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  1. 1. 61 The Potential of Aluminium Metal Powder as a Fuel for Space Propulsion Systems THE POTENTIAL OF ALUMINIUM METAL POWDER AS A FUEL FOR SPACE PROPULSION SYSTEMS ABDUL M. ISMAIL1* , BARNABY OSBORNE2 AND CHRIS S. WELCH3 1. Kingston University London, Faculty of Science, Engineering and Computing, Roehampton Vale Centre, Frairs Avenue, London SW15 3DW, UK. 2. Australian Centre for Space Engineering Research, School of Surveying and Geospatial Engineering, The University of New South Wales, Australia. 3. International Space University, Strasbourg Central Campus, 1 rue Jean-Dominique Cassini, Parc d’Innovation, 67400 Illkirch-Graffenstaden, France. Email: k0748324@kingston.ac.uk* JBIS, Vol. 65, pp.61-70, 2012 1. INTRODUCTION: JUSTIFICATION AND SUMMARY One of the principal limitations of long duration human spaceflight beyond cis-lunar orbit is the lack of refuelling capabilities on distant planets resulting in the reliance on con- ventional non-cryogenic, propellants produced on Earth. If one could develop a reliable propulsion system operating on pro- pellants derived entirely of ingredients found on nearby plan- etary bodies, then not only could mission duration be extended, larger amounts of payload could be ferried to and from the destination and eventually the cost of transporting propellant ingredients from Earth could be reduced, if not eliminated. Metal powder, in particular, aluminium burns energetically with oxygen and affords an impressive exothermic reaction that can be utilised for propulsive force. The oxidiser component of an aluminium powder based propellant can be in the form of pure oxygen, air, steam or a compound such as ammonium perchlorate (AP) making it ideal for terrestrial applications. So much so, that aluminium powder as a fuel has been investigated intermittently for over sixty years for military applications such as to propel torpedoes using an oxidiser in the form of steam produced by drawing in seawater, ram-air as an oxidiser for air- launched ram jet missiles and AP in powder form which when combined and ignited with aluminium powder results in com- bustion which can deliver a propulsive force suitable for long range surface-to-surface missiles. In terms of space applica- tions, the logic of employing oxygen/aluminium metal powder as an in-situ resource utilisation (ISRU) propulsion concept is due to the fact that the propellant combination are constituent chemicals that can be found on numerous planetary bodies and thus existing research could be adapted and enhanced for possi- ble employment in space based systems. Therefore the techno- logical readiness level is not as complex as if one were to develop an entirely new propulsion system. Living off the land to realise extended duration residency on extra-terrestrial bodies of interest is not exactly a revelation. Metal powder propulsion systems have been addressed intermittently since the Second World War, initially in the field of underwater propulsion where research in the application of propelling torpedoes continues until this day. During the post war era, researchers attempted to utilise metal powders as a fuel for ram jet applications in missiles. The 1960’s and 1970’s saw additional interest in the use of ‘pure powder’propellants, i.e. fluidised metal fuel and oxidiser, both in solid particulate form. Again the application was for employment in space-constrained missiles where the idea was to maximise the performance of high energy density powder propellants in order to enhance the missile’s flight duration. Metal powder as possible fuel was investigated for in-situ resource utilisation propulsion systems post-1980’s where the emphasis was on the use of gaseous oxygen or liquid oxygen combined with aluminium metal powder for use as a “lunar soil propellant” or carbon dioxide and magnesium metal powder as a “Martian propellant”. Albeit aluminium metal powder propellants are lower in performance than cryogenic and Earth storable propellants, the former does have an advantage inasmuch that the propulsion system is generic, i.e. it can be powered with chemicals mined and processed on Earth, the Moon and Mars. Thus, due to the potential refuelling capability, the lower performing aluminium metal powder propellant would effectively possess a much higher change in velocity (∆V) for multiple missions than the cryogenic or Earth storable propellant which is only suitable for one planet or one mission scenario, respectively. This paper reviews existing metal powder propulsion systems, while emphasising the option of oxygen/aluminium metal powder as a propellant, in order to highlight the potential of this near-term concept. Keywords: Metal, powder, fuel, ISRU, propulsion Paper presented at the 62nd International Astronautical Congress, Cape Town, South Africa, 3-7 October 2011. Paper No. IAC-11- D3.2.10.
