Engineering News 7386Engineering News 7386

Tight budgets don't stop aerospace

Newton, MA--Aerospace isn't dead. Granted, few projects as large as VentureStar^TM or the F-22 loom on the horizon. But interesting projects continue to challenge aerospace engineers. Here are a few examples.

To fly forever. Or at least for 2,000 to 5,000 hours. That's the goal of the ambitious solar-powered aircraft program underway at AeroVironment Inc., Simi Valley, CA. Recent test flights of a quarter-scale model of the Centurion, the next member of AeroVironment's family of solar-powered flying wings, answered many questions about the proposed vehicle's aerodynamics and stability.

Rik Meininger, AeroVironment's Centurion project manager, says the flights went well. "We had it designed so we could change some things while we flew it, like dihedral and wash-in, wash-out on the tip panels, move the center of gravity around--those kinds of things."

The question that initially concerned AeroVironment's 24-member project team (about two-thirds engineers, the rest assembly people) involved the aircraft's size. At 62.5-ft span and 2-ft chord, "the airplane is so large that if we did a really tight turn, would it be possible for us to stall the inboard wingtip? We went through that series of tests," says Meininger, "and we never did see a stall."

Engineers are incorporating data from the tests into the full-scale proof-of-concept Centurion aircraft, which will be able to reach 100,000 ft with a load of sensors and stay there for two hours. Centurion will be a big airplane. "Our sizing studies have it at around 210 to 230 ft in span," says Meininger. "Our design weights are looking to be in the neighborhood of 1,300 lb. It's just basically the model scaled up. And we'll fly at about 24 to 34 ft/second."

After Centurion, AeroVironment intends to push forward with a follow-on vehicle. About the same size as the Centurion, Helios will be capable of flying around the world. "The big issue with time aloft is systems reliability," Meininger explains. "People don't normally design servos and electronics to fly a single 2,000- to 5,000-hour mission, which is the ultimate goal."

AeroVironment team members are having fun and doing the job at relatively low cost. "We're doing this on a budget that most people would use to go out and do one flight," says Meininger. "I'm not old enough to have been in the business in the late forties to early sixties, the heyday of aeronautical development, but this is probably the closest you get to it in the modern age."

Make it better. When new projects disappear, one response is to find new markets for existing aircraft, and update them to attract new customers. An example: The veteran SH2G Super Seasprite helicopter, made by Kaman Aerospace, Bloomfield, CT, has begun to find foreign purchasers. The helicopter's main rotor uses metal blades. But an internal R&D project has developed a composite blade, the CMRB2, capable of superior performance and a blade life of 15,000 hours--four times that of the metal blade.

"We wanted it to have the performance of the existing metal blades at a low cost," says Cliff Gunsallus, Kaman's director of development programs. The prototype blade flew last year. "We flew it on an SH2 to the edges of the required flight envelope," he says, "and it demonstrated reduced vibration and improved performance." The next step involves modifying the basic blade for full production. Kaman's engineers intend to fly a blade this year that mirrors the final production blade.

The project has involved two designers, four to six manufacturing personnel, and several flight test people. Gunsallus expects to see more projects of this sort in the future. "I don't think there's new start money around for much of anything. Everybody will be pushing the platforms that exist," he predicts. Trying to get the cost out, and make a product that sells on the international market and has long-term value and supportability imparts incredible pressure, say company officials. As Gunsallus explains, "We've got to do eight times as much with a third of the resources to capture those niche positions on the international market." The program began last April, and Kaman expects to begin producing the CMRB2 blades in early 1998.

An old bird flies again. Back in the 1970s, the U.S. Air Force set up an Advanced Medium Short-Takeoff-and-Landing Transport program to produce a follow-on to the C-130. The program funded the construction of Boeing's YC-14 and YC-15, but the project never got past the technology demonstration phase.

