Liquid Rocket Engine Design for Additive Manufacturing

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Liquid Rocket Engine Design for Additive Manufacturing
Conference Paper · October 2018
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69th
International Astronautical Congress (IAC), Bremen, Germany, 1-5 October 2018.
Copyright ©2018 by the International Astronautical Federation (IAF). All rights reserved.
IAC-18-F1.2.3 Page 1 of 17
IAC-18,C4,5,10,x45214
Liquid Rocket Engine Design for Additive Manufacturing
Jan Fessla
*, Hanmo Shenb
, Nihar Patelb
, Tai Wei Chena
, Suyash Ghirnikarb
, Martin Van Den Berghec
a
Department of Astronautical Engineering, University of Southern California, 854 Downey Way, Los Angeles,
California, 90089, fessl@usc.edu
b
Department of Aerospace and Mechanical Engineering, University of Southern California, 854 Downey Way, Los
Angeles, California, 90089,
c
Department of Earth Sciences, University of Southern California, 3651 Trousdale pkwy, Los Angeles, California,
90089.
* Corresponding Author
Abstract
This paper discusses the processes involved in the additive manufacturing of a regenerative and film-cooled
liquid rocket engine with a thrust of 10 kN using Inconel 718, while detailing validation techniques. A
description of the objectives and design constraints provide the context and motivations. Computational Fluid
Dynamics (CFD) models were developed and provided the expected pressure and thermal regimes under
regenerative and film cooling. Additionally, Finite Element (FE) models were used to predict the capabilities
of the engine structure. A description of 3D printing methods highlights the benefits and limitations of the
technology, specifically the influence the design of liquid rocket engines. A pintle injector is used, printed as a
separate, easily removable and replaceable component. Issues related to overhangs, surface roughness, and
shrinkage; aspects related to post-print processing and the need to minimize machining are discussed. Results
from the CT scans of the engine and its components are presented. The paper also outlines the series of tests
that will be performed on this engine to verify its performance and provide design basis for future works This
engine will be used to power the reusable flight vehicle that is under development at the Kyushu Institute of
Technology in Japan. The student-led Liquid Propulsion Laboratory at the University of Southern California
is responsible for the work detailed below.
Keywords: Liquid Propulsion, Additive Manufacturing, Cooling Channels, Film Cooling, Pintle
Nomenclature
h = Heat Convection Coefficient
Cp = Specific Heat
μ = Dynamic Viscosity
ρ = Density
c*
= Characteristic Velocity
𝑚̇ = Mass Flow Rate
λ = Vaporization Heat
Pr = Prandtl Number
Re = Reynolds Number
Nu = Nusselt Number
T = Temperature
P = Pressure
R = Chamber Radius
D = Chamber Diameter
A = Chamber Cross-section Area
p = Chamber Circumference
S = Area Covered Gaseous Cooling Film
x = Distance Downstream the Coolant Injection
E = Young's Modulus
α = Thermal Expansion Coefficient
ν = Poisson's Ratio
σ = Stress
Subscripts
aw = Adiabatic Wall Condition
hw = Hot-side Wall Condition
cw = Cold-side Wall Condition
co = Coolant Property
c = Property of Combustion Gas
0 = Stagnation Condition
l = Property of Liquid Coolant
g = Property of Gaseous Coolant
s = Saturation Condition
I = Initial Condition
t = Throat Condition
i = Inner Side Chamber Geometry
o = Outer Side Chamber Geometry
eff = Effective Parameter
Θ = Tangential Component
r = Radial Component
x = Axial Component
Acronyms/Abbreviations
AM Additive Manufacturing
CA Center Annulus
CAD Computer Aided Drawings
CFD Computational Fluid Dynamics
LOX Liquid Oxygen

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IAC-18-F1.2.3 Page 2 of 17
REPDA Rocket Preliminary Design and Analysis
TMR Total Momentum Ratio
WFTS Water Flow Test Stand
WIRES Winged Reusable Sounding Rocket Vehicle
1. Motivation
This rocket engine, codenamed Balerion (Fig. 1),
was designed by the Liquid Propulsion Laboratory
(LPL) at the University of Southern California to meet
the requirements of the reusable flight vehicle called the
Winged Reusable Sounding Rocket #13 (WIRES #13)
(Fig.2), currently under development at the Kyushu
Institute of Technology by the Kyutech Space Club. To
meet the requirements, two engines with a total thrust
capacity of 20 kN and a burn time of 25 seconds have
been proposed. LPL is responsible for the development
of the propulsion systems which includes the engines
and propellant feed lines.
This paper will focus on the analysis, design,
manufacturing, and testing of the engine. Attention and
deliberation were focused on the use of additive
manufacturing to minimize the number of components
and gain a deeper understanding of the technology that
is sweeping the industry.
Table 1. Engine parameters
Propellants LOX/Kerosene
Chamber Pressure 375 psi
O/F ratio 1.5
Chamber Temperature 2464 K
Thrust 10 kN
Specific Impulse 241 sec
Total Mass Flow Rate 4.25 kg/s
Fig. 1. CAD model of Balerion engine
2. Theoretical Background
In the preliminary design and 1-Dimensional (1D)
analysis phase of Balerion, an in-house Rocket
Preliminary Design and Analysis (REPDA) MATLAB
code was used to determine all aspects of the thrust
chamber and nozzle design.
Fig. 2 CAD model of WIRES #13
2.1 Propellant and Material Selection
During the conceptual design phase, a propellant
trade study was conducted on two fuel types – Unleaded
kerosene (Jet-A) and isopropyl alcohol (IPA).
Ultimately, Jet-A was selected mainly because it has
more chemical and fluid properties readily available.
Due to the engine utilizing additive manufacturing
material selection options were very limited. Materials
that are available and applicable for a rocket engine
include high-strength nickel-base superalloy Inconel
718 and aluminium alloys, in particular AlSi10Mg. The
material properties of interest were thermal conductivity
for cooling performance and yield stress for structural
integrity. The comparison is shown in Table 2.
Table 2. Comparison of AlSi10Mg and Inconel 718
Property AlSi10Mg Inconel 718
Thermal Conductivity 173 W/(mK) 11 W/(mK)
Yield Stress 228 MPa 1241 MPa
The deciding factors between the two were the low
yield stress of AlSi10Mg and the higher availability of
Inconel 718 for printing. It was determined that
AlSi10Mg was not a viable option when it came to
withstanding the operational pressures during testing
and lacked detailed data sheets for its temperature
capabilities. However, Inconel 718 has more readily
available information regarding its properties. With
slight material modification (heat treatment) can handle
the required operational conditions of the engine. Table
3 shows the properties of Inconel 718 after undergoing
AMS 5662 heat treatment.
Table 3. AMS 5662 properties of Inconel 718. Physical
properties are at 992 K
Melting Point
Heat-treated
1571 K
1650 K
Thermal Conductivity 22 W/m/K
Yield Stress 1010±50 MPa
Young’s Modulus 2.25×105
MPa
Thermal Expansion Coefficient 1.51×10-5
K-1
Poisson Ratio 0.283
2.2 Engine Sizing using REPDA Software
For propellant analysis, the relationship between
O/F ratios at different chamber pressures and chamber

