qnli question: What engineer was recruited by Becker and Dornberger to join their secret program? sentence: The head of the German Army's Ballistics and Munitions Branch, Lieutenant Colonel Karl Emil Becker, gathered a small team of engineers that included Walter Dornberger and Leo Zanssen, to figure out how to use rockets as long-range artillery in order to get around the Treaty of Versailles' ban on research and development of long-range cannons.