Fast-forward to the present day. Both the Air Force and McDonnell Douglas (which recently merged with Boeing) saw the need for an aircraft that could provide a testbed for advanced technology. The YC-15 looked especially suitable for the purpose. "It was similar to the C-17," says George Sillia of McDonnell Douglas Military Transport Aircraft, Long Beach, CA, "so technologies proven on the aircraft would be in some cases directly applicable to the C-17. In other cases, the work may be applicable to future airplanes of any type."

McDonnell's engineers felt comfortable with the YC-15. "It was built by us, and had the same kind of characteristics--blown flaps technology, cargo airlift technology--as the C-17," notes Sillia. "It filled the bill very nicely at reasonably low cost to start up the program." Personnel from McDonnell Douglas and the Air Force's Aerospace Maintenance and Regeneration Center at Davis Monthan Air Base, Tucson, AZ, brought the YC-15 back to life. "It took us from April 1996 to February 1997 to refurbish it," says Sillia.

One area of work that the aircraft will test is open-architecture avionics. "The Air Force and McDonnell Douglas want to design cockpits that are easy to change as new avionics come along. In other words," explains Sillia, "the more you turn avionics into black boxes and components that slide in and out, the more rapidly you can upgrade your fleets." Other work planned for the YC-15 includes advanced fuel-tank inerting and tests of enhanced defensive systems and autonomous cargo handling.

McDonnell Douglas Military Transport Aircraft will operate the YC-15 under an eight-year lease from the Air Force. It's reportedly the first Air Force developmental aircraft leased back to a contractor under a cooperative research and development agreement.

The old gal has more than one dance left in her, and the YC-15 may be willing to change partners from time to time, says Sillia. "We're going to be making this aircraft available to others. And if outfits like NASA or DARPA have some technology they want to flight test, they may be able to rent or lease this airplane for their program."

Despite the cuts, despite the downsizing, there's plenty of excellent work going on in the aerospace industry.

Polymer enables self-heating beverage can

San Diego--Imagine this. A container with the dimensions and appearance of a standard 16-ounce beverage can, but with an added convenience--a self-contained heat source.

Ontro Inc., a developer of natural thermal technologies, will have such a container on the market shortly. Seamless, the container holds 10.5 ounces of coffee, tea, soup, baby formula, or any other suitable beverage. A push-button trigger built into the container activates a mixture of two natural materials that heat the liquid to a consumption temperature in five minutes.

For most products, the container will gain about 75 degrees Fahrenheit over the starting temperature, although the gain can be increased or decreased by adjusting the proportions of the heating materials. Once the container reaches the target temperature, it will keep the product warm for at least 20 minutes.

To meet the materials requirements of the container, Ontro selected ACCPRO Enhanced Polymer supplied by Amoco Polymers Inc., Alpharetta, GA. The resin combines high heat resistance, high strength, and ease in processing. It also has excellent heat-transfer characteristics, the ability to serve as an insulator for the container, and withstands the retort process required by the FDA for canning certain products.

The Ontro vessel fits standard fill lines and can withstand normal manufacturing procedures, including hot, pasteurized, and irradiate filling. It is expected to cost packagers only a modest amount more than standard cans.

Electrical isolators safeguard structural tests

Hampton, VA--When aircraft structures are load and thermal tested at the NASA Langley Research Center, all hell can break loose. The Combined Loads Test System (COLTS) now under construction will duplicate temperatures, forces, and internal pressures a future aircraft will encounter.

NASA had to ensure computers and data-acquisition circuits attached to strain gages and thermocouples were electrically protected if the structure failed--showering the test apparatus with shrapnel, and contacting it with high-voltage heaters. Circuitry beyond typical slow-blow fuses was needed.

Wyle Labs (Hampton, VA), Langley service-support contractor, and Phoenix Contact (Harrisburg, PA) devised a simple in-line circuit module between the sensors and data-acquisition equipment. These strain-gage and thermocouple isolation modules are mounted on DIN rails, giving room to mount a grounding block to ground each module. "It's critical to provide a good ground to assure a fast fuse opening," notes Byron Stonecypher, Wyle's head of hardware engineering.

