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6TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS 2015)
Copyright 2015 by Sebastian Soller. Published by the EUCASS association with permission.
Development of Liquid Rocket Engine Injectors Using Additive
Manufacturing
S. Soller*, S. Beyer**, A. Dahlhaus**, A. Konrad**, J. Kretschmer**, N. Rackemann*, W. Zeiss **
*Airbus Safran Launchers GmbH, D-81663 Munich
** Airbus DS GmbH, D-81663 Munich
Abstract
Airbus Defence and Space pursues a comprehensive approach to apply additive manufacturing to liquid rocket
engine injectors. The research and technology activities performed so far address the entire manufacturing process,
from material properties and design concepts to non-destructive inspection technologies to allow for the adequate
quality assurance to be applied in later production. With a stepwise approach to demonstrate the maturity of the
processes, Airbus Defence and Space has developed the technology to a status where the further qualification and
application into flightworthy components can be performed.
Abbreviations & Acronyms
aka
also known as
GH2
Gaseous Hydrogen
ALM
Additive Layer Manufacturing
LOX
Liquid Oxygen
AM
Additive Manufacturing
NDI
Non-Destructive Inspection
cD
Discharge Coefficient
pc
Chamber Pressure
CFD
Computational Fluid Dynamics
SLM
Selective Laser Melting
CT
Computer Tomography
UTS
Ultimate Tensile Strength
DLR
German Aerospace Center
YS
Yield Strength
1 Introduction
From a design and manufacturing point of view, a liquid rocket engine's injector head is one of the most demanding
subsystems of a liquid rocket engine. The injector head is subjected to extreme temperature and pressure gradients:
During startup, the propellant manifolds cool down to cryogenic temperatures within seconds, whereas the injector
faceplate has to withstand the thermomechanical load of the combustion, which takes place at temperatures above
3000 K and a pressure level of up to 200 bar. In today's rocket engines, the injector comprises several hundred piece
parts, which are manufactured to highest accuracy from different materials and subsequently integrated. The
integration process is characterised by numerous manual working steps, inspections and checks and thus significantly
contributes to the overall manufacturing cost of a rocket engine.
Additive manufacturing technologies offer the potential to reduce the production cost by eliminating several
integration steps, when an injector head is manufactured from two or three piece parts instead of several hundreds.
Additionally, additive manufacturing allows to optimize the design of the injector's housing according to reduce the
weight of the component. Typically, this part is milled from a cast raw part and the manufacturing steps to minimize
the wall thickness or locally remove material where it is not needed are skipped, as the potential weight savings
cannot balance the additional cost of manufacturing. On top of that, the material properties of parts produced in
additive manufacturing are known to be superior compared to cast parts, which additionally can be used to reduce the
weight of the parts.
Worldwide, the manufacturers of aerospace propulsion systems are developing additive manufacturing processes not
only for comparatively simple parts like brackets or fasteners, but also for more delicate and safety-relevant parts like
turbine blades or injectors for combustion chambers. SpaceX and Avio have proposed alternative designs for small
thrust chambers [1], [2]. Airbus Defence and Space is currently developing components of propulsion systems
produced with additive manufacturing, like valve housings, small thrusters or injectors of liquid rocket engines [3],
[4]. Likewise, Snecma is investigating the benefit and limitations of this manufacturing technology [5].
EUCASS-2015-031
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1.1 Processes to be developed and analyses to be performed for liquid rocket
injectors in AM
When developing a new manufacturing technology like additive manufacturing towards application in an existing
product, a number of tasks have to be addressed, which stretch well beyond the individual manufacturing step of
printing a part from a metallic powder bed. Figure 1 illustrates the sequence of different steps from powder
production up to surface finishing and non-destructive inspection of 3D-printed parts.
Figure 1: Manufacturing Process Chain in additive manufacturing from powder to finished product
The first fundamental decision to be taken is the selection of the most suitable alloy for the manufacturing process.
