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IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 122
DESIGN AND FLOW SIMULATION OF TRUNCATED AEROSPIKE
NOZZLE
Vinay Kumar Levaka1
, Srinivasa Reddy K2
1
Student, Aerospace dept., Aurora’s Scientific and Technological Institute, Hyderabad, Telangana, India
2
Professor, C.M.R. Engineering College, Hyderabad, Telangana, India
Abstract
Aerospike nozzles are being considered in the development of the Single Stage to Orbit launching vehicles because of their
prominent features and altitude compensating characteristics. This paper presents the design of aerospike nozzles using
characteristic method in conjunction with streamline function, and performance study through numerical simulation using
commercial Computational Fluid Dynamics (CFD) code ANSYS FLUENT. For this purpose nozzles with truncation lengths of
25%, 40%, 50% are choosen, because of the thermal and structural complications in the ideal aerospike nozzle. Simulation of the
flow is carried out at three different altitude conditions representing Under-expansion, Ideal, and over-expansion conditions of
the flow. FLUENT predictions were used to verify the isentropic flow assumption and that the working fluid reached the design
exit Mach number. The flow-fields obtained through the numerical simulation are analysed to know the effect of truncation on the
performance of aerospike nozzle. Optimum percentage of the truncation is selected by the comparison of nozzles with different
lengths of truncation under various altitude parameters. The results show that the flow pattern of the nozzles under different
altitude conditions are almost similar. The 40 % truncated nozzle is found to give optimum performance and it has achieved the
desired exit Mach number in all the three altitude conditions.
Keywords: Aerospike Nozzle, Single Stage to Orbit (SSTO), Linear Aerospike, Truncation and Rocket Nozzle
--------------------------------------------------------------------***----------------------------------------------------------------------
1. INTRODUCTION
Ever since jet and rocket propulsion systems have emerged,
researchers have invented and implemented many types of
nozzles, mainly to increase the thrust performance of
nozzles in off-design working conditions. Among these
various designs, features of the aerospike nozzle have
attracted researchers since mid-1950s. Many theoretical
studies of the aerospike nozzle have been carried out in
1960s. In early 1970s, thermal and strength problems of the
aerospike nozzle and development of more efficient
methods for fabrication of conventional nozzles led to a
decline in research activities in this field. Development of
the nozzle with the capability of producing optimum
amounts of thrust in wide ranges of altitude has been a
subject of continuous dedicated efforts within the
community of rocket propulsion.
The phenomenon of producing optimum amounts of thrust
by a rocket nozzle in off-design conditions is called as
altitude compensation. Nozzles with the altitude
compensation characteristics are basic feature in realizing
the development of Single Stage to Orbit (SSTO) vehicles.
Reusable SSTO vehicles offer the promise of reduced
launch expenses by eliminating recurring costs associated
with hardware replacement inherent in expendable launch
systems.The most popular altitude compensating rocket
nozzle to date is the aerospike nozzle, the origin of which
dates back to Rocketdyne in 1950s.
1.1 Aerospike nozzle
An aerospike nozzle has a spike in the center of the nozzle.
Aerospike nozzle can be described as an inverted bell nozzle
where the flow expands on the outside of the nozzle instead
of being completely constrained by the nozzle walls. From
the throat, the innermost streamlines of the flow follow the
contours of the spike, gradually being turned in the axial
direction. Aerospike rocket nozzles are designed for
consistent performance over a wide range of ambient
pressures.
Traditional converging-diverging nozzles have a single
ambient pressure at which the rocket exhaust gases are
neither over-expanded nor under-expanded. As the operating
conditions move away from the design nozzle pressure ratio
(NPR), a shock or an expansion fan will form at the exit
plane of the converging-diverging nozzle. These result in
reductions in the efficiency of the nozzle. An aerospike
nozzle does not have a solid geometry defining the outer
limits of the flow path in the supersonic region of the flow.
Instead it allows the exhaust gases to expand freely beyond
the throat, via the mechanism of a Prandtl-Meyer expansion
fan.
1.2 Advantages
Smaller nozzle: The truncated spike can be far smaller than
a typical bell nozzle for the same performance, as shown
below. In addition, a spike can give greater performance for
a given length.
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 123
Fig-1: Size comparison of a bell and a plug nozzle
Superior performance: Altitude compensation may result
in greater installed performance.
Lower vehicle drag: The aerospike nozzle fills the base
portion of the vehicle thereby reducing a type of drag called
base drag.
Modular combustion chambers: The linear aerospike
engine is made up of these small, easier to develop, less
expensive thrusters that give the engine greater versatility.
Thrust vectoring: Because the combustion chambers can
be controlled individually, the vehicle can be maneuvered
using differential thrust vectoring. This eliminates the need
for the heavy gimbals and actuators used to vary the
direction of traditional nozzles.
Fig-2: Aerospike thrust vectoring control
Lower vehicle weight: Even though the aerospike tends to
be heavier than the bell nozzle, it shares many major
structural elements with the vehicle reducing overall weight.
1.3 Disadvantages
Cooling: The central spike experiences far greater heat
fluxes than does a bell nozzle. This problem can be
addressed by truncating the spike to reduce the exposed area
and by passing cold cryogenically-cooled fuel through the
spike. The secondary flow also helps to cool the centerbody.
Manufacturing: The aerospike is more complex and
difficult to manufacture than the bell nozzle. As a result, it is
more costly.
Flight experience: No aerospike engine has ever flown in a
rocket application. As a result, little flight design experience
has been gained.
2. DESIGN METHODOLOGY
Design of the aerospike nozzle mainly refers to the design of
the central spike and the determination of angle of the
primary nozzle. Method of characteristics in conjunction
with the streamline conditions of A.H.Shapiro is used for the
design of aerospike nozzle contour. A point on the
characteristic line where it satisfies streamline condition will
be the point on spike contour. Since the method of
characteristics solution is based upon a start line slightly
greater than Mach number equal to one, it is apparent that
the complete supersonic flow field and the nozzle
performance is governed by this parameter.
The gas expansion process in the flow field of the plug
nozzle is assumed to be isentropic, adiabatic, and
frictionless. The method of characteristics is logically and
physically applicable for determining pertinent parameters
throughout the flow field of a supersonic isentropic plug
nozzle. Expansion process is determined by the Prandtl-
Meyer expansion function, which follows the below
equation
ʋ =
𝛾+1
𝛾−1
1
2
𝑡𝑎𝑛−1 𝛾−1
𝛾+1
𝑀2
− 1
1
2
− 𝑡𝑎𝑛−1
𝑀2
− 1
1
2
Because of the aerospike nozzle geometry, the physical
nozzle throat area is not normal to the engine centerline but
is inclined by a specific angle. Because of the nozzle
contour geometry, the combustion products are accelerated
differently such that an axisymmetric sonic line shape, even
with regard to the nozzle throat centerline, is not formed.
