Several similar conversions of the Space Shuttle's external tank (ET) were also studied. During launch the ET accelerated to about 98% of orbital speed before being dropped and deliberately spun in order to increase its drag. A number of proposals suggested keeping the ET attached to the Shuttle all the way into orbit, bleeding off any remaining propellant through the Space Shuttle Main Engines, which would have been "left open". One such test had been scheduled, but was canceled after the 1986 Space Shuttle "Challenger" disaster dramatically changed safety rules.