# The Application of Counter-Rotating Turbine in  Rocket Turbopump

Authors: Tang Fei; Zhao Xiaolu; Xu Jianzhong
Journal: International Journal of Rotating Machinery
 (2008)

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## Abstract

Counter rotating turbine offers advantages on weight, volume, efficiency, and maneuverability relative to the conventional turbine because of its special architecture. Nowadays, it has been a worldwide research emphasis and has been used widely in the aeronautic field, while its application in the astronautic field is seldom investigated. Researches of counter rotating turbine for rocket turbopump are reviewed in this paper. A primary analysis of a vaneless counter rotating-turbine configuration with rotors of different diameters and rotational speeds is presented. This unconventional configuration meets the requirements of turbopump and may benefit the performance and reliability of rocket engines.


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## Body

## 1. Introduction

Turbopump is one of the main modules of liquid rocket engine and
represents a large part of rocket engine overall cost. So improving the
efficiency of turbopump and reducing its weight are effective ways to improve
the performance of rocket engine [1]. Furthermore, to simplify the mechanical
architecture of turbopump may improve the reliability of rocket engine.Because of
difference in densities of propellants, oxidizer pump and fuel pump of rocket
engine require different rotational speed to avoid the caviation erosion.
Generally, oxidizer’s rotational speed is higher because the density of the oxidizer
is higher than that of the fuel. What’s more, for low density fuel, improving
rotational speed can reduce the fuel-pump diameter and volume remarkably.At early
age, some LH2/LOX engines with a single turbine drive the LH2 pump, and by
gearbox the LOX pump directly. But gearbox brings excess weight and reduces
credibility of engine in the cryogenic environment of LH2. Two turbines in
series driving pumps, separately, is another solution, however, this configuration
will bring excess losses and make the engine more complex and less credible.The
counter rotating turbine is an axial turbine with a first stator followed by a
first rotor, an optional second stator, and a second rotor, which rotates in
opposite direction of the first stage. In 1958, Wintucky [2] presented the basic analysis of counter rotating
turbine with restricted hypothesis like the same mean diameters for each rotor,
the same rotational speed, and constant axial velocity. In 1980s, the
scientific researchers made wide researches on counter-rotating turbine by
stimulation of IHPTET (the integrated high-performance turbine-engine technology
program), and now several countries have developed counter-rotating turbines
for aircrafts.Because of
its unconventional architecture, counter-rotating turbine offers advantages on
weight, volume, efficiency, and maneuverability relative to traditional multistage
turbines. It must be emphasized that configuration of counter-rotating rotors
can reduce the torque on the aircraft remarkably, so the maneuverability of
aircraft driven by counter-rotating turbine engine is much higher.These
advantages are also helpful for rocket engines. The counter-rotating turbine
with two rotors, which have different rotational speeds and powers to drive the
fuel pump and oxidizer pump, respectively, is proposed. This configuration can
avoid the disadvantages of gearbox and two turbines configuration is mentioned
in the above paragraphs (see Figure1).Figure 1
Counter-rotating  turbopump.


## 2. Review of Counter-Rotating Turbine for Turbopump

In 1984, Narou [3] of ISAS presented the idea of application of counter-rotating
turbopump on upper-stage LOX/LH2 rocket engine.In a research
of NASA ACE project (advanced chemical engine), Huber and Veres [4–6] design a counter-rotating turbine for the upper-stage
rocket engine. They take full-scaled experiments upon the turbine, and the experimental
data accord well with the theoretical prediction. An unconventional blade with a
160° turning angle is introduced into this counter-rotating turbine.
Earlier investigation on this blade in the National Launch System project [7, 8] indicates that it can improve turbine
efficiency and decrease blade number remarkably.In 1993, researchers
of NAL [9–13] designed a counter-rotating turbine experiment
facility and took a series of theoretical analysis, numerical simulations, and
experiments on the counter-rotating turbine. Their research included a
secondary flow loss of ultra-high-load turbine, feasibility of counter-rotating
turbine and rotor-rotor interaction. NAL counter-rotating turbine also adopts
UHLTC [10] (ultra-highly loaded turbine cascade) with 160° turning angle. Experimental data show the flow structures, and the loss
mechanisms in UHLTC were basically the same as those seen in the conventional
cascades, except for stronger-passage and leakage vortices. A large internal
loss generates in cascade passage, and a large downstream mixing loss is found.
Yamamoto figures out that those losses come from the strong passage vortices
and the associated laminar flow separation due to the extremely large turning
angle. Preliminary comparison between the experimental results and Hah’s
numerical predictions shows a good agreement [13].Pratt and
Whitney have developed a twin rotor turbopump (TRT) [14] for NASA ablative engine. The TRT is a liquid
LOX/Kerosene turbopump that incorporates a back-to-back counter-rotating
turbine. Although the counter-rotating turbine for a turbopump is theoretically
good, there is no flight-proven counter-rotating turbine for fluid rocket
engine despite of the above scientific projects.