Initial module input is through a 1A, 500V fuse. At the other end of the fuse is a gas discharge tube that breaks down above 90V, allowing up to 500A to flow within 10 to 1,000 musec. (The fuse typically blows before the first 100 musec after seeing 480 to 500V.) A current limiter (a resistor for the thermocouple module; an inductor for the strain-gage module), leading to a bidirectional transient suppressor diode, temporarily "blocks" the fast-changing voltage, absorbing energy until the fuse blows.

The designers had to find a supplier for the 1,536 custom modules for three test rigs. Only two of 12 vendors had any interest. NASA and Wyle Labs selected Phoenix Contact on the basis of lowest cost per channel.

Anti-icing fluid is good enough to eat

Mountain View, CA--An anti-icing fluid three engineers invented using non-toxic materials could soon see use in applications ranging from aircraft to automobiles.

Developed at NASA's Ames Research Center, the fluid is anti-corrosive and biodegradable. Ingredients include such non-toxic, FDA-approved materials as propylene glycol, which is edible. Other deicing fluids can contain diethylene and ethylene glycol, which are poisonous, according to NASA scientist Leonard Haslim. The non-toxicity of the fluid should help save money now being spent to meet the Clean Water Act, he says.

"Propylene glycol is used in stuff like candy bars and cough syrup," says Haslim. "Diethylene and ethylene glycol taste sweet to dogs and cats and may poison fish."

The Ames fluid forms a barrier to snow and ice on aircraft, and is pseudo-plastic. When the plane takes off, the fluid develops a thinner viscosity and blows off the wings so they are clean and have proper lift. "Its behavior is something like latex paint," Haslim says. "It goes on very thick, then thins out."

Technicians can apply the fluid using existing equipment, such as pumps. In automotive applications, the fluid can either be sprayed on the windshield with a sprayer or put into the car's washer fluid container. Drivers can apply it the night before a snowstorm and the next morning, Haslim says, one swipe of the wiper blade will remove all the accumulated ice.

Other possible applications include use as a replacement for road deicing salts to reduce bridge and automobile damage due to the fluid's non-corrosive characteristic. A version used for power lines, insulators, and microwave towers is being developed with a different viscosity.

The fluid will most likely see its first use in nonaviation applications because of strict aviation regulations, Haslim says. Government and industry are currently testing the fluid for aircraft use, and comparison tests against corrosive deicing salts as well as other deicing materials are in the works.

Longbow drawbacks countered by assembly analysis

by Michael Puttre Associate Editor

Orlando, FL--On the field of Agincourt, the longbow heralded the end of knights as the arbiters of battle in Europe, and with that, ended the justification for feudalism. Nobody is claiming the Longbow upgrades to the McDonnell Douglas AH-64 Apache helicopter will result in equally profound changes in society. However, the new technology is designed to deliver as nasty a shock to the modern equivalent of armored cavalry: tanks. Key to the upgrades: design for manufacturing and assembly (DFMA) analysis techniques and software.

The Longbow program is a two-phase engineering effort. Phase one involved developing fire-control avionics for the modified helicopter, designated AH-64D. Phase two revolves around modifying the Hellfire, an anti-tank missile that is the Apache's primary armament, into a millimeter-wave radar-guided version.

The original Hellfire is laser-guided and requires a spotter to hold an illuminating laser on the target. This is hazardous work for the spotter. The Longbow upgrade enables the Apache to acquire its own targeting information, and the missile is fire-and-forget--meaning the launching helicopter can scoot after loosing a shot. Also, the millimeter-wave radar works in all weather.

Lockheed Martin Electronics & Missiles, the builder of the Hellfire, was selected to perform the Longbow upgrade. Both the fire control and missile efforts required brand new assemblies. And in both cases, costs, parts counts, and assembly times were all reduced with the aid of DFMA analysis techniques and software.

Pioneered by Boothroyd Dewhurst, Wakefield, RI, the DFMA approach prompts engineers to analyze their designs as assemblies for the purposes of reducing the number of components and simplifying manufacturing and servicing. The company markets a family of DFMA software products that provides a framework for engineers to set up and run their analyses.