To be able to select the correct powder basis, the requirements which the later finished product has to meet have to
be mirrored against the material properties. Therefore, comprehensive material characterisation programmes are
needed to generate a sound data base. Airbus Defence and Space focusses on three different alloys which may be of
interest for the application in liquid rocket engine injectors and small thrusters: The Ni-based superalloy Inconel 718,
stainless steel 316 L and CoCr for high temperature and high-strength requirements. All these materials need to be
characterised not only with respect to -typically anisotropic- material parameters like yield and ultimate strength, but
also with respect to their material compatibility, e.g. hydrogen embrittlement.
The design of the parts to be manufactured also needs to take into account the constraints and specific requirements
of the build-up process. For example, the manufacturing parameters need to be adapted to the local geometry, and
vice versa, and the design has to take into account the limits of the manufacturing process with respect to wall
roughness or porosity. Another aspect to be taken into account during the development of an injector using additive
manufacturing is the increased wall roughness of the surfaces. The effect of this increased wall roughness needs to be
investigated with respect to the turbulence of the flow field and the overall injector pressured drop. If there are very
stringent requirements for the surface finish in terms of roughness or porosity, the correct treatment process has to be
selected from a variety of possible processes.
1.2 Injector Development Logic
Developing a new liquid rocket engine injector with additive manufacturing is not such a straightforward process as
it may seem from first sight. In the beginning, a design concept needs to be established, which minimises the use of
supporting structures and takes into account the geometrical limitations given by the powder-based process. For
example, there is a certain maximum angle up to which overhanging areas can be built without using supporting
structures, which would have to be removed by turning or milling afterwards.
Transferring the design of an existing liquid rocket injector to an additive manufactured design thus first starts with
design studies to take into account these manufacturing constraints (see Figure 2 for illustration). From these studies,
a baseline design is selected. Single element injector flow checks are used to characterise the injector's hydraulic
behaviour and to identify how accurately and reproducible the functional geometry like orifices or small-sized annuli
can be manufactured. The flow checks additionally give a first evidence of the mechanical stability of the injector
with respect to potential attack due to cavitation.
Once the single injector elements have confirmed the design parameters of the individual injector, subscale
demonstrators are manufactured and subjected to hot fire tests. Here, the injector is subjected to the full life cycle of
thermomechanical loads like in the later real application. The last step is the manufacturing of a full scale injector,
which is again flow checked and inspected before being operated in an full scale setup. The entire process is linked
to ongoing development programmes in order to provide the newly developed technology in time and according to
the later operating requirements.
Powder production
3D printing
Surface finish
Machining of functional surfaces
Non-destructive inspection & test
Soller et al.: DEVELOPMENT OF LIQUID ROCKET ENGINE INJECTORS USING ADDITIVE MANUFACTURING
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Figure 2: Additive manufacturing injector development logic
2 Development & Demonstration Activities
The following section gives more details on the individual development and verification steps which have been
described before.
2.1 Material Characterisation
The data base for the later design of lightweight liquid rocket injectors was established during a material
characterisation program, in which specimens were tensile tested in inert as well as in hydrogen atmosphere at
different temperature levels. Figure 3 exemplarily shows results of this characterisation programme. The samples
have been precharged in hydrogen atmosphere and then tensile tested at different outgassing levels.
Compared to samples made from cast materials the levels of tensile strength and ultimate yield strength are
approximately 20% higher for additive manufactured samples, giving additional potential for weight savings in the
later design. In hydrogen atmosphere, the typical reduction of ultimate tensile strength can be seen clearly for the cast
samples. Again, the material behaviour here is better for additively manufactured samples when compared to cast
samples: The reduction in ultimate tensile strength is less pronounced for the SLM sample. Similarly, the elongation
at rupture also gives evidence of the fact that additively manufactured samples are less susceptible to hydrogen
embrittlement.
Figure 3: Results for yield and ultimate tensile strength testing of Inconel 718 made from additive manufacturing
("SLM") compared to cast samples with different hydrogen precharging levels
unloaded
loaded outgassed 8h
loaded outgassed 3h
loaded
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
UTS SLM
YS SLM
UTS cast
YS cast
SLM : knock-down 0.95 on UTS
cast : knock-down 0.88 on UTS
no knock-down on YS
Injector design concept
Single element flowcheck
Subscale hot fire test
Full scale design,
manufacturing and test
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2.2 Single element characterisation
As first step to transfer the design of existing liquid rocket engine injectors to a design optimized for additive
manufacturing, several samples of individual injectors were manufactured and cold flow checked. The flow checks
used water and gaseous nitrogen as substitute fluid for the propellants. The back pressure was set to 50 bar to be
representative of an envisaged thrust chamber application. The mass flow rate was set such that the Mach and
Reynolds number were comparable to the expected hot fire operating conditions.
