The National Space Society: A Backgrounder
NASA and industry have joined in a partnership to build a half-scale test vehicle, dubbed the X-33, to determine the feasibility of a single-stage-to-orbit reusable launch vehicle (RLV). The spacecraft's design incorporates a broad range of advanced technologies to enhance safety and reliability, as well as reduce costs by improving the efficiency of operations and maintenance.
The program's aim is to provide a high level of confidence in the new technologies so industry can take the next step and independently finance, build and operate a full-scale RLV. Such a vehicle holds the promise of opening a new era in space transportation. It would provide America a competitive advantage in the launch of commercial payloads and fuel new space commercial enterprises.
The X-33 more closely resembles a plane than a rocket launcher. The 67-foot, unpiloted test vehicle, being built by the Lockheed Martin Skunk Works team, takes off vertically and lands horizontally. The X-33 will launch from Edwards Air Force Base in the California desert. The sub-orbital vehicle will reach speeds approaching Mach 15, then land at sites in Utah and Montana.
Ground and flight tests are slated to begin in July of 1999. Once completed, Lockheed Martin will decide whether or not to proceed with the development of a full-scale RLV, called the VentureStar. As now configured, the vehicle would transport 25,000 pounds to the International Space Station or 50,000 pounds to low-Earth orbit. The full-scale RLV could be ready for flight testing as early as 2004. It would be able to launch from numerous sites, including those in northern latitudes to efficiently reach the space station and along the equator for geostationary missions.
An army of 18,000 workers is employed to maintain and operate the Space Shuttle fleet. It takes 60 to 70 days to prepare a shuttle for a new mission. By contrast, the X-33 will require only about 50 people and have a 48-hour turnaround, and the VentureStar will require under 300 people and have a turnaround of less than one week. NASA and industry estimate the cost for each flight of a full-scale RLV will be about one-tenth as much as the Space Shuttle. The five-year X-33 program is budgeted at $941.5 million (includes ground testing of components for a full-scale RLV), with industry contributing close to an additional $300 million.
The X-33 demonstrator incorporates the many new technologies needed to build a single-stage-to-orbit (SSTO) vehicle to evaluate the performance of these combined technologies as an integrated flight system. The launch vehicle is being designed for 100 trips to space. Specific technologies and challenges in the X-33 program include:
Weight Reduction - The Space Shuttle uses brute force to catapult the vehicle to orbit, while the X-33 relies on finesse. To reduce weight, many structural elements are being built with lightweight graphite-composite materials. Its aerospike engines (see below) have fewer components than traditional rocket designs. The vehicle utilizes state-of-the-art avionics, and all crew life-support systems are eliminated since the vehicle is not human piloted;
Thermal Protection System (TPS) - Large, durable, easily-replaceable, metallic TPS panels have been developed to insulate the X-33 from intense heat during reentry through the atmosphere. By comparison, the Space Shuttle is equipped with thousands of ceramic tiles that are brittle and prone to damage. Some 17,000 labor-hours are required to inspect, replace, and waterproof the shuttle tiles after each mission. Internal temperatures in the X-33 during reentry simulation will not exceed 350°F, while external surfaces will heat up to temperatures in the range of 1,600 to 2,200°F;
Reusable fuel tanks - The fuel tanks for the X-33 are designed to withstand extreme temperatures and pressures during launch and reentry. Like the Space Shuttle, the RLV will burn a mixture of hydrogen and oxygen, both of which are cooled to a liquid state and require special handling. (Hydrogen liquefies at minus 423°F and oxygen at minus 298°F.) Tanks for the shuttle are made of aluminum. The X-33's hydrogen tank is built with graphite-composite materials. Although the vehicle's oxygen tank is aluminum, tests are underway to develop a graphite-composite oxygen tank for the VentureStar;
Linear Aerospike Engines - The X-33 will be powered by two linear aerospike engines which produce low drag and maximize thrust during the full launch cycle. Steering is accomplished by varying the thrust of individual thrust cells, instead of a heavy gimbal system used by conventional rockets. The aerospike engines are being designed to operate safely for 20 missions before inspection is required. By comparison, the shuttle engines are removed and inspected after each flight.
U.S. companies once dominated the global marketplace for launching commercial payloads to space. We now command less than 30 percent of the world market because of our nation's inconsistent investment in space transportation, which has fluctuated like a roller coaster. The National Space Society strongly supports the X-33 program, and the follow-on Future-X program, to provide on-going development and flight testing of space transportation technologies with a stable level of funding.
The National Space Society, founded in 1974, is an independent, nonprofit space advocacy organization headquartered in Washington, DC. Its 23,000 members and 90 chapters around the world actively promote a spacefaring civilization. Information on NSS and space exploration is available at <http://www.nss.org/>.