Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
28Combustion Stability Improvement of LE-9 Enginefor Booster Stage of H3 Launch Vehicle

DAIKI WATANABE*1

HIROYASU MANAKO*2

TADAOKI ONGA*2

TAKASHI TAMURA*2

KAZUFUMI IKEDA*3

MITSUNORI ISONO*4

The Space System Division of Mitsubishi Heavy Industries, Ltd. (MHI) is currently
developing a new flagship launch vehicle (H3) in cooperation with the Japan Aerospace
Exploration Agency (JAXA). As a test for the development of the LE-9 first stage engine for the H3launch vehicle, a full-scale combustion system test was implemented. Through improvement in theinjector and resonator by establishing and applying a proprietary combustion stability evaluation
tool, significant improvement in combustion stability was attained. Currently an actual engine isbeing designed and fabricated based on the test results, and a combustion test of the total engine
systems is planned to be implemented in 2017.

|1. Introduction
MHI is currently developing the H3 launch vehicle in order to reduce cost and improvereliability in comparison with the current H-2A/B launch vehicle. In the development of a launchvehicle, the launch vehicle engine is one of the important factors that affect the reliability, cost andperformance. As the first stage engine of the H3 launch vehicle, the new LE-9 engine is currently
under development. A launch vehicle engine generates propulsive power by increasing thepropellant pressure through the use of a turbo pump (rotating machine), injecting the boosted
propellant into the combustion chamber through the injector to burn it under a high-temperatureand high-pressure condition, and using the heat energy generated by the combustion to acceleratethe flow in the nozzle to supersonic speed. Figure 1 shows schematic views of the injector and thecombustion chamber structure. Because the combustion temperature of hydrogen and oxygen
exceeds 3000°C, cooling is provided through the use of cryogenic hydrogen flowing inside the wallof the combustion chamber. The internal surface of the combustion chamber is made of a thin wallwith a thickness of several millimeters in order to enhance the cooling efficiency. If the combustionis unstable, the pressure vibration is amplified and the quantity of heat transferred to the wall
increases to the level where the cooling capacity of the combustion chamber becomes insufficient,resulting in the erosion of the combustion chamber. Because the erosion of the combustion
chamber is a serious phenomenon that leads directly to engine breakdown, it is essential for thedevelopment of a launch vehicle engine to secure sufficient combustion stability.
The components that are important for securing combustion stability are the injector andresonator. The injector injects hydrogen and oxygen through hundreds of injecting elements inorder to prevent the local high heat load of the combustion chamber. The resonator, which islocated between the injector and the combustion chamber, suppresses the pressure fluctuation
through resonance phenomena in a space. This paper reports on the improvement in combustion
stability of the LE-9 engine brought about by changing the shape of the injecting element and theresonator.
*1

Space Vehicle & Equipment Department, Space Systems Division, Integrated Defense & Space Systems*2

Chief Staff Manager, Space Vehicle & Equipment Department, Space Systems Division, Integrated Defense & Space Systems*3Manager, Technology Planning Department, Technology Strategy Office*4Combustion Research Department, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
29

Figure 1

Structure of injector and combustion chamber|2. LE-9 engine
Table 1 compares the specifications of the LE-9 engine with existing engines. The LE-9engine adopts an expander bleed cycle similar to the LE-5B engine. Figure 2 compares theexpander bleed cycle and the two-stage combustion cycle. The expander bleed cycle uses part of
the hydrogen boosted by the turbo pump for the cooling of the combustion chamber and utilizes theheat energy acquired at that time to drive the turbo pump. Then, hydrogen with its pressure
decreased when driving the turbo pump enters into the nozzle, and flows along the adjacent of thewall surface while cooling it (film cooling). On the other hand, the two-stage combustion cycle
uses all the hydrogen for the cooling of the combustion chamber and the nozzle, then burns it in the
sub combustion chamber together with part of the oxygen, and drives the turbo pump through theheat energy generated by the combustion. Because all the propellant is burnt in the main
combustion chamber, the performance is higher than that of the expander cycle. However, thenumber of parts is increased due to the existence of the sub combustion chamber and the system iscomplicated, resulting in difficult engine control, and therefore this cycle is disadvantageous forcost reduction and reliability improvement. For that reason, we adopted for the LE-9 engine anexpander cycle similar to the LE-5B engine, aiming at an increase in the propulsive power of theexpander cycle utilizing the design technologies for the single-stage high-pressure engine that we
cultivated in the past.

