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X-33: TESTBED FOR THE NEXT SPACE SHUTTLE

Operating the Space Shuttle fleet today costs about $5 billion per year. Present transport costs into orbit amount to around $22,000 per kilogram of payload. This is why the US Congress initiated the development of a new re-usable launch vehicle in 1993. The new system is to reduce costs to a tenth, i.e. $2,200 per kg pay load. This is an important aspect when it comes to supplying the International Space Station, which is currently being assembled, and for which mainly American Orbiters will be used. Once the station is up and running, the shuttle, which is currently also used for scientific experiments, will only be used for transport flights.

In 1994 NASA put out to tender the development of the advanced technology demonstrator X-33 as a demonstrator for the new re-usable launch vehicle. The system was supposed to be a single stage to orbit design (SSTO) which meant that it should not have additional solid fuel boosters or an external tank, as is needed by the Space Shuttle today. NASA's X-33 program director, Gene Austin, says: "We were convinced that the SSTO technology was advanced enough that we can reach a commercially successful space transportation system through the X-33."

Lockheed Martin received the contract for their version of the X-33 in mid-1996. It is a lifting body with two short wing stumps and two vertical tails. The design of the 53-percent scale X-33 has many traceable elements to VentureStar, which Lockheed envisages as the follow-on Shuttle.

It remains open whether the tracability of X-33 and VentureStar was the deciding factor in NASA opting for Lockheed's design. An American magazine commented, that NASA had decided on taking the technologically most challenging and risky path. There is some truth in that as Lockheed is embarking on new territory in several key technology areas with the X-33, such as a composite main structure and composite cryogenic fuel tanks, a metal thermal protection system on the bottom of the vehicle along with a new engine technology.

NASA is financing the X-33/VentureStar project with $941 million through 1999. Lockheed Martin is contributing a further $212 million as main contractor and system integrator. About 80 percent of the entire budget will be spent on the X-33 itself. The remaining 20 per cent is going into preliminary design work for VentureStar.

Industry will have to (and already did) finance any program overruns since NASA's funding is limited to just under $1 billion. The reason being that NASA just wanted to initiate the technological development of a new reusable SSTO launch vehicle which, however, needs to be commercially successful. NASA's Gene Austin says: "We want to go away from being the launch vehicle operator and just become a customer for the transport services instead".

The Marshall Space Flight Center in Huntsville, Alabama, has the NASA X-33 program responsibility. The vehicle itself is being built at the Lockheed Martin Skunk Works located at the southern edge of Edwards AFB in Palmdale, California.

Almost 30 companies and organisations are involved in the X-33 program. NASA is mainly making the test and research capacity of its various centers available. Another main program partner on the industry side, along with Lockheed Martin, is Boeing's Rocketdyne division from Canoga Park. Rocketdyne is responsible for supplying the linear Aerospike engines for the X-33.

The technology carrier is powered by two XRS-2200 linear Aerospike engines, fed by a mix of liquid hydrogen and liquid oxygen. Each engine will generate 206,500 lb take-off thrust at sea level. The linear Aerospike works fundamentally different from conventional bell nozzle rocket engines. While with the latter hot gases expand within the bell shaped nozzle, in the Aerospike, gases expand from a battery of thrust cells against the external surfaces of a central engine plug to generate thrust.

The design has some major challenges, one being the fabrication of the large thrustramps which have to withstand a thermal strain of up to 4,500 *F from the hot gases. Because of the engine-related delays, the flight-ready engines will have to be installed with the flight vehicle already being at the launch pad.

About 70 per cent of the vehicle was completed by the end of 1998. The X-33's graphite expoxide composite structure is well visible at the Skunk Works. The liquid oxygen aluminium tank has already been integrated into the forward section of the craft.

The two liquid hydrogen tanks, which will fill the aft part of the X-33, are still in the manufacturing and testing phase. These fuel tanks, made from composite materials, are being manufactured by Lockheed's subcontractor Alliant TechSystems, Utah. According to Lockheed "no composite cryogenic fuel tanks have ever been done of this size and complexity".

The right hand tank was completed in January and, after some more testing at the Marshall Space Flight Center, will be fitted into the X-33. As far as the left LH2 tank is concerned, Lockheed Technicians have now found damage to one of its four walls. Initial assessment of the situation revealed that the lobe skin will have to be replace. Alliant has begun manufacture of the replacement lobe skin and crews are in the process of removing the damaged part.

