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X-craft hone technology's leading edge

X-craft hone technology's leading edge

NASA administrator Dan Goldin, when laying down "technology pillars" for the agency, said the organization would fill the sky with swarms of experimental vehicles. The purpose: advance technology on many fronts with relatively low-cost projects. While not exactly an aluminum overcast, these unmanned craft are now taking off in the hope that they will lead to more efficient access to space and high-speed flight.

Here's a roll call on some of these programs:

X-33. By this time next year, the first flight of the X-33, being built by Lockheed Martin's famous Skunk Works (Palmdale, CA), should be history. The triangular vehicle uses the lift developed mainly by its body shape, rather than wings, and will land like a space shuttle. Its body volume holds the needed liquid hydrogen and oxygen propellants for a vertical rocket-powered liftoff.

The 69-ft-long by 77-ft wide X-33 will not fly all the way to orbit, but is a demonstrator for the company's twice-the-size VentureStar, which will have a payload bay to ferry satellites to orbit and cargo and crew to the International Space Station. The X-33 aims at proving technologies for an operational single-stage-to-orbit (SSTO) vehicle a tall order since the payload of a viable SSTO booster is under 10% of the liftoff weight. Thus any deviations from optimum result in large decreases in payload expected capacity.

According to NASA X-33 Program Manager Gene Austin, "X-33 is a combination of a flying and ground-based technology demonstration." Key items include:

First flight of a linear aerospike engine. This "inside out" configuration allows the exhaust gas to fully expand at any altitude (see figure). The thrust is spread out over the base of the vehicle, reducing stresses on structure and decreasing drag.

  • High strength-to-weight carbon-composite liquid hydrogen tank structure is the largest ever made, says Austin. A composite liquid-oxygen tank will be ground-proofed.

  • Metallic thermal protection system, consisting of roughly 18 3 18-inch panels of titanium and Inconel attached with four screws at the corners. Unlike the roughly 41,000 non-identical shaped, non-metallic tiles mounted with adhesive on the space shuttle, these 17 basic sizes are easier to attach and require less maintenance, leading to lower operational cost and a more aircraft-like operation.

  • Autonomous flight-control software simplifies operation and reduces the army needed to support a flight. The X-33 will essentially fly itself. Ground controllers will monitor flights with backup capability to change flight modes, for example, if a change to an alternate landing site is required.

Engine, composite structures, and metallic thermal protection are key elements X-33 must prove.

Austin also highlights the X-33 operations facility at Edwards AFB, CA, which is a duplicate of an operational VentureStar site. Here, the concept is to simplify handling and cut launch costs. Once the ground crew checks out the vehicle, rather than disconnect umbilicals and roll it to a launch pad (with attendant rechecks), the building itself is rolled away. The cradle holding the X-33 then rotates in place to the vertical for fueling and launch, without having to disconnect any umbilical lines.

While X-33 weight is somewhat higher than planned, along with more aft center of gravity, it still should be able to demonstrate the necessary peak heating rate (at Mach 12), maximum dynamic air pressure and g levels, and quick turnaround times, according to David Urie, former Lockheed Martin Skunk Works SSTO director and X-33 patent holder.

X-34. Whereas X-33 is demonstrating technologies needed in vehicles delivering medium and heavy payloads up to space station altitudes, X-34 is geared to developing reusable boosters for launching small payloads to low orbits. The vehicle is slung under a specially adapted Lockheed L-1011 airliner, flown by Orbital Sciences Corp. (Dulles, VA), the X-34 contractor. The 58-ft-long, 28-ft-span craft is then air launched, where its liquid oxygen/kerosene engine boosts it upwards of Mach 8 and 250,000 ft before landing autonomously up to 500 miles downrange.

Orbital Lead Wing Engineer and Chief Materials Scientist Tom Dragone emphasizes that, unlike the X-33, all the X-34's primary and secondary structure is carbon composite. It's the first such extensive use of this material in a reusable booster.

Dragone adds the key for the effort was SDRC's (Milford, OH) I-DEAS Master Series CAD package for analysis of the composite structure. "This was used to take the aerodynamic shape and, from that geometry, 'build' the vehicle skins. The program allowed defining the composite ply layups for fabrication, and produced the engineering drawings for parts manufacture, as well as the stiffness and strength analyses for such items as wing deflection. The processes and procedures in designing this vehicle will be more widespread in the future."

