FIRST STEPS TOWARD SPACE - Smithsonian Institution Libraries

NUMBER 10 39
P
(atm)
3 J
/ (In,.,
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WM ' = = a -- ^
tf /.J /^ ?^ JP «
-
'
T (.01 sec)
FIGURE 8.—Pressure records (in atm vs. time in sec) of larger rockets, using cordite as propellant.
FIGURE 9.—Launcher for spin-stabilized rockets
hoped from the change was due to the different
composition, which led me to expect a smaller sensitivity
of burning rate to pressure. I had reached the
conclusion that the erratic behavior of the equilibrium
pressure was due both to the geometric irregularity
of the cordite and to its high pressure
sensitivity. This conclusion, subsequently confirmed
by the experiments, was based on simple calculations
showing that deviations within ±5 percent of
the dimensions of the tubular charges (that is of
the order ±0.5 mm, on the radius, a very realistic
deviation for cordite) would result in a range of
equilibrium pressures from 30 to 186 atm for a
nominal pressure of 100 atm if the burning rate
varied with the 0.875 power of the pressure, but
only in the range from 82 to 119 atm for an exponent
of 0.625.
The results of the constant-volume tests are summarized
in Figures 10 and 12. Figure 10 gives the
pressure history obtained with different amounts
of the two propellants. It was immediately evident
from the smaller curvature of the C-powder (Polvere
C) records that the pressure sensitivity is decreased.
Figure 12, however, shows that where the terminal
pressures are plotted against the charge weight, the
"effectiveness" of the two powders is very nearly the
same. For both powders the pressure sensitivity decrease
with decreasing pressure since the pressure/
density ratio appears to follow very closely a p 0 - 25
power law within the pressure range of these tests.
Tests conducted next in the chamber shown in
Figure 1, with the C-powder and an exhaust orifice
of 7 mm, confirmed with their high regularity the
superiority of this type of propellant as compared
to cordite. For example, Figure 11a shows two representative
pressure curves. Figure 116 shows a record
obtained in the small test chamber shown in Figure
5. This chamber, of all those employed, allowed the
highest charge density of 0.71 kg/dm 3 . In comparison,
the chamber in Figure 1 allowed a loading density
of 0.17 kg/dm 3 and the chamber shown in Figure
14, a charge density of 0.54 kg/dm 3
The excellent behavior of these tests encouraged
construction of the new test chamber (Figure 13)
for 300 g of nominal charge. For safety reasons an
expansion chamber 5 was attached to the test
chamber to provide additional volume for the gases
in case the pressure would rise beyond 500 atm and
break the diaphragm R. But this precaution was
proved superfluous by the great regularity of the
tests. The chamber B was mounted as a pendulum,
as indicated in Figure 14, to allow measurement
of the thrust.
The values of the burning rates obtained from
the tests on the two propellants are summarized in