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IAC-18-F1.2.3 Page 3 of 17
temperature or specific impulse was given by REPDA.
REPDA was specifically developed to size a rocket
engine chamber and nozzle based on its thermal
performance requirements. It builds upon the freely
available NASA Chemical Equilibrium with
Applications (CEA) software, and the NIST Reference
Fluid Thermodynamic and Transport Properties
(REFPROP) database. Examples of the REPDA outputs
include chamber pressure, propellant mixing ratio,
thrust chamber geometry, cooling channel geometry,
along with many others.
2.3 Thermal Analysis
To determine the optimal thermal properties of the
engine, various iterations of differing chamber
pressures, O/F ratios, and propellant mixtures were
used. REPDA made these iterations quick and easy due
to its user-friendly GUI system as seen in Figure 3.
Fig. 3. Showing Propellants Input to REPDA
2.3.1 Regenerative Cooling
Using REPDA, inputting the desired rib thickness,
hot-wall thickness, minimum channel width, and
channel depth provides the required number of channels
needed to keep the temperature under the materials
melting point and thermal stress below the materials
yielding point. Additionally, printing surface roughness
can be inputted to determine effects on cooling
performance. This can be seen in Figure 4. The
governing equation used in this 1D analysis is the Bartz
equation (1).
ℎℎ𝑤 = 0.026
𝐶𝑝 𝑐
𝑃𝑟𝑐
0.6 (
𝜇 𝑐
𝐷𝑡
)
0.2
(
𝑃𝑐,0
𝑐∗ )
0.8
(
𝐴 𝑡
𝐴
)
0.9
𝑆 (1)
Fig. 4. Input in REPDA for regenerative cooling
calculation
While seeking the optimal design point of the
regenerative cooling system, the convection coefficient
correlation specifically dedicated to RP-1 at
supercritical pressure [1] was embedded into the
REPDA subroutines and is referenced as Eq. (2). In
terms of the thermophysical properties RP-1, a
believable data source from NIST [2] was also merged
into a spreadsheet accessible through the code.
Nu 𝑐𝑜 = 0.012 𝑅𝑒 𝑐𝑜
0.879
𝑃𝑟𝑐𝑜
0.4
(1+
2Dℎ
𝐿
) (2)
The temperature profiles calculated and shown by
REPDA inherently contain ±20% of uncertainty
introduced by the Nu number correlation which
suggested another cooling method be used to assist
regenerative cooling and help promote full engine
reusability. This introduced the need for film-cooling to
the engine discussed in Section 2.3.2
2.3.2 Film Cooling
To model the liquid film-cooling and start the
parametric analysis, a film-cooling model developed by
Stechman [3] was implemented. This model was
determined to be applicable solely for small rocket
engine designs. Both liquid and gaseous phases of the
film-cooling were taken into consideration. All the
equations were based on the following assumptions.
First, the gaseous coolant film exhibits no mixing or
chemical reaction with the main combustion fluids.
Second, the coolant film temperature profile does not
change rapidly as it moves downstream. Third, the
gradients across the coolant film are small. Finally, heat
transfer to the wall is negligible compared to the heat
transfer from the main combustion fluids. The length of
the liquid-cooled region was calculated by Eq. (3), and
the heat convection coefficient for the liquid phase was
calculated by Eq. (4).
𝐿 =
𝜂𝑚̇ 𝑙 𝐶𝑝 𝑙(𝑇𝑠−𝑇 𝐼)
𝑝ℎℎ𝑤(𝑇 𝑎𝑤−𝑇𝑠)
+
𝜂𝑚̇ 𝑙 𝜆
𝑝ℎℎ𝑤(𝑇 𝑎𝑤−𝑇𝑠)
(3)
ℎ𝑙 = 0.0288
𝐶𝑝 𝑙
𝑃𝑟𝑙
0.667
𝜇 𝑙
0.2
𝑥0.2 (
𝜂𝑚̇ 𝑙 𝜌 𝑙 𝑢 𝑐ℎℎ𝑤 𝑃𝑟𝑐
0.667
𝜋𝑅𝐶𝑝 𝑐
)
0.4
(4)
As for the gaseous film cooling, effective gas
temperature Teff was introduced as an alternative to the
adiabatic wall temperature. This temperature was
calculated according to Eq. (5). The gaseous convection
coefficient, as mentioned earlier, was concluded from
Bartz equation, see Eq. (1). The film cooling inputs for
REPDA are shown in Fig. 5.
𝑇 𝑎𝑤−𝑇 𝑒𝑓𝑓
𝑇 𝑎𝑤−𝑇𝑠
= 𝑒𝑥𝑝 (
−ℎℎ𝑤 𝑆
𝜙𝐶𝑝 𝑔 𝑚̇ 𝑔
) (5)