The flow direction at the throat is set at an angle equal to the
Prandtl-Meyer expansion angle associated with the user-
defined exit Mach number. The Prandtl-Meyer expansion
fan is centered at the tip of the cowl and its location is user-
defined. Fig-3 shows the schematic of the aerospike nozzle
displaying the characteristic lines representing Prandtl-
Meyer expansion waves.
Fig-3: Schematic of the axisymmetric aerospike nozzle
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 124
The technique used in this paper to calculate the contours of
the aerospike nozzles are similar to the technique outlined
by Lee and Thompson, 1964 [4]. For the traditional
aerospike nozzle, the technique used in this paper defines
the location of the ends of the throat and sets the flow
direction at the throat equal to the Prandtl-Meyer expansion
angle associated with the desired exit Mach number. Unlike
the technique outlined by Lee and Thompson [4], the
calculations used in this paper step forward through the
expansion fan by a user-defined Prandtl-Meyer expansion
angle increment. The intersection of the characteristics
emanating from the expansion point on one end of the throat
and the Stream Function originating from the last point
calculated on the nozzle’s contour define the nozzle’s
contour. The Prandtl-Meyer expansion fan is stepped
through until the Mach number along the characteristic
being analyzed is greater than or equal to the desired user-
defined exit Mach number, in which case, the intersection of
the Stream Function and characteristic signify the location
of the last point on the nozzle’s contour. The geometry of an
aerospike nozzle and the parameters involved can be seen
from Fig-4.
Fig-4: Geometry of an aerospike nozzle
This method becomes more accurate as the number of
characteristics used in the calculation increases, which
means choosing less value for the change in Prandtl-Meyer
angle. The increased number of characteristics also results in
a smoother contour.
2.1 Mach Number Calculation
The work presented in this paper uses the Bisection Method
to calculate the Mach number associated with each flow
field point’s Prandtl-Meyer expansion angle. The program
sets the lower limit of the range to a Mach number of 1 and
the upper limit to a 100 times the desired exit Mach number.
The program then calculates an initial guess Mach number
by averaging the range’s limits.
𝑀𝑎 𝑔𝑢𝑒𝑠𝑠 =
𝑀𝑎 𝑢𝑝𝑝𝑒𝑟 + 𝑀𝑎𝑙𝑜𝑤𝑒𝑟
2
Calculate the Prandtl-Meyer expansion angle of the guess
Mach number using Prandtl-Meyer expansion function and
compares it to the Prandtl-Meyer expansion angle of the
point. If the Prandtl-Meyer expansion angle of the guess
Mach number is greater than the point’s Prandtl- Meyer
expansion angle, the program sets the upper limit of the
range equal to the guess Mach number. If the Prandtl-Meyer
expansion angle of the guess Mach number is less than the
Prandtl-Meyer expansion angle of the point, set the lower
limit of the range equal to the guess Mach number. If the
difference between the Prandtl-Meyer expansion angle of
the guess Mach number and the point’s Prandtl-Meyer
expansion angle is greater than abs(1e-10
), recalculate a new
guess Mach number using the new range limits. If the
difference in Prandtl-Meyer expansion angles is less than
abs(1e-10
), the guess Mach number will be the point’s Mach
number.
3. NUMERICAL SIMULATION
This section describes numerical modeling and analysis of
external flow of the aerospike nozzle with different plug
shapes using a commercial CFD code ANSYS FLUENT.
The objective of the analysis is comparison of flow patterns
produced by aerospike nozzles with different plug shapes.
Numerical modeling also helps us to validate the design of
the Aerspike nozzle by comparing the expected Mach
number with the Mach number obtained from the numerical
simulation
3.1 Geometry
Using the design theory mentioned in the previous section
Aerospike nozzle contours with the nozzle length
percentages of 25, 40, and 50 are designed. The parameters
used as input for the determination of the contour co-
ordinates are as follows
Expected exhaust Mach number: Me = 3
Propellant: Ethanol-Oxygen, =1.21
Throat Radius: rt = 0.0508 m = 2 inch
Change in Prandtl-meyer angle Δ = 0.005
Truncation values are taken in accordance with
considerations regarding the thermal and structural
capabilities of the nozzle. But the increased amounts of
truncation leads to the larger base radius which in turn leads
to the larger Throat radius, so the design uses constant throat
radius. The Co-ordinates of the expansion points and the
throat angle which is equal to the max are calculated using
the Prandtl-Meyer equation.
Expansion point: (1.778, 13.3099)
Throat Angle: t = 62.7508o
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 125
Fig-5: Geometrical model of the 25% nozzle
Fig-6: Geometrical model of the 40% nozzle
Fig-7: Geometrical model of the 50% nozzle
Using the co-ordinates of expansion point, nozzle contour
co-ordinates, and Throat angle that are calculated,
geometrical models are developed and meshed using the
GAMBIT, a commercial modeling and meshing tool. Co-
ordinate data obtained from the design calculations is to
create the contour surface of the nozzle. The primary nozzle
is created by using the two geometrical arcs approximating
the convergent section at the throat.
3.2 Mesh
The solution domain, in all the cases, is discretized using a
structured grid of quadrilateral cells. As the geometric
variations in regions surrounding to the nozzle surface make
it impossible to generate a structured grid with acceptable
quality, domain has been divide into two different faces.
Different geometries of the truncated aerospike nozzles
cause the solution domain to have different number of cells.
Total number of grid cells for the 25%, 40%, and 50% cases
are 6566, 7388, and 7241, respectively.
Fig-8: Grid for analysis of 25% nozzle
Fig-9: Grid for analysis of 40% nozzle
Fig-10: Grid for analysis of 50% nozzle
3.3 Boundary Conditions
The condition implied at the inlet of the convergent section
is inlet with specified mass flow, with the following
boundary values, these values are taken from the Reference
[5]
Mass flow rate (m) = 3.25757 kg/s
Temperature (T) = 1577.826 K
Pressure (P) = 2045430 N/m2
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 126
Fig-11 Solution domain and boundary conditions
Boundary values are calculated using the nozzle design
parameters mentioned below
Chamber Pressure: P1 = 2067857 N/m2
Design Altitude: h = 3657.6 m
Mass Flow: m = 3.25758 kg/s
Three different cases of atmospheric conditions (Patm/Pdes)
have been analyzed, which correspond to different working
conditions (including over-expansion, optimum and
underexpansion). Boundary values imposed at pressure
farfield and pressure outlet boundaries in the 3 cases are
presented in Table-1. Apart from case 2, which corresponds
to the nozzles’ optimum working condition, the other cases
have been chosen hypothetically to test the effect of
truncation on nozzle performance. Supersonic Mach
numbers of external flow has been selected for all cases in
order to facilitate convergence criteria. Atmospheric
conditions at the design altitudes are given as input pressure
and temperature at farfield and pressure outlet conditions.
