EXPLORATORY INVESTIGATIONS ON METAL-BASED FUELS FOR AIR-BREATHING PROPULSION

Air-breathing solid fueled propulsion devices represent rugged, cheap, and rather simple options, out of the ramjet (RJ) category, which can contribute to volume containment and structural weight reduction. In the case of a ducted rocket, a fuel-rich propellant is burned in a primary combustion chamber and part of the oxidizer is taken from the atmo-sphere to complete the combustion inside a ram burner before exhaust. The use of metal additives contributes to the development of high-energy density materials, featuring better volumetric speci¦c impulse. Metal powders are characterized by high energetic content per unit volume but can feature issues of di©cult ignition, generation of condensed combustion products (CCPs), and incomplete combustion. The present work discusses a series of exploratory investigations on aluminum-based pyrolants. Thermochemical analyses, calorimetric investigations, and combustion tests will be considered, looking at improvements introduced by metal addition.


INTRODUCTION
In some high-speed and launch propulsion applications, air-breathing technology can replace rockets, with consistent gains of payload mass and cost. Currently, there are e¨orts underway to develop reusable launch vehicles using more fuel-e©cient engine cycles during part of the ascent orbit and exploiting two or more separate propulsion systems that operate independently. An example of these devices, called combination propulsion systems (CPS), is the rocket RJ con¦guration which uses a booster to achieve the initial acceleration to speeds capable of sustaining RJ operation. These solutions combine the space-based performance of traditional rockets with the atmospheric performance of RJ/scramjet engines. The use of atmospheric oxygen ensures oxidizer mass savings, leading to incremented payload mass fractions. Although not straightforward for technological reasons (e. g., high stagnation temperature, performance variation with §ight conditions, or thrust control), some past studies have drafted the possibility to develop multistage launch applications based on air-breathing units [1]. Currently, airborne launch options for small payloads are either under development or operative, conferring possible gains to atmospheric or suborbital missions [2,3]. Moreover, recurrent cost reduction is envisaged by implementing reusable propulsion unit. Weight breakdown at takeo¨for generic air-breathing and rocket systems is reported in Table 1, as suggested by Heiser and Pratt [4]. Among the air-breathing propulsion options, the con¦guration of a ducted rocket represents a compromise. The unit is fueled with an oxidizer-de¦cient solid propellant (pyrolant) which burns in a gas generator. The combustion products are then mixed in a secondary combustion chamber (ram burner), where oxidation is sustained by atmospheric air, before nozzle exhaust. This con¦guration grants higher gravimetric speci¦c impulse than a solid rocket unit but lower than a pure RJ. However, it maintains a higher degree of simplicity with respect to a liquid-fueled propulsion system, reaching interesting level of speci¦c thrust.
The use of energetic ingredients for increment of gravimetric speci¦c impulse potentially o¨ers signi¦cant mission advantages, including missile range capability (5 to 1 increase according to King [5]), reduced vehicle weight, higher velocities, and/or lower time-to-target values. Metals supplying high combustion heat per unit mass are exploited to generate high energy-density propellants. Atmospheric propulsion units can also take advantage of incremented volumetric spe-ci¦c impulse, granting more compact design solutions and reduced drag. Among the others, beryllium (not used for toxicity reasons), boron, aluminum, magnesium, titanium, and zirconium are considered in the competent literature [59]. Some of these metals are di©cult to ignite and e©ciently burn inside the rocket, leading to potential losses due to exhaust of partially oxidized products. This work reports an exploratory investigation performed on some oxidizer-lean propellants, based on ammonium perchlorate (AP), hydroxyl-terminated polybutadiene (HTPB), and aluminum. This kind of pyrolants represent a class of fuel-rich propellants used for ducted rockets but can feature a di©cult combustion. On one side, the use of an inert binder limits the decomposition capability of the propellant bulk, once the AP is progressively reduced. On the other hand, aluminum is a metal featuring high heat of combustion (16.4 MJ/kg) but can agglomerate at the burning surface.

IDEAL PERFORMANCE ANALYSIS
Ideal performance computation has been conducted using thermochemistry. Both primary combustion chamber and ram burner are assumed subsonic, ex-erting stagnation properties and equilibrium reaction chemistry. NASA CEA (Chemical Equilibrium with Applications) code was used for these evaluations [10]. In this respect, computations assumed thermochemical properties at the standard state and initial condition of 300-kelvin temperature for propellant ingredients. The brute formula and the relevant HTPB formation enthalpy (C 7.075 H 10.65 O 0.223 N 0.063 , -h 0 f = −58 kJ/mol) were recovered from [8]. Total temperature of ram air referred to the respective §ight level and Mach number. Nozzle expansion was computed using hypotheses of frozen chemistry model and calorically perfect gas. Inlet air conditions were evaluated according to a sim-pli¦ed mission pro¦le featuring constant dynamic pressure of 95 kPa. This is referred by the competent literature as an admissible ceiling value for missiles and rockets [2]. The inlet was adiabatic, accounting for actual pressure recovery according to the MIL-SPEC-5007D [11]. Beginning of takeover speed and limit of hypersonic §ight were considered ( Table 2).
The increment of the volumetric speci¦c impulse is obtained by those compositions which have a higher metal-to-binder ratio thanks to a progressive increase of pyrolant density. The speci¦c impulse decreases with the increment of §ight Mach number due to both negative contribution of ram drag and of incremented air inlet temperature. This second parameter induces more dissociation in the ram burner. The use of a frozen expansion model enables the observation of such e¨ect since it does not implement any chemical species recombination during nozzle expansion.

