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AMERICAS X-PLANES

No other aircraft program illustrates the USA's strong will for the leadership in aerospace technology better than the X-Planes. In more than 50 years since the first supersonic flight of the Bell X-1, the X designation aircraft have become the technological spearhead in developing new aircraft and spacecraft.

Including a few classified projects there are currently 12 active or planned X programms which will fly in the medium term. However, this number includes single projects, such as Boeing's or Lockheed Martin's Joint Strike Fighter prototypes (X-32 and X-35) which are not X-Planes in the classical sense of technology driven experimental vehicles.

The X programm was initiated at the end of the Second World War as a joint effort from NASA (back then NACA, National Advisory Committee for Aeronautics) and the US forces. The goal of the project was the exploration of the back then still unexplored speed region of high subsonic (transsonic) and low supersonic.

When Chuck Yeager broke the sound barrier on 14 October 1947, such accomplishing the near-term program goals, the flight experiments did not stop but were intensified. Most interestingly the original XS aircraft designation (for eXperimental Supersonic) was changed into X already at the very beginning of the program.

The continous research of supersonic flight gave the Americans an immense Knowledge about the flight in the high-speed region, cumulating in the X-15 that reached a speed of more than six times the speed of sound and an altitude of more than 100 km.

This high-speed research was only the beginning. In the past 50 years, the most different aircraft configurations were tested under the X designation. The spectrum reaches from a tailsitting vertical lift-off plane via a nuclear powered bomber and a gyro copter to the spaceplane of the future. Engines used in the projects include jet, propeller and rocket motors.

Today, the role of the X-planes has changed significantly. While at the beginning their purpose was mainly to give the US a technological lead (especially in the military aircraft sector), today's experimental aircraft must have a commercial perspective as far as the potential application of the tested technology is concerned. There is an obvious shift from aviation to spaceflight in the research activities.

The USA see a big market potential in spaceflight. But in order to exploit that market, the cost of transportation must be reduced considerably. However, the necessary technologies are not yet or only little tested. Here, the industry needs support to take the technological risk. This is the part the X-Planes must play today.

NASA has developed the Future-X concept in the frame of its Space Transportation Program. According to this concept, new technologies are tested with demonstrator vehicles in two classes. First the pathfinder class vehicles which are focus on a very narrow technology. Accordingly, these demonstrators are designed for a very short development time of less than two years from program start to flight tests. Also, the program costs should be less than 100 million Dollars.

One example for such a pathfinder technology demonstrator is the X-34 which is currently undergoing integration tests with its Lockheed TriStar launch aircraft at NASA's Dryden Flight Research Center. The X-34 is supposed to start suborbital flights next year, reaching Mach 8. The technologies to be demonstrated include an autonomous flight control and landing system, as well as, an integrated health monitoring system that is supposed to allow quick turnarounds between flights.

While the pathfinders are focusing on specific technologies, larger vehicles, the so called trailblazers, will be used to validate the integration of various technologies and systems. These experimental demonstrators almost look like the real vehicles. For example, the X-33 is a half-scale VentureStar which Lockheed projects as one possible successor of the Space Shuttle. The X-33 already features several design components of the VentureStar, such as new Aerospike rocket engines and a metal thermal protection system.

With this approach, NASA together with the industry is making its way towards the next generation of new space transportation systems. This also includes cost-sharing aspects of industry and government. Meanwhile, Europe has recognized the new X-attitude of the USA. With its Future Launchers Technologies Programme, which includes small and large technology demonstrators, the European Space Agency wants to create a technological basis for a competitive and at least partially reusable space transporter.

From page 12 of FLUG REVUE 11/99