Effects of aluminum composites on the regression rates of solid fuels

Innovative, mechanically activated Al–polytetrafluoroethylene (PTFE) composites and ammonium perchlorate (AP) coated nano-sized aluminum (C-ALEX) were produced, characterized, and tested as solid fuel additives. The ballistics of fuel formulations based on hydroxylterminated polybutadiene (HTPB) was investigated in a microburner by a time-resolved technique for regression rate (rf) data reduction. Both Al-composites show promising results in terms of rf and mass burning rate enhancement. In particular, the C-ALEX showed a percent rf increase over the baseline (HTPB) of 27% at an oxidizer mass flux of 350 kg/(m2s), without requiring dedicated dispersion procedures. This performance enhancement was nearly constant over the whole investigated range.

Innovative, mechanically activated Alpolytetra §uoroethylene (PTFE) composites and ammonium perchlorate (AP) coated nano-sized aluminum (C-ALEX) were produced, characterized, and tested as solid fuel additives. The ballistics of fuel formulations based on hydroxylterminated polybutadiene (HTPB) was investigated in a microburner by a time-resolved technique for regression rate (r f ) data reduction. Both Al-composites show promising results in terms of r f and mass burning rate enhancement. In particular, the C-ALEX showed a percent r f increase over the baseline (HTPB) of 27% at an oxidizer mass §ux of 350 kg/(m 2 s), without requiring dedicated dispersion procedures. This performance enhancement was nearly constant over the whole investigated range.

INTRODUCTION
In a hybrid rocket engine (HRE), the fuel and the oxidizer are stored in different states of matter. In the most commonly implemented and investigated con¦gurations (direct hybrid), the oxidizer is liquid (or gaseous) while the fuel is solid. In a direct hybrid, the combustion can take place only when the oxidizer §ows over the solid fuel grain. The resulting di¨usion §ame is established in the boundary layer [1]. The heat feedback from the §ame to the grain sustains the fuel gasi¦cation, yielding the condensed fuel surface regression. When compared to mature liquid rocket engines and solid propellant motors, HREs o¨er interesting features as high operating §exibility and theoretical performance, increased safety, and reduced recurring costs [1]. In particular, the maximum theoretical speci¦c impulse of HREs based on liquid oxygen (LOx) HTPB is close to the one of LOxhydrocarbon liquid propellants [1]. In spite of this, the use of the hybrid con¦guration in operating missions is hampered by HREs low r f and combustion e©ciency. Both of these drawbacks of hybrid systems are related to the di¨usive nature of the §ame ruling the combustion process [1,2]. The limits of low r f (and ' m f ) and of the poor combustion e©ciency hinders HRE implementation in high-thrust applications (i. e., launch systems) and in-space missions (where high e¨ective speci¦c impulse is required to minimize launched masses).
This work focuses on the ballistics of HTPB-based fuel formulations loaded with Al-based composite additives. The metal composites are designed and developed to promote the ballistic performance of HRE fuel formulations in terms of r f (and ' m f ). The energetic additives were created by mechanical and chemical activation processes aiming at depositing an oxidizing species source (i. e., PTFE, AP) over the surface of an energetic fuel powder (i. e., Al). The oxidizing species deposition was executed by mechanical activation for AlPTFE additives, while a chemical approach yielded to the AlAP powders. The strategy driving the composite formulation and realization is based on the reduced di¨usion distances between the fuel (Al) and the oxidizing species sources (PTFE, AP) aiming at enhancing the composite reactivity. In HRE combustion, the fuel di¨usion in the reacting boundary layer dominates the heat feedback to the regressing surface. The use of AlPTFE and AlAP is a way to overcome this limiting process by yielding oxidizing species close to the metal particles and to the gasifying surface. This should promote the heat feedback to the condensed phase. Di¨erently from approaches where the oxidizer (usually, AP) is added as a powder in the solid fuel matrix, the presented strategy places a limited amount of oxidizer as close as possible to the energetic fuel (i. e., Al) with the aim of maximizing the composite e¨ects on r f , while limiting the in §uence on solid fuel grain intrinsic safety and exhaust composition.
The produced composites were characterized in terms of their particle size distribution, morphology, active metal content, and ignition temperature. The ballistic response of the investigated fuel formulations is analyzed by a timeresolved technique for r f data reduction. The solid fuel ballistics characterization is performed considering the e¨ects of the additives on the formulations r f and ' m f . Cured HTPB was taken as the baseline fuel for the relative ballistic grading of the tested composite-loaded formulations.

