Accepted Manuscript
Title: Development and Testing of an Additively Manufactured
Monolithic Catalyst Bed for HTP Thruster Applications
Author: K. Essa H. Hassanin M.M. Attallah N.J. Adkins A.J.
Musker G.T. Roberts N. Tenev M. Smith
PII: S0926-860X(17)30217-X
DOI: http://dx.doi.org/doi:10.1016/j.apcata.2017.05.019
Reference: APCATA 16241
To appear in: Applied Catalysis A: General
Received date: 20-2-2017
Revised date: 18-5-2017
Accepted date: 19-5-2017
Please cite this article as: K. Essa, H. Hassanin, M.M. Attallah, N.J. Adkins, A.J.
Musker, G.T. Roberts, N. Tenev, M. Smith, Development and Testing of an Additively
Manufactured Monolithic Catalyst Bed for HTP Thruster Applications, Applied
Catalysis A, General (2017), http://dx.doi.org/10.1016/j.apcata.2017.05.019
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Accepted Manuscript
1
Development and Testing of an Additively Manufactured
Monolithic Catalyst Bed for HTP Thruster Applications
K. Essa
(1)
, H. Hassanin
(2,3)
, M.M. Attallah
(3)
, N.J. Adkins
(3)
, A.J. Musker
(4)
, G.T.
Roberts
(5)
, N.Tenev
(5)
, M. Smith
(6)
(1)
School of Mechanical Engineering, University of Birmingham, UK, Email: k.e.a.essa@bham.ac.uk
(2)
School of Mechanical and Aerospace Engineering, Kingston University, Email:
h.hassanin@kingston.ac.uk.
(3)
School of Metallurgy & Materials, University of Birmingham, UK
(4)
DELTACAT Ltd and University of Southampton, UK, Email: tony.musker@deltacatuk.com
(5)
Aerodynamics & Flight Mechanics Research Group, University of Southampton, UK, Email:
gtr@soton.ac.uk
(6)
TEC-MPC, ESA/ESTEC, NL, Email: Matthew.Smith@esa.int
Keywords:
additive manufacturing, 3-D printing, selective laser melting, hydrogen
peroxide catalysis, green space propulsion
Abstract
Additive manufacturing (AM), also known as 3D printing, is a revolutionary
manufacturing technology that has attracted many industries in the past two decades.
This is because AM enables the manufacturing of complex-shaped geometries
without the limitations of other manufacturing techniques. In this paper, the design,
development and testing of additively manufactured, monolithic catalyst beds are
described. A novel design methodology was employed and achieved catalyst bed
designs with complex geometry and high geometrical surface area whilst achieving
an acceptable pressure drop. Catalyst bed samples incorporating alumina ceramic
lattices with strut diameters ranging from 0.15 to 0.30 mm were fabricated via AM
and a subsequent heat treatment. The surface areas of the samples were improved
using different wash coats, including the use of gamma alumina and a mixture of
gamma alumina and carbon nanotubes (CNT). Manganese oxides were used to coat
the catalyst bed and decompose hydrogen peroxide. Four full-scale catalyst beds
with the most promising candidate geometries and wash coats were then
manufactured and subsequently tested in a 20 N-class HTP (High Test Peroxide)
monopropellant thruster. The firing results show that the additively manufactured
catalyst beds generally outperformed the baseline catalyst bed containing ceria
pellets that were also coated with manganese oxides.