  2. 2. 62 Abdul M. Ismail, Barnaby Osborne and Chris S. Welch But the selection of which ISRU propellant is a contentious issue as proponents of numerous ISRU propulsion concepts vie to present their projects as superior to their competitors. The highest performance non-toxic chemical propellant combina- tion known is liquid oxygen/liquid hydrogen (LOX/LH2 ) where the erstwhile space shuttle main engines using this propellant delivered a vacuum specific impulse of 452.5 seconds (at 18.94 MPa). However, propellant performance is not always the most important selection criterion. Although cryogenic propellant delivers superior performance, production could prove to be a challenge, in-situ, and difficult to store since it would also mean producing liquid nitrogen. Propellant combinations like LOX/LH2 , LOX/liquid methane (LCH4 ) and LOX/liquid car- bon monoxide (LCO) are all restricted to Mars, as is the lower performing non-cryogenic carbon dioxide (CO2 ) and magne- sium metal powder. By comparison, LOX combined with alu- minium metal powder, where the carrier gas is assumed to be 2% by weight of the fuel, delivers a theoretical vacuum specific impulse (excluding losses) of ~315 seconds (at 3.45 MPa), Fig. 1, and are readily available ingredients on Earth, the moon and Mars making it a truly generic propellant combination requir- ing only refuelling stations located on planetary bodies of interest. This in essence is the justification of the selection of this propellant combination for the chosen field of research. One should note that LOX, albeit a cryogenic, is being considered as an oxidiser candidate primarily due to the as- sumption that in parallel to research into an oxygen/aluminium metal powder propulsion system, an investment will most likely be made in other ISRU activities such as the acquisition of oxygen for life support. Therefore, the process of producing LOX is not considered to be as technologically taxing as manu- facturing cryogenic fuels. Where a liquid engine consists of propellant feed tanks, inert gas and a separate tank for the pressurant, feed lines, injector and combustion chamber, metal powder propulsion systems have a similar schematic to that of conventional pres- surised liquid propulsion concepts albeit with a few novel subsystems, namely the powder feed system, injection mecha- nism and the layout of the combustion chamber. This paper aspires to review and briefly analyse what previ- ous metal powder propulsion researchers have accomplished over the years in order to highlight the potential of aluminium powder as a fuel for space propulsion applications. To achieve this objective, various past metal powder propulsion systems were broken down into several subsystems that includes fuel storage, feed mechanism, injector, igniter and combustor. Each element is addressed as an independent subsection and is fol- lowed by an initial assessment summarising the findings. Naturally, this concept can only become a feasible option for mission planners once a number of inherent technological difficulties have been addressed, specifically metal powder combustion and work in this field is still on going. 2. REVIEW OF EXISTING SUBSYSTEMS Metal powder as an in-situ fuel was first suggested by Rhein in 1967 [1]. Prior to that, metal powder was advocated as a fuel to propel underwater submersibles [2] and then as a high-energy density fuel for propelling missiles [3]. As a result, a wealth of examples exist dating as far back to the Second World War where one can extract and analyse pertinent information from a variety of metal powder propulsion concepts. Needless to say, given that the focus of research is oxygen/aluminium metal powder propellant, design principles from related powder pro- pulsion systems can be examined for possible employment. In some cases, specific details from past studies can be over- looked such as the issue of combustion where the propellant comprises of aluminium metal powder combined with one of the following oxidisers; steam, air or AP. Combustion for a different metal powder propellant that can be ignored is CO2 / magnesium metal powder but at the same time lessons can be learned from specific subsystems employed in this concept such as the powder storage techniques, powder transportation and injection for application in an oxygen/aluminium metal powder propulsion system. This initial review is structured by first addressing the issue of metal powder storage, followed by methods of transporting the powder and then injection proceeded by ignition, combus- tion and chamber design. 2.1 Metal Powder Storage/Powder Packing Unlike a fluid, metal powder will settle inside a tank in a way that leaves interstitial spaces, reducing the quantity of fuel that can be stored within the system. Although when comparing a fluid to a solid, the latter’s energy-density level is much greater, attempting to fill those interstitial spaces will increase the operational capacity of the propulsion system. This subsection reviews the past work performed in the field of increasing powder packing density. Increasing volumetric density greatly enhances a metal pow- der propulsion system’s operational capability by being able to store a larger amount of fuel within the constraints of the tank and thus it is in the best interest of metal powder propulsion system designers to maximise storage density. That being said, the enhancing of packing density is dependent on two overrid- ing factors; methods of packing and powder size but the choice of powder size(s) is in most part dictated by three issues, namely “flowability”, “injectability” and “combustibility”. Therefore the final selection of powder size is an iterative process. Sources dating back to the 1930’s [4, 5] presented results for theoretical and empirical packing density for one, two, three and four sizes of powder within a system. From the early 1960’s, metal powder packing became a focus of attention due Fig. 1 Theoretical specific impulse for Oxygen/Aluminium propellant.