Table-1: Values imposed at farfield boundaries
CASE
Patm/Pdes
P
(N/m2
)
Exit
Mach
Temp. at
Farfield
(K)
Outle
t
Temp
. (K)
1 1.57 101325 1.5 288.15 350
2 1 64434 3 264.37 480
3 0.10 6410 3 216.61 420
3.4 Initial Conditions
Numerical values of flow variables vary greatly in different
regions of the solution domain in analysis of a nozzle with
external flow. For example, while values close to stagnation
properties prevail at the inlet of the convergent section, the
external flow might involve substantially lower pressures
and higher (even supersonic) Mach numbers. In such
circumstances, initialization of a flow variable with a
constant value throughout the entire domain can make
convergence difficult or sometimes impossible. To deal with
this problem, properties at nozzle inlet, throat and exhaust
surface, which can be roughly estimated using one
dimensional isentropic flow relations, have been used to
define custom field functions describing initial values of
axial velocity, pressure and temperature in regions
surrounding the nozzle.
3.5 Analysis Features
The nozzle contours were built on the assumption of
inviscid, irrotational, isentropic flow. So In CFD Simulation
the fluid is considered as inviscid. The isentropic
assumption, which implies irrotationality, was achieved by
assigning the specific heat at constant pressure as a constant
property of the working fluid. Combustion products have
been assumed to behave as a compressible ideal gas
(P=RT). The coupled implicit method has been used for
solution of the four governing equations (continuity,
conservation of momentum in longitudinal and radial
directions, and conservation of energy), considering severe
compressibility effects existing in the solution domain.
Fluxes of convected variables at cell walls are approximated
by the first order upwind scheme. Courant’s number has
been set to 0.5 for all the cases. This criterion has been
posed for convergence. One is reduction of the global
residual of solution of all governing equations to the order
of 10-5
, and the other is establishment of mass balance
between inlet, far-field and outlet boundaries, which is
checked by integration of mass flow through the mentioned
boundaries at each iteration. In all cases, the solution
process has been continued until both criteria have been
satisfied.
4. RESULTS AND DISCUSSIONS
Total study is divided into three different cases which
represent to under-expansion, ideal/ designed conditions and
over-expansion conditions.
Exhaust flow of the aerospike nozzle is characterized by
formation of a series of expansion waves, which originate
from the upper lip of the convergent section. Since the
exhaust flow is not bounded by a solid wall, these expansion
waves can adjust their intensity and domain to match the
exhaust flow with the external flow, which gives an
advantage of the altitude compensation in contrast to the
conventional nozzle.
4.1 Over Expansion Conditions
This case corresponds to the simulation of 25%, 40%, and
50% nozzles in the over-expansion conditions where the
pressure ratio Patm/Pdes=1.57. In this case we can observe
that the expansion waves originating from the upper lip of
the convergent primary nozzle extend even after the surface
of the nozzle meeting at the midpoint of the base, and the
secondary expansion can be observed at the tip of the
truncated portions. These expansion waves continue their
way in spite of the larger amounts of the compression which
leads to the over-expansion conditions. The effect of over-
expansion can be clearly seen in Fig-12(b), 12(d), and 12(f)
showing path-lines of the exhaust flow in the region before
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 127
the stagnation point. After the stagnation point the exhaust
gases flow parallel to the axis of symmetry. The stagnation
point in all three nozzles, with varying truncation
percentages, is nearer to the surface of the truncation base
reducing the area of recirculation region where the two
vortices are formed. This phenomenon can be clearly
observed in the above mentioned figures.
An oblique shock can be seen in this case, which can be
observed in Fig-12(a), 12(c), and 12(e) showing the Mach
number contours, nearer to the surfaces surrounding the
upper lip of the primary nozzle. These waves are
characterized by high velocities and expansion. But in all
the cases the effect of the shock waves on the exhaust flow
is in negligible range. In comparison between all the three
nozzles, it is observed that effect of shock waves is more in
the 25 % nozzle.
In the Fig-12(a), 12(c), and 12(e) showing the contours of
Mach number it is observed that the velocity of the flow in
all the cases is within the expected range. Velocity of flow is
gradually increased to give the optimum Mach number at
the end of the nozzle after the stagnation point. Mach
number of the exhaust flow of the three cases are obtained
as 2.6 in 25 % nozzle, 3.15 Mach in 40 % nozzle, 3.2 Mach
in the 50% nozzle.
4.2 Ideal Conditions
In this case discussion is about the results of simulation of
aerospike nozzle with varying truncation amounts at design
altitude.
At this condition the flow density pattern is same to that of
the previous case but the flow keeps on diverging even after
the stagnation point. But this divergence is contained by the
surrounding domain. From the Fig-13(b), 13(d), and 13(f)
showing the path-lines colored by density it is observed that
expansion characterized by decrease in density occurs at the
tip of the primary nozzle and again at the tip of the truncated
portion of the nozzle. This expansion is followed by the
compression of exhaust gases by the surrounding domain.
From the Fig-13 the position of the stagnation point and the
recirculation area forming two vortices at the base of
truncated nozzle can be observed. And the expansion waves
are extended even after the end of the nozzle surface. But
from the contours of Mach number shown in the Fig-13(a),
13(c), and 13(e) it is observed that velocity of the exhaust
flow is gradually increasing even during the compression
phase reaching the maximum Mach number of 3.2, 3.7 and
3.5 Mach for 25 %, 40% and 50% nozzles respectively at
the exit of the nozzle.
4.3 Under Expansion Condition
In this case the expansion waves originated from the upper
lip of the primary nozzle face the truncated portion of the
plug. The flow facing the truncation first encounters a sharp
expansion, then by continuing its way to the centre of the
plug base. From this point flow passes through compression
by the atmosphere and the flow meets at the stagnation point
where the flow properties are nearer to the ideal flow
conditions. This phenomenon is due to the formation of two
symmetric vortices in the base of the plug, which counteract
the effect of each other at two locations, one of which is
located at the center of the plug base, where the stagnation
conditions prevail. This can be noticed in the Fig-14(b),
14(d), and 14(f) showing the pathlines.
In this case it is clearly visible that the series of expansion
waves started at the upper lip of the primary nozzle got
compressed by the atmosphere; this compression is
characterized by increase in the density and reduction in the
velocity of the exhaust flow. Effect of these expansion and
compression waves on pathlines is such that the exhaust
flow leaves the exit surface straight after the point of
stagnation the exhaust gases flow parallel to the axis leaving
no residuals, thus producing a great deal of thrust. This can
be observed in the Fig-14(b), 14(d), and 14(f) showing the
pathlines colored by density.
