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One tire exhibited a leakage which was traced to a valve stem torque requirement problem. This was corrected by and no additional leakage hazard specification limits have occurred. Also, 100 percent of the tires are subjected to coldsoak and leakage tests and are given a rollin prior to mounting on the Orbiter.

Tire wear was not a major concern until the experience on STS 51-D at KSC.  There, spinup wear was seen up to 3 cords deep. Crosswind wear was very noticeable. One tire eventually failed due to a brake lockup and skid. This excessive wear combined with the break failure brought program management attention to the issue of flight safety and KSC landings. KSC landings were halted until a fix was developed. The fix was to improve nosewheel steering so that it could be used by the crews for rollout directional control. This would provide a primary and reliable means for directional control rather than using differential braking as the only technique. This upgrade was tested on the lakebed prior to permitting return to concrete/KSC operations. At the time of the launch of STS 51-L, the nosewheel steering system was considered operational; however, it was not fail ops/fail safe. It actually was not fully fail operational since it failed over to a non-usable, yet passive, state.

Testing using the simulated KSC runway indicated the spinup wear by itself is not a tire blow concern; however, that will also be examined in greater detail in testing during the summer of 1986. Testing using the KSC test surface does indicate that the spinup path may grow when subjected to crosswind. This was also seen in the STS 51-D case. For this reason, the crosswind at end of mission is currently limited to 10 knots. This limit is also being reviewed for applicability to RTLS and TAL.

A new, thicker tread design is being evaluated. The concern is to avoid trading one problem for another. The current tread is thin to reduce the heat load in the tire. A thicker tread will compromise the tire's ability to transfer heat. It may be time to challenge the industry with a further push on the state of the art in the design.

The other tire concern is the reason for the criticality 1 rating. The Orbiter adds aerodynamic loads to the vehicle loads during the nosewheel touchdown maneuver. The total load on a single side can be nearly 200,000 pounds. A single tire cannot take this load. As a matter of fact, the touchdown loads can exceed the load bearing capability of a single tire. The obvious result is that, for a single tire failure prior to main gear touchdown or after main gear touchdown and before nosegear touchdown, the vehicle will have significant if not catastrophic directional control problems. Prior to nosegear touchdown, control is with the rudder/speedbrake which loses effectiveness when more speedbrake is being carried. Rudder use is further complicated because the rudder is slow acting and non-linear in response, making control more difficult. After nosegear touchdown, simulations have shown that directional control is possible with the nosewheel steering system, for all failure scenarios within the flight rule wind placards.

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