LITERATURE SURVEY
In direct HREs, the §ame zone is established in the boundary layer developed by the oxidizer §ow over the solid fuel grain. Fundamentals of turbulent combustion in the boundary layer are discussed in [35]. Under the hypothesis of exclusive convective heat transfer from the §ame to the regressing surface, the model developed by Marxman and Gilbert [3] shows . The in §uence of p c on the solid fuel r f is widely discussed in the literature, though a unique behavior cannot be identi¦ed [28]. A detailed survey of studies about p c in §uence on solid fuel r f is given by Chiaverini [2].
The relatively low r f of hybrid fuel formulations is usually considered as the main drawback of these systems, since HREs are usually considered for access-tospace systems where high-thrust requirements drive the design. Techniques for r f enhancement include solid fuel loading with energetic additives [9], entrainment of melt fuel droplets [10,11], and the implementation of nonconventional oxidizer §ow injections [12,13]. In spite of signi¦cant r f increase (34 times faster than the conventional fuels), entrainment-producing fuels (e. g., para©n waxes) usually exhibit poor mechanical properties [14]. This limits their possible applications in large-scale systems. In spite of their e¨ective r f enhancements at lab-scale level or in smallthrust systems [12,13], exotic oxidizer §ow inlet con¦gurations, as vortex §ow and swirl injections, could be ine¨ective on intermediate and large-scale motors due to the viscous dumping of tangential components of the §ow velocity. Solid fuel loading with energetic additives o¨ers possible performance enhancements while granting overall fuel grain mechanical properties. Moreover, this approach can be implemented for small-as well as for large-scale systems.
Energetic additives for the solid fuel loading include metal or oxidizer powders and hydrides [9,15]. Ballistics of HTPB-based formulations loaded with AP was investigated in details by George et al. [16] and Frederick et al. [17].
The fuel grain loading with oxidizer particles provides signi¦cant performance enhancements. Considering HTPB + 25 %(wt.) AP burning in gaseous oxygen (GOx) with G ox = 160 kg/(m 2 s), a percent r f increase of 200% with respect to pure HTPB can be achieved [17]. On the other hand, while granting a signi¦cant ballistic performance enhancement, the solid fuel grain loading with high mass fractions of oxidizing powders hinders the advantage of improved safety and HCl-free emissions of conventional direct hybrid systems. Hydrides are attractive energetic additives for fuel formulations performance enhancement [1820]. Nevertheless, these materials exhibit high chemical reactivity with commonly employed binders as HTPB [21,22].
Recently, studies on hydride e¨ects on para©n-based fuel ballistics have been conducted by Larson et al. [22] and DeSain et al. [23]. First studies on metal-loaded fuel formulation for r f enhancement date back to the 1960s [9]. In this early investigations, the tested metal additive particle sizes were in the micron-size range; thus, the achieved r f increases were mainly due to the augmented radiation heat §ux from the §ame zone to solid fuel surface [9]. Conventional micron-sized additives are characterized by S sp ≤ 1 m 2 /g, hindering their combustion close to the regressing surface. This limits their contribution to the r f enhancement. Nonisothermal oxidation studies (di¨erential scanning calorimetry / di¨erential thermal analysis thermogravimetry (TG)) reveal that only minor mass increases are achieved by µAl for temperatures < 1200 K [24]. The reactivity of conventional micronsized additives can be enhanced by activation processes [2528], or by nano-sized powders [7,29,30]. Nonconventional micron-sized additives include composite [31] and activated [32] materials. In the ¦rst case, the additive reactivity is tailored by a dedicated synthesis or coating process. As a result, the ¦nal product exhibits increased reactivity usually testi¦ed by a lower oxidation onset temperature or ignition temperature [31].
Activation processes can be classi¦ed into mechanical, chemical, and mechanochemical approaches. In the ¦rst case, a single or a combination of additives (usually, both in the micron-sized range) are treated by a mechanical milling process. The resulting composite powder exhibits a combination of the characteristics of the starting ingredients o¨ering the possibility of tailored reactivity and overall features. With this approach, as a consequence of the applied mechanical stresses, the ¦nal product S sp can be increased with respect to the starting materials. Sippel et al. [32] enhanced the reactivity of µAl (Valimet H30, mean particle size of 35 µm) by mechanical activation with PTFE (mean particle size of 35 µm). Under nonisothermal oxidation in ArO 2 , the activated powder exhibits a distinguishing behavior with respect to the simple mix of the two ingredients. In particular, the activated µAl/PTFE exhibits exothermic reactions below the aluminum melting temperature (933 K). A review of the mechanical milling e¨ects on the characteristics and thermal/combustion behaviors of the ¦nal products is reported in [33]. The chemical approach for the activation of energetic materials [25,26] is based on a chemical modi¦cation of the virgin material composition/characteristics (e. g., particle surface functionalization). The mechanochemical activated materials exploit a combination of the mechanical and chemical approaches [34], chemical reactions being promoted by the milling environment/process. Nano-sized particles exhibit enhanced reactivity with respect to conventional, coarser particles, as testi¦ed by simultaneous thermal analysis [35] and burning tests [9,29,36].
Thanks to their reduced size, nanoparticles exhibit high S sp (∼ 10 m 2 /g), in turn yielding a high reactivity possibly favoring the energy release close to the energetic material regressing surface. Thus, nanomaterials may result e¨ective for r f enhancement (at least under certain experimental conditions) [29, 37 39]. On the other hand, nanoparticles require proper dispersion techniques to contrast/limit cold cohesion/clustering of particles. Though no detailed study on this aspect is available in the open literature, additive dispersion is considered crucial in order to be e¨ective in ballistic performance enhancement [37,40]. Due to their enhanced reactivity, nano-sized particles can exhibit a high sensitivity to ageing processes [41]. In spite of this, few open-literature studies deal with ageing of nanometric energetic materials [4244].
Contrasting the nano-sized additive characteristics with those of activated micron-sized metal powders, some advantages of the latter arise. Activated micron-sized materials o¨er a larger size than the nanometric counterpart. In general, this implies a higher active metal content (i. e., higher energy content) and a smaller S sp that is in between the one of the conventional materials and that of nano-sized additives. The reduced S sp could yield positive e¨ects on the viscosity of the activated particle suspension during solid fuel manufacturing, while limiting the impact of ageing e¨ects of these powders.

