41

6 ROCKETS AND ARTIFICIAL SATELLITES

Rockets - single stage and multistage - orbital velocity and escapevelocity - launching of a satellite - geostationary satellite (elementarydescription only) - satellite communications - Indian space programmes.

REVIEWYou have learnt earlier that

! momentum of a body is the product of its mass and velocity.

! to every action, there is an equal and opposite reaction (Newtons third lawof motion).

! there are a number of artificial satellites launched in the last few decadesfor different purposes and they include Indian satellites.

! the acceleration due to gravity varies with the height above the earth.

! a body executing circular motion experiences a centripetal force f = mv2/rwhere m = mass of the body, v = orbital velocity and r = radius of the orbit

! the centripetal force is the gravitational force in the case of planets andsatellites.

6.1 INTRODUCTIONFiction writers like Jules Verne had thought about vehicles which would fly up

in the sky to reach the moon and beyond. But the question was how would oneleave the earth? The science of rockets began with this question.

The advent of television and internet are two examples of the impact of satellitecommunication on our life style. This remarkable progress in technology hasbecome a reality because of the efforts of great scientists. The most importantamong them are Konstantin Tsiolkovsky of Russia, Robert Goddard of USA andHerman Oberth of Germany.

42

6.2 PRINCIPLE OF ROCKETSAt first there appears to be little in common between the small fire work rocket

and the big rocket used in space programmes. But both the rockets work on thesame principle, the principle of conservation of momentum.

Principle of conservation of momentum: The principle states that the totalmomentum of a system is conserved (remains constant) when the netexternal force acting on the system is zero.

Let us consider the case of a bullet (massm) fired with a velocity (v) from a rifle (mass M).To start with, that is before firing, both the bulletand the rifle are at rest. Therefore, the totalinitial momentum is zero. When the bullet isfired, the bullet leaves the muzzle of the riflewith a momentum mv, the rifle recoils (movesbackwards) with a velocity V (Fig 6.1). As thetotal final momentum must be zero,

mv + MV = 0 or V = -mv/M ..................... 6.1A rocket works on the same principle. During firing of a rocket, the fuel burns andis converted into gases at high pressure. The gases escape from the nozzle ofthe rocket with a high velocity. The rocket acquires an equal momentum in theopposite direction.

6.3 WORKING OF ROCKETBurning of fuel in a rocket or a jet air craft results in hot gases which shoot

out backwards. The reaction of hot gases thrown backwards from the rocket orjet air-craft provides the forward thrust.

Rockets can operate in the vacuum of space because they carry their ownoxidant. The substance that provides oxygen to burn the fuel is called an oxidant.Both fuel and oxidant are together called propellants.

Fig. 6.1

Robert Goddard (1882-1945)was the pioneer in rockets. Heconducted experiments to achievethe goal of launching rockets. Hisideas were extensively used forrocket and space programmes afterhis death.

Konstantin Tsiolkovsky (1859-1935) is known for his remarkableachievements. The design of rocketsdeploying orbiting satellites andeven space travel had been workedout extensively by him.

43

If a rocket has to move vertically up with a recoil velocity, theaction should be towards the earth. This is achieved by burningfuel inside the rocket so that the exhaust gas can escape throughan exit towards the earth (Fig 6.2)

The mass (M) of a rocket includes the mass of the propellantsand the scientific instrument or satellite. The scientific instrumentor the satellite which is usually placed in the front portion of therocket is called payload. When fuel burns and releases theexhaust, there will be an increase in velocity of the rocket. Theacceleration of the rocket is decided by the amount of fuel burnt

and the exhaust velocity (Vex

). However, the mass (M) of the rocket also plays arole. Larger the mass of the rocket lesser the acceleration. It can be shown that

(rate of fuel consumption = (mass of the rocketx exhaust velocity) x acceleration)

The product (mass x acceleration) is called thrust on the rocket. Thrust = RVex = Ma ................................ 6.2

where R is the rate of fuel consumption.

