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The characteristic trend of modern technical and scientific development is expressed by the fact that, in most important inventions and discoveries the first experiments are preceded by a large amount of pure theoretical work, including generally a lot of mathematical speculations at a very high level; less opportunity being left to the mere ingenuity of the skilled technician, which was sufficient not so long ago to make very important discoveries and inventions by purely experimental methods with only a minimum of theoretical knowledge. The most conspicuous example of that kind of scientific discovery is, of course, the release of atomic energy, which was preceded by an enormous amount of theoretical work in mathematical physics. This assertion can be applied on a more modest scale to the recent development of the liquid fuel propelled rocket. Though the powder propelled rockets have a long story, which goes far back into history, we may consider the liquid fuel rocket as a new invention. It had been preceded by a quite considerable literature, and before the first successful bifuel rocket took the air, the theoretical aspect of the problem had been fairly well developed.