STRATEGY AND INVESTIGATED MATERIALS
In this section, the characteristics of the investigated additives and the details of the fuel manufacturing procedure are discussed. This work investigates metal oxidizer composite energetic additives for the performance enhancement of solid fuel formulations for hybrid combustion. The metal-based composites enable the solid fuel loading with small (and localized) amounts of oxidizer. The composite powders should yield bene¦ts to the solid fuel ballistic response thanks to the oxidizer availability close to the metal particles during their ignition and combustion.
The considered energetic materials are characterized in terms of their particle size, morphology, active metal content, and reactivity. The powder particle size was determined by laser di¨raction (Malvern Mastersizer 2000) or nitrogen desorption (deriving the particle radius from the S sp [45]). The particles morphology was investigated by optical microscopy and by scanning electron microscopy (SEM). The additives metal content was determined by a volumetric method based on the Al + H 2 O reaction [45]. The additive reactivity is evaluated by oxidation, monitoring the sample weight during heating (TG), and by the ignition temperature tests performed by a hot-wire technique [46]. In the TG analyses, the relative grading of the powder reactivity is performed considering the parameters identi¦ed by Ilyin et al. [47]. The nonisothermal oxidation tests are performed in air, with a heating rate of 10 K/min. The ignition temperature of the composites was investigated under ambient conditions (air, 0.1 MPa), with a heating rate of 250 ± 50 K/s.

Aluminum-Based Composites: Production and Characterization
This section provides a description of the production procedures of the investigated composites and of their characteristics. In this study, two di¨erent families of aluminum-based additives are considered. In the ¦rst one, an Al powder with nominal size of 15 µm (Al15) was used. The second family of energetic additives is manufactured from an ALEX powder with nominal size of 100 nm (ALEX100). Figure 1 shows optical and electron microscope images of Al15 and ALEX. Details on the Al15 particle size distribution are given in Table 1. The Al15 particles have a spherical shape, with an apparently smooth texture. The monomodal particle size distribution yields D 43 = 19.8 µm. The powder C Al is (99.5±0.4) %(wt.). The ALEX is produced by electrical explosion of wire (EEW) and is passivated by air. The as-received nano-sized particles are coated by cat-   echol (0.2 %(wt.)). The ALEX100 exhibits S sp = 10.7 m 2 /g, in turn yielding a s = 208 nm, and C Al = (88 ± 0.3) %(wt.). The micrometric aluminum is used as the base ingredient in Al-PTFE composites obtained by mechanical activation process. The nano-sized aluminum is coated by AP, yielding to a fueloxidizer composite material produced by chemical activation. The PTFE and AP mass fractions in the composites are heuristically selected, with the driver of a reduced overall oxidizer content (< 5 %(wt.)) in the ¦nal fuel grain.