There are other practical aspects that need to beconsidered. (i) As the rocket consumes fuel at everyinstant, its mass goes on decreasing. (ii) Further theacceleration due to gravity changes continuously asthe rocket ascends. (iii) The atmosphere offersresistance in the lower strata of the atmosphere.

The important features of a rocket are explained inFig 6.3. The chamber called engine is the placewhere combustion takes place and exhaust comesout of the exit. The ratio of payload mass (m) to fuelmass (M) is called payload ratio. This is needed forrocket design . The propellants are kept in separatechambers. They are supplied to the engine, incontrolled quantities with the help of valves

Know by yourself

What is the differencebetween a rocket and amissile?

Fig. 6.2

Fig. 6.3

propellants

valves

engineexhaust

44

NoteHistorians have recorded that Tipu Sultan had missiles which causedextensive damage to British army.

6.4 MULTI STAGE ROCKETSA single rocket can not by itself lift a heavy load. The load increases (mass

of propellants increases) for greater heights. Several rockets are joined togetherone on top of the other, to provide the necessary power. Rockets built in this wayare called multistage rockets.

By using multistage rockets, the demand on fuel consumption can be slightlyreduced. The rocket reaches a height of about 100 km, and gets rid of the firststage. The engine of the second stage begins action at this point; the mass isreduced because the propellants and their tanks of the first stage are removed.The second stage carries the rocket to a still greater height and detaches itself.The third now takes over and so on. This process increases the efficiency of therocket.

NoteThe word rocket is generally used for the vehicles which carry a payload suchas satellite or space vehicle. The devices which orbit the earth are termedsatellites, although they are artificial. The devices that ascape from the earthare called space ships or space vehicles.

Most of the rockets used to launch space vehicles, have a massive firstbooster stage and two smaller stages.

Rockets are used not only for atmospheric studies but also for launchingspace vehicles and missiles. In scientific investigations on the study of higherlevels of atmosphere, instruments in the rocket are equipped with radio transmitters.This helps in the immediate transmission of collected data to the ground.

6.5 ORBITAL VELOCITY AND ESCAPE VELOCITYOrbital Velocity: You have learnt in the previous chapter, that a body for

example the moon executing circular motion around the earth, has its centripetalforce provided by the gravitational force between the earth and the moon. Thevelocity of the body along the circular path is called orbital velocity.

45

You know that centripetal force = mvo2

/ r and gravitational force = GMEm/r2

where m = mass of the body (moon in this case)vo = orbital velocity, r = distance from the earths centreME = mass of the earth and G = gravitational constant

Therefore 2E2

O

rmGM

rmv

=

hence vo = )hR(GM

rGM EE

+=

.................................... 6.3

where R = radius of the earth and h = height from earths surface.When h is sufficiently small compared to R, h can be neglected. Then

vo = Rg ......................................... 6.4

where g = 2E R/GM

Escape Velocity: All slow moving projectiles fallback to the earth. Rockets used for launchingspacecrafts have to overcome earths gravitationalattraction. What is the velocity with which a rocketmust be launched in order to escape completelyfrom earths gravitational attraction? This velocityis called escape velocity. Therefore escape velocity is defined as that minimumvelocity with which a body must be projected so that it escapes from thegravitational field of the earth.

If a body has to be moved against gravitational force, work has to be done. Itcan be shown that the work that has to be done in pulling a body (mass m) outof earths attraction is equal to GMEm/R. where ME = mass of the earth and R =radius of the earth. If the body is projected with a kinetic energy greater than thisvalue it escapes into space. The escape velocity Ve is therefore given by

R/mGMmV21

E2

e = or Ve = RgRGME 2)/2( = .................... 6.5

ProjectileAn object which is given an initialvelocity in any direction and thenallowed to travel freely under theaction of gravity, is called aprojectile

46

Fig. 6.4

The escape velocity is independent of the mass of the body. In the case of theearth, escape velocity is 11.2 km/s.Relation between Orbital Velocity and Escape Velocity