"Meeting the cost goals was the most challenging aspect of the program," says Al Davidoff, manager of engineering practices at Lockheed Martin Electronics & Missiles. Although the Longbow upgrade started from existing systems, the program incorporated many new technologies. As the number of subcontractors and new suppliers increased, the program's complexity and cost began to rise.

"The government was in a position where it could not afford the Longbow Hellfire missile as designed," says Ron Coonrad, a group industrial engineer at Lockheed Martin Electronics & Missiles. "The U.S. Army formally instituted a cost-reduction program, and we made commitments to the Army to meet their goals. DFMA helped us keep those commitments."

Lockheed Martin Electronics & Missiles had experience with DFMA from when it was bidding on another project. Although it did not win that particular contract, engineers appreciated the benefits of analyzing an assembly. "My role was to bring the process over to the Longbow team," Davidoff said.

An antenna, for example, proved to be a particularly costly assembly. A DFMA workshop studying the assembly enabled the engineers to reduce the number of parts from 46 down to 13. The number of assembly operations was reduced from 97 to 64. The redesigned unit cost 20% less than the original.

Lockheed Martin performed 76 DFMA workshops, one-third of them for internal development teams and two-thirds with suppliers. Each workshop involved representatives of most every group involved on the assembly in question, from design and manufacturing engineers and managers to inspectors and operators. "It is important to get the operators involved," Coonrad says. "The engineer doesn't always know what the operator has to do."

"The suppliers can be a little skeptical at first," says Davidoff. "But they quickly came to understand the benefits." In one instance, TRW, which is building the missile transceiver, performed a DFMA analysis and as a result was able to cut the number of parts from 458 down to 171. This reduced the number of operations required to assemble the transceiver from 1333 to 686. In all, TRW was able to cut costs by 25%.

Davidoff cautions that intelligence must be brought to the DFMA process, it is not simply a push-button procedure: "You have to balance simplification against making the few remaining parts more complex."

McDonnell Douglas Helicopter Systems, Mesa, AZ, which builds the Apache, is modifying the crew stations for the Longbow upgrade. Alfredo Herrera, a technical lead on the program, says DFMA in concert with advanced manufacturing techniques can provide significant cost and weight savings. In a report delivered at the 1997 International Forum on DFMA in Newport, RI, last June, Herrera indicated that high-speed machining and composite materials enable manufacturers to make the more complex components generally required when reducing parts count.

"Implementation of DFMA is not an easy task," Herrera concludes. "It takes the correct attitude in order to successfully overcome the barriers created by people used to a different approach."

Urethane shields aircraft windshields from crazing

Sylmar, CA--The manufacture of durable, reliable aircraft windshields calls for high-performance materials and exacting assembly procedures. That's why Sierracin/Sylmar Corp. turned to a fast-setting, durable polyurethane adhesive to bond a nylon-reinforced acrylic and epoxy edge attachment onto vacuum-formed acrylic sheet. The result: high-quality, long-lasting windshields for both commercial and military aircraft.

Adhesive selection for the application proved critical. Sierracin had found that many products craze the acrylic sheet. These stress crazes, when subjected to the temperature extremes that aircraft windshields encounter, can produce cracks that ultimately cause the windshield to be scrapped. After evaluating a variety of adhesives, Sierracin chose Uralane(R) 5774-AB urethane adhesive from Ciba Specialty Chemicals Corp., Performance Polymers, Los Angeles.

"The Uralane urethane adhesive not only is easy to apply and resists crazing on acrylic windshield panels, but it provides us with the strength needed at elevated temperatures," says Connie Magalang, Sierracin's materials laboratory group leader.

The adhesive, a tough polyurethane, once cured exhibits a lap shear strength of 1,400 psi on polycarbonate at room temperature, and 900 psi at 180F (83C).