Mechanically activated micron-sized aluminum
The Al15 is activated by a low-energy mechanical process. The activation is performed by a centrifugal mill, with alternate clockwise-counter clockwise rotation. A bimodal distribution of stainless steel spheres was used to achieve a ball to powder ratio of 5 : 1. The process controlling agent was toluene (0.2 %(wt.) of the powder charge). The activation atmosphere was air and it was not controlled in temperature. The mechanical activation was performed on Al15 and Al15 + PTFE mixtures. The Al §uoropolymer mixtures were homogenized by a resonant acoustic mixer (Resodyn TM LabRam Mixer [49]) before the milling process. The §uoropolymer was in a dry disperse phase (mean particle size of 1 µm). Three activated composites are considered in this study. The Act-Al15 is the product of mechanical milling applied to the virgin Al15 without the addition of PTFE. The Act-Al15(10) and the Act-Al15(30) are the powders produced by the milling process applied to Al + PTFE mixtures with a §uoropolymer mass fraction of 10% and 30%, respectively.  Figure 2 shows the particle morphology after the activation process for the investigated additives, while other relevant size distribution and C Al data are reported in Table 1. The Act-Al15 SEM images presented in Fig. 2a show irregularly shaped particles. The shape alteration is induced by the milling process. Aluminum §akes with a rough surface texture can be recognized (Fig. 2b), though some particles with nearly spherical shape are still present in the additive. The Al §akes can be originated by the direct sphere-particle collisions, or by sphere-multiple particles impacts. According to the achieved particle size characterization data, Act-Al15 powder presents a bimodal size distribution, with peaks at 25 and 115 µm (Fig. 3). Under the considered operating conditions, a small particle fraction (∼ 1 %(vol.)) exhibits a particle size in the range 1 to 5 µm and probably results from the particle fragmentation due to the milling process. In spite of the presence of nearly-spherical objects, the particle size distribution of the Act-Al15 is significantly altered with respect to the virgin material case. So is its reactivity as  (2), mechanically mixed (no milling activation) Al15 + 10 %(wt.) PTFE (3), and Act-Al15 (10) (4). Data reduction algorithm considers spherical and smooth-texture particles, while Act-Al15 and Act-Al15(10) particles show irregular shape and texture testi¦ed by the data reported in Tables 2 and 3. In particular, under slow heating oxidation conditions, the Act-Al15 exhibits -m 1 = 4.1% and -m 2 = 9.1%, with enhanced performance with respect to the Al15. The Act-Al15 exhibits no mass loss during heating in the nonisothermal oxidation tests (i. e., no adsorbed species desorption was sensed by the TG). This evidence suggests a limited impact of the mechanical activation process on the powder S sp , since materials with high speci¦c surface area usually show a 1%2% mass desorption during heating.
The e¨ect of the milling process on the Al15 is in any case visible as a shape and texture alteration, with the presence of (few) nonmilled particles. The Act-  Al15 exhibits a lower metal content than the original Al15. This is due to the milling process impacts, promoting the cracking of the alumina shell covering the particles. As Al is exposed to the air environment, it is oxidized and this induces the C Al reduction. Under relatively high heating rates (250±50 K/s), the ignition of Act-Al15 occurs at 924 ± 22 K. Under the same operating conditions, the Al15 ignition temperature exceeds the experimental setup measuring range (limited by the use of S-type thermocouples) (see Table 3). The increased reactivity of Act-Al15 with respect to the virgin material is a consequence of the activation process. Particle shape alterations seem the main responsible of the observed behavior, since the absence of -m 0 in the TG analysis suggests that Act-Al15 is characterized by a relatively low S sp . The activated powders containing PTFE exhibit more irregular morphology and texture than the Act-Al15 (see Fig. 2). The C Al of both Act-Al15(10) and Act-Al15(30) is a¨ected by the milling process, with PTFE deposition/inclusion into the activated particles. The D 43 of the powder batches containing the §uoropolymer is reduced with respect to the Act-Al15 case (see Table 1). This is a possible result of the PTFE friction and wear-properties on the milling process [50]. Under the investigated conditions, Act-Al15(10) and Act-Al15(30) exhibit a monomodal particle size distribution (see Fig. 3). The nonisothermal oxidation data of Act-Al15(10) and Act-Al15(30) are presented in Table 2. Under low heating rates in air, the mechanically activated powders present an initial mass loss (due to the §uoropolymer degradation), followed by an oxidation phase. The values of mass losses for both the §uoropolymer-containing composites suggest the presence of residual PTFE decomposition products on the Al particle surfaces. This is the possible cause of the delayed T on,1 of Act-Al15 (10) and Act-Al15(30) with respect to Act-Al15.
A similar e¨ect is likely to cause an oxidation onset delay in §uorohydrocarbon-coated ALEX tested under similar operating conditions in a previous study [45]. The oxidation process shows a higher degree of reaction in the Act-Al15(10) than in the Act-Al15(30) (see Table 2). After an initial mass loss of 7.1%, the Act-Al15(10) exhibits a mass gain of 7.4%. The Act-Al15(30) exhibits -m 0 = −24.1%, followed by an Al oxidation yielding -m 2 = −21.1%, with a mass gain of 3%. Thus, both the §uoropolymer-containing composites exhibit a lower reactivity than the Act-Al15 under the investigated TG conditions.
The high-heating rate tests show an opposite behavior, with the Act-Al15(30) exhibiting T ign = 854.4 ± 67.1 K, and Act-Al15(10) igniting at 1025.3 ± 143.2 K. Under the investigated conditions, this result may be due to di¨erent e¨ects: the PTFE inclusion in the activated particles (as a consequence of the milling process), an AlPTFE reaction thanks to the reduced di¨usion distance, and a AlPTFE composite reaction with the test environment (in turn triggered by the PTFE thermal decomposition). At the moment, further considerations on the reaction path of the composite are not possible due to diagnostic limitations.