The single element flow checks demonstrated not only the feasibility of manufacturing different designs of the
individual injectors, but also provided a data base of manufacturing margin which has to be provided to account for
thermally induced shrinkage during the manufacturing process. This margin has to be set accurately in order to avoid
additional manufacturing steps and cost. The flow checks also provided information on the mechanical stability of
the injectors against any potential abrasive effect of local cavitation: Some injectors were tested at cavitating
conditions and the effect of cavitation on the discharge coefficient was measured.
The scatter in pressure drop data was used to assess the possibility of avoiding any additional post-treatment like
turning, abrasive flow machining or chemical polishing, which may be used to improve the surface finish. Data
recorded in numerous tests justifies the obsolescence of any of these surface finishing processes for liquid rocket
injectors. With a bench reproducibility of 0.2%, the overall scatter in the calculated discharge coefficient was below
3% for the fuel injectors and below 4% for the oxygen injectors. No correlation was found with respect to the
location at which the injectors had been built up in the powder bed (see Figure 4). Having investigated additional
parameters like the repeatability of manufacturing accuracy from batch to batch, a first setup was designed for hot-
fire testing.
Figure 4: Discharge coefficient for additively manufactured injectors from one batch
2.3 Subscale Testing
For reasons of risk mitigation, a subscale injector cartridge was manufactured and hot-fire tested in nominal
operating conditions. The tests were performed at DLR's research and development test facility P8 in
Lampoldshausen. Liquid oxygen (LOX) and gaseous hydrogen (GH2) were used as propellants. The experimental
setup allowed switching to a classically manufactured injector configuration in order to achieve a reliable data base
for comparing the injector's performance data with the classical design and manufacturing route. The tests were
performed at combustion chamber pressures ranging from 35 bar to 81 bar and a propellant mixture ratio ranging
from 4.4 to 7.1. The test data was analysed with respect to several objectives:
− Pressure drop characteristic
− Heat flux evolution
− Combustion efficiency
The recorded data shows that the different manufacturing route of the injector has no impact on the heat transfer
profile or on the combustion efficiency. The pressure drop is set by accurately defining the size of orifices taking into
account the different surface roughness of ALM parts.
Discharge coefficient cD LOX [-]
Mass flow rate m LO X [kg/s]
∆c
D LOX
< 4%
Soller et al.: DEVELOPMENT OF LIQUID ROCKET ENGINE INJECTORS USING ADDITIVE MANUFACTURING
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Having successfully demonstrated the capability to manufacture and operate the delicate injection system based on
additive manufacturing, the next step in the development increased the level of complexity. In the follow-on test
campaign, in integral injector head was hot fire tested. This injector, illustrated in Figure 5, also included the
manifolds for fuel and oxidiser and was used to demonstrate the capability to design and manufacture more complex
geometries. Different from the insert that was tested during the first test campaign, this setup was manufactured from
stainless steel 1.4401 (aka 316L). The number of individual piece parts was reduced in this design from 80 to 3.
Having hot fire tested this part successfully; sufficient experience has been gained for both materials, Inconel 718
and stainless steel 316 L to transfer the know-how to full-scale injectors for different applications.
Figure 5: Subscale integral injector manufactured from stainless steel 316L
2.4 Full Scale Demonstrators
With the know-how built up during the subscale test activities, two potential applications were selected to
demonstrate the benefits of additive manufacturing for liquid rocket engines, an injector head for an expander cycle
engine and a gas generator setup.
For an engine configuration like Vinci, a design study resulted in a potential reduction in weight of 3 kg for the LOX
manifold alone, which equals a reduction in weight by roughly 25%. Figure 6 shows the optimised design. With
additive manufacturing, locally optimised stiffening structures on a thin wall can be manufactured far more cost
efficient than with classical manufacturing processes and thus become economically interesting.