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IAC-18-F1.2.3 Page 4 of 17
Fig. 5, REPDA inputs for film cooling
2.3.3 Thermal Stresses
The last consideration of 1D analysis is running a
thermal stress load study on the thrust chamber, which
is caused by the temperature difference between the
inner and outer surfaces of the wall. The thermal stress
on the structure was modelled as a hollow cylinder
being heated up from inside. There were three stress
aspects considered: tangential, radial and axial. These
formed the effective thermal stress model according to
the Von-Mises theory. The calculation was executed
using Eq. (6).
𝜎𝑣 = √𝜎 𝜃
2
+ 𝜎𝑟
2 + 𝜎𝑥
2 − (𝜎 𝜃 𝜎𝑟 + 𝜎 𝜃 𝜎𝑥 + 𝜎𝑟 𝜎𝑥) (6)
2.4 Injector
An injector is one of the key components in a liquid
rocket engine as it determines how the propellants are
delivered into the combustion chamber as well as their
mixing and combustion efficiency. Among the various
types of rocket engine injectors that exist, the Balerion
engine uses the pintle as it has inherently strong
combustion stability, reliability, design simplicity, and
ease of manufacturing compared to the traditional
showerhead and coaxial injectors. The pintle injector
delivers one propellant radially through the pintle and
the other propellant axially in the shape of an annular
sheet through the injector body to impinge and form a
uniform conical stream. Due to this spray cone
formation, a recirculation zone is created right below
the pintle tip, which acts as a mixer for unburnt
propellant droplets, further improving the combustion
efficiency and stability of the engine [4].
2.4.1 Key Parameters
The most important parameter for an injection
system is the total momentum ratio (TMR), which is
defined as the momentum ratio of the radial-to-axial
propellant stream, seen in the following equation:
𝑇𝑀𝑅 =
(𝑚̇ 𝑈) 𝑟
(𝑚̇ 𝑈) 𝑧
(7)
It has been shown that pintle injectors with a TMR
close to 1 provide the most optimal performance.
Another key variable, known as blockage factor, is
defined as the ratio of total hole diameters to the pintle
circumference, with the equation as:
𝐵𝐹 =
𝑁𝑑0
𝜋𝑑 𝑝
(8)
where N is the number of orifices at pintle tip, d0 is the
orifice diameter, and dp is the pintle diameter. This
value pairs with TMR, orifice size, and pintle geometry
to affect the propellant mixing efficiency [5].
In addition, some recommended values of the pintle
injector dimensions are given. For instance, the ratio of
chamber to pintle diameter is recommended to be
between 3 and 5, while the skip distance, defined as the
axial distance between the annular ring exit and the
radial hole, is suggested to be a 1 to 1 ratio with the
pintle diameter [5].
2.4.2 Pressure Drop Consideration
Accurately predicting the pressure drop across the
injector is crucial for the engine to perform nominally. It
is recommended to have a pressure drop of at least 20%
of the chamber pressure on both propellant sides in
order to neutralize the possibility of chamber pressure
oscillations. This would create combustion instability
that can build up and severely damage the hardware if it
were to backflow through the injector [6]. On the other
hand, if the pressure drop is too high, it can place extra
design limitations on the propellant tanks since the tanks
need to be sized based on the internal pressure
requirements. The higher the pressure requirement, the
harder the tank will be to design and manufacture.
Additionally, this would cause issues for the feed
system that delivers the propellants because it is
specifically sized to maximize performance while
minimizing weight.
2.4.3 Injector Sizing
Normally, when sizing a traditional injector, the
discharge coefficient (Cd) value of the orifice type will
be used along with the following equation to determine
total orifice area at a given mass flow rate, propellant
density and desired pressure drop:
𝐶 𝑑 =
𝑚̇
𝜌𝑉̇
=
𝑚̇
𝜌𝐴𝑢
=
𝑚̇
𝜌𝐴√
2∆𝑝
𝜌
=
𝑚̇
𝜌𝐴√2𝜌∆𝑝
(9)
However, since the typical Cd value of the pintle
injector was not found in the initial sizing phase, a
different model, mainly utilizing Bernoulli's principle,
was developed to size the injector.
For an injector to perform optimally, the TMR value
needs to be as close to 1 as possible, which depends
heavily on the propellant outlet velocities since the mass
flow rates are fixed from initial engine sizing.
Therefore, a MATLAB code separate from REPDA was
developed to vary the number and size of the orifices at

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IAC-18-F1.2.3 Page 5 of 17
the pintle tip to alter the central propellant velocity
flowing out from the orifices. Total orifice area at the
pintle tip was taken into account by using Bernoulli’s
equation (10) in an attempt to find the pressure drop
across the orifices. Assuming the elevation is negligible,
the pressure difference can be found from the change in
area when flow exits from the orifice into the chamber.
After that, the fuel outlet velocity can be found from the
TMR equation (7) at a value close to 1, which
determines the fuel side annular gap thickness as well as
its pressure drop.
𝑣2
2
+ 𝑔𝑧 +
𝑝
𝜌
= 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (10)
3. Design
This section covers the basis upon which the engine
was initially sized using results generated by REPDA
and how these numbers were implemented into creating
CAD model.
3.1.1 Printing and Manufacturing Constraints
The major limitation to any design is its
manufacturability. With additive manufacturing, the
driving constraints were established by the volume of
the printing instruments; the total working volume is
250×250×325 mm3
(9.85×9.85×12.8 in3
). Other
prominent design constraints would be material
availability, minimal wall thicknesses (0.45 mm, 0.0177
in) and overhangs in the printing direction (≥ 45°) and
critical considerations in design geometries.
3.2 Input Parameters
The input for initial dimensions of the engine such
as dimensions of regenerative cooling channels, outputs
generated by REPDA were used.
3.2.1 Results using REPDA Software
Fig. 6 indicates that the specific impulse of
LOX/Jet-A propellant pair hits the theoretical maximal
value at O/F ratio around 2.2, at which the chamber
temperature also raises up to 3400 K. Given the
potential cooling challenge on the thrust chamber,
particularly with the low thermal conductivity of
Inconel 718, an O/F ratio of 2.2 was not considered
feasible, and eventually an O/F ratio of 1.5 was
determined to be an optimal trade-off between specific
impulse and chamber temperature. Chamber pressure
selection was chosen according to the supply pressure of
the propellant tanks inside the WIRES#013 vehicle, to
be 375 psia.
Fig. 6. REPDA results showing relation between O/F
ratio and pressure to Specific impulse and combustion
temperature.
3.2.2 Injector Sizing
Considering cooling systems as well as overall
integration purposes, it was decided early in the design
stages to have Jet-A be the regenerative coolant entering
the chamber down the annular ring, and LOX flow
through the pintle, entering the chamber radially from
the pintle tip. It was however discovered through CFD
analyses during the iterative design process that the
LOX pintle outlet geometry, while maintaining a TMR
of 1, would not meet the required pressure drop of 20%
greater than chamber pressure. As a result, an additional
“throat” feature was added at the inlet of the pintle with
the purpose of making the pressure regime in the LOX
system meet requirements.
This pintle pressure drop, as modelled in 3D
simulations, was later proven to be inaccurate compared
to empirical testing, underestimated the added effects of
possible frictional loss and energy dissipation during
fluid flow. While 3D modeling can take into account
such parameters, quantifying and inputting them into
such models is challenging as some of these effects
result from the printing process, and are generally still
poorly constrained. Nevertheless, CFD can still help
size unknown orifice geometries in a design iteration
process.
3.3 Balerion Development Engine Model
Fig. 7 is the final model of the Balerion
Development Engine designed using Siemens NX
Unigraphics. It stands at 448.31 mm (17.65 in) tall with
a max diameter of 218.44 mm (8.30 in) at the flange.

