International Journal of Mechanics and Solids.
ISSN 0973-1881 Volume 12, Number 1 (2017), pp. 125-138© Research India Publications
http://www.ripublication.com/ijms.htm

A Review on Material Selection and Fabrication ofComposite Solid Rocket Motor (SRM) Casing
Singuru Rajesh1, Gamini Suresh2 and R.Chandra Mohan2
1

Asst. Prof., Department of Mechanical Engineering, Vignan`s Lara Institute ofTechnology and Engineering, Guntur, India.
2
Asst. Prof., Department of Mechanical Engineering, Vignan`s University,
Guntur, India.
2
Chief Managing Director, M/S Resins and Allied Products,
Vijayawada, Andhra Pradesh, India.

Abstract
During world wars many innovations are came into existence in the world.
These wars has all along provided a sudden movement to scientific researchand technology innovations. Rockets and Missiles are used to destroy enemy
aircrafts, tankers etc.., At first Solid Rocket Motor (SRM) casings are made ofmetallic materials, which are more in weight. For decreasing the dead weightof missiles and rockets we can use newer & lighter materials in theconstruction of the missile system which are composite materials Composite
Materials have extensive engineering application where strength to weightratio, low cost and ease of fabrication are required. A review of technicalresearch on composite materials used in the manufacture of solid propellant
missiles has been conducted. This paper presents a review of the current statusof composite solid rocket motor casing materials technology, in terms ofmaterials available and properties, fabricating methods of composite casing,loads over cases and non-destructive inspection techniques
Keywords: Rockets, Metallic, Solid Rocket Motor, Motor Casing,
Composites, Non-destructive techniques

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Singuru Rajesh, Gamini Suresh and R.Chandra Mohan
INTRODUCTION:

Solid rocket motors (SRM’s), Fig.1., are used in launch vehicles, missiles, andspacecraft. In a solid propellant rocket, the propellant to be burnt is contained within
the combustion chamber or in case [1]. The propellant charge or the grain contains thechemical elements for complete burning. Once ignited, it burns at a designed rate till
the propellant is completely consumed. Solid rockets are relatively simple ascompared to the other systems.

Fig. 1. Solid Rocket Motor
2.

PRINCIPLE OF OPEARTION AND COMPONENTS OF SRM

2.1 Principle of operation of SRM
Initially the propellant is ignited and it burns at a rapid rate producing gases. These
gases develop pressure inside the combustion chamber and this pressure force thegases to pass through the only exit available, which is the nozzle. Nozzle first reduces
the area of exit in order to increase the velocity of exhaust gases and gases reachsupersonic velocity at the nozzle throat. Now the area is increased to further increase
the velocity, as according to supersonics increase in area produces increase in thevelocity.
2.2 Components of SRM casing
Propallant Grain in Fig.2., have fuel and oxidiser mixed together in a suitableproportion. Finished propellant body called grain have rigid shape and form as per
design. This shape is obtained by casting or extrusion under pressure. Propellant
grains may vary in size depending on application.

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Fig. 2. Components of Solid Rocket Rocket Motor (SRM)
Igniter is the device that helps to start the burning of the main propellant grain of therocket motor. Its function is for short interval (0.1-2 seconds depending upon size)
only but vital. The igniter for small motors will be a few grams of grains while it will
be a few hundred kilograms for large boosters. The initiation is done using electrical
power by heating a resistance wire and initiating a primary composition.
The nozzle is the component through which the hot gaseous mass in the motor case isexpelled out. This has to be designed to withstand high temperatures and flow ofgases at high velocities. The dimensions of the nozzle are critical for the performanceand efficiency of the rocket motor. Nozzles are also used for producing control force
for the missile.
3. SRM CASING
In all solid rocket motors, the casing is a pressure vessel designed and fabricated towithstand up to certain internal pressures [2]. Case design is usually governed by a
combination of motor and vehicle requirements. Besides constituting the structural
body of the rocket motor with its nozzle, propellant grain, and so on, the casefrequently serves also as the primary structure of the missile or launch vehicle. Thus
the optimization of a case design frequently entails trade-offs between case design
parameters and vehicle design parameters [3].
Generally, Casings are made of metallic alloys. For applications in smaller rockets,titanium alloys and aluminium alloys are used and for bigger rockets nickel alloysteels are used [4]. They are fabricated to give cylindrical shells with ends flared forjoints. Complex welding and heat treatment fixtures and processes have been
specially evolved for specific casing. The casings are subjected to a number of quality
assurance tests for strength, toughness, soundness of weld and hydraulic pressure. The
casings are provided with thermal insulation on their inner surface to protect themfrom hot gases. The casing has provisions for end covers, nozzle and handling, etc.