EXPERIMENTAL ANALYSIS
A series of pyrolants have been selected for experimental analysis. Three di¨erent families containing, respectively, 30%, 40%, and 50% by mass of AP, were produced. Inert HTPB binder cured with isocyanate was used. The work targeted self-sustained combustion limit and burning behavior of AP pyrolants enriched with aluminum. The e¨ect of ¦ne oxidizer fraction was assessed as well. Table 4 reports details of experimented formulations, including the amount of the ¦ne oxidizer fraction. The propellants were compounded using a Labram Resodyn resonant mixer and were tested for burning rate (between 10 and 50 bar) and CCPs up to 40 bar.
Burning tests were accomplished in closed vessels ¦lled with nitrogen atmosphere and pressure-regulated with servovalves. Sample size was 4 × 4 × 30 mm. Ignition was obtained with hot-wire technique.
The burning rate measurement was performed using a top-to-bottom combustion con¦guration inside a windowed vessel. Propellant sides were inhibited using a low molecular weight polymer. Digital video recordings of the tests were used to derive burning rate after proper distance calibration. The postprocessing was performed using the in-house Hydra software.
The collection of combustion residues was performed using a strand burner with quenching pool sketched in Fig. 3. The collecting medium was a halogenbased liquid hydrocarbon. Particles were initially washed with acetone in order

A. AP30 Group
The combustion of the AP30 compo- burning at 50-bar pressure sition was highly irregular within the tested range, from 10 to 50 bar. The §ame front progressed inside the propellant structure, without destroying the skeleton. Under these conditions, the measurement of the burning rate was meaningless. A residual carbonaceous structure, containing most of unburnt metal particles, was left on the strand holder for all pressures. On the sides of this combustion product, some agglomerates could be observed as a result of the exudate accumulation. The result from 50-bar combustion is reported in Fig. 4.

B. AP40 Group
From an ideal viewpoint, this kind of formulation represents a good compromise between oxidizer content and performance. Even though the increment of oxidizer amount was expected to improve combustion quality, the resulting §ame was irregular. In this case, the generation of the carbonaceous skeleton trapping both aluminum particles and pyrolysis residues of the binder appeared to be dependent on both pressure and ¦ne AP fraction.
In tested compositions, the skeleton structure tended to disappear gradually once the combustion pressure was above 30 bar (Fig. 5). At 50-bar pressure, the skeleton disappeared but large agglomerates were collected after the burning test. The size of these agglomerates increased with the ¦ne AP content.
The formulation without ¦ne AP showed powdered residues while compositions with 12% of ¦ne oxidizer demonstrated more vigorous combustion but also incremented size of the CCPs, which could become as large as some millimeters.
For the AP40 formulations, the skeleton structure was in §uenced by both composition and pressure. Whereas the increment of pressure played in favor of a reduced skeleton structure, the presence of ¦ne oxidizer fostered the generation of large metal agglomerates.

C. AP50 Group
This family of propellants is charac- Figure 6 The AP50 burning rate ¦tting: 1 ¡ 0% FAP; 2 ¡ 6% FAP; and 3 ¡ 12% FAP terized by a larger amount of oxidizer, thus implying a worse impact on the weight of AP to carry onboard but also a better behavior in terms of combustion and size of agglomerates. As for the AP40 family, three di¨erent alternatives regarding the ¦ne fraction of AP (FAP) content were tested experimentally. Measurement of burning rate and collection of CCPs were possible thanks to the higher quality of combustion. Figure 6 and Table 5 collect results for burning rate measurements between 10 and 50 bar. Although the quality of the combustion was enough to identify a regressing surface, regularity and overall reproducibility were fair testi¦ed by the incremented scattering of experimental data and by the reduction of R 2 . Increment of ¦ne AP fraction induced more vigorous §ame but also more irregular combustion process leading to incremented data scattering, mainly at higher pressure. Moreover, as ¦ne oxidizer fraction was incremented (reducing the coarse component), the burning rate exponent decreased from 0.31 to 0.20. A distinctive behavior could be identi¦ed for the formulation with 0% FAP. Rather, a peculiar trend for curves of 6% and 12% FAP was not properly obtained due to incremented data scattering, eventually showing the same burning rate at 50 bar.
The analysis of the CCPs was performed at the constant pressure of 40 bar. Powder size distributions are reported in Fig. 7. Each curve derives from the averaging of three samples burned per each tested batch. Table 6 reports the respective D 43 along with statistical error estimation. CCPs show a bimodal distribution for each tested propellant. The ¦rst peak is in the range between 4 and 6 µm while the second one is between 330 and 450 µm. Particles up to   submicrometric fractions were not identi¦ed by the instrumentation. Incremented size of agglomerates was observed for increased FAP content and was more pronounced between propellants AP50 (0% FAP) and AP50 (6% FAP) than among propellant with 6% and 12% of FAP particles. Also, in the present case, the level of uncertainty increases with the amount of FAP, con¦rming the reduced reproducibility of the combustion process.