From the contours of Mach number in Fig-14(a), 14(c), and
14(e) we can observe that velocity of the flow during the
expansion after leaving the primary nozzle is high and this
velocity is later reduced by the compression of flow
achieving a Mach number of 1.5, 3.5, and 1.72 at the nozzle
exit for 25%, 40%, and 50% nozzles respectively.
4.4 Comparison of Results
In all the cases exhaust flow of the aerospike nozzle is
characterized by formation of a series of expansion waves,
which originate from the upper lip of the convergent section.
Since the exhaust flow is not bounded by a solid wall, these
expansion waves can adjust their intensity and domain to
match the exhaust flow with the external flow, which gives
an advantage of the altitude compensation in contrast to the
conventional nozzle.
It should be pointed out that regardless of the amount of
truncation and the extent of the plug base area, the flow
parameter distribution pattern is the same. Figures showing
pathlines clearly show the above-mentioned process in form
of path-lines colored by density for different plug shapes. It
should also be noted that in spite of existence of rotational
flow at the base area, path-lines will continue their way
parallel to the axis of the plug after the longitudinal position
corresponding to the end of a virtual ideal plug, even for the
25% truncated aerospike nozzle. In order to understand the
concept of thrust delivery by different truncated aerospike
nozzles in under-expansion conditions, design conditions
and in over-expansion conditions, it is necessary to approach
the thrust components differently. Dividing thrust into the
three following components, explains this phenomenon
more clearly
1) Thrust produced by the nozzle convergent section
2) Thrust produced by the plug surface
3) Thrust produced by the plug base
In under-expansion conditions, when the plug is truncated,
its lateral area decreases. Therefore the pressure thrust
produced by the plug reduces. On the other hand, thrust
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 128
generated by the base region increases because of the
increase of the base area. These two effects compensate
each other, and the total nozzle thrust becomes almost the
same for different nozzle truncation. This effect can be seen
most clearly for the 25% plug.
But in over-expansion conditions, the situation is totally
different. In these conditions, as the nozzle length becomes
shorter, hence decreasing the plug area, thrust produced by
the plug still decreases, while as the atmosphere pressure is
higher than the exhaust pressure thrust produced by the base
pressure would have a negative value. So by increasing
truncation, the negative value of base thrust will increase,
hence decreasing total thrust in over-expansion conditions.
It can be concluded that for the 25% plug, total thrust is
lowest. At low altitudes (i.e., over-expansion conditions)
base pressure linearly increases as atmospheric pressure
increases. At high altitudes, pressure at the base remains
constant despite variation of altitude. As the altitude
increases, atmospheric pressure decreases and the difference
between base pressure and atmospheric pressure increases,
hence increasing the base thrust.
(a) Contours of Mach Number, 25% nozzle (b) Pathlines, 25% nozzle
(c) Contours of Mach Number, 40% nozzle (d) Pathlines, 40% nozzle
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(e) Contours of Mach Number, 50% nozzle (f) Pathlines, 50% nozzle
Fig-12: Flow pattern of the ideal and truncated aerospike nozzles in over-expansion conditions (Patm/Pdes = 1.57)
(a) Contours of Mach Number, 25% nozzle (b) Pathlines, 25% nozzle
(c) Contours of Mach Number, 40% nozzle (d) Pathlines, 40% nozzle
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(e) Contours of Mach Number, 50% nozzle (f) Pathlines, 50% nozzle
Fig-13: Flow pattern of the ideal and truncated aerospike nozzles in design conditions (Patm/Pdes = 1.00)
(a) Contours of Mach Number, 25% nozzle (b) Pathlines, 25% nozzle
(c) Contours of Mach Number, 40% nozzle (d) Pathlines, 40% nozzle
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
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(e) Contours of Mach Number, 50% nozzle (f) Pathlines, 50% nozzle
Fig-14: Flow pattern of the ideal and truncated aerospike nozzles in under-expansion conditions (Patm/Pdes = 0.10)
6. CONCLUSIONS
The results clearly indicate that the aerospike nozzle is
capable of producing the optimum performance at different
altitudes.
From the simulation results we know that the base pressure
compensates the loss of thrust in under-expansion
conditions, plug truncation has minor effect on the loss of
thrust in these conditions. But in over-expansion, thrust loss
will increase with the increase of truncation. Base pressure
thrust is closely related to variation of base pressure with
atmospheric pressure. Base pressure is constant in under-
expansion conditions, but increase with the increase of the
atmospheric pressure in over-expansion conditions.
Based on the observed behavior of the exhaust flow, it can
be concluded that the 40 % truncated nozzle is
recommended. Because its flow pattern shows the signs of
optimum performance and it has achieved the desired exit
Mach number in all the three altitude conditions.
REFERENCES
[1]. Angelino G., “Approximation Method for Plug Nozzle
Design”, AIAA Journal, Vol. 2, No. 10, Oct. 1964, pp. 1834-
1835.
[2]. Gross, Klaus W., "Performance Analysis of Aerospike
Rocket Engines," 1972.
[3] Lee, C. C., “Computation of plug nozzle contours by the
Rao’s optimum thrust method”, NASA CR-21914 R-61,
1963.
[4]. Lee, C. C., Inman. S. J., “Numerical analysis of plug
nozzles by the Method of characteristics”, NASA
TECHNICAL NOTE R-10, 1964.
[5]. Besnard, E., H. H. Chen, T. Mueller and J. Garvey,
“Design, Manufacturing and Test of a Plug Nozzle Rocket
Engine”, AIAA Paper 2002-4038, 2002.
[6]. Naghib Lahouti, A., Nazarinia, M. and Tolouei, E.,
“Design and numerical analysis of aerospike nozzles with
different plug shapes to compare their performance with a
conventional nozzle”, The Eleventh Australian International
Aerospace Congress, Melbourne, Australia, 13-17 March
(2005).
[7]. Tomita, T. et al., Nobuhiko, K. and Ogawara, A., "A
conceptual system design study for a linear aerospike engine
applied to a future SSTO Vehicle," The 46th
AIAA/ASME/SAE/ASEE Joint Propulsion Conference and
Exhibit, AIAA-2010-7060, 2010.
[8]. Sakamoto, H., Takahashi M., Sasaki, M., Tomita, T.,
Kusaka K. and Tamura H., “An Experimental Study on a
14KN Linear Aerospike Nozzle Combustor”, AIAA Paper
99- 2761, 1999.
[9]. Chang Hui Wang, Yu Liu, Li Zi Qin, “Aerospike nozzle
contour design and its performance validation”, Acta
Astronautica 64 1264-1275, 2009.
BIOGRAPHIES
L. Vinay Kumar pursuing his M.Tech. in
Aerospace Engineering from Aurora’s
Scientific and Technological Institute,
Hyderabad, India. He did his B.tech from
TKR College of Engineering, Hyderabad.
His research interests include
Computational Fluid Dynamics, H.V.A.C. Systems, clean-
room designing, Green building technologies.
