CHAPTER 1

INTRODUCTION

Standing pretty on the launch pad and awaiting the inexorable countdown, a launch

vehicle symbolizes the hopes & aspirations of thousands of men and women. To put it

more prosaically, every launch represents the culmination of efforts spread over

thousands of man years!

Yes there is thrill in every launch; but then there is also a finite amount of risk

involved in it. Every launch is potentially hazardous; for example, a rocket may pitch

down too much; the control systems may fail or a motor may even explode. Any such

mishap could, in principle, lead to a catastrophe! The damage potential of any mishap

depends on when what particular mishap occurs during the flight.

Under all circumstances, it is the moral, social and perhaps legal

responsibility of the launch agency to ensure safety of life & property of all

individuals, irrespective of whether they are in any way connected with the launch or

not. It should not only be ensured that a malfunctioning rocket is positively and

comprehensively destroyed, but it should also be guaranteed that a rocket behaving

normally is not inadvertently destroyed. Should you fail to destroy a misbehaving

rocket promptly; you run the risk of being called callous; if, on the other hand, you

destroy a normally performing rocket you would surely be branded an idiot –an

expensive one at that!

1

Hence careful planning at every stage, starting from the selection of launch site and

launch azimuth to monitoring the behavior of the rocket till the end of emission, is

needed to ensure ground & flight safety.

CHAPTER 2

GAS BOTTLES

One of the main classifications of rockets is based on the fuel used by them.

Accordingly there are certain rockets in which liquid fuel is used. These liquid fuel

rockets invariably need large quantity of gases at high pressure for their normal

operation. At the very start of the flight tremendous quantity of fuel should be burnt in

order to produce a high thrust. A high thrust means huge quantity of fuel should be

burnt. Once the rocket gets into a particular zone, it does not require the earlier thrust

for rest of the flight. So the fuel consumption should be controlled or should be

reduced.

Gas bottles are spherical containers used for controlling the fuel consumption

in rockets. The main function of gas bottles in rockets is to activate the control valve

of liquid flow. Gas bottles are usually filled with gases at high pressure for their

operation. The gases used in these gas bottles should be non-reactive gases in order to

reduce the chance of explosion, if any. Mostly in all gas bottles helium, oxygen,

nitrogen etc. are used. The most preferable gas used here is helium.

This is because the density of helium is low compared to other gases and

helium is highly corrosive resistant. Since it is highly non-reactive, it reduces the

chance of over burning and explosion inside the rockets. Sixteen such high pressure

2

gas bottles are required for the liquid stages of GSLV. The design and manufacture of

these gas bottles conforming to the stringent specifications are a complex task.

The motive of every aerospace systems designer is to keep the inert mass as low as

much as possible. Therefore the material used for the manufacture of the gas bottles

should be of good quality and the mass of the material should be considerably low.

Accordingly in this case, special type of alloy known as titanium alloy is used for the

manufacture of these bottles.

3

FIG 1.1 PLATE ROUTE GAS BOTTLE

CHAPTER 3

INTRODUCTION ABOUT TITANIUM ALLOYS

The main reason behind the selection of titanium alloy for the manufacture of

gas bottles is its high specific strength. It possesses high strength to mass ratio. The

high specific strength titanium alloy is extensively being used in aerospace area due to

4

its characteristics like high resistance to corrosion, good fatigue strength, high

temperature strength and less density. In addition, it retains excellent mechanical

properties, undergo very little creep and structurally stable up to a service temperature

of 550°C. Almost all fabrication techniques like machining, grinding, forming, heat

treatment, surface treatment, welding etc can be adopted to titanium alloys. However

unlike other metals, extra-ordinary precaution and adequate care must be diverted

while adopting these techniques to titanium alloys.

Titanium exists in two allotropic phases, that is α phase and β phase. The HCP

structured a is stable upto 882°C and transforms to BCC-b thereafter. By properly

alloying certain alloying elements, different types of alloys such as a, near α, αβ, near

β and β alloys can be produced.