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1. Introduction
Hydrazine and its derivatives, such as monomethylhydrazine (MMH) and
unsymmetrical dimethylhydrazine (UDMH), are the most commonly used propellants
for the propulsion and control of a wide range of spacecraft and satellites. They have
been the most favourable choice for aerospace applications for more than 50 years
as they show excellent flight performance. However, they are highly toxic and
carcinogenic. The use of these substances carries with it the burden of enhanced
health and safety protection for working personnel [1, 2]. Recently, low toxicity
(‘green’) storable liquid propellants have attracted a considerable amount of attention
as replacements for hydrazine based propellants. The movement towards the use of
green propellants is not only being driven by concerns regarding the toxicity of
hydrazine and its derivatives, but more importantly by the possibility of these
chemicals being banned [3, 4]. Hydrogen peroxide (H
2
O
2
) is a popular substance
used in many industrial applications such as food processing, cosmetics, and
wastewater treatment [5, 6]. High-test peroxide (HTP) is a highly concentrated
solution of hydrogen peroxide, with a concentration range from 85 % to 98 %. HTP is
considered to be a ‘green’ propellant because it only exhausts oxygen and water
upon catalytic or thermal decomposition. It is particularly attractive because of its
high density and low cost. HTP also promises considerable cost savings due to
simplifications in health and safety procedures during production, storage and
handling [7-9].
For an HTP thruster to work effectively the HTP must be decomposed catalytically to
produce superheated steam and oxygen, which can then be used either as the
exhaust stream in a monopropellant application, or as the oxidiser in a bi-propellant
application. In both applications, the thruster performance relies critically on the
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ability of the catalyst bed to decompose fully the HTP. The bed must be capable of
rapid and repeatable performance over the many operational cycles imposed by
typical mission profiles. A high-performing catalyst bed will offer a high surface area
(per unit volume of bed) and a low pressure drop, although these two requirements
are typically in conflict [10-12].
HTP catalyst beds often incorporate either metallic gauzes or screens (typically pure
silver, or silver-plated), or ceramic pellets coated with an active catalytic phase which
could be metallic or some type of metal oxide [13, 14]. To achieve the required
surface area per unit volume, the metal screens must by tightly-packed and therefore
usually exhibit relatively high pressure drops. Pellet-based beds usually have higher
surface areas and rather lower pressure drops, but the relative movement of the
pellets, caused by the vigorous decomposition of the peroxide, can cause
fragmentation and loss of pellets from the bed, possibly resulting in reduced lifetime
[15, 16]. An alternative to both the above types is a monolithic bed, which is
conventionally manufactured by extrusion of a ceramic paste through a die and then
coated with an active phase. However, the flow path for the propellant is through
straight channels which offer relatively low surface area so such beds are prone to a
phenomenon known as “flooding”, which quenches the decomposition reaction [17].
To prevent this, the bed loading (mass flow rate of propellant per unit cross-sectional
area) must either be kept very low, or the beds are made very long. This leads to
catalyst beds that are relatively large compared with beds that incorporate pellets or
metal screens, with attendant heat loss issues.
Additive manufacturing (AM) is a disruptive technology that is developing rapidly. It
promises to find widespread applications, especially in the aerospace industry.
Compared with conventional manufacturing techniques, the major benefits of AM are
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product design customisation for functionality, with reduced material wastage and
low energy usage, and reduced lead-time. However, perhaps the greatest benefit of
all is that AM enables design innovation, allowing the production of parts with
complex geometry that would be impossible to manufacture by conventional means
[18-21].
For these reasons, the European Space Agency (ESA) initiated a recent research
and development activity with the goal of enhancing HTP catalyst bed design
through the use of AM. A monolithic catalyst bed with a complex internal geometry
produced by AM could in principle overcome the limitations of the former types of
beds described earlier and is the subject of the study described in this paper. Here,
we describe the design and development of monolithic catalyst beds that are
manufactured using AM and tested in a relatively small (20 N class) monopropellant
thruster employing HTP. Computer Aided Design (CAD) and Computational Fluid
Dynamics (CFD) were employed for the optimisation of the catalyst bed geometry.
Alumina catalyst beds were fabricated using selective laser melting (SLM) and
subsequently coated using a manganese oxide catalytic layer. The performance of
the catalyst beds was initially evaluated by carrying out simple qualitative and semi-
quantitative tests on a range of small scale samples and then, after down-selection,
thruster firing tests on full-scale beds.
