From 1930, Sander (with other pyrotechnics factories) began secretly manufacturing rockets for military purposes, at the direction of Walter Dornberger. In 1936 he signed a contract for the sale of some of these weapons to Italy but was denounced as a traitor by the Nazis and imprisoned. During this time, the company was bankrupted, but upon his release later the same year, Sander founded a new company, Comet GmbH with the assistance of some of his previous suppliers. In 1938, the firm was nationalised and Sander, again imprisoned, was forced to sell his interests in it to the Nazi Party. He died while in custody.