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IAC-18-F1.2.3 Page 6 of 17
Fig. 7. Balerion Development Engine Model
In the initial concept design, the goal was to
minimize the number of components to reduce
complexity, thus improving reliability. This prompted
us to have only three components in total, two for the
injection system and one main engine body. However,
early on it was discovered that most printers under
service are restricted by printing volume, rendering the
unibody engine architecture unattainable. This led to the
decision to split the main body into two components and
join them with flange. One component became the
upper chamber body, while the second component
became the nozzle with the convergent and divergent
sections.
The two remaining components form the pintle
injection system. The pintle was designed to be
interchangeable so that a number of orifice
configurations, skip distances and pressure drops can be
tested during cold flows to select the most optimal
empirical based design model for engine performance.
The center annulus serves as a connecting adaptor for
the pintle and the chamber. Once inserted into the
chamber body, it forms an outer annular ring for the fuel
to be discharged into the chamber to form a propellant
mixture after it travels through the regenerative
channels. The pintle works on the concept of radial
injection while the outer annulus works on an axial
injection basis. The interaction of an axial fuel stream
combined with a radial outlet creates a conical spray
pattern.
More design changes were made based on analysis,
manufacturability considerations, legacy engine data,
and available resources. For more information on the
specific design features of the engine, refer to Section
3.4.
3.4 Design Features
As previously mentioned, many different
considerations were taken before reaching the model
shown in Fig. 7. The most critical considerations are
expounded upon in the following subsections.
3.4.1 Regenerative Cooling Channels
For the regenerative channels, the driving decision
for the geometry was based on the limitations of the
printer (For more information on the printer, refer to
Section 5.1.1.). The fluid acting as the coolant for
Balerion is Jet-A.
The channels are rectangular slots (shown in Fig. 8)
that run from the divergent section of the nozzle up into
the injector. The preliminary wall thickness of the
chamber separating the combustion gases and the
flowing coolant was 1 mm but was altered due to data
gathered on printing capabilities and referencing
existing printed components. The minimum wall
thickness is extremely case sensitive, and for Balerion,
the smallest possibility was around 0.3-0.4 mm.
However, to ensure that the print does not fail, 0.6 mm
was the decided minimum wall thickness.
REPDA was used to iterate the channel ribs and the
thickness ended up decreasing from 1 mm to 0.6 mm.
This automatically increased the number of channels
based on the engine cross-section which allows for
better cooling of the engine body. The number of
channels increased from 124 to 150. With a constant rib
thickness, the channel size varies while traveling axially
up the engine cross-section, thereby providing the
highest cooling at the throat due to that region having
the most restricted slot openings (0.65 mm). For a
smaller channel cross-section, the fluid moves at a
higher velocity which corresponds to having better
cooling as more energy is dissipated away from the hot-
wall into the coolant which is then injected back into the
chamber. The only unchanged parameter was the
channel depth, which remained constant at 1 mm. Refer
to Table 4 for more information on channel sizes.
Fig. 8. Regenerative Channels as shown in a
chamber cross-section.
Table 4. Final dimensions of regenerative cooling
system
Inner chamber wall thickness 0.6 mm
Channels depth 1 mm
Rib thickness 0.6 mm
Channel maximum width 2.9 mm
Channel minimum width 0.63 mm
Number of channels 150
While maximizing the effectiveness of regenerative
cooling through modelling and iterative design, it was
still apparent that the thermal stress caused by the steep
temperature gradients in the Inconel structure could

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IAC-18-F1.2.3 Page 7 of 17
potentially cause structural failures. REPDA analysis
indicated that the peak stress occurs at the engine’s
throat and is 130 ksi, which approaches Inconel’s yield
stress of 146 ksi at 922 K. For this purpose, film cooling
was added as a means to control the engine’s thermal
and stress regime.
3.4.2 Film cooling
The Film cooling orifices are located above the
flange on the chamber side. The orifices serve as a
thermal management system by injecting a pre-
calculated amount of fuel into the chamber in a fuel rich
setting. The orifices are 0.38 mm in diameter. The
number of orifices is the same as the number of
channels.
3.4.3 Sensors
To fully characterize the empirical performance of
the engine, sensors are placed in various places for
measuring pressure and temperature (Table 5). Testing
ranges from measuring the pressure drop on the LOX
and regenerative cooling channel while visualizing the
cone formation. LPL’s Water-Flow Test Stand (WFTS)
was used for testing [7].
Fig. 9. Sensor placement in the injector & chamber
The left cross-sectional view (Fig. 9) provides
insight into how these sensors are placed to measure
pressure and temperature in the most critical areas. On
the top, three sensor ports are located in the center
annulus. Two sensors measure pressure and the other
measures temperature. The same architecture is used for
the fuel side of the injector. The chamber has four
pressure transducers which are located evenly around
the circumference of the chamber and are used to
measure the chamber pressure. The multiple sensors
will help in determining pressure distribution in the
chamber and help to detect combustion instabilities.
The last group of sensors is located in the cavity at
the end of the nozzle. Five sensors are used in total to
fully characterize the flow properties in the nozzle
cavity. Two pressure transducers are located
symmetrically around the fuel inlet. The purpose of
Table 5. Sensor configuration on development engine
Position
of Sensors
Number of
Sensors
Purpose
Pressure Transducers
Nozzle exit 4 Coolant pressure
Chamber 4 Chamber pressure
Injector 3 Fuel & LOX pressure
Thermocouples
Nozzle
cavity
2 Coolant temperature
Injector 2
Fuel & LOX
temperature
Fig. 10. Sensor Placement in nozzle cavity
these sensors is to measure flow distribution on both
sides of the cavity to ensure they are even. The third
pressure transducer is located on the side opposite to the
fuel inlet. Fig.10 shows the variable cross-section
manifold from the fuel inlet toward the other side of the
nozzle. This is provided to keep the velocity of the fluid
constant as it is distributed to channels. Two
thermocouples are also located in the nozzle fuel
manifold. The value from these sensors will be
compared with the temperature readings downstream of
the regenerative cooling channels to determine the
cooling performance during the hot fire testing phase.
3.4.4 Constant Velocity Manifold
The constant velocity manifold acts as a flow
distributor to ensure that the fuel is traveling at the same
rate axially through the regenerative channels of the
engine. This is key for the ignition priming sequence
because the fuel needs to be discharged proportionally
as it will directly affect cone formation and thereby the
engine performance. Reference Fig. 11 to see the cut
away view of the constant velocity manifold.

