ABSTRACT

Supercavitation is the state of the art technology that may revolutionize underwater propulsion systems. Cavitation is a problem for most of the engineering application where as supercavitation is the booming scientific discovery for underwater automobiles, torpedo and propellers.

Cavitation is the process of formation of vapour bubbles of flowing fluid in a region where the pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour bubbles in region of high pressure. At first small vapour filled bubbles are formed that gradually increase in size. As the pressure of the surrounding liquid increases, the cavity suddenly collapses-a centimeter sized cavity collapses in milliseconds. These Cavities implode violently and create shock waves that dig pits in exposed metal surfaces thus causing damage to the material.

“Water limits even nature's strategies, and the fastest bird moves twice as quickly as the fastest fish”.

The phenomenon holding back the fish is the tremendous resistance that water offers to a moving object, called drag. The same drag acts on the bird as well, but the magnitude is considerably less owing to the lesser density of air. The human being has crossed the sound barrier in air and land, what about underwater.

Water is the most challenging environment for an Engineer. Being 1000 times denser than air, it offers resistance roughly 1000 times as high as that in air. Supersonic under Water Travel is the dream of scientists working on a bizarre technology called SUPERCAVITATION.

Supercavitation is the state of the art technology that may revolutionize underwater propulsion systems.

CONTENTS

CHAPTERS PAGE NO.

1. INTRODUCTION

2. DISADVANTAGES OF CONVENTIONAL

UNDER-WATER PROPULSION SYSTEM

3. HISTORY

4. CAVITATION

5. SUPERCAVITATION

6. SUPERCAVITATING PROJECTILE

7. MAKING A SUPERCAVITATING

PROJECTILE

8. OTHER CHALLENGES

9. EXISTING SUPERCAVITING WEAPONS

10. APPLICATIONS OF SUPERCAVITATION

11. ADVANTAGES OF SUPERCAVITATION

10. THE FUTURE

11. CONCLUSION

REFERENCES

CHAPTER 1

INTRODUCTION

Water limits even nature’s strategies, and the fastest bird moves twice as quickly as the fastest

fish. The phenomenon holding back the fish is the tremendous resistance that water offers to a

moving object, called drag. The same drag acts on the bird as well, but the magnitude is

considerably less owing to the lesser density of air. The human being has crossed the sound

barrier in air and land, what about underwater? Water is the most challenging environment for

an Engineer. Being 1000 times denser than air, it offers resistance roughly 1000 times as high as

that in air. Supersonic under Water Travel is the dream of scientists working on a bizarre

technology called SUPERCAVITATION. Supercavitation is the state of the art technology that

may revolutionize underwater propulsion systems.

CHAPTER 2

CAVITATION

Cavitation is the process of formation of vapour bubbles of flowing fluid in a region where the

pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour

bubbles in region of high pressure. At first small vapour filled bubbles are formed that gradually

increase in size. As the pressure of the surrounding liquid increases, the cavity suddenly

collapses-a centimeter sized cavity collapses in milliseconds. Cavities implode violently and

create shock waves that dig pits in exposed metal surfaces.

At first, the physical characteristics of boiling and cavitation are almost identical. Both involve

the formation of small vapour-filled spherical bubbles that gradually increase in size. However,

the bubbles produced by the two processes end in very different manners. In boiling, bubbles

are stable: the hot gas inside either escapes to the surface or releases its heat to the

surrounding liquid. In the latter case, the bubble does not collapse, but instead fills with fluid as

the gas inside condenses.

When it acts upon propellers, cavitation not only causes damage but also decreases efficiency.

The same decrease in water pressure that causes cavitation also reduces the force that the

water can exert against the boat, causing the propeller blades to “race” and spin ineffectively.

When a propeller induces significant cavitation, it is pushing against a combination of liquid

water and water vapor. Since water vapor is much less dense than liquid water, the propeller

can exert much less force against the water vapor bubbles. With the problems it causes, it is no

wonder maritime engineers try to avoid cavitation.

CHAPTER 3

SUPERCAVITATION

The scientists and the engineers have developed an entirely new solution to the cavitation

problem. Cavitation becomes a blessing under a condition called supercavitation, i.e., when a

single cavity called supercavity is formed enveloping the moving object almost completely. In

Supercavitation, the small gas bubbles produced by cavitation expand and combine to form one

large, stable, and predictable bubble around the supercavitating object.

Supercavities are classified as one of two types: vapor or ventilated. Vapor cavities are the pure

type of supercavity, formed only by the combination of a number of smaller cavities. In a

ventilated cavity, however, gases are released into the bubble by the supercavitating object or a

nearby water surface

Supercavitation is the use of cavitation effects to create a large bubble of gas inside a liquid.

