Prototypes of a lightweight bus could demonstrate the benefits of polymer composite technology to commuters in several American cities in 1998. Developers in November expect to adopt a final configuration for the federally funded Advanced Technology Transit Bus, and Northrop Grumman Corp. plans to begin fabricating the first prototype in April at a plant in El Segundo, Calif. Plans call for a low-floor, wheelchair-accessible vehicle with seats for 43 passengers, standing room for 29 and an engine using low-emission, compressed natural gas.
Standard E-glass and Dow Chemical Co.'s Derakane 441-400 epoxy vinyl ester resin form a sandwich over an inner core of DiAB Group's Klegecell foam to create the lower tub, ceiling and connecting ladders, the ATTB's major structural parts. In contrast, a conventional bus structure has as many as 250 parts.
An early design used posts as connectors.
``Rather than individual posts, they now use a ladder assembly that makes production of the vehicle easier,'' said Jim Pachan, supervisor in the equipment engineering section of the Los Angeles County Metropolitan Transportation Authority.
Pachan said the ceiling and two full-length ladders - attached ``like Tinkertoys'' - become a single component once the resin hardens. Likewise, resin seals the ceiling-ladder assembly and tub to form a 5,800-pound shell. Breastplates, suspension and wheels are attached to the shell.
Designers plan a 19,250-pound prototype and, eventually, 18,000-pound production models. A typical compressed natural gas bus weighs as much as 31,000 pounds, and a diesel bus about 28,000 pounds. Compressed natural gas, liquified natural gas, fuel cells or electricity could power production models. The ATTB uses four tires instead of six and automotive-style hydraulic brakes.
Pachan said that composites have fared better than expected during several tests conducted to validate the computer modeling of the composite structure. For example, no significant damage occurred during a 25-mile-per-hour side-crash test.
Other tests checked fastener torque and pull-out, sidewall bending and torsional bending, floor deflection and stress under a gravitational force of two, wheel-well impact, and delamination. Off-the-shelf generators and wheel motors failed to meet weight or size requirements, so custom work was needed to acquire these items for the prototypes.
Los Angeles-based Northrop Grumman is producing the ATTB's composite shell and enclosure panels.
``The real challenge is to build a vehicle of this size for a cost that is competitive with a normal metallic bus,'' said Dave DeMent, Northrop Grumman manager of ATTB integrated product development. ``We have procedures to reduce cost, and those are applicable to a vehicle like a bus.''
Northrop Grumman, a major aerospace firm, ``has spent billions developing composites technology,'' generally ``working with structures that cost hundreds of dollars per pound'' of material, DeMent said. ``The challenge is to find materials and processes that allow us to do this'' on a transit bus rather than an aircraft.
The company expects to complete the first prototype in late 1996 and follow with rigorous, year-long road and vibration tests.
Five subsequent prototypes will see regular service with transit agencies accommodating riders in the congestion of New York and Chicago, the dampness of Seattle, the humidity of Houston, the heat of Phoenix and the hilly terrain of San Francisco. Eighteen transit agencies in 12 states provided input on their requirements and, eventually, may buy production models.
In December 1992, the Federal Transit Administration awarded a $4 million ATTB grant to Los Angeles County, which competitively contracted with Northrop for the prototype design and development. The total cost of the project is estimated to be $48.5 million with completion in 1999. The FTA is an agency of the Transportation Department.
``This is an important project that extends over several years and one for which we have very high expectations,'' Lawrence Schulman, FTA associate administrator for research, demonstration and innovation, said in a telephone interview.
``This is an agency initiative, and we are committed to the project.''