  3. 3. 63 The Potential of Aluminium Metal Powder as a Fuel for Space Propulsion Systems to its applications in atomic energy and space research. McGreary covered a large number of powder sizes and found that the measured density of single sized spheres in an orthorhombic arrangement was 62.5% [6]. In perhaps the most detailed powder propellant study to date, Loftus, Montanino et al. concluded that maximising packing density is achieved by use of mainly large powders and the remainder with smaller powders; what is referred to as a bi-modal distribution [7]. The smaller of the two powders in a bi-modal distribution should be at least a factor of 7 times smaller than the larger powder. It was found that a 70/30 mass distribution of 30/3 µm spherical aluminium powder, respectively, provided the highest volumet- ric density at 1.98 grams/cubic centimetre which corroborated the results initially presented by Westman and Hugill. Loftus, Marshall et al. went on to highlight that increasing “sphericity”, surface treatment of the powder and introduction of moisture (or lubricant) enhances packing densities in smaller pow- ders [8]. But, Goroshin, Formenko et al. found that the powder’s dispersion characteristics, an understanding of which is necessary for powder injection, improved by drying the powder at 393 to 413 K for 24 hours prior to injection taking place [9]. This implies that any presence of moisture will act as an inhibitor to flowability. It also means that the heating of the powder and the subsequent increase in the powder’s surface temperature affects the surface condition, which assists flow and injection. These two studies collectively conclude that packing density can be increased by adding moisture but said moisture would then have to be removed in order to increase flowability and dispersion/injection efficiency thus complicating the system’s overall design; assuming one as- pires to obtain ideal operational parameters for every subsystem. Given the fact that this propulsion system is intended to operate in space or for planetary surface transportation where temperatures are expected to be extremely low, adding moisture to the powder would be considered illogical for obvious reasons unless an addi- tional component is added to the fuel tank where said moisture can be removed. This of course adds complexity to an already com- plex system. The vast majority of metal powder propulsion researchers however did not address packing density and focussed on an orthorhombic arrangement. One important finding from this subsection is the tem- peramental nature of powder characteristics and that a minor inconsistency in powder properties can affect an entire sys- tem’s design and performance. It also serves as a warning to future metal powder propulsion system designers that one should not take for granted that two different sources of the same size powder fuel will afford the same results. A mini- mum level of quality control in powder production is there- fore required in order to attain corresponding performance if the same propulsion system is to be employed in different planetary scenarios. 2.2 Metal Powder Feed Given the non-conformal behaviour of metal powder beds, a novel mechanism to transport the powder from the tank to the injector is required. The configuration or design element of this subsystem depends on the method used to transport the powder. A number of devices to feed metal powder from the tank and into the combustion chamber have been proposed over the past six decades, some of which were addressed in detail and others presented as a conceptual option. One method to feed metal powder was ignored since it is only suitable for low thrust electromagnetic accelerators [10-12] and not high thrust, as is desired in this study. Table 1 summarises the categories of powder feed systems, either proposed or used in past experiments. Where the auger and powder pump only have one possible configuration, the pressure fed system offered three possible configurations all of which were unrestricted which meant that the powder bed could move about freely within the tank. This option not only used over 5% by weight (of powder fuel) of fluidising gas [13], the powder would be fed in an inconsistent manner leading to some researchers to develop a method to constrain the powder bed as the powder exists the system. The solution was to employ a syringe (or piston/piston head) which can either be pressure fed and in doing so requires only 1% by weight (of powder fuel) of fluidising gas or powered by a mechanically actuated piston. This subsection will briefly address the category of powder feed devices that have been covered in past powder propulsion systems and highlights pros and cons associated with each configuration. The vast majority of researchers addressing metal powder propulsion systems chose to connect the powder fuel tank directly to the injector manifold which in turn was connected to the com- bustion chamber. While the clear benefit of eliminating the feed tubes reduces the complexity of the system and allows the re- searcher to focus on the feed mechanism, injection, ignition, com- bustion and cooling, two essential factors will not be addressed, namely the flow of powder through the feed lines which is required to determine and work towards counteracting problems associated with the resultant pressure drop but also a major engineering concern; notably wear and attrition. Fricke, Berman et al., on the other hand, stored their powder in a separate tank where the powder was fed into the combustion chamber via long lengths of tubing; perhaps one of the only studies to have done so [14]. The success of this positive expulsion fluidised bed (PEFB), as it was referred to, can be considered a pioneering piece of work and the inventors were awarded a Patent [15] for their efforts. Thereafter the vast majority of powder propulsion engineers chose to employ this specific subsystem or slightly adapted the design, as in the case of Miller and Herr [16], Fig. 2. TABLE 1: Categories and Configurations of Powder Feed Systems. Category of powder feed system Possible configurations Auger (also referred to as screw or worm feeder) One type Powder pump One type Pressure fed Conventional fluidised bed; Fluidised bed with a porous plate; Fluidised hopper Syringe (or piston) Pressure fed piston; Mechanically actuated piston
