Riemann's Contribution to Flight and Laser Fusion

Courtesy of Adolf BusemannThe seminal Volta meeting on supersonic aerodynamics in 1935. Busemann is second from the left, second row. To hisleft is Karl Wieselsberger, von Karman's successor at the University of Aachen. In the front row are Genera/ Crocco(right), president of the meeting, and his son.He had lots of people working on high speeds duringWorld War I. After the war was over, Germany wasn'tallowed to work on practical airplanes anymore. Ofcourse, in turbines, too, you very often have supersonicspeeds, at least in the first stages of the turbine—not inthe later ones. But Prandtl's researchers just wanted to getan idea of the compressible flow for any kind of application.The application for airplanes seemed out of reach orsuppressed.Question: Was there already research on supersonic flowin turbines, say in the 1920s?Yes. I worked as an engineer and had learned in college,of course, about steam turbines and things like that. Theywere already invented. Therefore, we wanted to see howto make them most efficient—how to get the most energyout, put the least into a reversed flow, and reduce energylosses. But Prandtl's researchers were a little bit spoiled,because at the first Mach, at around 1.6, when two linesindicating pressure changes are going through each other,they stay straight—it doesn't look like they interfere witheach other. They were spoiled because that was wherethe lines should have changed their curvature from oneside to the other, but near the turning point there wasjust no curvature at all.Therefore, when I came I wasn't so spoiled. I said whenthis is not so linearized, finite disturbances have to interferewith each other. When one goes through the otherone, it has to change its direction. And of course, theMach number in the wind tunnel that I had at that timewas already a little bit lower. There was already a visiblecurvature in a certain direction, and therefore when Icame in I worked a little bit more on how to produce apicture predicting the interference of two crossing Machwaves.Question: What year was that?I got my doctorate in 1924, and then I went to workwith Prandtl in 1925. I stayed with him until 1931, when he"sold" me to Dresden. Those were bad times.There was a young assistant there at Dresden who diedbefore he was 33, and then there was a free opening.Those were pretty bad years. Some people didn't know ifthey should change their subject of study. America alsohad very bad times then. But when you lose a war, youcan't be so much better—even when Germany had justrecovered in 1929-30 and therefore seemed kind ol highup, and America seemed to be very low.Because of the economy, Prandtl couldn't keep so manypeople at his institute. The ones who could got other jobs.At a certain institution for aerodynamic research, they toldthree people, "One of you has to go. You select him."The workers came back and said, "Well, we would rathertake two thirds of our salaries and stay on here."Question: When did you go to Dresden?Prandtl couldn't keep everybody at Gottingen, so I wentto Dresden. I had already agreed to a 10 percent smallersalary there. And then all of a sudden, one year or another,the governor of Saxony said that everyone had to give upone tenth of his salary. So I lost not only my voluntary 10percent at Gottingen, but another 10 percent. One or twopeople said it was illegal. But they said it couldn't behelped; it was the government's order. So I lost 20 percent.October-November 1981 FUSION 35