The basic concept continued to be studied through the 1960s and 1970s. A serious problem was that the interceptor missiles had to be very fast to reach the ICBM before its motor stopped firing, which required a larger motor on the interceptor, meaning higher weight to launch into orbit. As the difficulties of this problem became clear, the concept evolved into the "ascent phase" attack, which used more sensitive seekers which allowed the attack to continue after the ICBM's motor had stopped firing and the warhead bus was still ascending. In all of these studies, the system would require an enormous amount of weight to be lifted into orbit, typically hundreds of millions of pounds, well beyond any reasonable projections of US capability. The US Air Force repeatedly studied these various plans and rejected them all as essentially impossible.