Externally, Columbia was the first orbiter in the fleet whose surface was mostly covered with High & Low Temperature Reusable Surface Insulation (HRSI/LRSI) tiles as its main thermal protection system (TPS), with white silicone rubber-painted Nomex – known as Felt Reusable Surface Insulation (FRSI) blankets – in some areas on the wings, fuselage and payload bay doors.
FRSI once covered almost 25% of the orbiter; the first upgrade resulted in its removal from many areas, and in later flights it was only used on the upper section of the payload bay doors and inboard sections of the upper wing surfaces.
The upgrade also involved replacing many of the white LRSI tiles on the upper surfaces with Advanced Flexible Reusable Surface Insulation (AFRSI) blankets (also known as Fibrous Insulation Blankets, or FIBs) that had been used on Discovery and Atlantis.
Originally, Columbia had 32,000 tiles – the upgrade reduced this to 24,300.
The AFRSI blankets consisted of layers of pure silica felt sandwiched between a layer of silica fabric on the outside and S-Glass fabric on the inside, stitched together using pure silica thread in a 1-inch grid, then coated with a high-purity silica coating.
The blankets were semi-rigid and could be made as large as 30" by 30".
Each blanket replaced as many as 25 tiles and was bonded directly to the orbiter.
The direct application of the blankets to the orbiter resulted in weight reduction, improved durability, reduced fabrication and installation cost, and reduced installation schedule time.
All of this work was performed during Columbia's first retrofitting and the post-Challenger stand-down.

Despite refinements to the orbiter's thermal protection system and other enhancements, Columbia would never weigh as little unloaded as the other orbiters in the fleet.
The next-oldest shuttle, Challenger, was also relatively heavy, although 2,200 lb (1,000 kg) lighter than Columbia.
Externally, Columbia was the first orbiter in the fleet whose surface was mostly covered with High & Low Temperature Reusable Surface Insulation (HRSI/LRSI) tiles as its main thermal protection system (TPS), with white silicone rubber-painted Nomex – known as Felt Reusable Surface Insulation (FRSI) blankets – in some areas on the wings, fuselage and payload bay doors. FRSI once covered almost 25% of the orbiter; the first upgrade resulted in its removal from many areas, and in later flights it was only used on the upper section of the payload bay doors and inboard sections of the upper wing surfaces. The upgrade also involved replacing many of the white LRSI tiles on the upper surfaces with Advanced Flexible Reusable Surface Insulation (AFRSI) blankets (also known as Fibrous Insulation Blankets, or FIBs) that had been used on Discovery and Atlantis. Originally, Columbia had 32,000 tiles – the upgrade reduced this to 24,300. The AFRSI blankets consisted of layers of pure silica felt sandwiched between a layer of silica fabric on the outside and S-Glass fabric on the inside, stitched together using pure silica thread in a 1-inch grid, then coated with a high-purity silica coating. The blankets were semi-rigid and could be made as large as 30" by 30". Each blanket replaced as many as 25 tiles and was bonded directly to the orbiter. The direct application of the blankets to the orbiter resulted in weight reduction, improved durability, reduced fabrication and installation cost, and reduced installation schedule time. All of this work was performed during Columbia's first retrofitting and the post-Challenger stand-down.
Despite refinements to the orbiter's thermal protection system and other enhancements, Columbia would never weigh as little unloaded as the other orbiters in the fleet. The next-oldest shuttle, Challenger, was also relatively heavy, although 2,200 lb (1,000 kg) lighter than Columbia.
Space Shuttle Columbia