Coated nano-sized aluminum
The air-passivated, catechol-coated ALEX is taken as the virgin material for the creation of an innovative oxidizerfuel composite. An oxidizer coating is realized on the ALEX with AP by a lab-scale procedure developed at SPLab. After the coating deposition, the ALEX + AP rough powder is grinded and sieved to get a particle size ≤ 80 µm.
Two batches of material are considered in this study. The ¦rst one, C-ALEX(10), contains 10 %(wt.) AP. The second one, C-ALEX(15), contains an AP mass fraction of 15%. Under the investigated conditions, the TG of ALEX presents the typical behavior of nano-sized aluminum powders tested in air at low heating rates [47].
As shown in Table 2, after an initial mass loss (-m 0 = −1.3%) due to desorbed gases, the oxidation proceeds in a two-step process. Before the Al melting, the powder undergoes a mass gain of 27.1%. This ¦rst reaction is followed by a second oxidation process yielding -m 2 = 60.8%. Thus, the overall mass increase of the ALEX powder is 62.1%. The C-ALEX(10) presents a wide particle size distribution, with D 43 = 6.5 µm and a span of 9.06 (see Table 1). Being the measured particle size in the range 0.2 to 80 µm, the mass-mean diameter is in §uenced by the larger particles, thus resulting in a micron-sized composite. The C Al of the powder is 80.5 ± 2.0 %(wt.) immediately after the production. For the C-ALEX(15), the C Al results 74.8 ± 0.1 %(wt.). The TG analysis of the C-ALEX(15) reveals a -m 0 = −14.1% in the temperature range 473673 K (see Table 2). This initial mass loss is then followed by an oxidation phase whose onset temperature is the same as for the virgin ALEX powder and yielding -m 1 = 3.8%. The limiting mass gain achieved by C-ALEX(15) is 28.2% at T = 1223 K.
Under the investigated operating conditions, the data reported in Table 3 show that at high heating rates in air, the AP-coating can promote a small decrease of the powder T ign with respect to the virgin ALEX case.

Solid Fuel Manufacturing
The formulation of the HTPB binder used in this study is based on HTPB R45-HTLO (79.2 %(wt.)), dioctyl adipate (13.1 %(wt.)), and isophorone diisocyanate (7.7 %(wt.)). Dibutyltin diacetate is used as a curing catalyst and is added in excess to the formulation (0.05% by mass of the total weight of HTPB and isophorone diisocyanate). The fuel curing level (NCO/OH) is 1.04. Manufacturing is performed by a mechanical mixer. In spite of the use of nanosized additives, in this study, no dispersion protocol was implemented during the manufacturing. This choice is due to the fact that the Al-based composites investigated in this study are in the micron-sized range. The main advantage of additives in this size range is their limited sensitivity (if any) to particleparticle interactions. This simpli¦es the manufacturing procedure (easing possible largescale applications).

Experimental Setup
The relative ballistic grading of the investigated fuels is performed by burning tests conducted in a two-dimensional (2D) radial microburner. A scheme of the implemented setup is reported in Fig. 4, while a detailed description is given in [8,29]. Thanks to its peculiar design, the experimental rig enables a continuous visualization of the head-end section of the tested strand. This is done by the injector head implementation [8] and by a 45 degree mirror re §ecting the strand head-end image (see Fig. 1). A digital camera operating with frame rates up to 1000 fps (depending on burning run expected duration and operating conditions) captures the combustion images. The r f data are reduced by a nonintrusive optical technique. The 2D radial burner is designed to enable a direct control of the test operating conditions ( ' m ox and p c ). Oxidizer mass §ow rate and p c can be regulated independently due to the electrovalves acting on the dump line. These servoactuators are driven by a pressure controller whose action enables to reach a quasi-steady p c during the tests [8]. A digital §owmeter provides ' m ox measurement and control. Strand ignition is achieved by a pyrotechnic primer charge. The latter is ignited by a laser beam impinging on it. Primer charge combustion products, in turn, ignite the tested sample as