## 3. Vaneless Configuration of Counter Rotating Turbine

In order to obtain higher-engine performance, vaneless configuration of
counter-rotating turbine is proposed. In the following, paragraphs, this kind
of turbine will be called1+1/2 counter-rotating turbine. The “1” means a full high-pressure stage (HP)
with stator and rotor, while the “1/2” means the vaneless low-pressure stage (LP). This vaneless configuration offers
significant potential advantages.
(i)
The weight and volume of the engine is reduced remarkably because of the abolition of LP stator row.(ii)
Losses associated with LP stator blade are avoided.(iii)
The reliability of the engine is improved because the turbine structure is simplified.Although the
vaneless configuration may benefit turbine efficiency and engine reliability,
it will bring difficulties to organize the flow in low-pressure rotor passage. So
actually, it’s not easy to get theoretic high efficiency.In 1993, Huber
and Veres [4] conducted contrastive experiments on 1+1/2 and
1+1 counter-rotating turbines. As the experiments results show, the removal of
the LP stator is predicted to cause a significant increase in gas-incidence
angle entering the LP rotor. This off-design incidence is predicted to decrease
overall turbine efficiency approximately 3% at the design point. Performance
estimates based on their test results indicate that an LP rotor designed for 1+1/2
turbine would achieve an increase in efficiency of 2% relative to a 1+1 counter-rotating
turbine at the aero design point, and up to 7% higher efficiency at reduced-speed
operation.Yamamoto [12] also took experiments on vaneless counter-rotating
turbine to investigate the cascade internal flow and loss mechanisms under the
rotating condition. He found that low-energy fluids tend to migrate to the tip
endwall in the rotating case while to the hub endwall in the stationary case.Pempie and
Ruet [15] compared three solutions for LOX/LH2
turbopump: 1+1 counter-rotating turbine, 1+1/2 counter-rotating turbine, and two
turbines in series. As the results show, the configuration of 1+1 counter-rotating
turbine is considered the best choice because of its good integration of arrangement
and efficiency.Pempie and
Ruet also compared conventional turbine and counter-rotating turbine in another
application to LOX/Methane main rocket engine. According to the comparison
results, the counter-rotating turbine they design is smaller than the
conventional single turbine, and reduces mass-flow rate remarkably. As a result
of optimization, the rocket engine employing counter-rotating turbine obtains
the specific impulse of 1.7 second increases.Theoretical
analysis presented by Ji Lucheng [16] indicates that the work ratio (the ratio of HP
rotor work to LP rotor work) is propitious to obtain high efficiency. Generally,
high-work ratio is unsuitable for the aircraft engine. However, it meets the
requirements of the rocket turbopump by the square because the pumps of rocket
engines demand different input powers (see Figure 2).Figure 2
Influence of work ratio on HP efficiency.