Dr.K.Srinivasa Reddy completed his
B.E. in Mechanical Engineering with
honours from NIT Silchar, Assam. He
did his M.Tech. in IC Engines and Gas
Turbines from NIT Warangal, Telegana.
He was awarded with Ph.D. degree in
Mechanical Engineering in the year 2011 by JNTUH,
Kukatpally. He has 18 years of teaching experience and 2
years of R&D experience. His research interests include
fluid dynamics, Heat Transfer and development of non-
conventional energy utilisation technologies

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Design and flow simulation of truncated aerospike nozzle

  • 1. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 122 DESIGN AND FLOW SIMULATION OF TRUNCATED AEROSPIKE NOZZLE Vinay Kumar Levaka1 , Srinivasa Reddy K2 1 Student, Aerospace dept., Aurora’s Scientific and Technological Institute, Hyderabad, Telangana, India 2 Professor, C.M.R. Engineering College, Hyderabad, Telangana, India Abstract Aerospike nozzles are being considered in the development of the Single Stage to Orbit launching vehicles because of their prominent features and altitude compensating characteristics. This paper presents the design of aerospike nozzles using characteristic method in conjunction with streamline function, and performance study through numerical simulation using commercial Computational Fluid Dynamics (CFD) code ANSYS FLUENT. For this purpose nozzles with truncation lengths of 25%, 40%, 50% are choosen, because of the thermal and structural complications in the ideal aerospike nozzle. Simulation of the flow is carried out at three different altitude conditions representing Under-expansion, Ideal, and over-expansion conditions of the flow. FLUENT predictions were used to verify the isentropic flow assumption and that the working fluid reached the design exit Mach number. The flow-fields obtained through the numerical simulation are analysed to know the effect of truncation on the performance of aerospike nozzle. Optimum percentage of the truncation is selected by the comparison of nozzles with different lengths of truncation under various altitude parameters. The results show that the flow pattern of the nozzles under different altitude conditions are almost similar. The 40 % truncated nozzle is found to give optimum performance and it has achieved the desired exit Mach number in all the three altitude conditions. Keywords: Aerospike Nozzle, Single Stage to Orbit (SSTO), Linear Aerospike, Truncation and Rocket Nozzle --------------------------------------------------------------------***---------------------------------------------------------------------- 1. INTRODUCTION Ever since jet and rocket propulsion systems have emerged, researchers have invented and implemented many types of nozzles, mainly to increase the thrust performance of nozzles in off-design working conditions. Among these various designs, features of the aerospike nozzle have attracted researchers since mid-1950s. Many theoretical studies of the aerospike nozzle have been carried out in 1960s. In early 1970s, thermal and strength problems of the aerospike nozzle and development of more efficient methods for fabrication of conventional nozzles led to a decline in research activities in this field. Development of the nozzle with the capability of producing optimum amounts of thrust in wide ranges of altitude has been a subject of continuous dedicated efforts within the community of rocket propulsion. The phenomenon of producing optimum amounts of thrust by a rocket nozzle in off-design conditions is called as altitude compensation. Nozzles with the altitude compensation characteristics are basic feature in realizing the development of Single Stage to Orbit (SSTO) vehicles. Reusable SSTO vehicles offer the promise of reduced launch expenses by eliminating recurring costs associated with hardware replacement inherent in expendable launch systems.The most popular altitude compensating rocket nozzle to date is the aerospike nozzle, the origin of which dates back to Rocketdyne in 1950s. 1.1 Aerospike nozzle An aerospike nozzle has a spike in the center of the nozzle. Aerospike nozzle can be described as an inverted bell nozzle where the flow expands on the outside of the nozzle instead of being completely constrained by the nozzle walls. From the throat, the innermost streamlines of the flow follow the contours of the spike, gradually being turned in the axial direction. Aerospike rocket nozzles are designed for consistent performance over a wide range of ambient pressures. Traditional converging-diverging nozzles have a single ambient pressure at which the rocket exhaust gases are neither over-expanded nor under-expanded. As the operating conditions move away from the design nozzle pressure ratio (NPR), a shock or an expansion fan will form at the exit plane of the converging-diverging nozzle. These result in reductions in the efficiency of the nozzle. An aerospike nozzle does not have a solid geometry defining the outer limits of the flow path in the supersonic region of the flow. Instead it allows the exhaust gases to expand freely beyond the throat, via the mechanism of a Prandtl-Meyer expansion fan. 1.2 Advantages Smaller nozzle: The truncated spike can be far smaller than a typical bell nozzle for the same performance, as shown below. In addition, a spike can give greater performance for a given length.
  • 2. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 123 Fig-1: Size comparison of a bell and a plug nozzle Superior performance: Altitude compensation may result in greater installed performance. Lower vehicle drag: The aerospike nozzle fills the base portion of the vehicle thereby reducing a type of drag called base drag. Modular combustion chambers: The linear aerospike engine is made up of these small, easier to develop, less expensive thrusters that give the engine greater versatility. Thrust vectoring: Because the combustion chambers can be controlled individually, the vehicle can be maneuvered using differential thrust vectoring. This eliminates the need for the heavy gimbals and actuators used to vary the direction of traditional nozzles. Fig-2: Aerospike thrust vectoring control Lower vehicle weight: Even though the aerospike tends to be heavier than the bell nozzle, it shares many major structural elements with the vehicle reducing overall weight. 1.3 Disadvantages Cooling: The central spike experiences far greater heat fluxes than does a bell nozzle. This problem can be addressed by truncating the spike to reduce the exposed area and by passing cold cryogenically-cooled fuel through the spike. The secondary flow also helps to cool the centerbody. Manufacturing: The aerospike is more complex and difficult to manufacture than the bell nozzle. As a result, it is more costly. Flight experience: No aerospike engine has ever flown in a rocket application. As a result, little flight design experience has been gained. 2. DESIGN METHODOLOGY Design of the aerospike nozzle mainly refers to the design of the central spike and the determination of angle of the primary nozzle. Method of characteristics in conjunction with the streamline conditions of A.H.Shapiro is used for the design of aerospike nozzle contour. A point on the characteristic line where it satisfies streamline condition will be the point on spike contour. Since the method of characteristics solution is based upon a start line slightly greater than Mach number equal to one, it is apparent that the complete supersonic flow field and the nozzle performance is governed by this parameter. The gas expansion process in the flow field of the plug nozzle is assumed to be isentropic, adiabatic, and frictionless. The method of characteristics is logically and physically applicable for determining pertinent parameters throughout the flow field of a supersonic isentropic plug nozzle. Expansion process is determined by the Prandtl- Meyer expansion function, which follows the below equation ʋ = 𝛾+1 𝛾−1 1 2 𝑡𝑎𝑛−1 𝛾−1 𝛾+1 𝑀2 − 1 1 2 − 𝑡𝑎𝑛−1 𝑀2 − 1 1 2 Because of the aerospike nozzle geometry, the physical nozzle throat area is not normal to the engine centerline but is inclined by a specific angle. Because of the nozzle contour geometry, the combustion products are accelerated differently such that an axisymmetric sonic line shape, even with regard to the nozzle throat centerline, is not formed. The flow direction at the throat is set at an angle equal to the Prandtl-Meyer expansion angle associated with the user- defined exit Mach number. The Prandtl-Meyer expansion fan is centered at the tip of the cowl and its location is user- defined. Fig-3 shows the schematic of the aerospike nozzle displaying the characteristic lines representing Prandtl- Meyer expansion waves. Fig-3: Schematic of the axisymmetric aerospike nozzle