Regression Rate Data Reduction
An optical time-resolved technique is used for the r f data reduction [8,29]. The time-resolved technique is based on diameter sampling in time from the recorded combustion video. During the test, the strand head-end is visible and central port diameter is sampled along di¨erent radial directions (depending on combustion uniformity). Diameter sampling starts from the time the central port perforation becomes visible (after primer charge ignition). Sampling frequencies up to 10 Hz are achieved [8]. Usually, two di¨erently oriented local diameters are considered (e. g., vertical and horizontal). Three measurements are taken for each local diameter. The single local diameters sampled at time t i , D j,k (t i ), are evaluated by the di¨erent brightness between the §ame zone and the regressing surface. Starting from the local measured diameters, D i can be de¦ned as . (1) The sequence of the discrete D i de¦ned by Eq. (1) is then ¦tted to achieve a continuous history of space-averaged diameter evolution in time Diameter D(t) is valid for t ≥ t ign . The latter is an ad-hoc de¦ned parameter. The t ign is de¦ned as the time maximizing the data ¦tting of Eq. (2). A detailed description of the time-resolved technique developed at SPLab is given in [8,29]. Solid fuel ballistics can be completely de¦ned starting from Eq. (2), as discussed in [29]. In particular, solid fuel r f and G ox histories are de¦ned as The fuel mass §ow rate can be easily determined starting from the diameter history in time and the regression rate evolution as Results achieved by the time-resolved technique are checked comparing them to the corresponding thickness over time (TOT) data in order to gain information on data consistency between di¨erent reduction techniques: In the developed time-resolved technique, Eq. (2) is de¦ned for each performed test. In order to summarize results achieved under the same operating conditions for a given fuel, various D(t) are collapsed into an ensemble average curve. The latter is de¦ned by a power-law interpolation of time-trends identi-¦ed applying Eq. (2) to the performed tests. Ensemble curve enables de¦nition of proper error bars for D(t) and r f (G ox ). These are evaluated over the t (or G ox ) range where at least two of the performed single runs are de¦ned. Over this interval, the error bars are evaluated by con¦dence intervals centered on the average value resulting from single test diameters at a given time (for Eq. (2)) or regression rates at a given G ox (for Eqs. (3) and (4)). Con¦dence intervals are evaluated with 95 percent accuracy. In order to compare achieved results with literature data, a power-law approximation of analytical r f vs. G ox is proposed for each test and for ensemble curves:

EXPERIMENTAL RESULTS AND DISCUSSION
In this section, the experimental results concerning the relative ballistic grading of the HTPB-based fuel formulations loaded with aluminum composites are presented and discussed. The analysis focuses on the e¨ects of the energetic additives on the solid fuel r f . Cured HTPB is considered as a baseline for the evaluation of the additive ballistic e¨ect. In particular, the percent r f and ' m f enhancement over the baseline, are de¦ned at G ox ≥ G ox (t ign ) as The solid fuels are tested in GOx, with p c = 1.0 MPa. The oxidizer mass §ow rate is 5 g/s except where otherwise stated. An overview of the tested fuel formulations and of their actual densities is given in Table 4. The additive mass fractions of all of the additives but ALEX are de¦ned on the base of equal metal molar content. In the investigated formulations, the additive mass fraction is de-¦ned in order to get the same metal molar content as in HTPB + 10 %(wt.) µAl, with the micron-sized aluminum C Al = 99 %(wt.) (the remainder 1 %(wt.) being alumina). The r f e¨ects of the mechanically activated Al and Al + PTFE composites are introduced ¦rst. After this, the ballistic response of the AP-coated nano-sized aluminum is discussed.

