 Complete the following phrase:  During the flight, the crew shut down a thruster in the reaction control system RCS after receiving indications of a leak. An RCS heater also malfunctioned during the flight. Post-flight analysis of STS-28 discovered unusual heating of the thermal protection system TPS during re-entry, caused by an early transition to turbulent plasma flow around the vehicle. A detailed report identified protruding gap filler as the likely cause. This filler material was the same material that was removed during a spacewalk during STS-114, the Space Shuttles post- 