  4. 4. 64 Abdul M. Ismail, Barnaby Osborne and Chris S. Welch Experiments with the PEFB were conducted in both the horizontal and vertical to determine the effects of gravity and no such influences were recorded. Results also showed that supplying fluidising gas via the piston head during the powder expulsion process did not enhance performance due to the resistance afforded by the dense powder mass and gas flow. Although there was a continual supply of pressurant entering the system from the piston head, tank outlet pressure continued to slowly decay. By comparison, when fluidising gas was sup- plied only via the tank outlet during powder expulsion, near constant pressure was recorded. Although tank feed pressure registered a ~12% reduction, an almost constant flow rate of powder was observed for up to 15 seconds without adding any more fluidising gas. One could therefore surmise that the fluid- ising gas distributors at the exit end of the tank is all that is required and thus this concept is similar in configuration to that proposed by Akiba, Kohno et al. [17], Fig 3, coupled with a pneumatic syringe (i.e. a pressurised piston/piston head). Linne and Meyer point out that terrestrial powder flow techniques would be unsuitable for propulsion applications because they do not minimise carrier gas or cannot accurately control solids flow rate [18]. While these points are true, the technology which Linne and Meyer focus on is a system first introduced by Fricke, Berman et al. that was in fact adapted from terrestrial technologies. One therefore wonders if addi- tional concepts from the terrestrial bulk solids transfer industry were ignored in past metal powder propulsion research but which could, if found, be adapted for applications in a future metal powder propulsion system. Without much analysis, Akiba, Kohno et al. concluded that the added complexity of the powder fuel feed system leads to an increase in engine structural weight. This conclusion is dependent on a detailed trade-off which was not addressed and thus their conclusion was considered premature. The first and only known attempt at powder pumps for propul- sion was tackled by Tamura, Kohno et al. [19]. Empirical data did not correlate with the simple one-dimensional theory but this was to be expected since the authors make the statement “it has been proved that fluidised powder could be handled in the same way as fluid”. The first observation is that this was an assertion of previ- ous researchers and was not verified. The second point is that this assertion is not entirely correct. The conclusion that fluidised powder flows like a fluid, as inferred in a previous study [20] is a simplistic way of ignoring the complexities associated with pow- der flow in a carrier gas and by doing so, substituting complex two phase flow equations with existing fluid flow equations. Since in reality fluidised powder does not act like a fluid, it was no surprise that the theoretical results and empirical data did not match. The authors conclude that their initial study did not produce the desired results but given the potential benefits of a powder pump, the concept should not be dismissed outright. In terrestrial industries, an auger is employed to provide a reliable and continuous flow of bulk solids but it remains to be seen if such a method to transport metal powder in a propulsion system can prove beneficial.There are certainly logical reasons for its employment. Such a subsystem can be operated by an on-board power supply instead of using a pressurised fluidising gas, which in the case of a space-based propulsion system would either have to be obtained in-situ or brought from Earth.The latter is out of the question since the objective of this propulsion system for space applications is to completely eliminate the necessity of carrying any terrestrial chemicals to operate the engine.Also, employment of an auger could result in a reduction of structural mass associated with a pressurised system. Of course, the obvious draw-back to the use of an auger is wear and attrition given that the bulk material being transported will be metal powder. A method to ensure that the powder bed does not move about freely will be required, as was employed with the PEFB.This ‘piston head’can be connected to the threads of the auger and designed to provide continuous contact with the bed as the metal powder exits the system. An auger, therefore, presents itself as a desirable option and should be examined more closely. 2.3 Metal Powder Injection This subsection reviews the injection subsystem. A principal difference between conventional liquid rocket injector configurations and metal powder injectors is the metal powder fuel in the latter is pre-sized and thus a complex ‘shower head’ design is not required to produce a stream of fluid that will be distributed evenly throughout the combustor. There are three principal requirements of the metal powder injection system. The first is to provide a stoichiometric mix of metal powder fuel and either gaseous or cryogenic oxidiser to ensure successful ignition takes place. The second point is to avert powder re-agglomeration since agglomeration of powder reduces “ignitability” potential and in turn decreases combustion effi- ciency. The third is to minimise the use of fluidising gas. A number of different methods were identified from terres- trial powder propulsion concepts dating back to the 1940s until the 1970s as well as in metal powder for space propulsion Fig. 2 Positive expulsion fluidised bed. Fig. 3 Schematic of powder feed system.
  5. 5. 65 The Potential of Aluminium Metal Powder as a Fuel for Space Propulsion Systems during the 1980s until the present day. In addition to reviewing metal powder injection configurations of previous powder rocket engines, methods to aerosolise powders for metal powder com- bustion experiments were also examined since the latter has potential in propulsion applications and dispersion characteris- tics of the metal powder is pivotal to this sub-topic. In one of the first powder injection systems for a propulsion system, Branstetter, Lord et al. fed metal powder by a piston where injection of powder into the chamber was achieved via several sloped slots embedded in a rotating disk, which was powered by a small motor positioned on the other side of the chamber, Fig. 4.Aforeseeable complication with long-term use of this concept is the erosion of the slotted disk. Continual use would result in degradation and an increase in size of the slots causing an irregular flow of powder in consecutive tests. Regu- lar replacement of this component would therefore be required. There were no reported complications of the slotted disk con- cept during use but its employment was not repeated in any post-1951 research on metal powder propulsion systems. Injection by impingement was the most popular