DISCUSSION
From results obtained by AP40 and AP50 families, it appears that the amount of ¦ne oxidizer plays a role in the worsening of combustion quality. In both cases, the increase of FAP increments the size of the agglomerates. In AP50 group, levels of 6% and 12% of FAP increment the scattering of the burning rate results generating higher dispersion in burning rate data. Competent literature would suggest that increments both of burning rate and of pressure exponent should be observed for such kind of materials. Rather, obtained results show contrasting trends, obtaining an exponent of the Vieille£s law around 0.2 for propellants with the highest amount of FAP. Similar results have been reported in a conference paper by Nanda and Ramakrishna for oxidizer-lean formulations but the use of burning rate catalysts by the authors makes the direct comparison impossible [13]. However, in a report released by the Summer¦eld£s group, the authors underlined that for nonaluminized AP/PBAN (polybutadiene acrylonitrile) propellant, the incremented presence of ¦ner oxidizer fractions can lead to plateau burning or to propellant extinguishment at high pressure [14]. The same report also indicated that there was no in §uence introduced by the additional presence of aluminum. We should consider that the cited work of Summer¦eld£s group was not considering oxidizer-de¦cient compositions, which might alter the thermochemical balance at the burning surface.
An in §uence between ¦ne AP fraction and aluminum can be inferred if the pseudopropellant contained in the pocket structure is targeted, following the concept of Cohen [15] or of Maggi and coauthors [16]. The computed §ame temperature, considering binder and FAP as reacting mixture (assuming inert aluminum), are reported in Fig. 8. At 40 bar, the pocket temperature for material without FAP is slightly greater than 940 K, while for propellants AP50 (6% FAP) and AP50 (12% FAP) is, respectively, 1135 and 1118 K, above the melting point of the metal. It is evident from the bar plot that the larger increase is obtained by passing from 0% to 6% of FAP, while the growth is limited if the ¦ne fraction is doubled from 6% to 12%. Similar trend can be noted also looking at results in terms of CCP dimensions. There is a substantial increase of D 43 between AP50 (0% FAP) and AP50 (6% FAP) while the di¨erence between AP50 (6% FAP) and AP50 (12% FAP) is not remarkable.
It can be inferred that the increasing size of agglomerates with FAP content is the result of two concurrent processes, which are, respectively, the local surface temperature and the burning rate. The increase of local surface temperature of the pocket, mostly driven by reactivity of ¦ne AP and binder, may favor the sintering of aluminum, leading to large particles. At the same time, the burning rate is increased by the presence of FAP, reducing the residence time of metal particles. The resulting CCPs represent a compromise between these concurrent in §uencing factors. It appears from the aforementioned results that under such experimental conditions, the in §uence of FAP on the local pocket temperature is stronger than the e¨ect on the burning rate.

CONCLUDING REMARKS
The present exploratory investigation considered a set of pyrolants for powering ducted rockets. Oxidizer-lean formulations were developed on the basis of standard AP/Al/HTPB propellants. The investigation analyzed the in §uence of coarse and ¦ne oxidizer fraction, in presence of inert binder and micrometric metal particles. Propellants with oxidizer fractions lower than 40% by mass created a skeleton structure which resisted to combustion process. In §uence on pressure and FAP was observed. Propellants containing 50% of AP were capable of self-sustained combustion without skeleton structure. With the increment of the ¦ne oxidizer, the burning rate analysis showed a reduction of the pressure exponent and at the same time, an increment of the agglomerate size.
The resulting propellants featured a burning rate level which is about 20% 40% lower than a standard AP/Al/HTPB composition for launcher applications but are in line with results reported in the competent literature [5]. However, the decrement of the burning rate pressure exponent limits their applicability in ducted rocket technology. In fact, such value of n is suitable for systems where propellant mass §ow rate must be kept constant or within a limited interval (such as in ¦xed fuel- §ow ducted rockets) while are less suitable for nonchoked solid RJs, where propellant throttling capability is important for ram burner combustion at di¨erent Mach and §ight levels. The presence of large agglomerates discourages the use with variable- §ow ducted rockets since the pintle located in the primary combustion chamber may be damaged or clogged.
For the future, burn-rate modi¦ers, boron-based compounds, and nanometal particles can be tested to improve the behavior of the metallic fuel. On the other hand, other inert/active binders might be of interest, such as GAP (glycidyl azide polymer) or PPG (polypropylene glycol), for the reduction of metal agglomeration process. The tuning of the agglomerate size distribution should be a compromise between residence time needed for combustion and §ame stabilization granted by aerodynamic e¨ect of large particles.