5.2.1. Time-Resolved Regression Rate
Relative ballistic grading results are reported in
Figure 7 and
Figure 8 and
Table 6. All the ballistic data were defined by ensemble average curves summarizing, at least four tests per formulation. All the fuels but F3, F8–F10, and F15–F19 were tested with
. = 5 g/s; thus, their performance was evaluated taking F1A as baseline. For the tests with
= 6 g/s, F1B was considered as the reference for the relative grading. Data reported in
Table 6 feature relatively low
R2 due to the typical trend captured by time-resolved approaches, as originally reported by Evans et al. [
18] (see
Figure 4b,c). The time-averaged data of
Figure 8 were evaluated using Equations (5) and (6) at
= (250 ± 15) kg/(m
2∙s). The latter value was selected since it was common to all the tested formulations.
In spite of the (minor) differences in the operating conditions, solid fuel formulations loaded with μAl (F2 and F3) featured similar ballistic responses (see
Figure 7 and
Figure 8). The
rf(Gox) of F3 is shown in
Figure 9a. Fuels loaded with μAl exhibited no significant performance enhancement over the baseline for
Gox > 200 kg/(m
2∙s). This reflected the relatively low TG reactivity of these additives (see
Table 5), yielding limited heat transfer enhancement for high oxidizer mass fluxes (long burning/reaction time). Faint
rf and
increases were achieved for
Gox = 155 kg/(m
2∙s), thanks to the radiation heat transfer that mitigated the effects of convective heat transfer decrease (as testified by the
nr ~0.5 in
Table 6).
The F4 showed marked
rf and
increases over the baseline for relatively high
Gox (see
Figure 7). This performance was quickly lost as the convection decreased, as testified by the
rf(Gox) power law approximation reported in
Table 6, and by the low
Gox data of
Figure 7. The time-averaged data of
Figure 8 captured this effect, as testified by
= 26% ± 10%, in turn yielding
= 36% ± 14%. In the high-
Gox region, the augmented flame temperature obtained thanks to ALEX-100 combustion increased the convective heat transfer, and the emitting particles enhanced the radiation contribution.
Figure 10 shows images captured during F4 combustion. In the earlier phases of the combustion, small and bright slivers detached from the regressing surface and from the strand head-end (
Figure 10a,b). As the burning proceeded (
Figure 10c,d), the formation of a surface crust layer was observed at the head-end (but likely also on the regressing surface). The creation of this surface layer was not observed in μAl-loaded fuels (see
Figure 3). A more detailed discussion of this phenomenon is given in
Section 5.2, where Al aggregation/agglomeration phenomena are discussed.
Interestingly, the nAl-loaded formulation manufactured without additive dispersion techniques (F8) showed no performance enhancement over the corresponding baseline probably due to the metal particle clustering inhibiting Al combustion (see
Figure 7 and
Figure 8). The F5 showed
rf performance and
Gox sensitivity similar to F4, as testified by the data reported in
Figure 7 (note the relatively high error bars for
Gox = 325 kg/(m
2∙s)) and
Table 6. The stearic acid coating exerted no marked influence on the powder characteristics (
Table 4) and reactivity (
Table 5); this surface treatment was implemented mainly for storage purposes. The solid fuel formulations F6 and F7 exhibited a limited
rf dependence on
Gox (see
Table 6), while providing interesting instantaneous and time-averaged
. In particular, these formulations featured a nearly uniform
enhancement over the baseline for the whole investigated
Gox range. This suggests that, at high heating rates, the metal reaction with fluoropolymer decomposition products may enhance the metal combustion performance close to/at the regressing surface in both high- and low-
Gox regimes. Such an effect was not observed at slow heating rate (see
Table 5). Data reported in
Figure 7 showed that, for
Gox = 155 kg/(m
2∙s), the average
increase over the baseline of F6 and F7 were 53% ± 14% and 62% ± 14%, respectively, with marked differences with respect to the ALEX-loaded fuel.
The ballistic responses of fuel formulations loaded with VF-ALEX showed similarities with the burning behavior of F10. The AP-ALEX-100B-loaded formulation featured an average
increase over the baseline of ~37% over the whole investigated
Gox range (see
Figure 7). The
AP content in the solid fuel was ~2 wt.%; however, locating it in the composite powder enabled a maximization of the oxidizer impact formulation ballistic response. This was testified by the fact that F9 (same nominal composition of F10, but with AP and nAl that were added separately) showed no significant performance enhancement over the baseline (see
Figure 7 and
Figure 8). The performance difference between F10 and the VF-ALEX-100-loaded fuel was partially due to the higher SSA and reduced particle size of the latter, promoting the additive reactivity at fast heating rates and compensating for the higher flame temperature of Al + AP (see
Table 2).