method to mix the powder fuel with the oxidising fluid. Where the vast majority of concepts employed a non-reactive fluidising gas for powder entrainment, Dean, Keith et al. chose to entrain the aluminium powder with 10% of the gaseous hydrogen fuel and then the remaining 90% in three separate locations along the axis of the chamber [21], Fig. 5. Foote, Lineberry et al. on the other hand added 15% of the oxidiser upstream to enhance circulation of the combustion products [22]. By comparison, every other concept introduced 100% of the fluidising gas, be it reactive or non-reactive, along with the metal powder. It should be noted that the propulsion system developed by Dean, Keith, et al. employed metal powder as an additive to enhance the performance of an oxygen/hydrogen propulsion system and did not intend on the metal powder being the primary fuel compo- nent, as is desired in this specific study. The impingement angle used by Dean, Keith et al. was 60 degrees where the four injector ports are positioned on the hemispherical injector face, 30 degrees from the centreline. All other injectors apart from Meyer tended to have a 90 degree impingement angle including the concept addressed by Fricke et al. which used both fuel and oxidiser in powder form, Fig 6. Meyer chose to employ two injector configurations; a triplet (O-F-O) or a quadlet (O-F-O-F) but it was unclear if the metal powder fuel and oxidiser injectors were angled towards the combustor centreline. Albeit not acknowledged, all three O/F injector designs employed by Mistry and Coxhill [23] were very similar to a schematic initially proposed by Foote and Litchford presented a year earlier [24]. Both projects dealt with CO2 /magnesium metal powder propellant where the latter was conceptual. In the powder propulsion concepts that worked, there was general consensus that a low powder flow rate was a major culprit in causing powder blockage in the injector port. In conventional fluid injectors, reducing the inner diameter of the tube will reduce pressure resulting in an increase in injection velocity of the fluid but this scenario simply doesn’t work for powder flow and blockages increase. Wickman opted to cir- cumvent this complication by introducing magnesium metal powder fuel via feed lines minus an injector nozzle [25]. When compared to the PEFB, a conventional fluidised bed will have a finite amount of pressurant and while this approach is accept- able for the terrestrial bulk solids sector, for space based appli- cations it is not recommended given the large quantity of feed gas required to accompany the propulsion system. A noticeable similarity with various powder impingement concepts is that most fluidised powder was injected via the engine centre line and that the gaseous oxidiser flowing at a very high injection velocity from a specified angle would direct itself towards a point of impingement just inside the chamber leaving the remainder of the chamber free to allow an increased combustion residency time. The high velocity oxidiser jet strikes the fluidised powder stream and encourages turbulent flow which is desired during the mixing and combustion phase. The design of the injectors developed by Bell Aerospace, Fig. 7, were somewhat unique as the objective here was to mix the oxidiser and fuel, both of which was in powder form, prior to injection. One major conclusion from the study was that the Fig. 5 Rocket engine schematic diagram. Fig. 6 AP/Aluminium powder impingement. Fig. 4 Fuel injection by slotted disc.
  6. 6. 66 Abdul M. Ismail, Barnaby Osborne and Chris S. Welch flow behaviour of dense-phase gas flow mixtures roughly obeys the orifice liquid flow equations. BellAerospace presented two injector schematics. Where the mixing cup option is not en- tirely relevant to a propellant where one component is a fluid and the other is a solid (in this case, metal powder), the coaxial injector schematic which consisted of a central orifice with a vortex insert surrounded by an annular orifice, does show promise given that a similar schematic was employed by Foote et al. over two decades later. Out of all the engineering concerns associated with metal powder injection, the dispersion technique presented by Goroshin, Kleine et al. [26] seems to solve these problematic issues. The design is such that a limited amount of fluidising gas can be used to supply an extremely high velocity stream via a µm sized slot that would shear the oncoming metal powder several micrometre lay- ers at a time and in doing so, produce an evenly distributed dust cloud thus resolving the potential problem of re-agglomeration. This shearing process was referred to as an ‘aerodynamic knife’. Using this method, the oxidiser/fuel mixture ratio can be control- led with ease. As with all injector designs for aluminium metal powder propulsion systems, there is also a problem pertaining to the injector face. Without adequate protection, recirculation of metal oxide combustion products will settle around the injector port and solidify which will result in reducing the exit port area. The dispersion mechanism strives to solve this potentially serious problem by the supersonic jet which not only disperses the powder but also creates an aerodynamic buffer between the combustion products and the injector port. Figure 8 shows a close-up of the dust dispersion mechanism employed in laboratory experiments by Risha, Huang et al. [27], which was based on a design introduced by Goroshin, Kleine et al. A similar powder injector configuration was suc- cessfully used by Zubrin, Muscatello et al. in a metal powder rocket engine demonstration [28].” 2.4 Metal Powder Ignition and Combustion The metal powder is injected with a carrier (or fluidising) gas and mixes with an oxidiser. The fluidising gas can be inert such as nitrogen or in some cases hydrogen, helium or methane to enhance ignition and combustion. The approach taken by the vast majority of investigators was to avoid the complexity of a flight-ready system and simply provide sufficient energy to ignite the aluminium metal powder in order to conduct the propulsion experiment. Several methods of igniting the alu- minium metal powder in a rocket combustor have been used, namely an electric spark, a high energy squib or a flame torch using propane or hydrogen. An ideal igniter would be capable of multiple restarts, produces sufficient energy to ignite lean propellant mixtures and be able to withstand the high heat flux generated from the combustion process. Although ignition and combustion in metal powder propul- Fig. 7 Injector assemblies. Fig. 8 Particle dispersion sub-assembly.