The cavity (the bubble) reduces the drag on the object, since drag is normally about 1,000 times

greater in liquid water than in a gas. Current applications are mainly limited to very fast

torpedoes.

Cavitation happens when water pressure is lowered below its vapor pressure or vapor pressure

increases to equal water pressure. This often happens at extremely high speed although it can

happen at any speed and even when not moving. Cavitation occurs inside a pump or around an

obstacle, such as a rapidly spinning propeller or in a body of liquid (such as a kettle) due to

temperature/pressure change. The pressure of the fluid can drop due to its high speed

(Bernoulli's principle) and when the pressure drops below the vapor pressure of the water or

the temperature increases. When vapor pressure increases to water pressure, it vaporizes–

typically forming small bubbles of water vapor (water in its gasphase). In ordinary

hydrodynamics, cavitation is a mostly unintended and undesirable phenomenon: the bubbles

are typically not sustained but implode as they and the water around them suddenly slows

down again, with a resulting sudden rise in ambient pressure. These small implosions can even

lead to physical damage, for instance spalling damage to badly designed rotating propellers,

pumps, and piping.

Various underwater methods of propulsion have been proposed to reach the necessary speed,

with a possible concept being a rocket engine burning aluminium with water. As an example, a

conventional rocket engine is used to propel the Russian Shkval supercavitating torpedo.

CHAPTER 4

HISTORY

In the early 1960’s, Mikhail Merkulov at the Hydrodynamics Institute in Kiev realized that the

solution lies in a phenomenon called cavitation. It was a daring idea because naval architects

usually see cavitation as a menace, rather than something that works to their advantage.

CHAPTER 5

DIFFRENCE BETWEEN BOILING AND CAVITATION

At first, the physical characteristics of boiling and cavitation are almost identical. Both involve

the formation of small vapour-filled spherical bubbles that gradually increase in size.

However, the bubbles produced by the two processes end in very different manners. In boiling,

bubbles are stable: the hot gas inside either escapes to the surface or releases its heat to the

surrounding liquid. In the latter case, the bubble does not collapse, but instead fills with fluid as

the gas inside condenses.

Boiling takes place mostly when heat is given to the liquid. While in the process of cavitation

pressure drop is the main reason for the production of vapour of the liquid.

When it acts upon propellers, cavitation not only causes damage but also decreases efficiency.

The same decrease in water pressure that causes cavitation also reduces the force that the

water can exert against the boat, causing the propeller blades to "race" and spin ineffectively.

When a propeller induces significant cavitation, it is pushing against a combination of liquid

water and water vapour. Since water vapours is much less dense than liquid water, the

propeller can exert much less force against the water vapour bubbles. With the problems it

causes, it is no wonder maritime engineers try to avoid cavitation.

Cavitation is a problem for most of the engineering application where as supercavitation is the

booming scientific discovery for underwater automobiles, torpedo and propellers.

Whereas boiling does not create much problems in engineering applications and can be easily

controlled.

CHAPTER 6

DISADVANTAGES OF CONVENTIONAL UNDER-WATER PROPULSION SYSTEM

The problem holding back conventional propulsion systems from high speeds is drag. No matter

how streamlined an object is, it suffers resistance as it moves through a fluid. One source of

friction is skin friction, the force that is required to shear the thin layers of fluid lying against the

moving body’s surface. This happens in air too, but water being a thousand times as dense as

air generates a thousand times as much drag. Moreover the power required to overcome drag

is proportional to the cube of its velocity. So each incremental improvement in propulsion

technology produces only a meager increase in speed.

Torpedoes are mostly the fastest propelled objects moving under water. It is drag which is the

main factor that limits the speed of conventional torpedoes. At high speeds drag is so

enormous that efficiency of propulsion is so low. Modern torpedoes can reach speeds below

180 km/hr. As is the case of most bizarre ideas, the idea of an entirely new under water

propulsion system owes its birth to the cold war. In the early 60’s Russian torpedoes were

inferior to those of Americans in speed. Rather than push conventional technology, the

Russians decided to try to leapfrog the Americans with a radical solution.

CHAPTER 7

A SUPERCAVITATING PROJECTILE

For a start the body has to be cruising very fast at least 180km/hr, which is far faster than

ordinary torpedoes. The nose rather than being streamlined should be flat. Thus at high speeds

water is forced to flow off the edge of the nose at such an angle that it cannot wrap around the

surface of the body. As it passes over the edge it vaporizes due to high velocity. Thus a big

cavity is formed which encloses the front part of the object.