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IAC-18-F1.2.3 Page 8 of 17
Fig. 11. Constant Velocity Manifold / Fuel Inlet
3.4.5 Fuel Inlet
The fuel inlet (as seen in Fig. 11) is the latest major
addition to the engine. It is also printed out of Inconel
718 and its purpose is to inject the fuel into the engine
without having to drill directly into the nozzle. This
component was developed and integrated to address the
concern of clogging the channels from metal dust and
shavings during the subtractive machining phase. In the
preliminary design, there was a straight thread feature
that would connect directly with a flex hose which
would increase possibility of clogging the channels.
With the addition of the fuel inlet there is lower chance
of machining failure.
3.4.6 Flange
There were five major aspect to consider during the
design of the engine flange. The number of bolts,
location, seal, coolant transfer cavity, and alignment
(Fig. 12). There are a total of 18 bolts on the flange that
allow to reduce the preload per bolt and to gain a higher
structural factor of safety. The preload per bolt required
to ensure o-ring compression is 6357 N, providing a
factor of safety of 4.2.
The thermal and stress load on the flange was a
primary concern during design. Therefore, the flange
was placed right below the film cooling orifices in order
to maximize cooling effects on both the flange and
throat. For more information about the film cooling
holes, refer to Section 3.4.2.
After the location had been decided, sealing the fluid
in the regenerative channel while keeping the hot gases
inside the chamber was the next challenge. To solve
this, two metal o-rings were integrated on each side of
the coolant transfer cavity to seal off the various fluid
compartments (combustion and coolant). O-rings made
of Inconel 718 with a silver coating were chosen due to
their thermal capabilities.
The fluid transfer cavity through the flange was used
as a way to connect the 150 regenerative cooling
channels between the chamber and nozzle while
avoiding alignment issues. The fluid transfer cavity
serves as a ribless zone that pools the fluid from one
component to the other without needing a specific
orientation. Two chamfers were added onto the sides of
the cavity as an extra safety feature to ensure that after
torquing down all the bolts, the channels will not have
any flat overhangs that would cause a heavy pressure
drop during operation.
The need for an alignment feature was driven by the
1 mm opening of the fluid transfer cavity. With such a
small opening, it is important to ensure that they align
almost exacty to not affect engine performance. To
achieve this, precision dowel pins are used on two of the
eighteen bolt holes. The two bolts are oversized to 10
mm to ensure that they cannot be placed into any other
bolt hole.
Fig. 12 Engine Flange cross section
3.4.7 Pintle and Center Annulus
The Pintle and Center Annulus (CA) as seen in Fig.
13 are the main components of the injector. The key
design considerations were to make the pintle modular
to test various development models. Initial design
calculations were made to size the baseline model. For
more details on sizing, refer to Section 2.4. Through the
use of the WFTS, the pintle will undergo various tests to
determine the percent error of the design values, and
then iterated to achieve an optimal design. Details of the
tests can be found in Section 6.1.1.
The modularity of the pintle comes from its
interchangeability with the CA. As long as the threaded
section remains constant, various pintles with modified
skip distances, orifice diameters, or flow diverters can
be tested to empirically characterize the design. There is
a Teflon o-ring seal in between the pintle and CA and
also between the CA and chamber to prevent any
unwanted leakage of combustion fluids.
In order to switch out pintles with the CA, tooling
features were printed into both components. For the CA,
a hex feature was sized to ensure it can be placed within
a vice grip for fastening and worked on with a standard
wrench. The pintle bottom hex socket was designed to
be used with a standard torque wrench to achieve the
required preload which would be determined via testing.
3.4.8 Fitting Selection
All fittings consist of an MS Boss/SAE configuration on
the side that connects into the engine. The pressure
transducer and temperature sensor ports convert to
Swagelok™ while the

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Fig. 13 Pintle/Center Annulus
fuel and LOX inlet ports to AN fitting, to comply
with the requirements of the test setup. The reason for
using MS Boss fittings is to have a pre-integrated
sealing device on the fittings. This will prevent the
hassle of performing sealing studies and procuring
special sealing devices for the fittings. MS Boss fittings
are highly adaptable to different kinds of fittings and
come in a wide range of sizes.
3.4.9 Integration
For empirical testing of the engine at its design hot
fire conditions, an integration tool was required. The
tool would need to align and connect the engine to the
test setup and act as a thrust transfer mechanism. The
main failure mechanism for the part would be buckling.
The tool would also need to have enough clearance to
pass flex hoses and other plumbing connections while
providing easy access for assembly and disassembly.
The tool would have a bolt pattern that would mate with
adequate accuracy on to the engine. The bolt pattern on
the engine is shown in Fig. 14. There are 9 bolts in total
which are designed to handle the thrust and the
moments from the engine.
Fig. 14. Integration Bolt Pattern
4. Numerical Simulation
To confirm results obtained by REPDA, 3D CFD
analysis were performed on all engine components
using Ansys Fluent software and supercomputer
services provided graciously by Nimbix. These 3D CFD
analyses were used to understand the thermal regime of
Fig. 15. CFD temperature profiles of the regenerative
system (without film cooling). The curved plane shows
heat flux along the throat wall, the three straight planes
show cross-sections of temperature across the
combustion fluids, the Inconel and the coolant.
the solid engine structures as well as coolant and
combustion fluids. However, film cooling was not
included in thermal simulations du to the complexity
and computing requirements associated with such
complex 3D analyses.
4.1 Thermal Results
The thermal results without considering film cooling
described a very steep temperature gradient through the
throat wall at about 100 K per 0.1 mm, which was also
predicted by REPDA. Furthermore, analysis showed
that a thin veneer of the coolant was in contact with the
channel. The channel temperature was predicted to be
700 K, but the bulk coolant temperature never exceeds
460 K (Fig. 15 and 16). This predicts that at ~500 psia
the coolant will not boil.
CFD analysis could help predict the effectiveness of
regenerative cooling at the flange, an otherwise complex
structure which 1D analysis does not account for.
Despite the coolant being diverted away from the hot
wall through a flange manifold, the highest temperature
at the flange (without film cooling) is around 1150 K.
Fig. 16. Temperature profile of the engine hot wall
along the throat and flange, without film cooling.