  7. 7. 67 The Potential of Aluminium Metal Powder as a Fuel for Space Propulsion Systems sion systems are independent subjects, the topics are intricately tied together and thus are presented collectively within each different ignition option. 2.4.1 Electro Static Discharge A flight version of an ISRU metal powder propulsion system using oxygen and aluminium as a propellant will require a reliable ignition system capable of multiple starts. To this end, a number of investigators initiated research into electro-static discharge (ESD) or spark ignition. Meyer employed an oxygen/hydrogen augmented spark ig- niter to initiate combustion for the gaseous oxygen/aluminium metal powder engine; which to a certain extent parallels the idea presented by Dean, Keith et al. over two decades earlier. The conclusion based on Dean, Keith et al.’s preliminary tests was that both ignition and combustion were related to the flame temperature and dust cloud concentration. For open flame tests, ignition was achieved by supplying a 2000 Volt spark from two insulated wires that were attached to the injector face. Small- scale performance tests also employed a spark which initiated combustion with the oxygen/hydrogen mixture which in turn ignited the aluminium powder. Thus, the fluidising gas effec- tively acted as a primer for metal powder ignition. Shorr and Reinhardt’s effort was based on what was then on- going work by Bell Aerospace where this specific study ad- dressed the issue of spark ignition for fluidised powder bi- propellants [29].Albeit not aluminium metal powder, the report did provide an insight into AP and polyethylene powder igni- tion fluidised with air and hydrogen, respectively. In previous powder propulsion experiments, BellAerospace would employ pellets and hypergolic liquids as igniters but spark plugs were investigated for multiple start-stop. At the time of the Bell Aerospace research effort, the spark energy required to ignite fluidised powder propellants was unknown but common sense dictated that the spark should deliver sufficient energy to ignite the propellant which in turn would supply the required enthalpy to ensure continuous combustion. It was determined that a number of factors were involved in guaranteeing combustion by ESD; 1) required temperature of the metal powders to combust, 2) powder size, 3) space between the powders, 4) spark intensity and 5) heating time available (i.e. dictated by the chamber’s characteristic length or L*). Experiments by Malinin, Kolomin et al. were conducted to ascertain the ignition region of the primary flow of the mixture, the areas of flame stabilisation as well as combustion stability of both primary and secondary flows of the mixture, chemical and phase compositions, powder-size distribution of the con- densed products and combustion chamber efficiency with rela- tion to combustion parameters [30]. Two methods were identi- fied to ignite metal-air mixtures in the pre-chamber; an electric arc and high temperature flame of an igniter’s combustion products. It was found that the principal factors relating to ignition were pressure, air-to-fuel ratio and initial velocity. These results parallel the conclusion of Dean, Keith et al.. As with the previous effort, two types of igniters were examined by Xia, Shen et al. [31]; high temperature gas and a spark plug. Ignition of the propellant was achieved by use of a high energy spark plug and the employment of a ‘flame holding technique’ in order to sustain combustion was also proven. The authors confirmed the use of a spark plug as being suitable for multiple restarts without elaborating on the quantity of restarts that are possible and most importantly, the effectiveness of the ignition mechanism for future application. One should also note that, the conclusion by Xia, Shen et al. contradicts Branstetter, Lord et al., as explained in the next subsection. 2.4.2 Squib Ignition One of the reasons why Branstetter, Lord et al. chose to employ a gunpowder squib was due to the fact that all metal and ceramic parts such as thermocouples on the spark electrodes and flame holders would turn white hot before 10 seconds and then continue to burn through after 20 seconds. Accumulation of unburned powder and solidified combustion products would deposit on the spark electrodes rendering them useless for re- ignition. This was also a problem experienced by Wickman albeit those tests were conducted using CO2 /magnesium metal powder propellants where the unwanted deposits consisted of carbon. Loftus, Montanino et al. and Loftus, Marshall et al. focussed on the ignition of AP and aluminium, both in powder form. Igniting powder propellants required adapting technolo- gies from both liquid and solid propellant igniters. AP/alu- minium propellants requires ignition that initially produces sufficient thermal energy to decomposeAP (421.89 K) that will in turn provide the heat and oxidants to initiate aluminium powder combustion at 866.33 K. The idea would therefore be to select an igniter which can produce high temperatures at low chamber pressures. Malinin and Berkbek do not mention much about the igniter mechanism apart from the fact that ignition is achieved by use of a pyrotechnic igniter which is positioned in the pre-chamber [32]. Over 300 test firings were reportedly conducted using ASD-1 (25 µm) and ASD-4 (~9 µm) and both ignition and combustion proved reliable. Finally, Mistry and Coxhill who looked into a concept using CO2 /magnesium metal powder propellant quoted information obtained from the work of Malinin, Kolomin et al.. The proposal was to use a pyrotechnic igniter with oxygen to ensure combustion but no documented information on the successful operation of the igniter, ignition of the propellant or combustion data was presented. 2.4.3 Torch Goroshin, Bidabadi et al. [33], Goroshin, Fomenko et al., Foote, Lineberry et al., Goroshin, Higgins et al. [34], Foote,Thompson et al. [35] and Foote and Litchford all employed a propane-oxygen pilot torch, irrespective of the metal powder propellant combina- tion. Standard practise would be to switch on the torch and allow the metal powder fuel plus oxidiser to ignite until stable combus- tion is reached after which the torch is switched off. In the case of aluminiumpowdercombustion,preheatingthechamberwasfound to be a necessity in order for combustion stability to ensue. Miller and Herr chose to direct a hydrogen/oxygen torch towards the fluidised aluminium powder stream for just over 35 seconds in order to ensure continuous combustion. Foote, Lineberry et al. found that as the concentration of the aluminium dust cloud increases, the ignition time is reduced due to the additional heat supply. However, burning time in- creases due to depletion of the oxidiser in the gas stream. For example, at a gas temperature of 2400 K and a pressure of 50 atm, ignition time for 70 µm powders decreased from 5 ms to 1 ms when aluminium dust concentration increases from 0% to 10% weight where burning time increased by a factor of 5 to 8. 2.5 Combustor As for the combustor design, the schematic should be presented in a way that the L* is large enough to allow sufficient resi-
