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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.
JBIS, Vol. 65, pp.61-70, 2012
1. INTRODUCTION: JUSTIFICATION
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-
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
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/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 . Prior to that, metal powder was advocated as a fuel to
propel underwater submersibles  and then as a high-energy
density fuel for propelling missiles . 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
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
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% . 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 . 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 . 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 . 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-
The vast majority of metal powder propulsion researchers
however did not address packing density and focussed on an
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
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 ,
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
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
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 . 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  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 , 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
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. , 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 . 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. . 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  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.
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 , Fig. 5. Foote, Lineberry et al. on the
other hand added 15% of the oxidiser upstream to enhance
circulation of the combustion products . 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  were very similar to a
schematic initially proposed by Foote and Litchford presented
a year earlier . Both projects dealt with CO2
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 . 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.
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.  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.
, 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 .”
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.
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 .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 . 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. ; 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
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 . 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
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.
Goroshin, Bidabadi et al. , Goroshin, Fomenko et al., Foote,
Lineberry et al., Goroshin, Higgins et al. , Foote,Thompson et
al.  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
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.
As for the combustor design, the schematic should be presented
in a way that the L* is large enough to allow sufficient resi-
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
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.
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. )
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
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
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.
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
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
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 .
One pertinent study by Orbitec  should be in the public
domain yet for reasons unknown it is inaccessible and unob-
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.
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.
Fig. 10 Dense phase transport expulsion apparatus.
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* * *
(Received 3 February 2012)