## 4. Counter-Rotating Turbine with Rotors of Different Diameters

Previous analysis has indicated that the counter-rotating turbine with rotors, which
have different rotational speeds, can avoid the disadvantages of the gearbox
and two turbines configurations.In 2003, Pempie
and Ruet [15] presented their solution of the counter-rotating
turbine for the rocket turbopump. They introduced a counter-rotating turbine
with rotors of different rotational speeds and mean diameters, which is quiet
different to the existing one, and present a basic one-dimensional design
theory for this turbine basing on the traditional turbine design method.Two rotors
are used to drive the pumps, respectively, so rotational speeds and works of the
rotors are determined by requirements of pumps:(1)NA=NO,NB=NF,ΔhA*ΔhB*=PFPO=UA(VU1−VU2)UB(VU3−VU4).The velocity
triangle-calculation process of the counter-rotating turbine is similar to the traditional
multistage turbine except for the difference of rotors mean diameters. According
to the conservation laws of mass and momentum moment, parameters on LP outlet section
and HP inlet section must satisfy(2)ρ2DAHAV2M=ρ3DBHBV3MDAV2U=DBV3U.The
following equation expresses the efficiency of turbine:(3)η=UA(VU1−VU2)+UB(VU3−VU4)Δh.The degrees of freedom are the following:(i)
the mean diameters of rotors:DA, DB;(ii)
the mean blade speed:UA, UB;(iii)
the degree of reactionΩ and the pressure distribution through the turbine blade.


## 5. Primary Efficiency Analysis of the Counter-Rotating Turbine

In 2001, Lucheng [16] presented a primary efficiency analysis of 1+1/2
counter-rotating turbine basing on conventional multistage turbine-analysis theory.
Although his analysis does not take the difference of rotor diameters into
account,  the existing analysis method can be also suitable for this new-style
counter-rotating turbine after giving some modification on velocity-diagram
calculation according to Pempie’s one-dimensional design theory.In order to simplify the architecture of the rocket engine, the
simplest1+1/2
configuration is adopted, which consists of HP stator, HP rotor
for fuel pump, and LP rotor for oxidizer pump. Turbine inlet and
outlet flow directions are assumed to be axial. Velocity triangles
of the turbine are illustrated by Figure 3.Figure 3
Velocity  triangle diagram.The work-speed
coefficientλ is introduced to correlate turbine efficiency.  It's defined as
the ratio of the square of the blade velocity to the specific work of the stage
as follows:
(4)λA=UA2/ΔhA*=UA/ΔVAλB=UB2/ΔhB*=UB/ΔVB.Referring
to the velocity triangle diagram, the changes of circumferential absolute velocities
can be derived as follows:(5)V1U=W1U+UAV2U=W2U+UAV3U=W3U+UBV4U=W4U+UB=0⇒ΔVA=W1U−W2UΔVB=W3U−W4U=V3U.In this analysis, it’s assumed that row loss is
in direct proportion to average kinetic energy at row inlet, so the efficiency
of the high-pressure stage can be written as(6)ηA=ΔhA*ΔhA*+kSω(V02+V12)+kRω(W12+W22),
where ω represents the ratio of a blade-surface area to an equivalent
weight flow per unit-annular area, and as an assumption, the relation between
rotor-loss coefficient kR and stator-loss coefficient kS is obtained as kR=2kS. According to some experiential
relation expressions in [17], the expression of ηA can be modified to
(7)ηA=λAλA+0.01172⋅tan|α1|⋅ΠΠ=(1+Ψ)2(6ctg2α1+1)+2(1+Ψ−λA)2+2(Ψ−λA)2, Ψ=UB/UA⋅λA/λB.Expression ofηB can be obtained
by applying the method used in deriving ηA  as follows:
(8)ηB=λBλB+Ttan|α3|{4ctg2α3+2[(1−λB)2+λB2]}. The efficiency of turbine can be expressed as
(9)η=1/λA+(UB/UA)2(1/λB)1/λAηA+(UB/UA)2(1/λBηB).In order
to obtain these analytical expressions, some experiential relation expressions
are introduced, and reheat effect and the secondary flow loss due to the change
of rotors diameter are ignored. So the results of this primary analysis are
instructional to the selection of design parameters but not so exact.


## 6. Efficiency Optimization

According to the above-mentioned analysis, turbine efficiency is
expressed as terms of blade-velocity ratioUB/UA,
work-speed coefficient λA, λB, and flow angle α1 and α3. So the performance of turbine can be optimized by adjusting
these parameters, and the optimization processes are illustrated with contour
charts of efficiency in Figures 4–6. These charts are plotted on the condition
of α1=60°, α3=60°, UB/UA=−0.3, and more charts corresponding to various conditions are not shown here
for limitation on extent. According to the comparison of Figures 4–6, turbine
efficiency η  is mainly decided by the high-pressure stage.Figure 4
Contour of HP stage efficiency.Figure 5
Contour of LP stage efficiency.Figure 6
Contour of turbine efficiency.The
influence ofα1 on HP efficiency is illustrated by Figure 7 plotted
on the condition of λA=0.6, UB/UA=−0.3. When α1 satisfies the following equation, ηA can achieve maximum:
(10)α1=arctg6Ψ2Ψ2+2(1+Ψ−λB)2+2(Ψ−λB)2.Figure 7
Contour of HP stage efficiency.Whenα3 satisfies the following equation, ηB can achieve maximum:
(11)α3=arctgλBλB+0.09376λB2−λB+0.5.