  8. 8. 68 Abdul M. Ismail, Barnaby Osborne and Chris S. Welch dency time for the metal particles to fully combust. L* is defined by the volume of the chamber divided by the nozzle’s throat area and new chambers are usually sized by reference to previous experience with the same propellant. But since very little practical data for oxygen/aluminium powder propellant exists, combustor sizing will have to be determined empiri- cally. Given that complete combustion of aluminium metal powder is a lengthy process, the majority of powder propulsion system designers chose to add a pre-combustor to the primary combustor, indicated by the labels (2) and (3), respectively in Fig. 9. In the former, combustion takes place in ‘fuel-rich’ mode, i.e. a high fuel/oxidiser ratio, in order to ensure ignition and sustain combustion. The latter chamber operates in “lean” mode and is sized to ensure stoichiometry where additional oxidiser is added to stabilise combustion which in return is expected to reduce incomplete combustion and serve to minimise two phase flow loss. In reality, sub-micrometre particles are still ejected out of the system so losses have thus far been unavoidable. 3. INITIALASSESSMENT All past rocket engine tests using aluminium powder as a fuel delivered tangible results and by doing so, highlighted areas that require additional attention. Apart from one study (Mistry and Coxhill), all other CO2 /magnesium metal powder propul- sion system tests that took place prior to (Wickman as well as Zubrin, Muscatello et al.) and after (Szabo, Miller, et al. [36]) proved feasible and thus useful mechanisms could be extracted and analysed in order to determine which methods and ap- proaches would be best suited for employment in an oxygen/ aluminium metal powder propellant propulsion system. In terms of this initial study, numerous metal powder propul- sion concepts were broken down into subsystems and assessed as independent components while taking into consideration the effect that one subsystem would have on the performance of the overall engine design. It was found that packing densities can be best achieved when using spherical powder sizes in a bi-modal arrangement where one powder diameter is at least 7 times larger than the smaller particle. Larger particles were found to flow more easily but take longer to combust whereas smaller particles (<10 µm) combust quicker yet have a higher tendency to agglomerate, flow less readily and plug the feed lines and injectors. Ideal powder mixtures where each of the two powder sizes are perfectly identical, can afford a packing or solid mass density of 82% but in reality the figure stands between 70 and 75%, which translates to 1.9 and 1.98 cubic grams per centimetre, respectively. It was clear, however, that character- istics played a big part in bulk powder properties, most notably powder size, smoothness, roughness of surface, shape (spherical or irregular), type of structure (crystalline or amorphous), compo- sition (organic or inorganic) and dielectric constants. Additives enhance flowability and even though some of these compounds like silicon dioxide could be acquired and produced, in-situ, on the lunar and Martian surface, combustion instabilities increase as a result of these additives coating the surface area of the metal powder, inhibiting ignition and combustion. However, with refer- ence to propulsion systems, bi-modal powder sizes is not a well- researched concept and even if one were to attain high packing density, it does not necessarily mean that the selection of powders will produce the most efficient combination for powder flow, injection or combustion. It remains to be seen if the aforemen- tioned combination (70/30 mass distribution of 30/3 µm spherical aluminium powder, respectively) would be suitable since the alu- minium powder in the quoted study combined with AP and there- fore the fuel may have been sized specifically for this propellant combination. Oxygen/aluminium is less energetic and is slower burning than AP/aluminium and therefore the aluminium powder sizes for the former would most likely have to be smaller. This has yet to have been examined. The fact that the PEFB developed by Bell Aerospace Com- pany in the late 1960’s/early 1970’s consistently provides metal powder researchers with a reliable flow of powder leads one to conclude that for propulsion applications, this is the subsystem of choice. Figure 10 shows the PEFB system apparatus during engine calibration. However, there is still the question of whether or not a mechanical auger could replace a pneumatic feed system, if only to reduce structural mass and eliminate the necessity to carry an inert fluidising gas. It is clear that in order to avoid re-agglomeration, the injector design should possess a characteristic that ensures the powder disperses in a turbulent fashion throughout the chamber while at the same time, takes into consideration the fact that the pressurant is finite. Goroshin et al.’s “aerodynamic knife” achieves just this, as was proven in numerous laboratory experiments as well as in one metal powder rocket engine demonstrator. For the sake of static experiments, squibs that release large amounts of heat for a fixed duration proved to be the most reliable way to ignite metal powder propellants but the empha- sis of a few researchers was to take the more complex route and focus on ignition by ESD. Igniting powders by spark affords an option for continual engine restart but combustion bi-products either coat the inner chamber or completely melt the ignition components, deterring re-ignition. This issue has yet to have been resolved. The introduction of a highly reactive, low mo- lecular weight, fluidising gas such as hydrogen, helium or methane greatly assists ignition and combustion but that would then mean having to rely on chemicals from Earth which de- feats the primary objective of this effort by aspiring to develop a propulsion system that can operate 100% on in-situ resources. Therefore a major challenge would be to achieve multiple ignition and steady combustion without having to rely on ter- restrial chemicals.Additional methods to ignite powder propel- lants such as pellets, hypergolic liquids and electrically heated wire are known to exist but they were not addressed due to insufficient information required for comparative analysis. Given the complex combustion mechanism associated with aluminium metal powder, a pre-combustion chamber is consid- ered a prerequisite for an oxygen/aluminium metal powder Fig. 9 Experimental reactor highlighting pre-combustor and main combustor.
