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X-34: NASA'S RLV TECHNOLOGY DEMONSTRATOR

With the X-34 experimental vehicle, NASA is focussing mainly on one goal: to significantly reduce the launch costs by achieving a high degree of reusability at the same time. When NASA and Orbital Sciences signed the agreement for the development of the X-34 in 1995, the vehicle was supposed to become a commercial launch vehicle for payloads up to 2000 lbs into low Earth orbits along with being a technology testbed for future reusable launch vehicles. Only a year later the industry raised concerns about the commercial viability of the project. The original concept, which had envisioned a 140000 lbs vehicle and was to be launched from the back of a Boeing 747 airliner, was reduced significantly.

Today's X-34 has a maximum mass of approximately 17000 lbs plus 30000 lbs of fuel. NASA's main goal is the evaluation of new and cost-efficient technologies for future reusable launch vehicles, such being able to make a decision on the Space Shuttle successor at the beginning of the next century.

Unlike the larger X-33, which in its technological and operational concept is already very close to the projected VentureStar orbiter, the X-34 is mainly used "to demonstrate technologies which can not be fit into the Space Shuttle's cargo bay", says Mike Allen, X-34 deputy program manager at NASA's Marshall Space Flight Center in Huntsville, Alabama.

The main operational difference between the two current NASA experimental vehicle programs: While the X-33 is a single-stage to orbit system, the X-34 will be airlaunched by a Lockheed TriStar airliner.

Wherever possible commercial off-the shelf equipment was used for the X-34. For example: The nose gear comes from the Saab 2000 regional turboprop airliner, the main gear from the Northrop F-5 fighterbomber. The navigation and flight guidance computer is a derivative of the system used in Orbital's Pegasus expandable launch vehicle which is also airlaunched by the company's TriStar. A sensor package includes a flush air data system, a Litton inertial navigation system and differential GPS.

The vehicle will be controlled in flight by hydraulically actuated control surfaces (elevons, body flap and an all-movable vertical tail) along with hydraulically actuated thrust vector control of the main engine. The vehicle also has an attitude control system (nitrogen cold gas), a brake chute, main wheel brakes and a nose wheel steering system.

The fuselage, wings and rudders are made of an aluminium honeycomb structure lined with carbonfibre composite sheets. The kerosene tank is also made of graphite expoxy material while the two liquid oxygen tanks are made of aluminium. The lay-out of the tanks in various compartments is to protect the vehicle from a center of gravity migration during flight and to avoid slushing.

The X-34's nosecap, wing and vertical tail leading edges are made of SIRCA (Silicon Impregnated Reusable Ceramic Ablator). This impregantion which is covering the ceramic tiles is charring and ablating when exposed to the high temperatures during reentry. The SIRCA impregnation is to allow a reusability for several flights without having to replace any tiles.

The major part of the vehicle is covered with a flexible external surface insulation, "for the first time also at the windward side of a reentry vehicle", says Mike Allen. The bottom of the X-34 is covered by so called AFRSI (high temperature advanced flexible reusable surface insulation) which is supposed to withstand temperatures up to 2000 degrees Fahrenheit.

NASA's largest program share is the engine technology. In 1996, the Marshall Space Flight Center began the development of the Fastrac (fast track) engine as part of the agency's advanced space transportation program. The simple rocket engine is built to deliver 60000 lbs of thrust at low costs. According to NASA, a Fastrac engine currently costs about $1 million which is only one seventh of the costs for comparable engines. In the long term, as technology improves, NASA forecasts a price of only $350000.

The main factors contributing to the lower price are a significantly reduced number of parts along with the utilization of simple production methods. "The technology is simple enough so that the engine can be built by any company", says Danny Davis, manager of NASA's low cost technologies project. Since there are no industry proprietory rights on the Fastrac technology, the data are available to any interested party.

Fastrac only has one turbopump unit (one pump for the kerosene and one for the liquid oxygen) and does not have complex and sophisticated engine control electronics. The engine's mixture ratio is adjusted in ground tests before launch and will then operate on the preselected setting during the flight.

The inside of the one-piece thrust chamber is lined with layers of silicate reinforced phenolic material. There is no complex tubing running through the chamber wall for cooling purposes like in other engines. Instead, the composite lining charres and decomposes during engine operation, such cooling the walls (ablative cooling). The majority of the engine parts is reusable. Only the thrust chamber has to be replaced (because of the damage from the ablation) and the hypergolic cartridge, which is used to ignite the engine, must be refilled. The first full-power hot test of the Fastrac engine was accomplished just recently at the Stennis Space Center.

At the end of February, Orbital Sciences delivered the first of three vehicles to NASA. The first vehicle is a structural test article which is used at the Dryden Flight Research Center for vibration tests and for the integration and FAA certification flights of the Lockheed-TriStar-Airliner.

The delivery of the first flight vehicle is scheduled for mid-1999. It will be delivered to the White Sands test facility in New Mexico for the initial phase of the flight test program. Since at White Sands the X-34 is limited to a maximum speed of Mach 2.5, the flight campaign will be moved to NASA's Kennedy Space Center in Florida as soon as the vehicle has proven its reliability.

At the Florida location, the X-34 will be carried by the TriStar to a distance of approximately 900 kilometers over water before releasing the vehicle. The vehicle will then fly back to the base, reaching speeds of up to Mach 8 and altitudes up to 250,000 ft. In the second part of the flight test program, the X-34 is supposed to demonstrate safe subsonic flying characteristics in adverse weather conditions such as rain and fog and be able to safely land in crosswinds up to 20 kts.

At the end of 1998, NASA decided to build a second flight vehicle. This is supposed to ensure the program continuity even if one of the vehicles is lost during the test flights. There are currently up to 25 flights budgeted over a one-year period.

From page 44 of FLUG REVUE 5/99