Figure 2Engine cycle schematic view

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)30

Table 1EngineEngine cycle
Propulsive power(under vacuum) [kN]
Engine mixture rate [－]LE-9 engine specifications

LE-9
Expander bleed cycle

LE-7A
LE-5B
Two-stage combustion cycle Expander bleed cycle

1471

1098

137

5.9

5.9
12.3 (main combustionchamber)
440
47
1.8
3.7
H2A/H2B launch vehicle
1st stage

5.0Combustion pressure [MPa]10.0Specific impulse [sec]Nozzle expansion ratio [－]Weight [ton]Length [m]

426
37
2.4
3.8
H3 launch vehicle
1st stage

Application launch vehicle

3.6448110
0.3
2.8
H2A/H2B launch vehicle
2nd stage

For the LE-9 engine, we increased the size of the combustion chamber to a length
approximately twice that of the LE-7A to secure the turbo pump driving energy required to increase
the propulsive power. The combustion chamber is long in the axial direction, and therefore theaxial directional acoustic mode frequency in the combustion chamber is low. Because acoustic
modes in the axial direction, radius direction and circumferential direction are coupled in thecombustion chamber, the mode density for a frequency becomes high when the axial directional
acoustic frequency is low. This is an important characteristic for securing combustion stability as
described below.

|3. Combustion system test
3.1

Purpose of test

We started the development of the LE-9 engine with various element tests for understanding
the characteristics of the phenomena occurring inside the engine, and established and improved thedesign analysis tools. In addition, material tests and new manufacturing technology development
tests to accumulate base data for design and manufacturing were executed. Based on the data,
prototype combustor system components were designed and manufactured. While thecharacteristics of the combustor system described below are difficult to evaluate in sub-scaleelement tests because of the scale effects, they are directly linked to the engine performance.
Therefore, we set development steps where direct evaluations and verifications through full-scale
combustion system tests through the use of a prototype engine were executed. Table 2 summarizes
the explanation and the purpose of testing for each characteristic.
Table 2 Purpose of combustion system test (full-scale test)Item

Engine transient property

Combustion efficiency(combustion performance)

Combustion stability

Cooling property of
combustion chamber

Purpose of combustion system test (full-scale test)
These properties cannot be understood by a sub-scale test
Properties of change with time inbecause they largely depend on the flow path volume andpropulsive power, pressure andthe heat capacity (easiness to be heated and cooled) oftemperature at startup andcomponents. Therefore, it was necessary to obtain data
shutdown
for analysis and verification in full-scale tests.
Index that indicates the ratio ofEvaluation in the order of 0.1% is necessary and therefore
the amount of heat energy
correct evaluation through analysis is difficult. This item
actually generated fromcannot be evaluated by a sub-scale test due to the effects
complete combustion of theof the diameter and the length of the combustion chamber.
propellant to the theoreticallyThus, it was necessary to perform verification throughgenerated heat energy amount.
full-scale tests.
- This item cannot be evaluated by a sub-scale test
because the response characteristics change dependingon the shape of the hundreds of injection elements, thearrangement of the resonator and the diameter of theCombustion pressure fluctuation
combustion chamber. Thus, it was necessary to perform
level
verification through full-scale tests.
- It was necessary to obtain data for improving theprediction method in order to understand the tendencyof the test.
The coolant temperature distribution (where the coolant
Combustion chamber wall
temperature becomes higher at the inner circumference
temperature, heat amountside and lower at the outer circumference side) formed inacquired by coolant (hydrogen),the flow path grows in the axial direction. Thus, it wasand pressure drop between
necessary to obtain data for analysis and verification
coolant inlet and outlet
through full-scale tests.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)31

3.2

Test diagram

Figure 3 shows the test diagram. The test does not include the turbo pump as the target, andis specialized for the downstream combustor system. Liquid hydrogen and liquid oxygen werepressurized in the tank on the upstream side of the engine and supplied to the engine in ahigh-pressure state. There was no turbo pump and therefore the turbine driving gas was discharged
through the branch pipe. This test was implemented at the MHI Tashiro test facility (Figure 4).