Computer simulation may lead to hardened aircraft

Long Beach, CA--Commercial aircraft may someday be more immune to onboard explosions thanks in part to computer simulations. Legislation following the PanAm Flight 103 bombing over Lockerbie, Scotland, mandated the FAA to look at ways to prevent bombs from getting onboard in the first place, and to improve the survivability of aircraft should a blast occur.

As a first step, engineers at McDonnell Douglas have applied PAM-SHOCK and PAM-FLOW, two finite-element programs from ESI North America (San Diego, CA), to the task of modeling the blast and the resultant fuselage damage. They chose these programs after looking at others because of the powerful coupling algorithm between the computational fluid dynamic (CFD) and computational structural dynamics (CSD) codes.

"Other codes required node-to-node coupling, so that the fluid mesh almost has to be designed to match your structure, which is very painful to do," says Victor Chen manager of structural mechanics at McDonnell Douglas Aerospace. "PAM allows the fluid boundary to sit on the structure surface."

Chen also liked the fact that the programs would automatically refine the mesh across high-pressure gradients and "unrefine" it when the event has passed, maintaining accuracy and computational speed.

Initial models look at a bare charge in a simplified fuselage structure, ignoring luggage and containers. Chen found that cabin pressure differential plays a major role in the amount of damage, driving any cracks much further than from the blast alone.

As for what changes might be made to aircraft structure or cargo containers, for competitive reasons, Chen isn't saying. Interesting areas to investigate, however, are the fuselage liner, a typically fiberglass covering that acts as a sail to catch some of the explosion force and possibly doing more damage, and hardened cargo containers.

Spacecraft sports 'out-of-this-world' composite

Dulles, VA--Putting a costly communications satellite in orbit is a major design achievement. Assuring that the spacecraft can withstand all the environmental conditions it encounters while making its worldly rounds is equally critical.

That's why Orbital Sciences Corp. combined the best attributes of two vastly different metals to achieve a structural material that meets requirements for its newest ORBCOMM communications satellite system. AlBeMet(R) composites, developed by Brush Wellman Inc., Elmore, OH, consist of beryllium and aluminum. Compared to pure beryllium, often used for structural members in a spacecraft because of its high strength and low weight, the new composite is easier to form and can be dip brazed. Moreover, it costs slightly less than beryllium. In this case, however, raw material costs proved minor compared to the extensive production requirements.

Engineers at Brush Wellman had the task of marrying this "odd couple" of metals for the new composite. The resulting material is 25% lighter than aluminum, and has a 50% lower coefficient of thermal expansion, 20% higher thermal conductivity, and 50% higher fatigue strength. The two AlBeMet grades, 140 and 162, have moduli in the range of 21 to 26 Msi, density between 0.076 to 0.082 lb/inch³, and a specific stiffness 2.5 to 4.0 times that of aluminum. These properties are isotropic, making them ideal for the structural requirements of spacecraft.

Working with materials experts from Brush Wellman and beryllium fabrication experts from Peregrine Falcon Corp., Fremont, CA, Orbital Sciences design engineers put the materials through a rigorous qualification testing program. Tests included: static loading, vibration testing typical of a launch, separation shock loading to over 600g, and thermal cycling simulating the space environment. The structures survived with no cracking, permanent deformation, or degradation of material properties.

AlBeMet components on the ORBCOMM satellite system include:

A flat 41-inch-diameter, 6-inch-high housing that contains the craft's computers, attitude-control systems, propulsion system, and communications payload.

The primary spacecraft ring consists of three arched, 1-inch-thick AlBeMet aluminum honeycomb sandwich panels fastened by three AlBeMet separation brackets. The brackets incorporate 1/2-inch-thick upper and lower flanges that hold the separation bolts and the shear bearings that provide a stiff load path between structures when stacked eight high for launching. Vacuum-brazed AlBeMet brackets connect the payload shelf to the primary spacecraft ring. Like bonded components, the brackets form the rectangular tube that supports the antenna boom segments.

For the launch mode, the solar panels and antenna boom are folded into the structure, allowing the stacking of multiple spacecraft. Nonexplosive, separable bolts attach the units to each other at three points on the interface.