Ti6Al4V is the most widely used titanium alloy in aerospace due to its good

fabricability and strength. It is an a-b alloy containing about 6% aluminium (a

stabilizer) and 4% vanadium (b stabilizer). Its ultimate tensile strength is around 900-

1000MPa and yield strength ranges from 830-920MPa with around 14% elongation.

Its fracture toughness is around 44-66MPa and impact strength is 19-25 joules in

annealed condition.

5

CHAPTER 4

FABRICATION OF TITANIUM ALLOYS

4.1 MACHING & GRINDING

The fact that titanium is sometimes classified as difficult to machine by traditional

methods can be explained with its physical, chemical and mechanical properties as

below:

6

Titanium is a poor conductor of heat. Heat generated by the cutting action,

does not dissipate quickly, resulting most of the heat concentrated on the cutting edge

and tool face.

Titanium has a strong alloying tendency or chemically reactivity with cutting

tools and elements in the cutting environment at tool operating temperature. This

causes galling, welding, and smearing along with rapid destruction of the cutting tool.

Titanium has a relatively low modulus of elasticity, thereby having more

springiness than steel resulting the work to move away from the cutting tool unless

heavy cuts are maintained or proper backup is provided. Slender part to deflect under

tool pressure causing chatter, tool rubbing and tolerance problem. Rigidity of the

entire system is very important together with the use of sharp tools.

Titanium and its alloy can be machined successfully on

convectional and CNC machine tools provided that certain requirements are satisfied.

In all machining operations rigidity of both work piece and cutting tool is desirable.

Best results can be obtained if the cutting tool is ground to a good surface finish. Due

to titanium metals tendency to gall or smear on to other metals, sliding contact

between the work piece and its supports should be avoided, and the use of roller-

steadies and running centres is recommended.

4.1.1 TURNING

Turning can be easily performed on titanium alloy with low cutting speeds and feed as

course as practicable. A good surface finish can be obtained with everycourse feeds by

using suitably shaped tools with a large nose radius. This will be limited by the work

7

piece rigidity as a large nose radius causes increased toolloads and work piece

deflection. Due to the lower elastic modular of titanium, these deflections are greater

than that would occur on steel work pieces. For a given metal removal rate, use of

heavy feeds and low speeds give longer tool life than light feed and fast speed. Light

finishing cuts, particularly less than 0.13mmdeep should be avoided, as tool wear can

be excessive. Tops rakes for tungsten carbide tools should be from 0o to 60 positive,

depending on the severity of the operation. Cast alloy tools operate best with 5o

positive rake, while HSS tools with up to 15o positive rake. A relief angle of

approximately 70 is always desirable. Cutting speed of the order of 6-12m/min can be

employed for HSS tools and 30-36m/min can be used for tungsten carbide rods.

4.1.1. TOOL LIFE &CUTTING SPEED

The FIG1.2 shows the graph detailing relation between tools life (in minutes) with

cutting speed in (m/min) for a given cutting tool material at a constant feed and depth

of cut for Ti6Al4V. It can be seen that at a high cutting speed, tool life is extremely

short. As the cutting speed decreases, tool life dramatically increases.

From the figure it can be observed that titanium alloys are

very sensitive to change in feed.

When cutting titanium, a high shear angle is produced between the work piece and

chip, resulting in a thin chip following at high velocity over the tool face. High

temperature develops and since titanium has lowthermal conductivity, the chips have a

tendency to gall and weld them to the cutting edge. This speeds up tool wear and

failure.

8

FIG 1.2 GRAPH SHOWING TOOL LIFE AND CUTTING SPEED

9

4.1.2 CUTTING TOOLS

Major improvements in the rate at which work piece are machined usually result from

the use of proper cutting tools. The tungsten carbide cutting tools, typicallyc-2 grades,

performed best in operations such as turning and milling while high cobalt high speed

steels were most applicable in drilling and dripping of titaniumalloys.

K20 and h13a carbide tools of sandvik/iscarmake proved to be the best for

turning operation and k20grade sub-micron solid carbide end mills for milling are

recommended. For finish machining poly crystalline diamond tools are used. In recent

years ceramic tools have been used successfull in machining titanium alloys.

4.1.3 CUTTING FLUIDS

Cutting fluids in machining titanium alloys require special consideration because

chlorine ions have, under certain circumstances, caused stress - corrosion cracking.

