International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
Control of Response at Payload Base using Isolation System
for a Typical Launch Vehicle with Strap-On-Boosters
Jiji Jolly#1, Rajesh A.K*2
#
*
M tech student, Ilahia college of Engineering and Technology
Asst. Prof. Ilahia college of Engineering and Technology
Abstract: Large solid motors are found to produce oscillations in chamber pressure resulting in thrust
fluctuations during their operation. If the frequency of these oscillations matches with any of the
longitudinal/lateral modes of the launch vehicle, then large responses are possible on the launch vehicle and at
satellites. Generally isolation systems are designed as solution to such problems. The magnitude of response
depends on the amplitude of thrust oscillation .Dynamic response studies are carried out with finite element
model of the launch vehicle. Analysis is also carried out by incorporating an isolation system at the strap-on
core attachment location and the results are compared with that of non-isolation system which shows the
effectiveness of the isolation system. The pressure and thrust data obtained from static firing test of solid motor
are analysed for characterizing the input excitation for the dynamic response studies. From this study,
frequency and the amplitude of the thrust oscillation was obtained. This study shows that the acceleration
responses at satellite locations with and without isolation system.
Keywords: LV (Launch Vehicle), SC (Space craft), Thrust, Pressure, Oscillation, Acceleration, Frequency .
I. INTRODUCTION
Satellites are perhaps among the most
amazing products in use today, used for many
purposes from communications to reconnaissance
to weather prediction, and much more. Excessive
dynamic and shock loads during ascent can be a
satellite killer causing permanent damage to
electronics, optics, and other sensitive equipment.
In order for the spacecraft to survive a trip to orbit,
one of two choices must be made: (1) design all
structure, payloads, and systems on the spacecraft
to be strong enough to survive the high launch
loads, or (2) reduce the magnitude of the high
launch loads. The former is not a good choice
because it typically requires additional cost,
schedule, and weight. The latter is the preferred
choice, launch dynamic loads can be reduced
through the use of vibration isolation.
The goal of a vibration isolation system is
to reduce the dynamic loads on a payload by
reducing the transmission of dynamic loads to the
payload. Whole-spacecraft vibration isolation
systems significantly reduce the dynamic loads in
both the low frequency range (coupled loads
analysis range) and in the high frequency range
(shock loading range).
The isolation system explained in above
literature review has some limitations. The above
papers explain the isolation system which is
provided on top of the launching vehicle. This only
protects the parts above the isolation system. But in
my study we are giving isolation system on the
connecting element between strap on boosters and
the core. It will protect all the sensitive packages in
ISSN: 2231-5381
the core vehicle from the thrust oscillation of the
strap-on-boosters.
II. DETAILS OF THE STRUCTURE
In present study, a typical launching vehicle is
considered. The total height of the launching
vehicle is approximately 54 m. It consists of two
solid boosters which having the height of 27.5 m.
The diameter of the solid boosters is 3 m and the
diameter of the core is 3.8m at the bottom and 4.8
m at the top. The material used for core is
aluminium and for solid booster is steel. Stiffeners
are provided at the inter stages and also ring
stiffeners are provided on both of the solid boosters
at an interval of 5 m. Rods are used for the
connection between core and solid boosters. The
weight of the solid booster is 230T and weight of
the liquid in the four tanks is 150 T, 100 T, 30 T
and 20 T respectively. The layout of the launching
vehicle is shown in Figure 1.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
Fig 2: Isometric view of a launch vehicle
III. NORMAL MODE ANALYSIS
There are many reasons to compute the natural
frequencies and mode shapes of a structure. One
reason is to assess the dynamic interaction between
a component and its supporting structure. The
mode shapes of the launch vehicle obtained from
normal mode analysis are as shown in Figure 3.
These results characterize the basic dynamic
behavior of the structure and are an indication of
how the structure will respond to dynamic loading.
Each mode shape is associated with a specific
natural frequency. Table 1 shows the mode number
and frequency from Normal Mode Analysis of the
launch vehicle.
Fig 1: Layout of the launch vehicle
Table I
Mode Number and Frequency from Normal Mode
The launching vehicle was modelled with the help
of the software MSC.Patran. The geometry of the
structure was modelled as shown in Figure 2.
Points and curves were used to create the geometry
of the structure. Once the geometry of the structure
was created, finite element descritization was done
followed by meshing. Once the model was
generated, material of the structural elements was
assigned. The material used for core is aluminium
and for solid boosters are steel. Rods were used for
the connection between core and solid boosters.