Mechanically Activated Micron-sized Aluminum
Under the investigated experimental conditions, the ballistic response of the HTPB + Al15 testi¦es limited in §uence of the micron-sized additive due to its   Table 7 Relevant parameters and data ¦tting of the power law approximation of r f (Gox) (see Eq. (5)). The poor data ¦tting is due to the marked initial r f values [29] Fuel relatively low S sp . As shown by the data presented in Table 5, the µAl-loaded fuel delivers a r f detriment with respect to the baseline at high G ox , and -r f,% = 14.6% at G ox = 150 kg/(m 2 s). The data shown in Table 6 highlight the increase in the fuel mass burning rate, reaching 22.1% for G ox = 150 kg/(m 2 s). The resulting power law approximation of r f (G ox ) yields n r = 0.477 ± 0.001, due to the high radiation heat transfer e¨ects toward the end of the combustion ( Table 7). The ballistic response of HTPB + Act-Al15 is shown in Fig. 5. The ensemble average curve shows instantaneous r f values higher than those of the baseline over the whole investigated G ox -range as testi¦ed by the data presented in Tables 5 and 6. At G ox = 300 kg/(m 2 s), the -r f,% of HTPB + Act-Al15 is 37.4%. The percent mass burning rate increase over the baseline ranges from 46.3% at G ox = 300 kg/(m 2 s) to 24.7% at 150 kg/(m 2 s). While the Act-Al15 T ign is characterized by a reduced data scattering (see Table 3), the ensemble r f (G ox ) of the formulation shows relatively high error bars. This suggests that the bare additive characteristics are uniform, being other e¨ects as the particlebinder interactions responsible for the observed ballistic responses.
The burning behavior of the HTPB loaded with the mechanically activated Al + PTFE composites is presented in Fig. 6. As testi¦ed by the data reported in Table 6, the HTPB + Act-Al15(10) exhibits no signi¦cant r f improvements with respect to the baseline for G ox > 250 kg/(m 2 s). On the other hand, an r f decrease is achieved in the low oxidizer mass §ux region.
For G ox = 200 kg/(m 2 s), -r f,% = −5.1% and for G ox = 150 kg/(m 2 s) the percent r f decrease is 10.1%. Thanks to the increased density of HTPB + Act-Al15(10) with respect to the baseline (see Table 4), the fuel mass burning rate is augmented with respect to the HTPB reference until G ox = 180 kg/(m 2 s). The formulation loaded with Act-Al15(30) o¨ers enhanced performance with respect to both HTPB + Act-Al15(10) and the baseline (see Tables 5 and 6).
The -r f,% performance over the baseline decreases during the combustion reaching 0.8% for G ox = 150 kg/(m 2 s). Thanks to the fuel formulation density, this performance corresponds to a percent mass burning rate increase of 10.8%. As for the HTPB + Act-Al15, both fuel formulations loaded with Act-Al15 (10) and Act-Al15(30) present a relatively high data scattering, possibly related to the additive dispersion and to the fuel manufacturing process.
The data presented in Table 7 show that the fuel formulations loaded with additives based on Act-Al15 present the same r f (G ox ) power law dependence. Nevertheless, for the HTPB + Act-Al15, this is the e¨ect of enhanced r f over the baseline in the whole investigated range. The augmented performance of the fuel is related to an increased additive reactivity in the high G ox -range (thanks to a lower T ign with respect to Al15, see Table 3) and to an intense radiation heat transfer in the low G ox region. It is, moreover, possible that Act-Al15 plays a role in increasing the regressing surface roughness. The HTPB + Act-Al15 behavior in the initial phases of the combustion run can be directly related to the mechanical activation process altering the particle shapes and the alumina shell characteristics (a similar behavior is observed for micron-sized thermites [36]). Under the investigated conditions, the higher the PTFE mass fraction, the stronger the additive e¨ects on the mass burning rate when passing from Act-Al15(10) to Act-Al15 (30). Nevertheless, no bene¦ts are achieved by these Al + PTFE composites in the low G ox region. Neither the additive combustion nor a reaction between the binder and the PTFE decomposition products can provide bene-¦ts to the solid fuel r f . This behavior contrasts with experimental evidences from the combustion of §uorocarbon and §uorohydrocarbon coated nAl [47] and requires further investigations.

Coated Nano-Sized Aluminum
The ballistic response of HTPB-based fuels loaded with nAl and nAl-based composites is presented in Tables 57 and in Fig. 7. These two fuel formulations were tested in GOx, under an oxidizer mass §ow rate of 6 g/s, and Tests performed with an initial oxidizer mass §ow rate of 6 g/s: 1 ¡ baseline (HTPB); 2 ¡ HTPB + C-Alex (15); and 3 ¡ HTPB + C-Alex(15), Eq. (5) p c = 1.0 MPa. The relative ballistic grading of the nAl-loaded formulations is performed taking a HTPB baseline burning under the same operating conditions. Both hereby-considered nAl-doped formulations were manufactured without a dedicated dispersion protocol. This was done in order to evaluate the bene¦ts of the C-ALEX(15) over the virgin additive with a standard manufacturing procedure. Under the investigated operating conditions, the absence of a dedicated dispersion protocol for the HTPB + ALEX manufacturing yields a moderate increase of the r f with respect to the baseline. The maximum performance enhancement is achieved in the ¦rst phases of the combustion. The ensemble average data exhibit a -r f,% = 14.9% at G ox = 300 kg/(m 2 s), reducing to 6.0% at 150 kg/(m 2 s). This result is possibly due to two main reasons: the absence of a nanoscale dispersion of the additive and the insurgence of aggregation/agglomeration phenomena in the condensed phase during the combustion [29]. The ballistics of HTPB + C-ALEX(15) exhibits signi¦cant improvements over the one of the ALEX100-doped fuel. A mass burning rate percent enhancement > 30% is achieved in the whole investigated G ox range. For G ox > 250 kg/(m 2 s), the error bars of the loaded-fuel ensemble average are partially overlapped to those of the baseline data, while for G ox ≤ 250 kg/(m 2 s), the uncertainty intervals of the two formulations are distinct. Under the oxidizer mass §ux of 300 kg/(m 2 s), the HTPB + C-ALEX(15) yields a -r f,% = 26.5%, corresponding to a fuel mass burning rate percent enhancement of 38.8%. This result shows the attractive features of the proposed coating strategy. The APcoating of ALEX100 yields augmented r f over the baseline, without the implementation of dedicated manufacturing procedures.

