In the summer of 1928, a young engineering student named Eugen Sänger submitted his doctoral thesis to the Technical High School in Vienna. The subject of his thesis was the problem facing high-altitude rocket plane flights. He described his study as comparing:
The different ways of advancing into space; it calculates the most economical and safest method (aerospace transporter-space station-space ship) and supplies a complete theory of this method . . . The conquest of space with minimum energy display will proceed according to the following principles:
1. Transport to the altitude of a space station by means of the special aerospace-plane; further advances into space will use modified spacecraft.
2. Ascent of the aerospace-plane according to the principle of minimum energy expenditure.
3. Descent of the craft as a glider, without energy expenditure.
Sänger was told by one of his instructors that if he tried to get a doctoral degree in spaceflight, he would most likely be "an old man with a long beard before you have succeeded in obtaining your doctorate."
Sänger abandoned his original paper and submitted a more conventional paper for his degree.
But he didn't forget what he really wanted to do.