During machining, cutting fluid supply should be flood type. Usually the heavy

chlorine bearing fluids excel in operations such as drilling' tapping and broaching

If chlorine-bearing (or halogen containing) cuttingfluidsare used on certain

circumstances, the job need to be subjected to controlled washing after machining in

order to remove the effect of chlorine. FIG1.3, shows the effort of various cutting

fluids on tool life in drilling Ti6AL4V'

10

FIG 1.3 GRAPH SHOWING THE EFFECT OF CUTTING FLUIDS

4.1.2 Milling.

In milling, the chief problem arises from chips welding on the teeth resulting in cutter

chipping and breakage. This is minimized with climb milling, in which the tooth

finishes its cutting stroke when moving parallel to the feed. Absolute rigidity is

necessary to avoid Chatter, but the chip is only attached to the tooth by thin sliver,

which is easily broken off. Typical machining parameters are used to machine

4.1.3 Drilling.

11

Titanium may be drilled with short high -speed drills; the holes should be as

shallow as possible. A continuous feed of about 0.05-0.13mm/revolutions for small

size or 0.13-0.23mmlrevo.lutions for larger size should be maintained. Flood

(lubrication with heavy Chlorinated cutting fluids reduces fabrication troubles but this

will invite stress corrosion Risks. Fig: explains the effects of various cutting fluids on

tool life with different cutting Speeds on Ti6AI4V. Carbide drills proved to yield

better results in drilling titanium alloy.

4.1.4 Threading.

The various manufacturing process and related processing techniques in

connection with the fabrication of titanium alloy is described below. The anticipated

problem and possible remedies in each case is also discussed. Single point screw

cutting is preferable than threading with a die. Conventional method of Screw cutting

can be used, but success can also be achieved when increments of cut of 0.25-0.50mm

are applied at right angle to the axis of the component. Cuts of les6 than 0.13mtn

should be avoided. Machine tapping with cutting speeds up to 6mm/min is preferable.

Generous lubrication with heavy chlorinated oil is recommended. The lubricating Oil

should be removed with a decreasing agent such as acetone, immediately after use.

4.1.5 Grinding.

Care must be exercised in grinding of Titanium alloys to avoid loss of surface integrity, which otherwise cause dramatic loss of mechanical properties especially fatigue. Even proper grinding practices using conventional parameters (wheel speed, down feed, etc.) may result in appreciably lower fatigue strength due to surface damage. A reduction in wheel Speed to half or one-third of the normal speed, together with the use of suitable coolant, will usually achieve an acceptable grinding ratio. Water based soluble oils results in poor wheel Life but some chlorinated /sulphurised grinding oil and solutions of vapour -phase rust Inhibitors of the nitride amine type are satisfactory .Vitrified bond A60M wheels can be used at the surface speed of 500m/min and grinding ratio of 10 or more with a metal removal rate Of 1.3 cm3imin. Abrasive out off is a simple method of parting small bars and rods provided that the

work is covered with a flood of coolant with a wheel. Table Table

12

suggests recommended surface speed for different types of wheels fordifferent grinding operations.Fine dry titanium swarf exposed to naked flame may ignite and burn fiercely.Covering it with a mixture of dry asbestos wool and chalk powder can effectivelylocalize the fire. Do not attempt to put out burning titanium with water or with anyextinguisher but dry powder type .4.2 Effect of Various Machining Methods on Titanium.The surface of Titanium alloys is damaged by some traditional machiningoperations. Damage appears in the form of micro cracks, built up edge, plasticdeformation, heat affected zone and tensile residual stresses. In service this can leadto degraded fatigue strength and corrosion resistance. Fig: shows the effect ofvarious machining methods on high cycle fatigue behavior of Ti6AI4V.The basic fatigue properties of Titanium alloys rely on a favorable compressivesurface stress induced by tool action during machining. Electro mechanical removalof material, producing a stress — free surface can cause a reduction in fatigueproperties. From the Fig: it can be seen that , in operations like end mill cutting andturning , the fatigue strength is on the higher scale than other operations , possiblydue to residual compressive stresses

13

14