 ft
Takeoff weight: 2,186,000 lb
Fuel: LH2/LO2
Fuel weight: 1,929,000 lb
Main propulsion: 7 RS2200 Linear Aerospikes
Takeoff thrust: 3,010,000 lb
Maximum speed: Orbital
Payload to low-Earth orbit: 45,000 lb
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The X-33 is the half-scale model of Lockheed Martin's VentureStar, whose specifications are listed here.
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The design of the X-33 is based on a lifting body shape with two "linear aerospike" rocket engines (described in the previous issue of Aerospace Engineering) and a rugged metallic thermal protection system. Other X-33 features are lightweight components and fuel tanks that conform to the vehicle's outer shape.
X-33 operations will consist of unpiloted flight beginning with a vertical takeoff and ascending to an altitude of 50 mi. It will reach air speeds of Mach 15 and land horizontally like a typical airplane.
Test flights are planned to begin in 1999 at Edwards Air Force Base. As greater confidence is established during testing, flights will gradually expand to longer distances. Short-range flights are planned to cover 110 mi. for 18 min. and reach a top speed of Mach 4. Mid-range flights will cover 450 mi. for 24 min. at a top speed of Mach 9 and the long-range flights will travel 950 mi. at a top speed of Mach 15 for 28 min. These flights will normally be conducted on seven-day intervals, with one of them coming after a two-day turnaround.
For NASA to maintain its goals of an X-33 flight in 1999, it had to discard many traditional concepts and practices involved in research and development. Joe Ruf, fluid dynamics engineer at NASA's Marshall Space Flight Center, added, "In the past we would have conducted a careful, systematic set of experiments on the X-33 test model, looking at the different effects of each test, and refining the model and test techniques as we went. With the first test flight scheduled for 1999, we could not follow those practices with the X-33. So we have performed much of our design, testing, and analytical work in parallel. It is a practice called 'concurrent engineering' -- a practice dictated by schedule and costs."
Wind tunnel testing, which is traditionally done, was conducted on 25 different configurations of the X-33 demonstrator. Over 2500 test runs have been completed since December 1996.
Another methodology used in conjunction with wind tunnel testing is computational fluid dynamics, which is an analytical prediction of a fluid's behavior. During a computational fluid dynamics analysis, physical and thermodynamic laws that describe the fluid's behavior—whether air, liquid, oxygen, liquid hydrogen, or other substances—are written in computer code. The shape of the model is analyzed and described to the computational fluid dynamics code by a mesh or grid. The grid is a series of points in space with which the code predicts the fluid's behavior.
"You can do really rapid analysis with computational fluid dynamics, quickly gaining an idea of what a change in a vehicle's configuration will do," said Ruf. "This can be accomplished before you try to justify spending a lot more money on additional wind tunnel tests."
Computational fluid dynamics allows NASA engineers to go beyond a wind tunnel's capabilities and perform tests that would otherwise be more expensive and dangerous in a wind tunnel. One example is the testing of the effects of rocket engine exhaust gas plumes on the vehicle's base pressure or forebody aerodynamics. Such testing could prove to be too expensive or dangerous if conducted in a wind tunnel.
Another approach to this testing is to substitute a cold plume (500 °F) for a hot plume (5000 °F) in a wind tunnel test. However, the effects of the hot plume versus the cold plume cannot be properly scaled in this testing. A computer can model hot plume gases using computational fluid dynamics, which will keep wind tunnel costs low and eliminate the hazards associated with the 5000 °F gases.
The X-33's aerodynamic loads or pressures are also being determined by computational fluid dynamics to analyze the vehicle's critical body components. Traditionally, NASA engineers had to perform calculations on maximum airload or maximum dynamic pressure before testing it in the wind tunnel. By using computational fluid dynamics, wind tunnel testing began before the launch trajectory and final configuration were established. Other areas in which analytical predictions from computational fluid dynamics proved useful were engine performance and the vehicle's sonic boom strength.
Even though computational fluid dynamics plays a significant role in the X-33 development, it does not wholly replace wind tunnel testing. It was through wind tunnel tests in December 1996 that instability problems were discovered in an early vehicle configuration. By testing new configurations, controls were found to stabilize the vehicle, allowing X-33 development to continue.
These new testing methods allowed the X-33 to pass a comprehensive design review, which gives the go ahead for the fabrication of all remaining components, completion of subsystems, and assembly of the subscale prototype launch vehicle. The review served as an opportunity for program officials to announce the resolution of issues such as vehicle weight, modifications to the canted and vertical fins, and plans to use densified propellants to carry additional fuel.
The two-volume X-33 Environmental Impact Statement, which studied issues such as public safety and environmental effects, and included public meetings and hearings near the proposed launch sites, was completed in November 1997. The next goal in the X-33 program is the construction of a launch facility at Edwards Air Force Base, which is scheduled to be completed within the year.
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