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Casings can also be made of a composite material such as fibre reinforced plastic(FRP). Such a casing has low weight and high strength.
NASA SP-8025 [5] provides that basic motor case design will be based on motor,
vehicle and mission design parameters which results in maximum performance andmaximum cost effectiveness. The basic case-design parameters include case internalpressure, motor case inertia and flight loads, maximum case limit, length-to-diameter
ratio (l/d ratio), propellant mass fraction, internal and external envelope constraints.
l/d ratio influences the capability of holding these loads. With l/d ratio b/w 2 to 5 will
gives the best case design. The increase in the values of l/d ratio produces a longvehicle violates constraints and result in severe case buckling and bending problems.
When the case parameters are not specified the case design selection will be dependon most influencing parameters motor internal pressure, thrust loads and loads
resulting from particular motor. Propellant Mass Fraction Parameter is the measure of
motor design efficiency and defined as ratio of mass of initial propellant to mass of
total motor. For SRM it varies from 0.3 to 0.96 (PMFP).
Following are main considerations to be done in casing design. The case end closure
shall be the minimum size and optimum shape required to satisfy propellant grain
design and clearances for auxiliary equipment. The minimum mechanical propertiesof the case material shall not be less than needed for structural loading at the criticaloperating temperature, as imposed by fracture mechanics theory and design safety
factors. The material strength to density ratio shall result in a case with in the weight
limit defined by the required propellant mass fraction for the motor. Case materials
shall not experience brittle and ductile failure at a case loading less than the design
load. Case-material mechanical and physical properties shall be within establisheddesign limits after exposure to the intended fabrication process. The case material
shall withstand any harmful environment encountered during fabrication, processing,storage and service and also withstand the thermal, moisture, corrosive effects etc.,
The case material shall with stand both low and high-cycle fatigue induced either by
the predicted thermal cycling, by the predicted pressure cycling or by the worst
combination.
The case loading profile includes all individual design loads or the combination ofdesign loads such as Attachment loads (Motor Igniter, Single or Multiple Nozzles,
Thrust-vector-control System, Motor Thrust Skirt, Clustering Structure, Motor
Staging, Thrust-Termination or Thrust-Reversal Hardware, Aerodynamic Control
Surfaces, Instrumentation, Electrical and Destruct-System hardware), Internal Loads(Internal Pressure, Axial Thrust, Thrust Misalignment, Thermal Stress) and External
Loadings (Ground Handling, Launch Pad Loads, Flight Loads). The case design
Stress shall not exceed the allowable stress, whether yield or ultimate: and the
maximum deflections shall not exceed allowed deflections. The case fabrication shallbe most reliable, least time consuming and cost effective. The fabrication shall not
produce undesirable effects. Destructive Testing and Non Destructive testing(Inspection Plan, Inspection Process, Hydrostatic Proof Test).

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4. MATERIALS IN SRM CASING
The selection of materials for SRM components is done on the basis of high specificstrength, high specific modulus, fabrication easiness, easy availability, criticalrequirements and service conditions [6] shown in Fig.3. The classification of materialsof rocketry are shown in Fig.4., as follows structural metallic materials, composite
materials, thermo-structural materials, thermal protection materials, special materials
and chemicals. Table 1 depicts different materials and their applications in SRM
casing.