As soon as both tanks and their supporting structure have been fitted to the X-33, the bottom of the X-33 will be covered with thermal protection panels. While with today's Space Shuttle mainly the more fragile thermal protection tiles are used, metal panels are utilized on the X-33, (Inconel and titanium), which are directly fastened to the fuselage structure.

The 18" by 18" panels are overlapping each other. They contract on the ground, while the frozen fuel is on board and expand during flight due to the thermal loads. For the X-33 flights, Lockheed Engineers expect approximately 870 *C on the bottom, 1,200 *C on the top and 300 *C on the top of the vehicle.

This is why the top of the X-33 will be covered in a flexible low-temperature external heat insulation similar to the one used on the Space Shuttle. The areas exposed to the highest temperatures, i.e. the nose cap and the leading edges of stabilizers are made of Carbon-Carbon.

The metallic thermal protection system (TPS) has one big advantage: The panels don't have to be glued to the vehicle surface as is the case with the Shuttle today. After every Shuttle flight about 17,000 man-hours have to be spent to fix the thermal protection. Exchanging the individual TPS panels on the X-33 supposedly takes only five minutes. For this reason a full-size reusable launch vehicle VentureStar will be completely covered with the new metal TPS panels.

Along with the metallic TPS, the X-33 leads to the VentureStar in other technological applications also. This includes the use of Carbon-Carbon, advanced graphite-epoxy composites along with the entire vehicles aerodynamic concept and the integration of the systems.

While two downsized linear Aerospike engines are powering the X-33, VentureStar will be powered by seven RS-2200s, each delivering more than twice the thrust of the XRS-2200. In today's Space Shuttle operations, the Shuttle main engines have to be removed after every flight to replace the turbo pumps. VentureStar's RS-2200 are supposed to be certified for 100 flights and, according to Lockheed's present concept, will only have to be removed every 20 flights for minor maintenance and every 60 flights for a major overhaul.

Still, a go-ahead to the VentureStar program will heavily depend on successful test flights of the X-33. Due to the various program delays the first flight is now scheduled for the end of 1999. The launch complex in the northeast of the Edwards AFB range area was completed in January on time and 13 months after groundbreaking. The launch site consists of liquid hydrogen, liquid oxygen and nitrogen fuel tanks, along with a movable hangar, the so called translation shelter, which covers the vehicle during horizontal launch processing and is removed when the X-33 is raised vertically for launch. On site there is also a water tower holding 250000 gallons to feed the noise suppression system.

Two flight profiles are planned for the intended 15 test flights: The first two flights are planned to the Michael Army Airfield in Utah, located in a distance of 720 km from the launch site. According to present calculations, the X-33 will reach a maximum speed of about Mach 8 and a maximum altitude of 165,000 ft in this profile. A further flight profile takes the vehicle to Malmstrom Air Force Base, Montana, which is 1,500 km away. Here, the flight demonstrator will supposedly reach a maximum altitude of 260,000 ft and a max speed of Mach 13.5. During its suborbital flight, the X-33 will reach about half the altitude that VentureStar will reach when delivering payload to low-earth orbit.

The flight of the unmanned X-33 will be fully automatic. After its hypersonic flight the vehicle will touch down on a conventional F-15 and F-16 derived landing gear. In the ascent powered flight phase the vehicle's flight control is mainly based on thrust vectoring from the Aerospike engines. In the higher density of the lower atmosphere (initial launch phase and landing approach) the thrust vector control is complimented by elevon and rudder control surfaces. For the suborbital, unpowered cruise phase, the vehicle also has an attitude control system with eight thrusters available.

An important part of the flight test phase is the demonstration of aircraft-like operations. For this, two turn-arounds are to be accomplished within seven days. One of the turn-arounds is supposed to take only two days. These times do not include the time required to transport the vehicle from the landing site back to Edwards via truck.

Another goal of the operational part of the tests is that a maximum of 50 people will be allowed to work on the X-33 during the turn around. The reason being the tracability to VentureStar which is intended to operate aircraft-like from a space port (with the runway being right next to the launch pad) and will be turned around at a minimum effort and time. Today's Shuttle turn-around takes up to 80 days and involves thousands of people.

VentureStar, like the X-33, is will be operating fully automatic. However, while X-33 is an unmanned vehicle, VentureStar will probably be certified for manned flights to fulfill the needs of the Space Station logistics. Still, even when being certified for manned flight, VentureStar will probably not have pilots: "We have actually been out of the flying business for quite some time", says NASA's Gene Austin, "really the only times that the Shuttle pilots touch something is when lowering the landing gear and for the flare."

From page 10 of FLUG REVUE 3/99