Propellant Formulation and Mixing

Propellant formulations were chosen largely due to legacies from previous

experiments. Work performed by Hedman et al. at Purdue University used propellants

with 80% solids loading (SL) 1:1 coarse-to-fine (C/F) ratio propellant with 200 μm or

400 μm AP as the coarse AP and 20 μm AP as the fine AP [67-70]. The C/F ratio and

solids loading were chosen in part to isolate the coarse AP crystals for easier

determination of flame height above the individual coarse particles. These experiments,

have adopted the 80% SL 1:1 C/F 400 μm/20 μm formulation as a baseline propellant.

Other formulations considered are a variation off this baseline.

The 400 μm AP was purchased from Firefox Enterprises, while the 20 μm AP was

obtained from Alliant Techsystems (ATK). Particle sizes were obtained by dry-

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Analyzer (GmBH). Particle size distributions and other details of particle size

measurement are reported in Ref. [67] and are given in Figure 2.1. Four methods of

varying the propellant formulation were undertaken: monomodal propellants, changing

the coarse-to-fine ratio, adding catalyst to the propellant, and changing the coarse

oxidizer from AP to another energetic material. The AP for the monomodal propellants

were selected from the AP in stock at the Maurice J. Zucrow Laboratory and typically

came from Firefox Enterprises. To ensure AP particle sizes were known, the AP was

sieved into a series of bins. The sieving bins are shown in Table 2.1 as are the

specifications for the sieves used (VWR International). The average particle sizes were

then determined by dry-measuring the AP particles using forward light scattering on a

Sympatec HELOS Particle Analyzer (GmBH).

Figure 2.1. Normalized size distribution for AP crystal diameters for monomodal propellants.

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The coarse-to-fine ratio propellants contained nominally 400 μm coarse AP and 20

μm fine AP. The solids loading was held at 80% for all propellants. The propellant formulations can be seen graphically in Figure 2.2. The propellants are described by the

percentage of coarse AP (% cAP) present in the mix. An increase in % cAP is equivalent

to an increasing coarse-to-fine ratio.

For some propellants the coarse

AP in the baseline propellant was

replaced with an alternative coarse

oxidizer, such as ammonium

dinitramide (ADN) or ammonium

nitrate (AN). The material particle size

was typically around 400 μm. The ADN

(China Lake NAWCWD) was sieved

using the 355 μm sieve described in

Table 2.1. The AN was used as received

Table 2.1. Monomodal propellant particle size sieving bins and sieve designation.

Average Particle Size (μm) Sieve Bin (μm) VWR Sieve Designation 22 μm < 25 μm 57334-602 46 μm 25 μm < x < 53 μm 57334-602 57334-594 125 μm 75 μm < x < 106 μm 57334-594 57334-586 219 μm 106 μm < x < 355 μm 57334-586 57334-572 456 μm -- As received 802 μm -- As received

Figure 2.2. Coarse-to-fine ratio propellants formulations.

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and is shown in Figure 2.3. Note the

spherical shape of the AN particles. The

image was taken using a Hirox KH-8700

microscope with an OL-350-II lens. The

AN-based propellant, was an 85% solids

loading, 1:1 C/F ratio of coarse AN

particles and 20 μm fine AP particles in an

HTPB binder. The ADN propellant had an

80% solids loading with a 1:1 C/F ratio with coarse ADN (average diameter 230 μm) and

20 μm fine AP particles. Instead of HTPB, PBAN was used in an effort to forestall any

compatibility effects that have been reported by previous researchers. The ADN-based

propellant was hand-mixed and cured at 60°C for seven days prior to use. Neither AN nor ADN was observed to fluoresce on the surface under the laser light, in contrast to AP.

However, the fine AP in the propellant enabled the surface location to be determined.

Catalysts used in the propellants were either mixed into the binder directly or

encapsulated into the fine AP. For a further discussion of the latter, see Section 2.2. Two

catalyst sizes were used: nominally 53 µm (Firefox Enterprises) and 3 nm (Mach I Inc.).

Particle size distributions are given in Ref. [70] and Ref. [133], respectively. The catalyst

percentage in the propellant was driven by the amount of iron oxide captured in the

composite particles.

The HTPB binder used was 72.9% R45-M prepolymer (Firefox Enterprises),

1.0% Tepanol HX-878 (3M Corporation) as a bonding agent, 14.6% icodecyl pelargonate

(RCS RMC) as a plasticizer, and 11.5% Desmodur E744 (Bayer Corporation) as a Figure 2.3. Representative image of

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curative. When PBAN was used the composition was 79% PBAN resin and 21% D.E.R.

331 epoxy resin (Dow Chemical). The solids loading was ideally held at 80% for all

propellants except where noted. After mixing, the actual solids loading was measured and

found to be on average 79.83% ± 0.18%. The average and standard deviation were found

by looking at over fifty propellant mixes. The propellants skewed to being more fuel-rich

than desired as small amounts of extra binder ingredients cause relatively large changes

in the overall binder percentages, while small amounts of AP or other oxidizers did not

produce as large of a change due to larger percentage of AP in the propellants.

All propellants but a 6% coarse AP (cAP) propellant were mixed by hand. The 6%

cAP propellant was mixed on a LabRam resonant mixer (Resodyn Acoustic Mixers, Inc.)

to more adequately disperse the large amount of fine AP in the formulation [24]. The

mixed propellants were degassed under vacuum and cast into 6.35 mm diameter plastic

molds 80 mm in length. Propellants were allowed to cure for at least seven days prior to

use in experiments. In some cases an additional propellant mix was used to check data

trends. In these cases no statistical differences were found for flame heights, particle

lifetimes, and ignition delays between propellant batches.