The ORBCOMM satellite system comprises a constellation of 26 small satellites in low earth orbit that provide seamless, two-way, real-time messaging between any two points on the globe. With sophisticated global positioning navigation technology, users can accurately determine their position or that of remotely located objects.

The first two ORBCOMM satellites have been launched on a Orbital Sciences' Pegasus small launch vehicle. The satellites are fully operational, but are providing limited service. The company plans three more Pegasus launches this year, each consisting of eight ORBCOMM spacecraft in a vertical stack.

Software improves modeling and assembly of large structures

Los Angeles--The latest release from The MacNeal-Schwendler Corporation--MSC/SuperModel Version 2.0--offers new capabilities in component modeling and assembly and in interpretation of analysis results.

The software supports the processes engineers typically use in the design of complex structures such as aircraft, jet engines, satellites, and launch vehicles. It provides an integrated engineering environment that helps coordinate design models and analysis results between project teams.

Highlights of Version 2.0 include: automated attachment procedures that simplify the integration of externally supplied component models such as those from subcontractors, and the ability to manage an arbitrary number of such files. The new release also boasts performance improvements in component assembly, as well as in the extraction of component models and their associated analysis results.

Version 2.0 of the MSC/SuperModel system comprises three modules: modeling and simulation, assembly and management configuration, and file manager. The availability of separate modules will enable users to more easily tailor the software to meet specific needs, says MSC's manager of aerospace products, Greg Sikes.

Locking structure strengthens Eagle feathers

Irvine, CA--Tolo is providing U.S. Air Force F-15E Eagles with a robust replacement Grid-Lock(R) vertical-tail aluminum structure. The piece supplants a honeycomb one exposed to the high vibration loads and harsh conditions at the trailing edge above the movable rudder. F-15Es with this and other parts will roll off the assembly line next year. Even sooner, the Air Force may order these pieces as spares for currently operational aircraft.

At up to Mach 2.5, the vertical tails are subject to harmonic vibration at 32 Hz, and the twin-engine afterburners acoustically pound the parts with noise up to 165 dB. Skin friction can heat-load the pieces to 300F. Such stresses can debond honeycomb skins. This weakens shear strength--which comes from the joining adhesive along the numerous thin bond lines formed by the cell walls--and allows corrosion to spread though the internal structure.

The Gird-Lock replacement has the same 8-lb weight but is 20% stronger. Its two skins feature integral machined stiffener ribs having grooves that hold the internal rib formers. This tongue-in-groove arrangement interlocks the components, rather than loading the adhesive bonds, as with honeycomb.

Curtis Lockshaw, Grid-Lock manager at Tolo, notes that the ribs, unlike the smaller honeycomb cells, can be designed to incorporate flanges and bosses or strengthened to accommodate specific loading characteristics. On the F-15 part, rib stiffeners on the skin underside between the internal ribs allow it to resist buckling during high-speed flight. These stiffeners were optimized for weight and flight and acoustic loads.

The Grid-Lock concept reduced the parts count from around 50 to about 20--simplifying design, construction, and repair. And the large, interlocked cells tend to isolate any damage and permit drain holes for moisture and pressure relief--eliminating further bondline stresses inherent in honeycomb cells.

A similar Grid-Lock fairing below the rudder is now being installed on F-15s for Israel and Saudi Arabia. McDonnell Douglas Group Manager Ed Schaefer says this piece proved the concept which precludes honeycomb problems. He adds that water intrusion into honeycomb--with subsequent freezing and thawing under flight conditions--shortened part life.

Laser cuts cost of new stereolithography machine

Valencia, CA--3D Systems Inc. has developed a new stereolithography machine that the company says will save money on installation and cooling while providing more accurate parts.

The SLA 5000 is 40% smaller than its predecessor, the SLA 500, and requires no external facilities such as a chiller for cooling. Engineers say the cooling is all internal.