CONCLUDING REMARKS AND FUTURE DEVELOPMENTS
This work deals with the production, preburning, and combustion characterization of aluminum-based energetic additives. Two main families of aluminum composites were considered in the analysis, focusing on their use in HTPB-based hybrid solid fuel compositions. The ¦rst additive family is based on the mechanical activation of a spherical 15-micron aluminum (Al15). The second group of additives is based on the use of nAl. The µAl was treated by a low energy ball milling process, with the possible addition of PTFE during the activation. As a result, three di¨erent additives were produced: Act-Al15, a mechanically activated variant of the virgin Al15, and two Al + PTFE composites with §uorocarbon content of 10 and 30 %(wt.). The nAl-based composite is a coated variant of an air-passivated ALEX powder. The investigated additive is labeled C-ALEX (15) and has a 15 %(wt.) coating of AP. The strategy based on the use of PTFE and AP aims at realizing energetic additives enhancing the combus-tion reactivity of the hybrid fuels by the presence of fueloxidizer compositions. The ballistic response of the fuels was investigated under reference conditions (p c = 1.0 MPa, GOx §ow) in a 2D-radial burner. Data reduction was performed by a time-resolved approach. Preburning characterization of the additives was performed by SEM, nonisothermal oxidation (TG), ignition temperature, and C Al determination tests. The SEM images of the mechanically activated aluminum show the Act-Al15 morphology modi¦cation (see Figs. 1 and 2). The milling process yields §akes and particles with irregular textures. The TG runs (air, 10 K/min) evidenced the enhanced reactivity of the Act-Al15. Under the investigated conditions, nonisothermal oxidation runs on this powder yielded a limiting mass gain of 9.1% at 1223 K, while for the virgin µAl, -m 2 = 5%. The thermal behavior of the PTFE-containing additives revealed a delayed oxidation onset and a reduced mass gain with respect to the Act-Al15 (see Table 2). The reduced mass gain of the PTFE-composites over the Act-Al15 is related to two main possibilities. First, at low heating rates, the PTFE decomposition and the Al particle oxidation occur as separated reactions. Second, decomposition residuals of the PTFE can hinder the metal oxidation at higher temperatures. In any case, further investigations are needed to fully understand the behavior of the composite. The TG runs performed on the C-ALEX(15) show a higher reactivity of this composite over the µAl-based counterpart. The AP-coated additive reaches -m 2 = 28.2%. Considering the coating degradation, the overall mass gain results in 42.3% (see Table 2). The data presented in Table 3 show the result of the ignition temperature tests performed with heating rate of 250 ± 50 K. Under the investigated conditions, Act-Al15(30) exhibits a marked reactivity, with ignition occurring at 854.4 ± 67.1 K. Only C-ALEX(15) yields better results, with T ign = 772.8 ± 13.3 K, while the Act-Al15(10) ignites at 1025.3 ± 143.24 K. The latter result is in §uenced by two di¨erent e¨ects. The ¦rst one is the reduced impact e¨ects during the milling process due to the presence of a relatively soft material (i. e., PTFE) (see Fig. 2). The second one is the limited §uoropolymer dispersion in the ¦nal product, low oxidizing contribution to the composite ignition. While for Act-Al15(30) the PTFE dispersion and mass fraction promotes the reactivity limiting the negative e¨ects of the PTFE softness, this does not happen for Act-Al15(10). The mechanically activated Al-based additives exhibit interesting performance in terms of the solid fuel r f and mass burning rate enhancement. The result presented in Tables 6 and 7 show that Act-Al15(30) yields a stronger e¨ect on the fuel mass burning rate than Act-Al15(10). This highlights the possible bene¦ts of the PTFE-addition to the Al mechanical activation process. Act-Al15 shows a marked e¨ect on the r f enhancement over the baseline at high oxidizer mass §uxes, yielding -r f,% = 37.4% at 300 kg/(m 2 s). The result is only partially reduced by the G ox decrease. The combustion of HTPB + C-ALEX (15) showed the bene¦ts of AP-coated nAl on the ballistic response of the solid fuel. The composite has a relatively large particle size (D 43 = 6.7 µm); nevertheless, it provides -r f,% > 20% over the whole investigated G ox -range without requiring a dedicated fuel manufacturing/casting procedure. According to the experimental r f data, the AP-coating seems to hinder the aggregation/agglomeration processes in the fuel condensed phase, thus limiting the G ox -dependence observed in nAl-doped fuels burning under similar operating conditions [29,39,47].
Recommended future steps of this work should focus on a detailed investigation of the in §uence of the e¨ects of process parameters on the characteristics of the ¦nal mechanical and chemical activated composites (e. g., Al and PTFE initial particle sizes, milling time and conditions, techniques for coating deposition). At the same time, an assessment of the possible end-user issues due to the composite nature of the powders should be performed. Focusing on §uoropolymer-containing additives, the path of the Al-PTFE reaction should be analyzed in details. In light of the C-ALEX(15) bene¦ts to the HTPB ballistic response, the possible in §uence of the coating on the ¦nal particle size and active metal content stability in time should be investigated to increase the powder r f -e¨ect while avoiding/limiting possible additive ageing and degradation.