Original Date: 10/20/1997
Revision Date: 01/18/2007

Northrop Grumman is in the process of using aerospace-type materials, construction techniques, and defense conversion technologies for its commercial Advanced Technology Transit Bus (ATTB) program. To date, the company has achieved impressive results from its ATTB prototypes and test beds.

The ATTB program began in 1992 with the objective of developing a lightweight, low floor, low emission transit bus (Figure 3-1) by using proven, advanced technologies developed in the aerospace industries. This vehicle is designed to meet federal, state, and local (southern California) axle weight and clean air requirements. In addition, the ATTB will meet or exceed the Americans with Disabilities Act requirements through the use of a low, flat floor and a simple ramp system that is more reliable than current wheelchair lift technology.

The ATTB is an all-wheel drive vehicle with drive motors for each wheel. These motors reverse to generate electricity as a braking action which will considerably reduce the wear on brakes. To meet emissions requirements for urban buses (2.5 gm/bhp-hr NO_x, 0.05 gm/bhp-hr PM), the ATTB uses a hybrid propulsion system that incorporates an internal combustion engine-generator, set in concert with a high power-density flywheel, for driving the electric wheel motors from natural gas. The 40-foot long bus can accommodate two wheelchair positions, and 43 seated and 29 standing passengers. The ATTB’s curb weight is 10,000 pounds less than an equivalently configured conventional transit bus.

The ATTB’s user-friendly design features wide doors and a low, flat floor (15-inch maximum floor height) for shorter dwell times and easy boarding/deboarding of passengers. The lack of internal steps or risers eliminates significant tripping hazards. The driver’s station is ergonomically designed and modular to achieve driver- commanded high quality performance. All ATTB components are modularly designed for easy and quick swap-out repairs. This design facilitates low cost repair and operational costs, and will accommodate returning buses into service within one shift. The ATTB body, fabricated from fiberglass and foam, is substantially more impact resistant than conventional transit buses. The unit cost of an ATTB will be in the same range as existing conventional transit buses.

Northrop Grumman also developed full-scale Structural and Mobile Test Beds for its ATTB program. The Structural Test Bed was used to validate the design of the ATTB’s composite structure. The testing program included a side impact crash test with a 4,000-pound car traveling at 25 mph, which resulted in only cosmetic damage to the test bed. The Mobile Test Bed was developed to evaluate the performance and handling characteristics of the vehicle such as its propulsion, control, braking, dynamometer, and suspension systems. Both test bed models (Figure 3-2) successfully completed their testing programs.

Northrop Grumman has completed six prototypes in its ATTB program. One prototype is currently being field- tested at the Pennsylvania Transportation Institute, Altoona Bus Testing Center for one million miles of test driving and vibration testing.

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