FIRST STEPS TOWARD SPACE - Smithsonian Institution Libraries

218 SMITHSONIAN ANNALS OF FLIGHT
Vienna—because his goal was to develop an aircraft
engine—limited his efforts to testing propulsion
units, but he proceeded very systematically and
obtained valuable test data.
Though Sanger started his tests later than the
German group and worked mainly by himself, he
was in a more advantageous position: from the
very beginning, because of his different objective,
he devoted more attention to the problem of cooling
his rocket engine than the German group did.
The latter, when not simply relying on the heat
capacity of the combustion chamber walls, placed
the engine into a container filled with a stagnant
coolant, although Oberth had already proposed in
his book a regenerative cooling process. Since
Sanger wanted to develop a rocket engine for a
manned aircraft and not a ballistic projectile to be
launched vertically, he had to build his engine so
carefully and so safe that it would be reusable
many times. Thus, he initially concentrated on the
development of the engine itself, without facing
the complex problems of producing a flightworthy
overall system. Consequently, he could devote more
attention to the so-called "braking tests" in a
ground test facility than other researchers did who
were interested in reporting as fast as possible on
flight altitudes and ranges obtained by their ballistic
rocket models. (He called these "braking tests" in
analogy with tests of internal combustion engines,
in which torque is braked and measured, whereas
in his tests the thrust was being sustained and
measured.) Sanger prepared his tests in a most
systematic and logical way and, especially in studying
cooling problems, took advantage of tapwater
available from a stationary source. From the very
beginning, Sanger's tests aimed at obtaining high
exhaust velocities which are accompanied by high
combustion temperatures and chamber wall stresses,
whereas Oberth and his followers tried to achieve
first of all simply a "functioning" of the rocket engine
and artificially lowered the combustion temperatures
by water injection.
To aid in understanding the development approaches
taken in the early German and Austrian
rocket projects described herein, a systematic synopsis
of possible and so far known cooling methods
for rocket engines is being attempted. In principle,
two methods can be distinguished for cooling
rocket engine parts exposed to combustion gases—
capacity (capacitance, or heat-soak) and dynamic
cooling.
Capacitance cooling is a static process whereby
heat flowing from the combustion chamber is stored
by the solid chamber walls and—if they are present—also
by the walls of a cooling jacket surrounding
the thrust chamber. The heat thus received is
continually collected within the coolant material,
but this method does not result in an equilibrium
condition and is useful for a limited time only, i.e.,
until the heat storage capacity is exhausted. A limit
case of capacity cooling is represented by ablation
cooling; here heat is dissipated by successive melting
or subliming of a suitable protective layer
(e.g., nylon, phenolic resin, or graphite) covering
parts endangered by heat.
The term dynamic cooling covers any method
using conduction, convection, or radiation to dissipate
from the endangered zone that amount of
heat which cannot be stored by the combustion
chamber walls. There are two ways to accomplish
this:
1. To minimize heat transfer from the combustion
gas into the heated wall side either by reducing
the temperature difference between both (high
wall temperatures with refractory wall materials,
artificial reduction of combustion temperature
by water injection, etc.) or by influencing the
boundary layer (coolant mist, optically reflective
wall surfaces, electrical fields for sufficiently
ionized combustion gases, etc.).
2. To dissipate as Tapidly as possible the heat contained
in the chamber wall by maximizing heat
transfer from the cooled chamber-wall side into
an adjoining suitable and efficiently ducted
coolant.
Gartmann proposed the terms "internal," and "external"
cooling for these two methods.
Film cooling, invented by Oberth and achieved
by injecting water into the boundary layer, is an
example of internal dynamic cooling. In the case of
external dynamic cooling—where the amount of
heat from the hot combustion gases, passing to and
across the combustion chamber wall and then into a
flowing coolant, is carried off with the coolant—a
state of equilibrium can be obtained if the coolant
can be ducted in such a way that the heat amount
received by the heated chamber wall side equals