If we could make this cavity enclose the entire body most of the drag could be eliminated. This

is possible by two ways. If the body is fast enough so that the entire length of the body passes

through before the cavity collapses, it will appear as if the cavity is traveling along with the

body. If the object is not fast enough to travel through the vapour cavity before it collapses,

then artificial ventilation into the cavity can keep it open until the object moves past. Once a

super cavity is formed which completely encloses the object, the drag force is nearly eliminated

as the only portion in contact with liquid is the nose. Only the leading edge of the object

actually contacts liquid water. The rest of the object is surrounded by low-pressure water

vapor, significantly lowering the drag on the supercavitating object. With an appropriate nose

shape and a speed over 180 km per hour, the entire projectile may reside in a vapor cavity.

Since drag is proportional to the density of the surrounding fluid, the drag on a supercavitating

projectile is dramatically reduced, allowing supercavitating projectiles to attain higher speeds

than conventional projectiles. In water, a rough approximation predicts that a supercavitating

projectile has 200,000 times less skin friction than a normal projectile. The potential

applications are impressive.

CHAPTER 8

MAKING A SUPERCAVITATING PROJECTILE

Although the idea may seem simple, making a supercavitating projectile is a daring challenge.

The technological hurdles to be overcome are many. The most important question is how to

propel the body if no other part except the nose is in contact with the surrounding fluid. Also

the enormous drag exerted on the blunt nose would literally crush any material.

(I) PROPELLING THE OBJECT

When a supercavitating projectile is enclosed by a cavity conventional propulsion techniques

cannot be used. A rocket engine is a solution. As the cavity encloses the vessel it is similar as

flying in the air. Therefore by using a rocket engine high speeds can be attained which in turn

helps for retaining the cavity.

When the projectile is fired from above water it pulls along with it a ventilated cavity which is

unstable but as supercavitation starts this ventilated cavity is converted to vapor cavity. Then

the rocket motor is fired and using the exhaust the cavity can be stabilized. A rocket motor also

provides an immensely powerful thrust, enabling the object to achieve high velocities. The

overall drag reduces enormously once you reach the supercaviting regime and then increases

only linearly with speed.

An aluminium burning rocket is an answer to a compact and efficient propulsion system. It

would use water as its oxidiser and so would not need to carry oxygen. The problem with

aluminium has been that unreacted fuel quickly becomes coated with aluminium oxide,

inhibiting any further reaction. To avoid this, powdered aluminium can be injected to a vortex

of water, which keeps the molten drops apart.

Using a rocket motor has another advantage. The exhaust from the motor can be used to

ventilate the cavity and stabilize it. The exhaust can be ducted round from just behind the nose

which strengthens the existing cavity and expands it to a bigger one. Thus the cavity can be

retained much longer.

(II) THE NOSE

The nose being the only part in contact with water it is subjected to extremely high stresses.

Ordinary materials under these conditions will buckle and eventually crush. So inorder to

withstand such high stresses nose must be made of materials hard as well as light weight. Light

weight materials like carbon composites in honey comb structure can be used.

Unlike conventional noses, a supercavitating body has a rather blunt nose. Water is forced to

flow off the edge of the nose at such an angle that it cannot wrap around the surface of the

body.

If the projectile is of the correct shape, a bubble of air starts to form around the object... This

extends to cover the entire projectile, and hence the cavitating object is no longer moving

through water, but through air which creates but a fraction of the friction! Hence

supercavitating projectiles can travel as fast as above the surface.

CHAPTER 9

OTHER CHALLENGES

At the present time only supercavitating weapons are under development. Supercavitation

occurs when an object moving though water reaches speeds in excess of 100 knots. The speed

at the initial state is currently unattainable using the conventional methods. For firing the

missile from above the water a mechanical catapult offers a simple solution. A Mechanical

catapult is the one which is used in the aircraft carrier for launching aircrafts airborne. It is

powered with the pressure exerted by the compressed fluid in two long cylinders. It will take

the aircraft to about 250 km/hr through less than 50 m path. Using a modified version of this

type of catapult we can attain the initial velocity for the projectile. By this same type of catapult

we can attain a higher initial velocity underwater for the projectiles firing from submarines etc.

The major challenges in the implementation of this technology are the following. Inside the

cavity the projectile is very unstable. A projectile dropped into water draws a column of air

down with it, creating a temporarily ventilated cavity that reduces drag on the torpedo. The air

eventually leaks out, but if the torpedo is moving fast enough the collapsing ventilated cavity is

replaced by a vapor cavity.