Data for F11 burning with
Gox = 325 kg/(m
2∙s) showed that the
enhancement of μAl15 was improved by the LE mechanical activation (see
Figure 7 and
Figure 8). Similarly to F4, F11 showed reduced
rf and
performance as
Gox decreased. The TG analyses suggested a slightly improved reactivity of LE-μAl15 with respect to the starting μAl (see
α (933 K) and
α (1273 K) in
Table 5), although the SSA of the powders featured no significant differences. The morphology of LE-μAl15 was the likely reason for the augmented reactivity of the powder at the high heating rates encountered in the combustion process. In addition to this, the irregular shape of particles of LE-μAl15 may augment the gasifying surface roughness when protruding from the regressing fuel grain. As an effect, the increased surface roughness may have played a minor role in the
rf enhancement of F11.
The ballistic effects of the fuels loaded with Al + PTFE composites obtained from LE mechanical activation showed no differences for
Gox = 155 kg/(m
2∙s). On the other hand, for
Gox = 325 kg/(m
2∙s), the ballistic response of F14 exhibited a
rf increase over the baseline of 44% ± 13%. This corresponded to an
enhancement of 59% ± 15%. In
Figure 7, the uncertainty interval of F14 data at high
Gox partially overlapped those of F12 and F13. In spite of this, it should be noted that the average
rf and
increases over the baseline of the fuels loaded with PTFE–μAl composites from LE activation showed higher values as the fluoropolymer content was risen and the particle size of the starting Al was reduced (see
Table 4 and
Table 5 and
Figure 7). Under the investigated conditions, the presence of a particle fraction of sub-micrometric size probably played a key role in the effectiveness of the activation process of μAl7.5, compensating for the lubricant effect of the PTFE.
The use of an HE process enabled the production of Al + PTFE composites by more intense activation stresses. These, in turn, permitted higher fluoropolymer mass fractions in the composites than the LE procedure. Under the investigated conditions, the solid fuels loaded with additives produced by HE mechanical activation featured the highest
performance enhancements (see
Figure 7 and
Figure 8). F15 showed no significant
enhancement over the baseline, since the exploitation of PTFE as the oxidizer required the presence of additional metal ingredients (acting as fluorine scavengers [
10,
24,
87]). Formulations in which the Al–PTFE reaction was exploited showed enhanced ballistic performance over non-loaded HTPB in terms of both
rf and
. For F17,
rf enhancements were mainly observed for
Gox < 250 kg/(m
2∙s), due to a relatively low impact of the Al + PTFE reaction on the metal combustion under intense convection. This was possibly related to the slow heating rate behavior of HE-μAl15-T45 (see
Table 5). This powder showed a relatively low
α (1273 K), and a higher
Ton,1 with respect to LE-μAl7.5-T30 and nano-sized additives featuring strong
rf increases at high
Gox. The F16 and F18 formulations featured the same composition, although the former was loaded with mixed ALEX-50 + PTFE (separated and not mechanically activated powders), while the second was loaded with HE-ALEX-50-T45. The ballistic response of F18 was similar to that of F16, as reported in
Figure 7. Both formulations featured strong data dispersion in the early phases of the combustion, with ensemble uncertainty intervals close to those of the baseline formulation. For this reason, in
Figure 7, the data for
rf and
increases of F18 at 325 ± 20 kg/(m
2∙s) are presented without error bars. As combustion proceeded, data scattering was reduced, and F18 showed percentage
rf enhancements over the baseline of 54% ± 13% at 155 ± 10 kg/(m
2∙s). Under these operating conditions, the uncertainties in the performance enhancement were as for the other formulations, showing interesting performance with an
increase over the baseline of 141% ± 20% (see
Figure 7). F16 and F18 showed similar ballistic responses. This was partially due to the high PTFE and nAl loads providing good fluoropolymer and metal dispersion (fuel slurry was treated by ultrasound irradiation, thus mitigating metal particle clustering). Thus, the relative grading was performed in conditions favoring F16. In addition to this, three points should be highlighted to understand the advantages offered by F18: (i) the implemented HE activation procedure parameters used in this study aimed at a simple diffusion distance reduction between Al and PTFE, without pursuing further reactivity increases, (ii) the preparation of this fuel formulation proceeded in a simpler way than that of F16 since the micron-sized HE-ALEX-50-T45 was added to the formulation as a single ingredient that did not require ultrasound irradiation for effective dispersion and reduced the fuel formulation viscosity (see PSD and SSA data in