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Fig. 17. CFD model of LOX pressure drop through the
pintle injector.
4.2 Fluid Results
To ensure that combustion products do not backflow
into the feed system, CFD helped define an improved
pintle geometry that would provide a 19% pressure drop
(Fig. 17) for the LOX flow. This was achieved by
including a throat feature upstream to achieve an
effective LOX pressure regime while maintaining the
desired TMR. 3D CFD analyses has helped in the
design of an appropriate injector capable of providing a
22% pressure drop for the kerosene entering the
chamber, while similarly providing appropriate relative
mass flow rates between the outer annulus and film
cooling (86% and 14% of total kerosene mass flow rate
respectively) (Fig. 18).
Along with helping narrow down design geometries,
CFD proved effective at providing test system
requirements to meet propellant flow criteria. The
pressure drop in a full-length cooling channel under
nominal hot fire conditions was quantified at ~100 psia
(Fig. 19). These results illuminated benchmark
requirements for feed system to maintain the desired
engine performance under hot fire conditions.
Fig. 18 Pressure drop through the outer annulus for
fuel side of injector
Fig. 19. Nominal pressure drop along the entire length
of the regenerative cooling channel.
5. Manufacturing
This section describes the basic features of the
additive manufacturing (AM) post-printing process and
inspection of printed components
5.1 Additive Manufacturing
Selective Laser Sintering (SLS) technology printers
have reached a stage where they are capable of printing
complex structures using a wide variety of materials.
This manufacturing process may not be cost effective
but saves a large amount of time, effort and material.
The overall benefits not only include resource savings
but also allows the organization to research and validate
additively manufactured structures for service. AM does
come with inherent disadvantages, which lead to design
complications. These problems were interpreted during
the preliminary design and helped save excessive
changes to CAD during the final stages.
5.1.1 Printer Specification
The engine was printed on an EOS M290 machine.
Industrial 3D printers are limited on build volume and
accuracy so as to maintain a large amount of reliability
and precision.
Table 6. Printer Specification [13]
Parameter Value Units
Build Plate Size 250×250×325 mm
Resolution ±40 μm
Roughness ±15 μm
Min. Wall Thickness 0.3 ~ 0.5 mm
Laser Speed 7 m/s
Laser Power 400 W
Focus Diameter 100 μm
5.1.2 Print Inputs
The most common AM constraint is the angle at
which an object can lie when unsupported from beneath,
an issue of overhang. All the surfaces that do not have
supports underneath them would need be placed at an

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angle greater than 45º to the horizontal. With any angle
less than 45º to the horizontal, the material would need
to be supported by a mesh or solid structure. This
severely restricts the geometry that you can have given
that surfaces need to have a good surface finish or are
thin wall sections that cannot be machined to remove
supports. The flange section of the chamber was
provided with a 45º solid support section because the
chamber was printed flange up. The reasons for printing
the chamber flange up are mentioned in the upcoming
sections. The nozzle, however, was printed flange down
and by doing this no sections were at an angle less than
the recommended limit. The center annulus was also
supported via a 45º ribbed support.
Another consideration for manufacturing is the print
direction. This refers to the orientation of the parts and
how they will print as the deposition goes on. The parts
can be printed at various angles and do not need to have
a flat plane on the build plate. This is particularly
helpful as the print direction dictates the strength of the
parts. In the case of Balerion, the chamber was printed
flange down to prevent the need of a support structure
on the interior of the chamber because the thin walls of
the section would not have been able to withstand the
stress. This meant that a solid support was needed to
provide stability for the flange.
Fig. 20 Printed on parts on build platforms
Studies have shown the a relationship between print
orientation and mechanical properties [14]. The
placement of parts on the build plate largely affects the
success rate of the print. This means that parts must be
properly spaced to avoid a print failure which has a very
high monetary impact. The pintle was printed in its
functional orientation with the injection holes closer to
the build plate. The same applies to the center annulus,
which was printed with the pintle mating face on the
build plate. These directions were all in the +90º or -90º
directions which ensured consistent mechanical
properties.
The print success rate is also highly affected by the
amount and direction of heat transfer that can occur on
the build plate. This is the reason why no two parts were
placed on the same build plate. The pintle and the
nozzle were printed on the same plate initially and it
was noticed that the surface quality of the pintle was
poorer than that of the nozzle. A new plate was
configured to accommodate the small parts such as the
center annulus, the pintle and fuel inlet on the condition
that they could be spaced at a sufficiently large spacing.
The files were provided to the print company in the
.STL file format. The parts were modeled in NX 11,
which allows users to set the resolution on the generated
file. To ensure maximum resolution and accuracy from
the printer the file resolution was set to 0.0001. This file
format is capable of converting a solid body file into a
mesh-based file for the printer to interpret.
5.2 Post–Printing Inspection
In addition to AM, several post-processing
technologies, including surface and heat treatment are
required for the final product to accomplish the required
thermal, mechanical and surface properties. Inconel-718
when heat treated as per AMS 5664 develops a hardness
close to 47 HRC. For the main design considerations,
some of the thermal and mechanical properties of AMS
5664 are listed in Table 3. To verify that all critical
features were printed as intended various techniques
were used. They are enumerated in the following
sections.
5.2.1 Powder Removal
The AM process leaves a large amount of unbound
powder in the channels and closed sections which needs
to be removed. There are multiple methods which are
recommended for powder removal. For this specific
application, the parts were first placed on a shaker table
and close to 80 % of the loose powder was removed by
vibrational means. The shaker table is initially set to a
low-frequency setting and then the frequency increases
to remove more powder. After this, the parts underwent
a compressed air blowdown to remove any stray powder
stuck in any low points of the system.
The parts were then ultrasonically cleaned in an
industrial grade ultrasonic cleaner for 1 hour each. The
parts were CT scanned before and after the ultrasonic
cleaning to determine the effectiveness of the procedure.
5.2.2 Heat Treatment
The printed parts do not have the surface
properties that are recommended for liquid rocket
engines. The heat treatment process was composed of
solution treatment and annealing at the recommended
temperatures and times. Before heat treatment, the
product was closer to a light gray color; after heat
treatment, a discoloration of the parts to a blue-gray
tinge was noted. The heat treatment process is done
before part removal to prevent excess distortion and
warpage of parts during the heat treatment process. This

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Fig. 21. Regen Channels at the throat
also ensures that any small particles of powder that
are still present in the channel do not affect the Wire
EDM process.
5.2.3 Part Removal
The smaller parts were separated from the build
plate using a band saw. The more critical and bigger
parts such as the chamber and the nozzle were separated
from the build plates using Wire Electrical Discharge
Machining (EDM) with 0.1 mm tolerance. EDM
was used due to the large surface area involved along
with the high risk of blade breakage if a band saw was
used.
5.2.4 CT scan
The internal structure was inspected by using
Computed Tomography (CT) technology. The primary
dimensions for inspection were that of the
regeneratively cooled channels. Multiple planes were
created at inspection locations and measurements were
taken at those locations. The width of the channels
varies from the end of the nozzle to the top of the
chamber. The width of the ribs, however, is constant.
On Fig.22 the dimensions of one of the regenerative
Fig.22. Regen Channels Inlet
Fig. 23. Planes for channel inspection on the
chamber
cooling channels inlet is shown. The average width
was determined to be 1.87 mm. The design value was
1.8 mm. The difference in dimension is attributed to
shrinkage.
The channels were then inspected at the throat. The
dimension of the channels is the smallest at the throat
due to its relatively small diameter (Fig. 21).
Fig. 24 Detail view on side of the chamber channels
Table 7. CAD/CT dimensions comparison (mm)
Location Parameter CAD CT
Nozzle base Width 1.8 1.87
Throat Width 0.67 0.63
Chamber top view Width 2.62 2.34
Chamber top view Depth 1 1.01
Chamber side view Width 2.62 2.35
Chamber side view Thickness 0.6 0.82
Pintle top view Diameter 1.8 1.73
Pintle side view Diameter 1.8 1.59