  • 3. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 124 The technique used in this paper to calculate the contours of the aerospike nozzles are similar to the technique outlined by Lee and Thompson, 1964 [4]. For the traditional aerospike nozzle, the technique used in this paper defines the location of the ends of the throat and sets the flow direction at the throat equal to the Prandtl-Meyer expansion angle associated with the desired exit Mach number. Unlike the technique outlined by Lee and Thompson [4], the calculations used in this paper step forward through the expansion fan by a user-defined Prandtl-Meyer expansion angle increment. The intersection of the characteristics emanating from the expansion point on one end of the throat and the Stream Function originating from the last point calculated on the nozzle’s contour define the nozzle’s contour. The Prandtl-Meyer expansion fan is stepped through until the Mach number along the characteristic being analyzed is greater than or equal to the desired user- defined exit Mach number, in which case, the intersection of the Stream Function and characteristic signify the location of the last point on the nozzle’s contour. The geometry of an aerospike nozzle and the parameters involved can be seen from Fig-4. Fig-4: Geometry of an aerospike nozzle This method becomes more accurate as the number of characteristics used in the calculation increases, which means choosing less value for the change in Prandtl-Meyer angle. The increased number of characteristics also results in a smoother contour. 2.1 Mach Number Calculation The work presented in this paper uses the Bisection Method to calculate the Mach number associated with each flow field point’s Prandtl-Meyer expansion angle. The program sets the lower limit of the range to a Mach number of 1 and the upper limit to a 100 times the desired exit Mach number. The program then calculates an initial guess Mach number by averaging the range’s limits. 𝑀𝑎 𝑔𝑢𝑒𝑠𝑠 = 𝑀𝑎 𝑢𝑝𝑝𝑒𝑟 + 𝑀𝑎𝑙𝑜𝑤𝑒𝑟 2 Calculate the Prandtl-Meyer expansion angle of the guess Mach number using Prandtl-Meyer expansion function and compares it to the Prandtl-Meyer expansion angle of the point. If the Prandtl-Meyer expansion angle of the guess Mach number is greater than the point’s Prandtl- Meyer expansion angle, the program sets the upper limit of the range equal to the guess Mach number. If the Prandtl-Meyer expansion angle of the guess Mach number is less than the Prandtl-Meyer expansion angle of the point, set the lower limit of the range equal to the guess Mach number. If the difference between the Prandtl-Meyer expansion angle of the guess Mach number and the point’s Prandtl-Meyer expansion angle is greater than abs(1e-10 ), recalculate a new guess Mach number using the new range limits. If the difference in Prandtl-Meyer expansion angles is less than abs(1e-10 ), the guess Mach number will be the point’s Mach number. 3. NUMERICAL SIMULATION This section describes numerical modeling and analysis of external flow of the aerospike nozzle with different plug shapes using a commercial CFD code ANSYS FLUENT. The objective of the analysis is comparison of flow patterns produced by aerospike nozzles with different plug shapes. Numerical modeling also helps us to validate the design of the Aerspike nozzle by comparing the expected Mach number with the Mach number obtained from the numerical simulation 3.1 Geometry Using the design theory mentioned in the previous section Aerospike nozzle contours with the nozzle length percentages of 25, 40, and 50 are designed. The parameters used as input for the determination of the contour co- ordinates are as follows Expected exhaust Mach number: Me = 3 Propellant: Ethanol-Oxygen, =1.21 Throat Radius: rt = 0.0508 m = 2 inch Change in Prandtl-meyer angle Δ = 0.005 Truncation values are taken in accordance with considerations regarding the thermal and structural capabilities of the nozzle. But the increased amounts of truncation leads to the larger base radius which in turn leads to the larger Throat radius, so the design uses constant throat radius. The Co-ordinates of the expansion points and the throat angle which is equal to the max are calculated using the Prandtl-Meyer equation. Expansion point: (1.778, 13.3099) Throat Angle: t = 62.7508o
  • 4. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 125 Fig-5: Geometrical model of the 25% nozzle Fig-6: Geometrical model of the 40% nozzle Fig-7: Geometrical model of the 50% nozzle Using the co-ordinates of expansion point, nozzle contour co-ordinates, and Throat angle that are calculated, geometrical models are developed and meshed using the GAMBIT, a commercial modeling and meshing tool. Co- ordinate data obtained from the design calculations is to create the contour surface of the nozzle. The primary nozzle is created by using the two geometrical arcs approximating the convergent section at the throat. 3.2 Mesh The solution domain, in all the cases, is discretized using a structured grid of quadrilateral cells. As the geometric variations in regions surrounding to the nozzle surface make it impossible to generate a structured grid with acceptable quality, domain has been divide into two different faces. Different geometries of the truncated aerospike nozzles cause the solution domain to have different number of cells. Total number of grid cells for the 25%, 40%, and 50% cases are 6566, 7388, and 7241, respectively. Fig-8: Grid for analysis of 25% nozzle Fig-9: Grid for analysis of 40% nozzle Fig-10: Grid for analysis of 50% nozzle 3.3 Boundary Conditions The condition implied at the inlet of the convergent section is inlet with specified mass flow, with the following boundary values, these values are taken from the Reference [5] Mass flow rate (m) = 3.25757 kg/s Temperature (T) = 1577.826 K Pressure (P) = 2045430 N/m2
