After graduation, Korolev worked with some of the best Soviet designers at the 4th Experimental Section aircraft design bureau OPO-4 headed by who emigrated to the USSR from France in the 1920s. He did not stand out in this group, but while so employed he also worked independently to design a glider capable of performing aerobatics. In 1930 he became interested in the possibilities of liquid-fueled rocket engines to propel airplanes while working as a lead engineer on the Tupolev TB-3 heavy bomber. Korolev earned his pilot's license in 1930 and explored the operational limits of the aircraft he piloted, wondering what was beyond his plane's altitude limit and how he could get there. Many believe this was the start of his interest in space.