  9. 9. 69 The Potential of Aluminium Metal Powder as a Fuel for Space Propulsion Systems propulsion system. This is due to the fact that aluminium metal powder tends to require relatively large characteristic chamber lengths to ensure near-complete combustion. There has been little in terms of research into alternative schematics of cham- bers based on varying powder size(s) which highlights a major gap in knowledge of this concept. 4. CONCLUSION A wealth of information on metal powder propulsion systems exists but a comprehensive review of past concepts has never been addressed. This initial study was considered a prerequisite to the development of any type of metal powder propulsion system. The approach taken was to acquire all publicly available material, extract relevant information based on subsystem categories and to briefly review said information.The subsystem categories include powder storage, powder transportation, powder injection, powder ignition and powder combustion as well as the combustor design. A realistic volumetric density of 75% is attainable via bimodal particle distributions as long as there is a factor of 7 between the two powder sizes.Asuitable combination is a 70/30 mass distribu- tion of 30/3 µm spherical aluminium powder, respectively, which provided a volumetric density at 1.98 grams/cubic centimetre. That being said, minor discrepancies in the powder characteristics can affect the way the powder rests in a powder bed and flows through the feed lines and injector. An oxygen/aluminium metal powder propulsion system will most likely have to use smaller aluminium powder sizes since the powder mass distribution and powder sizes were extracted from a project which used AP as the oxidiser, which is more energetic and provides a higher heat of reaction than pure oxygen. The PEFB which was invented by Bell Aerospace Company engineers during a 6-year program consist- ently proved to be the most efficient way to transport powder from a tank to the combustion chamber while using minimal amounts of fluidising gas. However, the auger or a powder pump has yet to have been addressed in detail and thus an in-depth trade-off is not possible until they too have been investigated.An auger would rely on an on board electrical power source to feed the powder and would all but eliminate the necessity of a pressurant and associated subsystem; as would a jet pump, hence its appeal. The aerody- namic knife concept proposed by Goroshin et al. solves the three major issues of powder propulsion systems; supplying a super- sonic jet stream to break up the metal powder in a turbulent flow, deter re-agglomeration and minimise use of fluidising gas. Ignition of aluminium metal powder is complex and a reliable, multiple restart, ignition system has yet to have been developed primarily due to a number of technological challenges, namely heat flux that melts internal chamber components and combustion residue de- posits in the form of aluminium oxide, coating the electrodes.Two phase flow and incomplete combustion can be reduced but not entirely eliminated by presenting a staged combustor where the pre-combustor operates in fuel-rich mode to ensure ignition and steady-state combustion and the primary combustor would operate in lean-mode, i.e. injecting the remaining oxidiser to achieve stoichiometry. Preliminary findings show promise but it remains to be seen if the positive elements of each subsystem, when combined together, can deliver a workable oxygen/aluminium powder propulsion system. 5. ADDITIONAL COMMENTS This paper is based on the literature review element of a Masters by Research thesis (titled “Oxygen/aluminium metal power space propulsion system: A literature review and trade- off analysis”) and the information contained within the study was based on publically available material. Additional sources of information exist but since metal pow- der as a fuel has primarily been under investigation for poten- tial military applications such as for propelling torpedoes and also ram-jet powdered missiles, access to some work [37, 38] was denied. Restrictions also apply to research in the field of metal powder propulsion from the former Soviet Union [39]. One pertinent study by Orbitec [40] should be in the public domain yet for reasons unknown it is inaccessible and unob- tainable. Heat flux generated by oxygen/aluminium metal powder combustion is very high and cooling is a major cause for concern. However, most historical aluminium metal powder propulsion tests were sufficient to attain steady state combus- tion but not long enough to require the inclusion of an active cooling mechanism. Therefore, there was insufficient informa- tion on cooling subsystems to critically analyse, hence the omission of this important topic. ACKNOWLEDGEMENTS The author(s) wish to thank Dr. Bryan Palaszewski (NASA Glenn Research Center), John Wickman (Wickman Spacecraft and Propulsion Company), Dr. Evgeny Shafirovich (University of Texas at El Paso), Dr. John Foote (NASA Marshall Space Flight Center) and Dr.Andrew Higgins (McGill University) for their useful insights, observations and assistance during the course of this study. 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