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Fig. 25. Top view of regen channels in the chamber
The dimensions of the regenerative channels on the
straight part of the chamber were also measured. Two
planes one on the top view and the other on the side
view (Fig. 23 Blue & Green plane) were used to
measure the width of the channels. The details views are
shown in Fig. 24 and 25.
The last part that was inspected was the pintle
injector. The most critical dimension being the orifice
diameters around the pintle tip. They were inspected
from (Fig. 26) top view (blue plane) and side view (red
plane). Detailed views are shown in Fig. 27.
5.2.5 Machining
The goal of the engine design was to limit the
amount of machining required because heat treatment
increases the difficulty of Inconel 718 machining. The
features that absolutely required machining were
connection features, sealing grooves and alignment
features.
With surplus machining features on this material
type, there is a lot of accrued cost when it comes to
tooling. Tools used for cutting and drilling easily get
worn down and need constant replacement, which adds
time to machine setup.
A wax will be employed to intentionally clog the
internal channels prior to machining the critical features.
By pooling melted wax into the internal sections of the
nozzle and chamber, it will prevent any foreign object
debris (FOD) from getting inside the
regenerative channels. If the channels somehow get
Fig. 26. Two planes for orifices inspection on the
pintle
Fig. 27. Top (top left), side (top right) view of pintle
orifices, detail top
clogged, it can have a detrimental impact on the
engine during cold-flows and hot fire testing. Therefore,
by utilizing wax, any object that attempts to penetrate
the internal features will get stuck and picked out before
melting out the wax.
For the external surfaces of the engine that cannot be
machined, bead blasting will be used to clean out any
printing defects. This mostly includes down skin caused
by higher laser speeds when printing close to 45o
structures. It is possible to print the features, but the
high laser speeds leave a very rough surface finish that
needs to be cleaned up because it can injure the handlers
of the component. Also, it can affect the performance of
the engine based on the location of the down skin.
Fig. 28 Full assembly of Balerion before Machining
6. Testing
The purpose of the testing campaign is to
characterize the design parameters of the hardware at
the component level, which can provide information on
selecting the propellant inlet conditions at the hot fire
test. The campaign includes water flow tests on

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individual components and subsystems, proof and
leak tests, cold-flow test and eventually the hot fire test.
The objective and method of each test will be explained
extensively in the following section.
6.1 Component Water Flow Testing
Subsystem water-flow testing is essential as it
provides information on the performance of designed
parts. The results can be interpreted to validate the
design parameters and analysis performed, as well as
adjust the testing conditions accordingly in the
following testing campaign. In Balerion’s testing
campaign, USC LPL’s water flow test stand will be
utilized to perform the water flow testing [7]. The
components that require water flow tests include the
pintle injector, both LOX and fuel sides separately and
together, the regenerative cooling channels and the film
cooling orifices.
6.1.1 Pintle LOX Side Water Flow Test
The objective of this test is to verify if the current
pintle tip geometry will provide a sufficient pressure
drop at the designed operating condition and validate
the pressure drop analysis result from engine sizing and
ANSYS.
The water flow test stand can measure the mass flow
rate and the pintle orifice upstream pressure, which can
be utilized to derive the flow coefficient, Cv, and the
discharge coefficient, Cd, of the pintle tip geometry.
Both non-dimensional values can serve as a tool to
determine the pressure drop across the orifices at
operating conditions, but the discharge coefficient can
further provide information in the future redesign of the
pintle tip geometry.
The Cv value was found using the following
equation:
𝐶𝑣 = 𝑄√
𝑆𝐺
∆𝑝
(7)
where the Cv is the flow coefficient, Q is the
volumetric flow rate in GPM, SG is the specific gravity
of the fluid, and ∆𝑃 is the pressure drop across the
testing article. For the determination of the Cd value of
the pintle tip, equation (9) was used.
Fig. 29. The testing article assembly Pintle + Center
Annulus
Multiple tests with different upstream pressures have
been performed to mitigate measurement uncertainty
and result computation error. The average Cv and Cd
values were found to be 3.714 and 0.689, with a
standard deviation of 0.007 and 0.001 respectively.
By substituting the operating condition into the Cv
equation, the pressure drop across the pintle tip at hot
fire test is estimated to be approximately 98.7 psi, which
is 26% of the chamber pressure, higher than the
previous predicted value, 18%. Some potential reasons
for the unmatched result might be the friction loss from
the unpredictable surface roughness from the 3D
printing and un-modelled turbulence that cause the
additional pressure drop. However, this result is deemed
acceptable since it should not affect the performance of
the engine. Also, with the acquired Cd value, the lab
will have a better understanding in sizing the pintle tip
geometry to obtain a desirable pressure drop.
6.1.2 Engine Fuel Side Water Flow Test
The goal of this test is to characterize the design
parameters on the engine’s fuel side flow. Watched
parameters include Cv and Cd values across the
injector’s fuel ring outlet and the regenerative cooling
channels, as well as the mass flow rate through the film
cooling orifices. The Cv and Cd data at the fuel ring
outlet can ensure the fuel side injector has a pressure
drop at the designed operating condition. For the
regenerative channels, the data can offer information on
the fuel side propellant inlet condition during hot fire
testing.
The film cooling orifices mass flow rate is the most
crucial data to obtain before the hot fire test. Initially,
12% of total fuel flow was figured to be the desired
amunt to be used as the film cooling coolant. In the
post-printing section, the orifices were too small to be
inspected by the CT scan, thus no information was
available on whether the orifices were clogged or had
shrunk from printing. By knowing the actual mass flow
rate coming of the film cooling orifices, the firing
operation conditions can be re-evaluated.
By performing water flow tests with the full
Balerion assembly on the water flow test stand, the Cv
and Cd values at the injector fuel outlet and the
regenerative cooling channels can be calculated. The
flow diverter, shown in Fig. 30, was designed to
temporarily store the main injector fuel flow, while
isolating the film cooling flow which can be measured
by the water flow test stand. This setup allows the
determination of the film cooling mass flow rate and the
calculation of the injector’s main fuel flow rate. The
acquired Cv and Cd values can be substituted with the
operating condition to determine the actual injector fuel
side pressure drop and the propellant inlet condition at
the firing. The film cooling mass flow rate can also be