A conventional bell nozzle (left, top) may have flow separation under higher pressures found at low altitudes, but may not allow complete expansion within the bell at higher altitudes to extract maximum thrust. An aerospike engine (bottom), linear on the X-33, allows the exhaust plume to fully expand at any altitude, pushing on the inner ramp for maximum thrust.

Engineers avoided one potential problem with the large scale of the structure, Dragone notes. Since Orbital is a design and integration house, it doesn't manufacture composite parts. Different fabricators build different subsystems. But each fabricator uses different tooling and processes, such as ovens vs. autoclaves. Thus the company had to choose between requiring a different set of specifications for each supplier or else finding a common material. Fortunately, LTM45 low-temperature cure epoxy resin from Advanced Composites Group (Tulsa, OK) enabled "one system with enough flexibility for oven or autoclave curing and could be used on metal, composite, or wood tooling," he says. "We thus have a single material data base and processing spec."

Another material highlight is the thermal protection system made up of ceramic matrix composite tiles fastened with a screw at each of four corners.

X-37. This demonstrator is part of NASA's Future X effort. First out of the blocks is the X-37. This Boeing craft is unlike the X-33 and X-34 in that it is a space-flight system, taken into orbit in the space shuttle cargo bay, beginning in 2002. It can also be lofted by expendable boosters and its technology may be more applicable to upper stages of rockets rather than boosters.

The 12,000-lb X-37 will take up half the length of the shuttle cargo bay at 27 ft long, and it has a 14-ft wingspan. Once released, a 7,000-lb vacuum thrust rocket engine will maneuver the craft and de-orbit it to re-enter and glide to a landing. The vehicle can function as an orbiting test bed (it has a 4 3 7-ft payload bay) and investigate the re-entry realm of spaceflight.

According to John London, NASA Future X Pathfinder program manager, plans are to use the all-composite structure X-37 to develop thermal protection systems in a real re-entry environment. "While the shuttle can't take risks, the X-37 can fly more demanding attitudes and velocities and the vehicle can handle very high heating rates," he adds.

London notes the X-37 will be the first U.S. spacecraft to make an autonomous runway landing from orbit. The kerosene and hydrogen peroxide engine, while not used for landing after re-entry, will give experience in flying with more environmentally friendly propellants, says London.

Hyper-X. No, it's not a previously undiscovered nervous condition but stands for Hypersonic Experimental. Now designated X-43, next year it will be boosted to high-speed in the upper atmosphere to investigate supersonic hydrogen ramjet (scramjet) combustion, with applications to future high-speed aircraft and space boosters. MicroCraft (Tullahoma, TN) is building three vehicles for two flights to Mach 7 and one to Mach 10. After airborne launch from a B-52 on a Pegasus booster, a 12-ft-long, 5-ft-span X-43 will climb to 100,000 ft for a 10 sec combustion test run before diving into the ocean without being recovered.

As flight speeds increase to high Mach numbers, air moving through an airbreathing engine starts to become supersonic. But this means each "slug" of air is only in the engine a short time for fuel to enter and be completely combusted. Thus, only hydrogen is able to react fast enough to produce complete, efficient burning.

NASA Hyper X Program Manager Vince Rausch says the big question to be answered is, "Can enough thrust be produced after boost to overcome drag?" The challenge with using such a small vehicle lies in the ability to integrate the systems necessary into a tight package and accelerating it to test conditions. If the first test flight dictates changes needed, "There is flexibility in the software for fuel control and aerodynamic control adjustments," he notes.

X potential spinoffs

X-33: Thermal protection metals may find use in turbine and other engines; woven composite joints, rather than metallic, for joining large composite tank skins could see other structural applications

X-34: Ceramic matrix composites could work their way into more efficient auto engines; embedded health monitoring for large structures under stress in aerospace, auto, and marine applications

X-37: Autonomous landing from space will apply to control of future robotic spacecraft and vehicles

X-43: Dynamic models with 15 degrees-of-freedom can serve in modeling complex systems; computational fluid dynamics can apply to non-Newtonian fluid flows in the human body; exotic materials may be used in artificial joints and surgical instruments

Engineering challenges

  • Structures light enough to allow a single stage to drive a payload to orbit

  • Systems, such as thermal protection materials, that are rugged enough for space flight but require minimal maintenance

  • Software for automated vehicle checkout and control, leading to small operational crews

  • New, more efficient propulsion that maximizes payload in lightweight vehicles

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