Chemistry Reference
In-Depth Information
propellant burning. They can be exemplified by the so-called “two-temperature pos-
tulate” formulated by Schultz and Dekker [4], a theory of the thermal layer devel-
oped by Chaiken [5], and others. Well-founded ideas about the burning mechanism
of composite solid propellants were formulated based on new results (including
ones obtained by the author of this monograph) in recently published papers (for
example [6, 7]).
Due to reasons given in the Introduction, neither data on the kinetic constants
of the high-temperature decomposition of ammonium perchlorate (AP, the main ba-
sic oxidizer) and typical binders nor even qualitative data on the macrokinetics of
the process have been available. The study of the burning mechanism of a composite
solid propellant is an extremely challenging task, since in real systems the decompo-
sition of the individual components occur in parallel with interaction between them
(which are complicated due to the heterogeneity of the condensed phase), dispersion
and heat losses from the gas phase.
Experiments described in the literature were carried out using mixtures contain-
ing model binders and AP. Most of the data available corresponds to the burning of
AP-polymethyl methacrylate (PMMA) mixtures (for example [2]). For this reason,
we studied the kinetics of the high-temperature decomposition of model composite
solid propellants using these two compounds.
Experiments on the linear pyrolysis of AP were performed using installations
LP-1, LP-2 and LP-3 and samples in the form of a cylinder (10 mm in diameter) or a
plate (18
×
×
3 mm) pressed from a standard commercial crystal powder at heater
temperatures in the range of 280-1050
◦
C (in some experiments, samples were made
from specially prepared doubly recrystallized AP. However, since at
T
0
=
const
the
scattering of the linear pyrolysis rate data for standard AP samples and specially
prepared ones was less than 15% given the experimental accuracy, the main sets of
experiments were carried out using standard commercial AP.
Depending on
T
0
(and, to a lesser degree, on the sample thickness), linear pyrol-
ysis occurs in two qualitatively different modes: with burning at
T
0
≥
16
520-560
◦
C,
10
−
6
ms
−
1
)
without burning, the evolution of gaseous products (brown nitrogen oxides) was
observed, while a thin layer of white AP deposit formed on cold parts of the in-
stallation (the latter formed due to the recombination of the products of equilib-
rium dissociative sublimation). In the experiments on very “slow” linear pyrolysis
(
U
<
10
−
6
ms
−
1
), the formation of the AP deposit did not take place.
First, let us consider the qualitative characteristics of AP linear pyrolysis without
burning. In this case, the shape of the pyrolysis surface depends on the heater tem-
perature, the linear pyrolysis rate and the ambient pressure. At
T
0
≤
and without burning at lower
T
0
. At high rates of linear pyrolysis (
U
≥
450
◦
C (heater
was Al/mica, see below) and
U
<
10
−
6
ms
−
1
, the pyrolysis surface is slightly con-
cave at atmospheric pressure and at
P
∞
= 4kPa.At
T
0
>
480
◦
C and
U
>
10
−
5
ms
−
1
the pyrolysis surface becomes flat at atmospheric pressure and slightly convex at
P
∞
0
.
1MPa.
We were the first to study the catalytic effects of almost all previously used
heater materials (more precisely, the oxides formed on the heater surface) on the
rate of AP linear pyrolysis [8, 9]. The temperature dependencies of the rate of AP