The diameter of the rod was 0.040 m. Ring
stiffeners were provided on solid boosters as well
as the core and straight stiffeners were provided at
inter stages. Ring stiffeners were provided as
channel sections and straight stiffeners were
provided as hat sections.
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Analysis
Mode numbers
Frequency (Hz)
1
2.16
2
2.64
3
3.05
4
4.02
5
4.65
6
4.70
7
5.60
8
6.18
9
6.53
10
7.33
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
IV. MODELING OF ISOLATION SYSTEM
In present study, Steel laminated rubber bearings
are used to protect sensitive electronic packages
from deleterious effects of vibration during flight.
Steel rubber is known for its very good damping
and isolation characteristics. Steel rubber laminates
could be easily compounded to give a high value of
hysterics damping. It exhibits little change in
transmissibility and resonant frequency. The
isolator is modelled using Bush element
(CBUSH).The bush element was introduced at the
connecting element between solid boosters and
core. Initially they are connected by using beam
sections. This beam section was replaced by the
isolator. The isolator is designed based on the
excitation frequency. The excitation frequency is
from 20 Hz to 30 Hz. The force applied was unit
force at the base of the strap on boosters. Isolator
performs best when the excitation frequency is
greater than
.Where
is the excited
frequency. Stiffness of the isolator is determined by
the equation,
(1)
Mode Shape 1
Mode Shape 2
In this study, the isolator was designed for
of
the first excitation frequency,20 Hz(125.78
rad/sec).Mass of the solid boosters is 230 T.
Stiffness of the isolator, k = 8.0 *107 N/ m.
V. FREQUENCY RESPONSE ANALYSIS
Frequency response analysis is a method used to
compute structural response to steady state
oscillatory excitation. In frequency response
analysis the excitation is explicitly defined in the
frequency domain. All of the applied forces are
known at each forcing frequency. Forces can be in
the form of applied forces and/or enforced motions.
The important results obtained from a frequency
response analysis usually include the displacements,
velocities, and accelerations of grid points as well
as the forces and stresses of elements.
Frequency response analysis was carried out for
different cases. The different cases were
For 230T solid boosters with unit force.
For 100 T solid boosters with unit force.
For actual force (30 T) at the time of
ignition.
Mode Shape 3
Mode Shape 4
Fig 3: Various Mode Shapes
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All the above conditions were checked with
isolation system .First two cases were done for
finding the effective stiffness of the isolator. At the
first stage of the trip, solid boosters was with a
weight of 230 T, so the first condition was checked
and found the reduction in the amplitude. At the
second stage the weight of the solid boosters was
reduced to 100 T, so the second case was
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
considered and evaluates the results. According to
this reduction in amplitude, the effective stiffness
of the isolator can be found out.
VI. RESULTS AND DICUSSIONS
The structure is analyzed in MSC. Nastran
2012 to find out the displacement and acceleration
response after introducing the isolation system .The
structure was analyzed with different conditions.
After the Normal mode analysis, next step is
Frequency response analysis In this study, the
Frequency Response analysis was carried out to
compute the structural response under thrust
oscillation. The response was taken at the node
56633; the node having the satellite weight is
concentrated. The variation in the acceleration
without and with the isolation system is explained
by using graphs.
The excitation frequency was taken around
the range of 20H z to 30 Hz. For each point the
corresponding acceleration frequency response was
found out. The maximum acceleration occurred at
24.6Hz.The amount of reduction in the acceleration
in 20 Hz was about 59 %.So by using isolation
system, the acceleration in each point can be
reduced.
CASE II
For 100 T solid boosters with unit force
CASE I
For 230T solid boosters with unit force
Acceleration-Frequency response for 100 T
without isolator
Acceleration-Frequency response
without isolator
Acceleration-Frequency response for 100 T with
isolator
Some parts of the launch vehicle were dispatched
few seconds after ignition. The weight of solid
boosters was reduced approximately to 100T. So
100 T condition was checked and there was a large
reduction in displacement. So the selected stiffness
value is suitable for all conditions. Again it is
checked for different damping ratios, while the
selected bush having small percentage of damping.
Acceleration-Frequency response
with isolator
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
CASE III
For actual force (30 T) at the time of
ignition.
Table 2 and Table 3 shows the percentage of
reduction of acceleration with and without isolation
system. From the above graph we can conclude that
by using isolation system on the launch vehicle
interface spacecraft, there is a large reduction in the
axial vibration of the launch
vehicle.