Table 4), and (iii) the use of a micron-sized composite based on nAl reduced the risks of particle suspension in air due to the use of a nano-sized ingredient. In spite of the high metal load, neither F16 nor F18 showed evidence of regressing surface phenomena inhibiting the nAl combustion. F19 showed a ballistic response that was not dependent on
Gox, while, for the other PTFE-loaded formulations, the
rf(Gox) power law approximations yielded
nr ~0.8 (see
Table 6). Thanks to this, the
enhancement over the baseline of F19 exceeded 600% for
Gox = 155 kg/(m
2∙s). These interesting results show the possibilities offered by fuel-rich composite additives, although the optimization of their performance requires future investigations to evaluate the impact of the high Al + PTFE mass fraction on the nature of the formulation combustion behavior and, in particular, on its PDL.
5.2.2. Combustion Surface Visualization
Combustion surface visualizations were performed on selected fuel formulations (F1, F3, F4, F8, F17, F18, and F19). The F1 was tested to provide details on the non-metallized baseline behavior, while the surface behavior of F3 provided insight into the solid fuel surface phenomena in the presence of the standard μAl. A comparative analysis of F4 and F8 enabled a comparison of the burning behavior of nAl-loaded fuels with and without sonication for additive dispersion. The PTFE-containing formulations F17, F18, and F19 were tested to evaluate the surface phenomena characterizing the fuels featuring the highest metal powder load and ballistic performance.
Figure 5 shows a representative frame for the combustion of F1. The burning proceeded uniformly along the visible sample length. High-speed visualization of the HTPB binder burning with 100 kg/(m
2∙s) ≤
Gox ≤ 400 kg/(m
2∙s) revealed the detachment of small fragments from the fuel grain [
63]. This phenomenon was not observed under the investigated conditions. The image sequences reported in
Figure 11 and in
Figure 12 show the combustion of F3 and F4, respectively. The fuel loaded by the micron-sized additive featured a combustion surface similar to that of F1, but with the detachment of small elements of intense brightness (see
Figure 11). These were Al particles or agglomerates released by the burning surface. The composition of these elements included Al, Al
2O
3, and (probably) binder decomposition products. The image sequence of
Figure 11e–g shows the protrusion from the regressing surface of an apparently non-ignited spherical aggregate (apparent size of ~200 μm). The latter was then inflamed during its flow in the boundary layer, as testified by the growth of its brightness and by the appearance of a diffusion trail.
Under the investigated conditions, the velocity of the gaseous mass blown from the regressing surface was relatively slow (approximately one order of magnitude lower than in solid propellant formulations), while the surface layer exerted relatively intense retention forces due to the viscosity of the pyrolyzing fuel. As a consequence, μAl-7.5 showed a faint activity at the gasifying surface (where temperature should be in the range of the Al melting point). On the one hand, this low activity limited the particle aggregation at the regressing surface and, in particular, the creation of a crust layer hampering the combustion. On the other hand, the metal powder characteristics hindered the additive enthalpy release close to the regressing surface once the particle was captured by the oxidizer stream. The condensed products leaving the regressing surface featured an apparent particle size of 100–200 μm (this observed size is not statistically relevant, and it should be taken as a rough estimation). Such a particle size range suggested relatively long burning times with energy release occurring far from the fuel grain (see
Figure 11a and, for a convenient comparison with the burning tests,
Figure 3). F4 showed a different burning behavior with respect to both F1 and F3. In the early phases of the combustion, the gasifying surface of HTPB + ALEX-100 showed the insurgence of a marked nAl aggregation (see
Figure 12a–d, with the white arrow in
Figure 12a highlighting the point the surface layer formation started at). This phenomenon probably began in the sub-surface layer of the pyrolyzing fuel. The insurgence of this surface layer was the likely cause of the fast decrease of the
rf and
performance observed in the F4 combustion tests (see
Figure 7,
Figure 8, and
Figure 12). Under the investigated conditions, the time-resolved
rf of F8 showed no significant performance enhancement with respect to the baseline. Observing the image sequence reported in
Figure 12i–l, a reduction in the aggregation phenomena characterizing F4 was noted for this fuel. The similarities between the high-speed surface visualization of F3 and F8 captured the effect of the reduction of nAl reactivity due to clustering, with a limited
rf effect caused by the relatively large size of the Al agglomerates detaching from the surface (see
Figure 12j).