3.3

Operating conditions

Figure 5 shows an example of the engine operation history. The names of the valvescorrespond to Figure 3. To prevent insufficient combustion chamber cooling, the CCV (CoolantControl Valve) was opened first to start the cooling of the combustion chamber, and then the MOV(Main Oxidizer Valve) and the MFV (Main Fuel Valve) were opened to start the rated operation.
As shown in the combustion pressure history, the combustion pressure increased and decreasedsmoothly and there was no problem with the transient properties at startup and shutdown. The test
time was determined by the restricted pressure tank capacity, but Figure 5 indicates that both thepressure and the temperature could maintain a steady state.

LOX: Liquid oxygen
GH2: Gaseous hydrogen
LH2: Liquid hydrogen

Figure 3 Combustor unit test diagram

Figure 4

Photograph of combustion test

Figure 5Engine operation history

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
323.4
(1)

Improvement in combustion stabilityProblem
Figure 6 shows the measurement results of the pressure fluctuation in the initial testconfiguration. The figure indicates that the combustion pressure fluctuated at a higher level
than the LE-7A at a certain frequency and there was a strong peak on the oxygen side flow pathinside the combustion chamber at the same frequency. In addition, the acceleration of theengine was greater in comparison with the conventional model and improvement in the stability
was necessary.

Figure 6(2)Pressure fluctuation (initial shape)

Cause of instability
Considering the intensive fluctuation found on the oxygen side of the injector, theinstability is suspected to have been caused by coupled phenomenon of the oxygen side
acoustic mode inside the combustion chamber and that inside the injector.

Figure 7 Cause of unstable combustion (initial shape)

Figure 7 shows the acoustic admittance inside the oxygen side injection element of theinjector. The acoustic admittance is an index that represents the susceptibility of the pressure toacoustic fluctuation and can be calculated manually. The larger the acoustic admittance, thehigher the tendency for instability. This figure indicates that the secondary mode frequency
corresponded well to the peak of pressure fluctuation shown in Figure 6. For the primary andtertiary modes, only a slight peak is found in Figure 6. Therefore, it is considered that theMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)33

fluctuation is determined not only by the acoustic characteristic of the injector, but also by thecharacteristics of the combustion chamber.
On the right side of Figure 7, the change with time in acoustic mode frequency inside theoxygen side injection element and inside the combustion chamber is plotted and the intensity of
the combustion pressure fluctuation is shown using a color contour diagram. While the acoustic
mode frequency of the injection element increases gradually as the speed of sound increases
along with the lowering of the oxygen temperature, the acoustic mode frequency inside thecombustion chamber decreases gradually as the speed of sound in the combustion chamberdecreases along with the lowering of the mixture rate. The figures indicate that the combustion
pressure fluctuation increased when the acoustic mode frequencies of the two approached eachother. According to the above results, it was assumed that the instability was caused by thecoupling of the oxygen side acoustic mode inside the injection element and that inside thecombustion chamber.
(3) Improvement in shape(a) Improvement in injection element
The acoustic admittance characteristic of the injection element was changed byreconsidering the flow path shape. As the principle of this change, two approaches wereconsidered: displacing the admittance peak frequency and lowering the peak height. It was
important to note the characteristic that the acoustic mode frequency in the axial direction
was low and the mode density was high because the combustion chamber had a long shape.
As shown in Figure 7, it was difficult to eliminate all coupling between the combustion
acoustic modes even when the injection element acoustic admittance peak frequency wasdisplaced, because many combustion chamber acoustic modes exist in a narrow frequency
band due to coupling with a low-order L mode. Therefore, the shape was reconsidered inorder to lower the admittance peak. Figure 8 shows the injection element admittance of eachconfiguration. The figure indicates that the acoustic admittance peak was separated intomultiple parts by this change in the injection element shape that enhanced the dispersion andeach peak height could be lowered.
(b) Improvement in resonator
Because a relatively large peak was also found for the tertiary mode of the injection
element acoustic admittance as described below, the damping function of the resonator wasenhanced as a countermeasure. The resonator is located on the circle between the injector andthe combustion chamber as shown in Figure 1. The resonance frequency (dampingfrequency) of the resonator can be adjusted by changing the internal volume, and therefore
resonators that are effective for multiple frequencies can be designed by combining andplacing resonators with different shapes on the circle. Figure 9 shows the absorption rate
calculated by acoustic analysis. The higher absorption rate represents a higher-performanceresonator with a high damping effect. Although conventional resonators concentrated theeffects on the secondary mode of the injection element acoustic admittance as shown in thefigure, the combination of shapes was reconsidered so that the resonator also became
effective for the tertiary mode.