  • 5. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 126 Fig-11 Solution domain and boundary conditions Boundary values are calculated using the nozzle design parameters mentioned below Chamber Pressure: P1 = 2067857 N/m2 Design Altitude: h = 3657.6 m Mass Flow: m = 3.25758 kg/s Three different cases of atmospheric conditions (Patm/Pdes) have been analyzed, which correspond to different working conditions (including over-expansion, optimum and underexpansion). Boundary values imposed at pressure farfield and pressure outlet boundaries in the 3 cases are presented in Table-1. Apart from case 2, which corresponds to the nozzles’ optimum working condition, the other cases have been chosen hypothetically to test the effect of truncation on nozzle performance. Supersonic Mach numbers of external flow has been selected for all cases in order to facilitate convergence criteria. Atmospheric conditions at the design altitudes are given as input pressure and temperature at farfield and pressure outlet conditions. Table-1: Values imposed at farfield boundaries CASE Patm/Pdes P (N/m2 ) Exit Mach Temp. at Farfield (K) Outle t Temp . (K) 1 1.57 101325 1.5 288.15 350 2 1 64434 3 264.37 480 3 0.10 6410 3 216.61 420 3.4 Initial Conditions Numerical values of flow variables vary greatly in different regions of the solution domain in analysis of a nozzle with external flow. For example, while values close to stagnation properties prevail at the inlet of the convergent section, the external flow might involve substantially lower pressures and higher (even supersonic) Mach numbers. In such circumstances, initialization of a flow variable with a constant value throughout the entire domain can make convergence difficult or sometimes impossible. To deal with this problem, properties at nozzle inlet, throat and exhaust surface, which can be roughly estimated using one dimensional isentropic flow relations, have been used to define custom field functions describing initial values of axial velocity, pressure and temperature in regions surrounding the nozzle. 3.5 Analysis Features The nozzle contours were built on the assumption of inviscid, irrotational, isentropic flow. So In CFD Simulation the fluid is considered as inviscid. The isentropic assumption, which implies irrotationality, was achieved by assigning the specific heat at constant pressure as a constant property of the working fluid. Combustion products have been assumed to behave as a compressible ideal gas (P=RT). The coupled implicit method has been used for solution of the four governing equations (continuity, conservation of momentum in longitudinal and radial directions, and conservation of energy), considering severe compressibility effects existing in the solution domain. Fluxes of convected variables at cell walls are approximated by the first order upwind scheme. Courant’s number has been set to 0.5 for all the cases. This criterion has been posed for convergence. One is reduction of the global residual of solution of all governing equations to the order of 10-5 , and the other is establishment of mass balance between inlet, far-field and outlet boundaries, which is checked by integration of mass flow through the mentioned boundaries at each iteration. In all cases, the solution process has been continued until both criteria have been satisfied. 4. RESULTS AND DISCUSSIONS Total study is divided into three different cases which represent to under-expansion, ideal/ designed conditions and over-expansion conditions. Exhaust flow of the aerospike nozzle is characterized by formation of a series of expansion waves, which originate from the upper lip of the convergent section. Since the exhaust flow is not bounded by a solid wall, these expansion waves can adjust their intensity and domain to match the exhaust flow with the external flow, which gives an advantage of the altitude compensation in contrast to the conventional nozzle. 4.1 Over Expansion Conditions This case corresponds to the simulation of 25%, 40%, and 50% nozzles in the over-expansion conditions where the pressure ratio Patm/Pdes=1.57. In this case we can observe that the expansion waves originating from the upper lip of the convergent primary nozzle extend even after the surface of the nozzle meeting at the midpoint of the base, and the secondary expansion can be observed at the tip of the truncated portions. These expansion waves continue their way in spite of the larger amounts of the compression which leads to the over-expansion conditions. The effect of over- expansion can be clearly seen in Fig-12(b), 12(d), and 12(f) showing path-lines of the exhaust flow in the region before
  • 6. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 127 the stagnation point. After the stagnation point the exhaust gases flow parallel to the axis of symmetry. The stagnation point in all three nozzles, with varying truncation percentages, is nearer to the surface of the truncation base reducing the area of recirculation region where the two vortices are formed. This phenomenon can be clearly observed in the above mentioned figures. An oblique shock can be seen in this case, which can be observed in Fig-12(a), 12(c), and 12(e) showing the Mach number contours, nearer to the surfaces surrounding the upper lip of the primary nozzle. These waves are characterized by high velocities and expansion. But in all the cases the effect of the shock waves on the exhaust flow is in negligible range. In comparison between all the three nozzles, it is observed that effect of shock waves is more in the 25 % nozzle. In the Fig-12(a), 12(c), and 12(e) showing the contours of Mach number it is observed that the velocity of the flow in all the cases is within the expected range. Velocity of flow is gradually increased to give the optimum Mach number at the end of the nozzle after the stagnation point. Mach number of the exhaust flow of the three cases are obtained as 2.6 in 25 % nozzle, 3.15 Mach in 40 % nozzle, 3.2 Mach in the 50% nozzle. 4.2 Ideal Conditions In this case discussion is about the results of simulation of aerospike nozzle with varying truncation amounts at design altitude. At this condition the flow density pattern is same to that of the previous case but the flow keeps on diverging even after the stagnation point. But this divergence is contained by the surrounding domain. From the Fig-13(b), 13(d), and 13(f) showing the path-lines colored by density it is observed that expansion characterized by decrease in density occurs at the tip of the primary nozzle and again at the tip of the truncated portion of the nozzle. This expansion is followed by the compression of exhaust gases by the surrounding domain. From the Fig-13 the position of the stagnation point and the recirculation area forming two vortices at the base of truncated nozzle can be observed. And the expansion waves are extended even after the end of the nozzle surface. But from the contours of Mach number shown in the Fig-13(a), 13(c), and 13(e) it is observed that velocity of the exhaust flow is gradually increasing even during the compression phase reaching the maximum Mach number of 3.2, 3.7 and 3.5 Mach for 25 %, 40% and 50% nozzles respectively at the exit of the nozzle. 4.3 Under Expansion Condition In this case the expansion waves originated from the upper lip of the primary nozzle face the truncated portion of the plug. The flow facing the truncation first encounters a sharp expansion, then by continuing its way to the centre of the plug base. From this point flow passes through compression by the atmosphere and the flow meets at the stagnation point where the flow properties are nearer to the ideal flow conditions. This phenomenon is due to the formation of two symmetric vortices in the base of the plug, which counteract the effect of each other at two locations, one of which is located at the center of the plug base, where the stagnation conditions prevail. This can be noticed in the Fig-14(b), 14(d), and 14(f) showing the pathlines. In this case it is clearly visible that the series of expansion waves started at the upper lip of the primary nozzle got compressed by the atmosphere; this compression is characterized by increase in the density and reduction in the velocity of the exhaust flow. Effect of these expansion and compression waves on pathlines is such that the exhaust flow leaves the exit surface straight after the point of stagnation the exhaust gases flow parallel to the axis leaving no residuals, thus producing a great deal of thrust. This can be observed in the Fig-14(b), 14(d), and 14(f) showing the pathlines colored by density. From the contours of Mach number in Fig-14(a), 14(c), and 14(e) we can observe that velocity of the flow during the expansion after leaving the primary nozzle is high and this velocity is later reduced by the compression of flow achieving a Mach number of 1.5, 3.5, and 1.72 at the nozzle exit for 25%, 40%, and 50% nozzles respectively. 