Fig. 3. Selection of Materials and Types of materials used in Rocketry

Fig. 4. Classification materials in SRM casing

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Singuru Rajesh, Gamini Suresh and R.Chandra Mohan
Table 1- Different materials and their applications in casings

S.No

Material(s)Application1.

Low case Carbon Steel (15 CDV6)

Used in solid rocket motor case2.

Maraging Steel (M250) with highstrength and high toughness

Used in booster solid rocket motor case3.

Titanium Alloy (Ti-6Al-4V)

Used in high pressure gas bottles

4.

Aluminium Alloys(AA 2219, AA2014, AA6061)

Used in liquid propellant tanks, engine
Components, airframe in reusable
Launch vehicles.

5.

Magnesium/ Mg-Lithium alloys

Used in upper stage structure likepayload adopter, avionic decks,
equipment bay structure.

6.

Carbon F/Kevlar F- Epoxy resin

Used in solid motor case, pressure
vessel, inter stages, payload adopter.

7.

Carbon Fibre / Silicon Carbide
Ceramic Matrix

Used in nose-cap of heat-shield, leadingedge and control surface of Rocket
Launching Vehicle (RLV)8.

SiC F/SiC Ceramic Matrix
Composite

Used in high temperatures / hot
structures

9.

Silica tiles: (Silica Fibre in Si gel)used in space shuttle and SRE (capsule)mission10.

Different coating materials

Used for protection and imparting
certain characteristics to surfaces,
potting of connectors

George P. Sutton et.al. [7] mentioned that certain metallic casings have stresscorrosion cracking problem which can result in spontaneous failure without any visualevidence of impending catastrophe. Importance given to lightweight thin metal cases
aggravates stress corrosion and crack propagation, often starting from a flaw in themetal, with failure occurring at a stress level below the yield strength of the metal.
Composites are used for the cases and their principal advantage is their lower weight
S.T. Peters et al. [8] compared different motor case materials with composites shownin Table 2.

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Table 2 - Different Materials and their applications in Casing
D6AC
Steel

Maraging Glass-filament
steel
composite
(s-2)

230
250 (1724)
Tensilestrength, (1586)
(MPa)
489
499 (8.00)
Density,(7.883)
g/cm3
202.5
215.5
Strength/density
Modulus, 29(199.5) 27 (189.1)
106 Gsa

170 (1172)

Organicfilament
Composite(Kevlar)120 (827.4)

Intermediatemodulus
polyacrylonr
carbon fiber
225 (1551)

124 (1.993)

85 (1.356)

97 (1.550)

588.1

610.2

1000.65 (31.72)

11 (75.84)

25(172.4)

5. COMPOSITE MOTOR CASING
Composite materials are widely used in SRM’s because of their intrinsic material
characteristics, such as high strength-to weight-ratio, high thermal resistance, andability to be tailored for a specific application [9]. They are primarily used forstructural components such as solid rocket motor casings, inter stages, nose cones,
nozzle support structures and nozzle insulators. While these materials exhibit better
performance than metals, they often require advanced analytical methods tocharacterize their behaviour under complex mechanical and thermal loading
environments. They are also susceptible to developing defects or damage during
manufacturing, storage, handling and transportation.
The choice of composite materials for rocket motor case is done by using higher
performance factor, N which is defined as N = P V/ W where P = internal pressure, V= internal volume W= weight of the motor case. By comparing performance factors ofdifferent isotropic and composite materials the composite materials have the goodvalue of performance factor.
Table 3- Comparision of Performance factors of Isotropic and Composite Materials
Parameter

Performancefactor (km)Mass per unit
length(Kg/m)
Wall
Thickness(mm)Isotropic Materials
7075-T6
D6AC 6 Al, 4V
Aluminum Steel
Titanium
7.0
9.5
9.0