However, the behavior of the cavity’s tail end becomes a problem. The supercavity’s tail end

may splash violently around the projectile’s rear, causing significant structural damage to

control and propulsive surfaces.

The splashing tail of a sphere dropped into water.

This splashing tail problem can be solved by making the cavity by ventilating it by the exhaust of

the rocket engine from just behind the nose at the front and from the rear. But my making the

cavity bigger we are increasing the instability. Since only the nose touches the water the

maneuvering is so tough.

Another big challenge is how to steer a supercavitating vehicle. Specially designed retractable

control fins that come in contact with water only when required to steer are a solution.

However there are technological hurdles yet to be overcome, towards realizing this. The

advanced thrust vectoring technology is another possible solution at the present time. The

pressure that the nose has to withstand at high speeds will be very high. So the right selection

of the material is another challenge. The use of composite light weight materials like graphite

epoxy or aluminum honeycomb will be effective.

CHAPTER 10

EXISTING SUPERCAVITATING WEAPONS

Now that a fundamental idea about supercavitation is obtained we can go further by analyzing

the Shkval Torpedo. The Shkval Torpedo of the Russians is equipped with the Supercavitation

technology. The original VA111 Shkval is capable of cruising at 500 km/h which is about 5 times

faster than the existing fastest torpedo. It is launched from the Akula and Oscar class

submarines. The Shkval is propelled forward by a solid rocket motor. Traveling at over 300 mph

(500km/h) the Shkval is so fast that (despite being equipped with one) it does not even require

a warhead! Its sheer mass and velocity is enough to sink an opposing submarine.

Shkval torpedo (not realistic)

Other than the Shkval torpedo U S Navel undersea warfare centre was successful in firing a

supercavitating bullet at 5800 km/h (mach 5) to destroy the underwater mines. The bullet fired

from a helicopter-mounted with standard Gatling gun supercavitates through the water and

detonates the mines with little cost and even less risk of human life. Unpowered bullets have

already broken the sound barrier underwater.

Very little is known about the Russians advancements in supercavitating torpedoes project.

May be in the future there may be a supercavitating torpedo/rocket which would race towards

a target underwater and then become airborne once nearing a coastline would render any kind

of anti ballistic missile shield useless.

CHAPTER 11

APPLICATIONS OF SUPERCAVITATION

Used for very high performance propellers and also for control surfaces such as rudders.

In 2004, German weapons manufacturer Diehl BGT Defense announced their own

supercavitating torpedo, Barracuda

RAMICS (Rapid Airborne Mine Clearance System), based on a supercavitating projectile stable in

both air and water.

Iran has claimed to have successfully tested its first supercavitation torpedo.

The Kursk submarine accident is rumored to have been due to a faulty Shkval torpedo

ADVANTAGES OF SUPERCAVITATION

Comparing the efficiency of the different methods of the hydrodynamic drag reduction with

efficiency of the artificial and vapor supercavitation, one can see its advantage.

Reduction of water adhesion to the surface or slippage;

-Effect of the movable surface (the gas flow velocity in the clearance may be equal to the water

flow Velocity

- Change of the physical constants of BL (here, the medium density reduction);

- Gas blowing into the boundary layer (BL).

Here, - in the case of the artificial cavitation, the gas is blown through the solid surface of the

object

CHAPTER 10

THE FUTURE

Far from now this simple cavitating theory could bring us the ultimate fighting machines. Sub

fighters racing around beneath the waves at thousands of kilometers per hour... Massive

underwater sub fighter carriers silently gliding through the deep blue... It may seem like

fantasy, but it's the future...

Future doesn’t ends with underwater sub fighters there will be supersonic fighters cruising

underwater at thousands of kilometers per hour maneuvering like aircrafts performing

dogfights and when nearing land they can be airborne and be a fighter aircrafts and can again

be back to the water. They can be launched either from a subcarrier or from land. They rules

air, land and water. They are the ultimate future.

CHAPTER 11

CONCLUSION

As is the case of most cutting edge technologies, supercavitation is largely concentrated around

military developments and applications. Very little is known outside about the recent

advancements in detail, as they are closely guarded military affairs. However this technology is

sure to revolutionize underwater weaponry and travel. Under water bullets have already

broken the sound barrier in water. The day is not far away when pencil shaped, rocket powered

vehicles break the sound barrier underwater.

REFERENCES

1. “Faster than a speeding bullet”, by Graham Rowe.

The New Scientist July 2000

2. “Studies on dynamics of Supercavitating projectile”

by S.Kilkarni, R.Pratap, Applied Mathematical Modelling

3. “Warp drive under water”, by S.Ashley

Scientific American, May 2001

4. http://www.onr.navy.mill