FIRST STEPS TOWARD SPACE - Smithsonian Institution Libraries

134 SMITHSONIAN ANNALS OF FLIGHT
A
FICURE 1.—Model B rocket. From Oberth, Wege zur Raumschiffahrt,
1929.
they provide a greater specific impulse. From the
requirement for a lightweight rocket structure came
the realization that ceramic materials could not be
used for rockets with liquid propellants. In these
rockets the combustion chamber has to be light in
weight and must have a thin wall, and the walls
have to be kept cool by flowing the fuel around
them. With this method, regenerative cooling is
accomplished. The pressure in the combustion
chamber must not be too low, otherwise the gas
exhausts at too low a velocity. The tanks, however,
should be under low pressure so that the walls do
not have to be too thick. From this consideration
the need for fuel pumps resulted. By the way, the
paper also contained a relatively simple mathematical
criterion for determining the advantage or
disadvantage of using a device which increased the
exhaust velocity but diminished the mass ratio of
empty rocket to filled rocket.
In spite of regenerative cooling I did not want
the temperature of the combustion chamber to be
too high, especially because the titanium and vanadium-steel
alloys of today were not known at that
time. A means of decreasing the temperature of the
combustion chamber without reducing the exhaust
velocity is available by adding another element to
the propellant which does not burn but only evaporates,
thus creating a specifically light vapor. When
combining hydrogen and oxygen, for example, the
excess of hydrogen creates that effect (Esnault-
Pelterie called it the "Oberth-effect").
In this way I had, in the 1920s, experimentally
achieved a propulsion system which reached exhaust
speeds of 3,900 m/sec to 4,0000 m/sec. I used
the propellants, however, in their gaseous state
because in Transylvania I could neither obtain
them in liquid form nor find means to liquefy them.
I wrote about this to some friends in Vienna;
whereupon a professor of the Vienna Technical
University answered that I must be a fraud. He had
calculated that hydrogen and oxygen, even when
used in their stoichiometrically correct proportion,
could not provide more than 3,200 m/sec because of
dissociation. However, he did not think of the fact
that dissociation is practically zero because of the
excess of hydrogen and the low temperature. Pure
hydrogen can be lighter and achieve a greater exhaust
velocity at low temperature than dissociated
or even undissociated H20 vapor at high temperature.
Today the Americans use H2 and 02 in their