Applying additive manufacturing to the entire injector head can significantly reduce the manufacturing cost due to
the fact that the effort for manufacturing, inspecting and installing the fuel injector sleeves can be reduced when
using an integral design approach. Figure 8 shows the full scale injector insert manufactured for a Vinci-like engine
demonstrator. This is currently being prepared for material inspection and flow check tests, before being used to
extract material samples and weld samples. A second piece part will be manufactured and prepared for hot fire tests.
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Figure 6: Design study for weight-optimised LOX dome
Figure 7: Full scale manufacturing demonstrator for expander engine application
2.5 Development of suitable non-destructive inspection technologies
Compared to parts made from forged or cast raw material via milling, turning and electronic discharge machining,
additive manufacturing requires a higher level of inspection in order to detect any potential material flaws like pores
or areas of insufficient bonding, which may have a deteriorating effect on the material properties. A 100% inspection
of such parts is desirable, but not fully feasible. For example, computertomographic (CT) inspection can only
provide information on defects down to the size of the minimum
resolution of the CT scanner's sensors. Additionally, local gradients in
wall thickness complicates CT inspections and may result in
"overexposed" or "underexposed" images with a poor signal-to-noise
ratio and blurred edges. Figure 8 shows a computertomography image
of the integral injector head which was presented in Figure 5. As of
today, post processing of the data allows to detect material flaws down
to 0.1 mm in size, which is sufficient to verify that cleaning procedures
to remove remnants of powder have been performed successfully and to
compare the actually manufactured geometry or orifices or flow
passages to the design values. Nevertheless, further advancement in the
computertomopgraphy technology is needed to allow a detection of
potential failures. Together with Fraunhofer Institute in Fürth and
Airbus Group Innovation Airbus Defence and Space works on the
improvement of NDI technologies and elaborates a catalogue of
defects, based on which the criticality of potentially undetected material
defect can be assessed.
Figure 8: Comnputertomographic
inspection of multi-element injector
Soller et al.: DEVELOPMENT OF LIQUID ROCKET ENGINE INJECTORS USING ADDITIVE MANUFACTURING
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2.6 Update of hydraulic and CFD modelling for additive manufacturing
In parallel to printing and testing of injector setups with additive manufacturing, reference test specimens have been
built to characterise the influence of the increased wall roughness and the specific geometry on the hydraulic
characteristics of the injectors. Compared to classically built injectors, the wall roughness of the additively
manufactured parts contributes more significantly to the pressure drop of an injector. In order to accurately predict
the pressure drop of liquid rocket injector setups made from additive manufacturing, the models to describe the
pressure drop caused by the combination of successive area reduction and increase, flow redirection and roughness
were adapted. Such, an accurate prediction of the hydraulic behaviour of the setup is ensured. Figure 9 shows several
samples, which were built to assess the effect of wall roughness and or orifice area ratio on the overall pressure drop.
Dedicated flow checks have been performed to provide a sound data base for the accurate modelling of the injector
hydraulics. With this updated data base, the pressure drop behaviour of the hot fire tested components could be
predicted with high accuracy (see Figure 10).
Figure 9: Samples to characterise wall roughness
Figure 10: 3D-CFD simulation of flow-field in LOX injector
3 Conclusion
Airbus Defence and Space pursues a comprehensive approach to apply additive manufacturing to liquid rocket
engine injectors. The research and technology activities performed so far address the entire manufacturing process,
from material properties and design concepts to non-destructive inspection technologies to allow for the adequate
quality assurance to be applied in later production. With a stepwise approach to demonstrate the maturity of the
processes, Airbus Defence and Space has matured the technology to a status where further qualification and
application into flightworthy components can be performed Full scale demonstrator hardware is being built and will
be tested in late 2015 and early 2016, respectively.
EUCASS-2015-031
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4 Acknowledgements
Part of this work was performed within the National technology program TARES. This program is sponsored by the
German Space Agency, DLR Bonn, under contract No. 50RL1210. ESA supports the activities to design and
manufacture liquid rocket engine injectors within its future launcher preparatory programme FLPP3 executed under
ESA contract 40000109198/13/F/MT. The authors would like to thank all colleagues for their contribution to the
achievements presented. The support of P8's bench operations team during the test campaigns is highly appreciated.
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