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The tire has been difficult to model with respect to side loads due to test facility limitations. Two math models were developed from early dynanometer tests plus extrapolation from nosewheel tire tests. New dynamic tests of main gear tires now show that a more flexible side response is proper. Since this model is from actual test data, it is the best obtainable and should result in some stability in the tire model community. Tire model accuracy impacts crew training, test conclusions, and nosewheel steering gain adjustments. As the community accepts the new tire model, gain adjustments to the nosewheel steering computer are made to assure that the Orbiter stays on the runway for severe limit cases. At some point, flight data should be collected to assure that the tire models being used in the simulators and, hence, the critical nosewheel steering gain adjustments are correct. Additionally, the Ames Research Center landing rollout simulator is the only accurate simulation of the landing and rollout dynamics. It is not generally available as a training device but as an engineering tool. It supports many programs other than the Space Shuttle Program. 

The current state of the tire program is that the tire meets specifications and has been certified through testing; however, testing accuracy and actual KSC landings indicate that the tires should undergo an additional test program prior to increasing the current crosswind limits. To date, the only blown tire has been caused by a brake lockup and the resulting skid wear after lockup. No tire failures have occurred due to tire problems alone. It should be noted that all runways at EAFB contribute much less wear to tires than at KSC.

Future considerations to eliminate the highspeed blown tire case are the addition of a skid at the bottom of the strut to take the major peak load during nosegear touchdown, to increase tread depth, and to add a roll-on-rim capability to the main gear wheel. None of these have been funded and have only had little study support.

In summary, the double blown tire case prior to nosegear touchdown could be catastrophic no matter what runway it occurs on. Simulation results say that the best place to take that case is to EAFB runway 22 and try to get a crosswind from the side with the failure. According to simulations, the worst runway is probably a lakebed runway. Testing is planned in the fall of 1986 to examine actual tire/wheel/strut failures to better understand this potential failure scenario.

XVII. ORBITER BRAKES

The Orbiter brakes are provided by B. F. Goodrich. The Orbiter brake design chosen in 1973 involved using beryllium rotors and stators with carbon lining to save some Orbiter weight and volume. However, the actual Orbiter weight continued to grow. The response from the brake world was not a redesign but an extension of the runway requirement from 10 000 feet to 12 500 feet.

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