Composite Materials (Epoxy resin)
EK-49
T-300
T-1000Glass Kevlar Carbon Carbon
4.0
12.0
11.0
25.0

360

290

295

650

220

250

100

22

6

11

44

26

21

11

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Devon K. Cowles [10] designed a ground launched rocket booster to meet specificmission requirements viz., minimum speed and maximum flight altitude. The motor
parameters such as nozzle size, expansion ratio, propellant size and shape aredetermined through an iterative process. The initial motor casing design made of alight weight casted aluminum later compared to a design made of a fiber and resin
composite material. Composite casing has more flight performance than Aluminum
casing shown in Fig.5.

Fig. 5. Aluminum vs composite casing flight performance
G.Avinash et al. [11] deals with the design of composite rocket motor casing andcompared results with metallic casing to quantify the improvement of performancefactor. A systematic approach is adopted for modelling of various components(composite casing shell, igniter end, nozzle end, igniter skirt end and nozzle skirt end)of composite rocket motor casing. The maximum expected operating pressure(MEOP) is considered for the design of the rocket motor casing and axial load as wellas pressure is used for design of the igniter and nozzle end skirts.
Vinay K. Goyal et al. [12] described Structural analysis of components in solid rocket
motors (SRM’s). Modelling simplifications are required to make the analysis of
SRM’s practical. These simplifications are illustrated using the main components ofan SRM: the composite motor case membrane, the joint regions, the nozzles and thepropellant. These simplifications enable the practical use of progressive failure
analysis (PFA) and discussed with examples when PFA is necessary to effectively
account for the load redistribution that occurs as a component is damaged.
Sidhant Singh [13] had proposed an Experimental Sounding rockets for research in
the field of aerospace engineering. The paper is focused on design and construction ofa solid rocket motor that can be utilized as the main propulsion unit in experimental
sounding rockets by researchers. Initially, basic designs were evaluated and thedifferent concepts of propellant configuration were observed. The availability, ease of
manufacturing and casting of propellants was a major factor in determining thesuitable propellant.

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STRAND Company`s [14] Proposal provided the values of different SRMs forlaunching vehicles. They also stated that Composite filament cases, successfully
employed in applications for more than 40 years, have created a small revolution in
SRM design e.g. the case's performance criterion. Pressure and internal volume/solid
propellant case mass for a composite case is 5 times a metallic one's.
M. Madhavi et al. [15] involves material characterization of FRP of carbon
T300/Epoxy for various configurations experimentally determined using filament
winding and matched die mould technique. Mechanical and physical properties are
obtained in the design of the composite shell. Netting analysis is used for the
calculation of hoop and helical thickness of the shell. Methodology of composite
pressure vessel design, calculation of necessary hoop and helical thicknesses of theshell. Progressive failure analysis over geodesic domes is carried out.
Nagesh [16] initiated with different types of composite failures and gave a fullfledged information about Progressive Degradation. A degradation model (S44)
incorporated into the finite-element analysis of the pressure vessel based on a
progressive failure criterion is discussed.
Alexis A. Krikanov [17] encompasses a new method to design laminated composite
pressure vessels under strain and strength constraints. Graphical analysis is presented
to find optimum layer thicknesses for given fiber orientations. Minimum pressure
vessel mass is determined from active execution of two constraints. By replacing
circumferential layer by second helical layer is suggested as a new way of strain
suppressing among the commonly used ways for strain suppressing such as (1)addition of extra plies and (2) use of composite material with a higher stiffness which
are supported with numerical results and graphs.
Shaik Shaheen et al. [18], Rao et al. [19], Mahesh Babu et al. [20], Rajesh et.al [21],Sheikh Naunehal Ahamed et.al [22] did design and analysis of composite motor
casing using different composite materials. Composites rocket cases are pressure
vessels, which are axis-symmetry in nature. Problems involving three-dimensional
axisymmetric solids or solids of revolution, subjected to axis-symmetric loading,
reduce to simple two-dimensional problems. Thus the problem is needs to be looked
at as a two-dimensional problem
6. FILAMENT WOUND FABRICATION OF CASES
Filament-wound composite pressure vessels, which utilize a fabrication technique ofﬁlament winding to form high strength and lightweight reinforced plastic parts, are amajor type of high pressure vessel and are widely used in the commercial andaerospace industries such as fuel tanks, portable oxygen storage, rocket motor casesand so on. These kinds of vessels consist of a cylindrical drum and dome parts, justlike typical pressure vessels [23]. Design of ﬁlament wound composite pressure vessel
comprises of, design of composite material, design of metallic polar boss and rubber
lining design [24].