## 7. Influnce of Rotors Diameter Change

For a
certain design project, pump rotational speed ratioNO/NF and pump work ratio PF/PO are given by requirements of
pumps, so blade velocity ratio UB/UA can be expressed as
(12)UBUA=NBNADBDA=NONFDBDA.Combine definition
expressions ofλA and λB to yield
(13)λB=(ΔhA*ΔhB*)(UBUA)2λA=(PFPO)(NONFDBDA)2λA.Then the
turbine efficiency can be expressed as terms of rotors diameter ratioDB/DA,
λA, α1, and α3. Referring to the requirements
in Table 1 and condition of α1=50°, α3=60°,
contour charts of efficiency are plotted as Figures 8 and 9.Table 1
Requirements of LH2/LOX pumps.
LH2 PumpLOX PumpWork2500 kW390 kWRotational speed91000 rpm18800 rpmFigure 8
Contour of HP stage efficiency.Figure 9
Contour of turbine efficiency.As these
figures show, high-rotor-diameter ratioDB/DA may benefit
the performance of turbine, but actually, excessive DB/DA may make the duct
geometry between two rotors unconventional
and bring some excess losses.The
primary fluid mechanical problem of this type of duct is caused by the tendency
of the boundary layer to separate from walls if the channel curvature is too
high. The result of too high curvature is always large losses. On the other
hand, if the curvature is too low, the fluid is exposed to an excessive length
of duct and friction losses.Duct pressure loss, defined as the following
expression, is introduced to evaluate the losses in duct:(14)LossD=p2*−p3*p2*.Duct total
pressure loss is a function of the duct geometry and the inlet dynamic pressure
head [18] as follows:
(15)LossD=L(p2*−p2),
where duct geometry is accounted by a loss coefficient L. Value of L
for a given geometry must initially be determined from experience and by using commercially
available correlations. The variation of L with inlet swirl angle is illustrated
by Figure 10. So once duct geometry has been set, then total pressure loss only
varies with inlet dynamic head and swirl angle.Figure 10
Effect of inlet swirl on loss coefficient (P. P. Walsh, P. Fletcher,Gas Turbine Performance).Considering
the complicated influence of duct geometry on turbine efficiency, to select a moderate
diameter ratio is very important for optimization of turbine performance. In
order to reduce duct losses, the duct geometry between two rotors has to be
designed carefully, and as shown in [18], total pressure loss of well-treated swan-neck
duct is no more than 2.5%. However, if DB/DA is too high,
boundary layer separation may occur and secondary flow loss will be high. At
this time, the 1+1 counter-rotating turbine will be more suitable because the
LP stage stator can benefit the flow field organization.Finally, it
must be emphasized that this evaluation of duct losses is just approximate and
only for primary analysis.


## 8. Conclusion

Counter-rotating turbine can improve performance and reliability of rocket
engine because of its special architecture and high performance. The counter-rotating
turbine with two rotors of the different diameters and rotational speeds can optimize
performance and configuration of rocket engine. Review of research on counter-rotating
turbine for rocket turbopump is presented in this paper.Primary
analysis basing on conventional turbine-design theory for this unconventional1+1/2
counter-rotating turbine has been proposed. The influence of optimization parameters
on turbine efficiency is illustrated by contour charts and analytical
expressions. For a certain project, free design parameters are DB/DA,
λA, α1, and α3, and as the results show, the
overall turbine efficiency is determined mainly by the high-pressure stage. Finally,
influence of rotors diameter changes, and duct geometry between two rotors on
the turbine efficiency is discussed. To reduce losses in the duct, moderate
diameter ratio and well-treated duct geometry are necessary. LP stator is also
helpful to avoid the secondary flow in the duct, if necessary.



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