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used to evaluate whether the cooling system can achieve
its designed thermal performance.
Fig. 30. Flow diverter in full engine assembly
6.1.3 Cone Formation
It is critical to visualize the spray pattern of the
pintle injector to determine if the flow from the LOX
side and fuel side collide and form a spray cone which
indicates mixing between the propellants. Also, this will
provide a data point to determine the relationship
between the total momentum ratio (TMR) and the spray
angle, since currently the spray angle is presumed to be
60 degree with the designed TMR value of 1 due to
momentum conservation [2], when in actuality the spray
angle needs to be tested and verified.
The injector testing unit, a testing tool, shown below
in Fig. 31, is designed to visualize the spray cone. This
test article is designed to have a similar fuel outlet
geometry as the engine to simulate the fuel flow while
allowing the actual pintle injector to be mounted on to
mimic the flow pattern. This test will be conducted on
the water flow test stand with varying flow rates on both
sides to achieve a TMR of 1 to investigate the spray
angle of the cone pattern. This information will aid the
lab in the future when designing pintle injectors as well
as provide modelling inputs in ANSYS to more
accurately simulate combustion in the chamber.
Fig. 31. Injector testing unit CAD model
6.2 Proof & Leak Test
To ensure the seals in between components’
interfaces are effective, proof and leak test are
conducted to identify the potential leakage points which
can be evaluated and mitigated before the firing. The
major locations to check for leaks are the pintle to
center annulus interface, the center annulus to chamber
interface, the engine flange interface, and the sensor
ports on the engine assembly.
For the full engine proof and leak test, a testing tool
is developed to seal the throat internally so that the
pressurization of the chamber and regenerative cooling
channels is possible. This testing tool, named throat
plug, is a modification of an off-shelf expansion plug
that utilizes the preload with center annulus in addition
to the friction between the expanding rubber and nozzle
inner wall to seal and retain itself against the pressure
load of the fluid. The section view of the throat plug
with the engine is shown below in Fig. 32.
Fig. 32. Throat plug assembly on engine
For the pintle and center annulus interface, in
addition to the regular proof and leak tests, a separate
cryogenic leak test needs to be conducted because in the
firing sequence, the interface will be pre-chilled to
cryogenic conditions. The purpose of this cryogenic test
is to investigate under cryogenic temperatures, if the o-
ring seal will still be effective and hold pressure. A
blank pintle, see Fig. 33, was designed and
manufactured to allow for pressurization of the
assembly and test the sealing at the interface.
Fig. 33. Blank pintle with center annulus assembly
CAD model

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6.3 Cold-Flow Test
The cold flow test of the engine should be conducted
after results from the aforementioned tests are reviewed
and accepted. The objective of the cold flow test is to
characterize the test stand’s feed system with the engine
integrated to supply the desired inlet propellant
conditions at the hot fire test. For the test, water will be
used on the fuel side since its property is similar to that
of Jet-A, while for the LOX side liquid nitrogen will be
used to simulate the cryogenic condition the engine will
experience.
In addition to characterizing the feed system, the
cold flow test is useful in determining the chill-down
procedure. It is desired to have the LOX side injector at
correct thermal condition so that when LOX exits the
injector it remains in the liquid state. The cold flow test,
with the help of the thermocouple placed upstream of
the injector, will help to determine if the proposed chill-
down procedure needs to be modified.
Furthermore, a timing test needs to be conducted to
determine the time taken between sending the valve
actuation command and when both the propellants reach
the chamber. This will provide information on setting
the ignition system because ideally the propellants can
be ignited now when they exit the injector into the
chamber. If not, there is a higher potential for the engine
to over-pressurize at ignition, known as a hard start, due
to the pooling of propellants in the engine, which can
damage or even destruct the engine. Therefore, it is
critical to perform the timing test to finalize the firing
sequence while minimizing the time difference between
when propellants enter the chamber and when ignition
starts.
6.4 Hot Fire Test
The main objective of the hot fire test is to verify the
engine performance including thrust, cooling
effectiveness, combustion efficiency and general
reliability. A pre-fire inspection will be conducted and
documented to compare with a post-fire inspection of
the engine to investigate if the engine’s inner wall burns
through during the firing. The temperature sensors
located outside of the engine can help in constructing a
thermal map across the engine’s wall to verify if the
regenerative cooling channels and the film cooling
orifices achieve the desired performance.
The four pressure transducers located around the
chamber allow the data acquisition system to collect
pressure oscillation during the firing, thus provide
information about whether the combustion is stable or
not.
Initially, short-duration firings (3 sec) will be
conducted to determine if the ignition sequence is
proper and to characterize the start transient condition of
the engine. In addition, the engine shutdown procedure
needs to be verified to ensure the safety of both the
engine and the test stand. The shutdown sequence needs
to be proved as reliable and repeatable under different
firing conditions. Then, longer-duration firing (6-15 sec)
can be conducted to verify the engine’s steady-state
performance.
7. Conclusion
Through the progress made on Balerion, additive
manufacturing has proven to be very advantageous
compared to conventional manufacturing methods. It
has provided the capabilities to significantly reduce
process time while allowing for increased complexity.
However, with any new method, there will always be
limitations. The main being the building volume
availability in commercially available 3D metal printers.
Additionally, a design meant for additive manufacturing
must comply with constraints such as overhang and
shrinkage that would not be considered in traditional
manufacturing methods.
Though additive manufacturing has progressed quite
rapidly over the years, more advances still need to be
made before it is capable of completely replacing
conventional manufacturing. Critical features such as
sealing surfaces, threads, etc. still rely on traditional
machining methods as additive manufacturing cannot
meet the required tolerance and surface finish needed
for optimal system performance.
As a student group, LPL has made substantial
progress on the design and manufacturing of a fully 3D
printed engine. The testing section (Section 6) lists the
next major steps in the design verification of the
Balerion engine. LPL has accumulated a lot of
knowledge during the printing process and will continue
to do so during the machining and testing phases. The
goal of LPL is to build upon the lessons learned as to
increase the skillset of its members in order to design
better systems in the future.
As mentioned, since the testing campaign has yet to
be conducted, a future publication will be submitted to
conclude the work on the Balerion Development
Engine.
Acknowledgements
We would like to thank Department of Astronautical
Engineering at the USC Viterbi School of Engineering
for supporting LPL with infrastructure and safety
oversight.
Next, we would like to thank USC Centre for
Advanced Manufacturing (CAM) for enabling us to use
metal additive technology and the USC Machine Shop
for providing expertise for technical drawings and
machining our components.
For CT scan images we would like to thank
Aerospace Corporation. Extensive numerical
simulations were possible thanks to sponsorship from
Ansys and Nimbix.

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Finally, we would like to thank the current USC
LPL team and alumni network for their willingness to
push the boundaries of what a student-led group is
capable of accomplishing.
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rocket thrust chambers aspects of modeling, analysis,
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