4.4 Comparison of Results In all the cases exhaust flow of the aerospike nozzle is characterized by formation of a series of expansion waves, which originate from the upper lip of the convergent section. Since the exhaust flow is not bounded by a solid wall, these expansion waves can adjust their intensity and domain to match the exhaust flow with the external flow, which gives an advantage of the altitude compensation in contrast to the conventional nozzle. It should be pointed out that regardless of the amount of truncation and the extent of the plug base area, the flow parameter distribution pattern is the same. Figures showing pathlines clearly show the above-mentioned process in form of path-lines colored by density for different plug shapes. It should also be noted that in spite of existence of rotational flow at the base area, path-lines will continue their way parallel to the axis of the plug after the longitudinal position corresponding to the end of a virtual ideal plug, even for the 25% truncated aerospike nozzle. In order to understand the concept of thrust delivery by different truncated aerospike nozzles in under-expansion conditions, design conditions and in over-expansion conditions, it is necessary to approach the thrust components differently. Dividing thrust into the three following components, explains this phenomenon more clearly 1) Thrust produced by the nozzle convergent section 2) Thrust produced by the plug surface 3) Thrust produced by the plug base In under-expansion conditions, when the plug is truncated, its lateral area decreases. Therefore the pressure thrust produced by the plug reduces. On the other hand, thrust
  • 7. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 128 generated by the base region increases because of the increase of the base area. These two effects compensate each other, and the total nozzle thrust becomes almost the same for different nozzle truncation. This effect can be seen most clearly for the 25% plug. But in over-expansion conditions, the situation is totally different. In these conditions, as the nozzle length becomes shorter, hence decreasing the plug area, thrust produced by the plug still decreases, while as the atmosphere pressure is higher than the exhaust pressure thrust produced by the base pressure would have a negative value. So by increasing truncation, the negative value of base thrust will increase, hence decreasing total thrust in over-expansion conditions. It can be concluded that for the 25% plug, total thrust is lowest. At low altitudes (i.e., over-expansion conditions) base pressure linearly increases as atmospheric pressure increases. At high altitudes, pressure at the base remains constant despite variation of altitude. As the altitude increases, atmospheric pressure decreases and the difference between base pressure and atmospheric pressure increases, hence increasing the base thrust. (a) Contours of Mach Number, 25% nozzle (b) Pathlines, 25% nozzle (c) Contours of Mach Number, 40% nozzle (d) Pathlines, 40% nozzle
  • 8. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 129 (e) Contours of Mach Number, 50% nozzle (f) Pathlines, 50% nozzle Fig-12: Flow pattern of the ideal and truncated aerospike nozzles in over-expansion conditions (Patm/Pdes = 1.57) (a) Contours of Mach Number, 25% nozzle (b) Pathlines, 25% nozzle (c) Contours of Mach Number, 40% nozzle (d) Pathlines, 40% nozzle
  • 9. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 130 (e) Contours of Mach Number, 50% nozzle (f) Pathlines, 50% nozzle Fig-13: Flow pattern of the ideal and truncated aerospike nozzles in design conditions (Patm/Pdes = 1.00) (a) Contours of Mach Number, 25% nozzle (b) Pathlines, 25% nozzle (c) Contours of Mach Number, 40% nozzle (d) Pathlines, 40% nozzle
  • 10. IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 11 | Nov-2014, Available @ http://www.ijret.org 131 (e) Contours of Mach Number, 50% nozzle (f) Pathlines, 50% nozzle Fig-14: Flow pattern of the ideal and truncated aerospike nozzles in under-expansion conditions (Patm/Pdes = 0.10) 6. CONCLUSIONS The results clearly indicate that the aerospike nozzle is capable of producing the optimum performance at different altitudes. From the simulation results we know that the base pressure compensates the loss of thrust in under-expansion conditions, plug truncation has minor effect on the loss of thrust in these conditions. But in over-expansion, thrust loss will increase with the increase of truncation. Base pressure thrust is closely related to variation of base pressure with atmospheric pressure. Base pressure is constant in under- expansion conditions, but increase with the increase of the atmospheric pressure in over-expansion conditions. Based on the observed behavior of the exhaust flow, it can be concluded that the 40 % truncated nozzle is recommended. Because its flow pattern shows the signs of optimum performance and it has achieved the desired exit Mach number in all the three altitude conditions. REFERENCES [1]. Angelino G., “Approximation Method for Plug Nozzle Design”, AIAA Journal, Vol. 2, No. 10, Oct. 1964, pp. 1834- 1835. [2]. Gross, Klaus W., "Performance Analysis of Aerospike Rocket Engines," 1972. [3] Lee, C. C., “Computation of plug nozzle contours by the Rao’s optimum thrust method”, NASA CR-21914 R-61, 1963. [4]. Lee, C. C., Inman. S. J., “Numerical analysis of plug nozzles by the Method of characteristics”, NASA TECHNICAL NOTE R-10, 1964. [5]. Besnard, E., H. H. Chen, T. Mueller and J. Garvey, “Design, Manufacturing and Test of a Plug Nozzle Rocket Engine”, AIAA Paper 2002-4038, 2002. [6]. Naghib Lahouti, A., Nazarinia, M. and Tolouei, E., “Design and numerical analysis of aerospike nozzles with different plug shapes to compare their performance with a conventional nozzle”, The Eleventh Australian International Aerospace Congress, Melbourne, Australia, 13-17 March (2005). [7]. Tomita, T. et al., Nobuhiko, K. and Ogawara, A., "A conceptual system design study for a linear aerospike engine applied to a future SSTO Vehicle," The 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA-2010-7060, 2010. [8]. Sakamoto, H., Takahashi M., Sasaki, M., Tomita, T., Kusaka K. and Tamura H., “An Experimental Study on a 14KN Linear Aerospike Nozzle Combustor”, AIAA Paper 99- 2761, 1999. [9]. Chang Hui Wang, Yu Liu, Li Zi Qin, “Aerospike nozzle contour design and its performance validation”, Acta Astronautica 64 1264-1275, 2009. BIOGRAPHIES L. Vinay Kumar pursuing his M.Tech. in Aerospace Engineering from Aurora’s Scientific and Technological Institute, Hyderabad, India. He did his B.tech from TKR College of Engineering, Hyderabad. His research interests include Computational Fluid Dynamics, H.V.A.C. Systems, clean- room designing, Green building technologies. Dr.K.Srinivasa Reddy completed his B.E. in Mechanical Engineering with honours from NIT Silchar, Assam. He did his M.Tech. in IC Engines and Gas Turbines from NIT Warangal, Telegana. He was awarded with Ph.D. degree in Mechanical Engineering in the year 2011 by JNTUH, Kukatpally. He has 18 years of teaching experience and 2 years of R&D experience. His research interests include fluid dynamics, Heat Transfer and development of non- conventional energy utilisation technologies