Figure 8Improvement in injector element acoustic admittanceMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)34

Figure 9(4)Broadening of resonator

Verification of shape improvement effectFigure 10 shows the combustion pressure fluctuation in a test where improvements in (a)injection element and (b) resonator shape are applied in sequence. By improving the injection
element, the large fluctuation that was found with the initial shape around the dimensionlessfrequency of 0.6 was suppressed. The relatively large peak remaining around the dimensionlessfrequency of 0.9 was suppressed by improvement in the resonator. In this way, bothimprovements in the injection element and the resonator are effective for enhancement incombustion stability. Finally, the fluctuation is suppressed to the same level as the current
LE-7A or smaller, and thus a plan to secure the combustion stability of the LE-9 engine wasestablished.

Figure 10 Verification results of improvement in combustion stability(5)

Establishment of combustion stability prediction method
Based on the test results, a combustion stability evaluation tool developed uniquely byMHI was established and verified. Figure 11 shows the analysis flow diagram. The governing
equation was an acoustic wave equation and a heat rate fluctuation characteristic equation. The
heat rate fluctuation characteristic was set by combining the injection element acoustic

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)35

admittance characteristic and the transfer function between the emission intensity (heat rate)
and the pressure fluctuation obtained through a visualization combustion test that wasimplemented separately. The acoustic characteristic of the resonator was taken into
consideration by setting the response characteristic obtained by acoustic analysis as theboundary condition. The speed of sound in the chamber, the density and the heat rate
distribution were set using steady state combustion CFD. A complex eigenvalue analysis wasperformed using the above settings to calculate the resonance amplification ratio of the wholesystem. The resonance amplification ratio is an inverse of the damping ratio, and a higher
resonance amplification ratio represents an unstable state. The right of Figure 11 compares thetest results and the analysis results. The analysis captured the unstable frequencies and themagnitude relation between the peaks of the resonance amplification ratio was similar to thetrend of the pressure fluctuation peaks observed in the test. For the LE-9 engine that is currently
under development, the combustion stability can be predicted by performing similar analysis
and comparing and evaluating the results.

Figure 11

Combustion stability evaluation method (N-τ acoustic FEM analysis)|4. Conclusion
As a test for the development of the LE-9 first stage engine for the H3 launch vehicle, afull-scale combustion system test was implemented. Although an unstable phenomenon caused by
the coupling of the acoustic mode inside the injection element and that inside the combustion
chamber occurred in the test, the combustion stability was significantly and successfully improved
due to improvement in the injector element shape and the resonator shape. As a result, theapplicability to an actual engine was confirmed. In addition, a combustion stability evaluation tool
was built, and it was confirmed that the tendency of the test could be reproduced. Currently theactual LE-9 engine is being manufactured, reflecting the improvements of the injector andresonator. A total engine system test is planned to be implemented in 2017.

References
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Kawashima, H. et al., Development of LE-9 engine, Space Propulsion (2016), 3124726
Negoro, N. et al., OVERVIEW OF LE-9 ENGINE DEVELOPMENT FOR H3 LAUNCH VEHICLE,
67th IAC (2016), IAC-16.C4.1.2x35064
Watanabe, D. et al., Hot-fire Testing of LE-X Thrust chamber assembly, 30th ISTS (2015), 2015-a-53