Acceleration-Frequency response without
isolator
Acceleration-Frequency response with isolator
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
Table II
Reduction in acceleration without and with isolation system for 230 T
Solid boosters with unit force
Sl No:
Acceleration without
Acceleration with isolator
Frequency (Hz)
isolator (m/s2)
(m/s2)
Reduction in %
1
20
5.15*10-5
2.11*10-5
59.02
2
20.5
5.13*10-5
2.24*10-5
56.33
3
21
5.23*10-5
2.44*10-5
53.33
4
21.5
5.49*10-5
2.70*10-5
50.81
5
22
5.97*10-5
3.10*10-5
48.07
6
22.5
6.79*10-5
3.73*10-5
45.06
7
23
8.21*10-5
4.76*10-5
42.02
8
23. 5
1.09*10-4
6.79*10-5
37.70
9
24
1.77*10-4
1.21*10-4
31.63
10
24.5
4.05*10-4
2.82*10-4
30.37
11
25
2.23*10-4
1.29*10-4
42.15
12
25.5
1.05*10-4
7.003*10-5
33.30
13
26
7.16*10-5
5.27*10-5
26.39
14
26.5
5.02*10-5
4.17*10-5
16.93
15
27
3.84*10-5
3.67*10-5
4.42
16
27.5
3.4*10-5
3.10*10-5
8.82
17
28
3.8*10-5
2.64*10-5
30.59
18
28.5
4.71*10-5
2.28*10-5
51.59
19
29
7.53*10-5
2.05*10-5
72.77
20
29.5
1.92*10-5
1.09*10-4
43.22
21
30
4.58*10-5
1.96*10-5
57.20
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 8 - October 2015
Table III
Reduction in acceleration without and with isolation system for 30 T force
Sl No:
Acceleration without
Acceleration with isolator
Frequency (Hz)
isolator (m/s )
(m/s2)
Reduction in %
1
20
1.55
0.634
59.03
2
20.5
1.54
0.674
56.13
3
21
1.56
0.732
54
4
21.5
1.64
0.812
50.4
5
22
1.79
0.932
47.93
6
22.5
2.03
1.11
45.32
7
23
2.46
1.42
42.27
8
23.5
3.28
2.03
38.10
9
24
5.31
3.63
31.6
10
24.5
12.15
8.46
30.37
11
25
6.70
3.89
41.94
12
25.5
3.15
2.10
33.33
13
26
2.15
1.58
26.51
14
26.5
1.50
1.25
16.67
15
27
1.15
1.10
4.34
16
27.5
0.931
0.910
2.25
17
28
0.784
0.656
16.32
18
28.5
0.684
0.601
12.13
19
29
0.617
0.587
4.86
20
29.5
0.577
0.506
12.3
21
30
0.563
0.499
11.4
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International Journal of Engineering Trends and Technology (IJETT) – Volume X Issue Y- Month 2015
VII. CONCLUSIONS
Application of the isolation system is one of the most effective
used in launch vehicle for controlling the dynamic response.
From this analytical study of a launch vehicle whose natural
frequency is around 2.16 Hz. It was observed that
The thrust oscillation produced by at the time of
ignition was very significant without damper.
From the analytical study it was observed that the
isolation system reduces the response by about 59 %
for unit application of force.
With original force of 30 T, the acceleration of
launch vehicle without isolation system is 10.29
m/s2 and the acceleration of launch vehicle with
isolation system is 8.96 m/s2.so the reduction is
12.96%.
By providing isolation system, the response is
obtained as 8.96 m/s2 at 24.5 Hz ,that is the
acceleration is reduced below 1g m/s2 (10 m/s2).
There by we can protect all the sensitive packages in
the core vehicle from the thrust oscillation of the
strap-on-boosters.
spacecraft.launch vehicle”, Smart structure and
Material passive damping and isolation Newport Beach
C A March 2000.
[7] Paul S. Wilke, Kenneth R. Darling;”Whole-spacecraft
vibration isolation flown on the launch vehicle”,
Aerospace Mechanisms Symposium, Langley
Research Center, Sepember 12-18,2009.
ACKNOWLEDGEMENT
I would like to express my heartfelt gratitude to Mr.
Rajesh A K, my faculty guide, Assistant Professor,
Department of Civil Engineering, Ilahia College of
Engineering and Technology, Muvattupuzha for providing his
valuable suggestions during my research.
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