134Singuru Rajesh, Gamini Suresh and R.Chandra Mohan

Motor cases are realized by the filament winding process. A combination of hoop andhelical winding is employed. The layer sequence and angle of winding are arrived atbased on design and analysis. The shell portion and ends have different layer
sequence and thicknesses. Composite motor cases will have metallic end fittings andattachments to facilitate integration/interfacing with other systems .These end fittingsare embedded / attached to the composite part at the different stages of manufacturing.
Carbon fiber with epoxy resin system is mostly used as material.
The major steps involved in the processing of Composite Motor case are represented
in a flowchart. Composite Nozzles are also manufactures in this process.

Fig. 6. Flow chart showing the steps in fabrication of composite casing

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Fig. 7. Filament wound case and Nozzle manufacturing process steps

Fig. 8. Filament case manufacturing over drum and mandrel
V.V.Vasiliv et al. [25] presented over composite pressure vessels made by continuous
winding of fibrous tapes reinforced in longitudinal and transverse directions andproposed winding of unidirectional fibrous band instead of instead of isotensoid
vessels for minimum mass.

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7. TESTING AND INSPECTION METHODS
The solid rocket motor cases, considered as pressure vessels, are designed with verysmall safety margins. The cracks in solid rocket cases are very small, on the order of
0.025 mm, and difficult to detect. Different types of defects in composite cases are
voids, propellant wound with tails, cluster of voids, propellant entrapment, loose flap
filled resin and separation between propellant & insulation may result in a
catastrophic failure of the flight [26]. Different NDT methods used in crack detectionof composite rocket cases are (i) Ultrasonic, (ii) Radiography and (iii) Thermography[27-28].
The detection of cracks in small and narrow diameter cylindrical structures is a big
problem. Pipelines, rocket cases, boilers, pressure vessels, chimney etc... are hollow
cylindrical in nature. Cracks in these will results in the loss of strength, which could
lead to failure of component and its function. In generally, Non-destructive testing
methods in Fig.9., are widely available for quality assurance and fault diagnostics[29]. Various inspection and testing are Dimensional inspection, NDT of the casing –UT and Radiography, Proof pressure testing, Post PPT NDT, Load testing and Burst
test for validation of design margins.

Fig. 9. NDT Inspection of casings
8. CONCLUSION
In the paper for the design of Solid Rocket Motor casing, a comparision of different
properties of carbon composites with Metal casings (D6AC Steel alloy, Maraging
Steel, Titanium etc.) shows that carbon fiber composites are best to use as materials infabrication of SRM Casing. On the other hand Carbon - Epoxy composite materials
with their higher specific strength, moduli and tailoribility characteristics will result in
reduction of weight of the structure. Filament winding is best manufacturing method
for fabricating conposite casings and are inspected through different NDTTechniques. In this we discussed design considerations of the composite shell is
described in detail. Finally, if motor case must be stiff, able to tolerate high pressuresand light in weight we should choose composites.

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ACKNOWLEDGEMENTFirst author thankfully acknowledges the support provided by M.Tech students;
faculty of Mechanical Engineering Department, Vignan University, Guntur and M/SResin & Allied Products Ltd., Nidamanuru, Vijayawada, Andhra Pradesh.
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