Under the tested
Gox conditions, these aggregates were not detached from the fuel grain and gradually covered the entire gasifying surface of the port. As a result, the vaporization surface was shielded from the flame by a layer of unreacted (or partially oxidized) metal. This condition is shown in
Figure 12d. Under these circumstances, the heat feedback to the solid fuel grain was reduced by the missed/incomplete metal oxidation.
The highly loaded fuel compositions with Al and PTFE featured a regressing surface with a glowing appearance (see
Figure 13). This was a possible effect of the reaction between Al and the fluoropolymer. Independently from the micro- or nano-metric size of the Al particles embedded in the formulation, F17, F18, and F19 featured a surface layer that was apparently weaker than that formed in F4. This surface layer was also easily exfoliated by the oxidizer flow in the (relatively low)
Gox conditions tested in the high-speed visualizations. The weakness of the surface layer limited (or avoided) the inhibition of heat feedback to the solid fuel grain encountered with F4. As a result, metal combustion occurred in more favorable conditions and, therefore,
rf enhancement was achieved.
5.2.3. Concluding Remarks
The screening of different fuel formulations was performed starting from the pre-burning characterization of a variety of Al-based energetic fillers. Composite additives were investigated together with air-passivated μAl and nAl. Effects of ingredients as AP and fluoropolymers on the metal ignition and combustion in the oxidizer-lean conditions encountered at/close to the regressing surface of burning fuel formulations were investigated.
Under the investigated conditions, F4 showed an
rf enhancement over the non-metallized baseline of 59% ± 10% at 350 kg/(m
2∙s) and a marked
rf(Gox) sensitivity. The ballistic performance of the nAl-load formulation worsened as the oxidizer mass flux decreased. The
rf increase over the baseline reduced to 45% ± 10% at 325 kg/(m
2∙s) and were absent at 155 kg/(m
2∙s). The resulting
at
= 250 ± 15 kg/(m
2∙s) was 36% ± 14%, and the power law approximation of
rf(Gox) yielded
nr = 0.959 ± 0.022. The latter value highlighted other effects on the combustion evolution than the convective heat transfer decrease. High-speed visualizations of the burning surface suggested that F4 burning performance loss was mainly caused by metal aggregation occurring at the regressing surface/subsurface. This phenomenon yielded the formation of a shield of unreacted/partially oxidized Al that limited the heat feedback toward the surface. Combustion tests results and surface visualizations supported the idea that high
Gox promoted Al particle/aggregate removal from the surface (and their combustion), as shown by
Figure 10 and
Figure 12. On the other hand, under 145 kg/(m
2∙s) ≤
Gox ≤ 160 kg/(m
2∙s), the aggregates resided on the fuel surface, creating a crust of accumulated material. This was a likely effect of subsurface aggregation creating a relatively strong web with good cohesion (see
Figure 10 and
Figure 12). Such behavior was not observed for fuels loaded with μAl (F2–F3) and for F8 (that was prepared without nAl dispersion procedures). These fuels exhibited no significant
rf and
enhancements over the baseline at high
Gox, while at 155 kg/(m
2∙s), μAl-loaded fuels showed increased performance over both F4 and F8. For F2–F3, F4, and F8, relatively large aggregates were observed to detach from the surface. The use of AP- and fluoropolymer-containing composites based on Al contrasted the
rf detriment observed for ALEX-100, thanks to the metal reaction with the oxidizer/coating decomposition products. In spite of an increased additive (and metal) mass fraction with respect to the other investigated fuels, combustion surface visualizations of F18 showed the build-up of a surface metal layer of reduced cohesion. This was the likely effect of the partial reaction between the metal and the PTFE. The resulting surface layer was easily exfoliated by the oxidizer flow, and faster
rf was, therefore, achieved by a combination of convective and radiation effects (see
Figure 7 and
Figure 8).