Additionally, says Diana Kalisz, 3D's senior director of stereolithography programs, the SLA 5000 lowers power costs. It runs 220V ac single phase on a 20-amp breaker.

Key to the SLA 5000 is a solid-state neodymium YAG laser from Spectra-Physics Inc. that operates at 355 nm at a pulse rate of 40 kHz. 3D's engineers worked closely with Spectra-Physics to refine the laser design, exchanging prototypes, models, and CAD files.

"This solid-state laser is the biggest of its kind in the world yet draws little electricity," says Kalisz. "Because the entire machine has a smaller footprint, users will save on installation."

Minimum layer thickness of parts produced by the SLA 5000 is 2 mils, which enables higher vertical accuracy and less time for finishing patterns for tooling, engineers say. Applications include fine patterns for tooling, patterns for rubber molds, and the Direct AIM (ACES Injection Molding) process for quickly building prototype parts without production tooling.

Software gives rocket a boost

Turin, Italy--On April 16, 1997, the European Space Agency's (ESA) Ariane 5 "Evolution" rocket roared from its launching pad in Kourou, French Guiana, South America. Design optimization and verification using ANSYS software from ANSYS Inc., Canonsburg, PA, helped ensure the rocket functioned more precisely than its predecessor, which failed due to a simultaneous failure of the rocket's two inertial reference systems.

Fiat Avio, Turin, Italy, makes the twin booster rockets and the turbine-driven cryogenic fuel pump to feed liquid oxygen (LOx) into Ariane 5's single huge engine, the Vulcain. Vulcain is 18x20 ft, weighs 33,000 lb, and develops 220,000 lb of thrust.

Engineers at ESA performed a major upgrade of the LOx turbo-pump design with ANSYS software. "The goal of analyses is to ensure the turbo-pump meets its demanding specifications," says Andrea Tasselli, project leader at ESA. The engineer and his team of five analysts used the software for design validation and analysis of various rotating parts, including the pump's turbine disks and their blades.

"ANSYS showed us that we had to revise some of our basic design solutions because some stress concentrations were too high," continues Tasselli. These included achieving maximum stiffness for the central housing to which all the other components are bolted. "After exploring several design options," he says, "we doubled stiffness in some components."

Tasselli also found that he is able to optimize the turbine disk with ANSYS's multiphysics capabilities. First he analyzed the disks. He then thickened them in a few carefully pinpointed areas to ensure that the required structural toughness remained at the pump's high operating temperatures.

Plastics give birth to new stop sign

Coral Springs, FL--Weighing in at just 10 lb, ALLSign is said to be the world's first all-plastic, retroreflective stop sign.

"When I developed the concept for this product, I wanted to make it durable, attractive, and a marked improvement over existing sign technology," says George Kochanowski, president of AllSign Products Inc., the sign's creator. "I selected polycarbonate because it is one of the most durable engineering thermoplastics available, and it will allow our product to provide exceptional optical performance and a much longer service life than current stop signs."

The polycarbonate that Kochanowski refers to is Makrolon(R) resin from Bay Corp.'s Polymer Div., Pittsburgh. It's covered with a protective layer of Bayer's Makrofol(R) EPC (enhanced performance composite) polycarbonate film.

Like any stop sign, the ALLSign product is a regulation, 30-inch-diameter octagon. But unlike other stop signs, it has a slight crown, which helps reduce glare. When assembled, the sign measures about one inch thick at the center and 3/4 inch thick around the edge.

The sign's interior presented the most difficult design and molding challenge. Like the taillight of a car or a bicycle reflector, the sign is retroreflective to make the most use of all light that strikes it. Numerous precisely engineered and molded cube corners, or reflects, line the inside of the sign's face.

Light that strikes these cubes reflects away from the sign at an angle equal to the incidence angle. Any light that penetrates past the cubes hits the rear of the sign, which reflects the light back into the cubes and the plastic lens. Light can also enter the sign from the clear edge or the translucent back. As a result, the sign is illuminated by sunlight during the daylight hours, and by automobile light, streetlights, and other light sources at night.