The Space Shuttle Solid Rocket Boosters (SRBs) are the pair of large solid rockets used by the Space Shuttle during the first two minutes of powered flight. They are located on either side of the orange external propellant tank. During launch they provide 83% of its liftoff thrust. Each SRB produces 1.8 times more liftoff thrust than one F-1 engine – the most powerful single-chamber liquid-fueled rocket engine ever flown – 5 of which powered the Saturn V "moon rocket's" first stage. The SRBs are the largest solid-fuel rocket motors ever flown, and the first to be used for primary propulsion on human spaceflight missions. The spent SRBs are recovered from the ocean, refurbished, reloaded with propellant, and reused for several missions.