Orbital velocity, vo = Rg ......................................... 6.4

Escape velocity, ve = Rg2 ......................................... 6.5

ve = 2.vo

......................................... 6.6

6.6 LAUNCHING OF A SATELLITEMultistage rockets are used to launch artificial

earth satellites. (Fig 6.4). At start the rocket must begiven sufficient velocity to rise to the desired height.When it has ascended the required height, thesatellite (last stage) is placed in to orbit with sufficenthorizontal velocity. If the velocity is greater, thesatellite moves into a larger orbit or escapes fromearths gravity. If the velocity is lesser than the propervalue, the satellite falls back to the earth.

6.7 GEOSTATIONARY SATELLITESSince 1956, artificial satellites have been placed in different orbits around the

earth. In recent times, a very important development has taken place in the areaof communication technology. A number of artificial satellites are launched sothat they remain in fixed positions relative to the earth, at a specific height abovethe equator. For such a satellite, the period of revolution is the same as that ofthe earth. Then the satellite appears to be stationary. These are called geostationarysatellites. These satellites provide relay facilities for international communication.

It is possible to connect any part of theglobe to any other part by placing threesatellites in proper positions (Fig 6.5). Themain objective is establishment ofcommunication link. They have been successfulin uniting the people as one entity. Thesesatellites are referred to as communicationsatellites. We are able to see events happeningat another part of the world as live because ofthese satellites.

Fig. 6.5

47

6.8 INDIAN SPACE PROGRAMMESYou have read about Indian space programmes in VII standard. However, a

brief account is given here. The organisation Indian Space Research Organisation(ISRO) was established in 1969, for the progress of space research and technology.Indian space programme was started in 1963 with the launching of an Americanresearch rocket, from Thumba.

India entered the scene of satellite technology in the seventies. Thefore-thought of Dr. Homi. J. Bhabha and Dr. Vikram A. Sarabai, put our country inthe front line. The programme has been furthered by Prof. Satish Dhavan, Prof.U.R. Rao and Dr. Kasturi Rangan with the launching of various types of satellites.

The first rocket was launched from Thumba, in Kerala as early as in november1971. This rocket was a sounding rocket used for atmospheric studies. The rocketRohini RH-75, had a single stage with only one instrument. Now multistagerockets are being used.

The first satellite Aryabhata, weighing 360kg was launched in 1975. Launchvehicle called CI intercosmos of the former USSR, put it into orbit at a height of550km. It worked efficiently for 6 months.

Bhaskara was the first among the Remote sensing satellites (IRS); It wentinto orbit in 1979. After Bhaskara, sophisticated IRS satellites are being launched.The monitoring of the behaviour of the satellites is achieved at the Master controlfacility at Hassan.

Technological advancement has made it possible for us to successfully buildsatellite launch vehicles. Four launch vehicles namely Satellite Launch Vehicle-3(SLV-3), Argumented Satellite Launch Vehicle (ASLV), Polar Satellite LaunchVehicle (PSLV) and Geo Synchromes Satellite Launch Vehicle (GSLV) have beenbuilt and successfully tested.

The Polar Satellite Launch Vehicle, PSLV-C5, placed the heaviest eversatellite into orbit on 17th October 2003. The satellite weighing 1380kg was putinto orbit at a height of 821 km from Sri harikota range. The satellite is IRS-P6(Indian Remote Sensing Satellite). The launch vehicle PSLV-C5 is dependable andcan be used in all weather conditions.

The geostationary satellite : APPLE, went into orbit in 1981. It was launchedby a space vehicle called Ariane from French Guiana. The INSAT series ofgeostationary satellites have revolutionised our communication capabilities. SatelliteINSAT-A was put into orbit on 19th April 2003. Indias latest communication

48

satellite INSAT-3E was successfully launched from French Guyana, on 28thSeptember 2003, using Ariane space vehicle. The 27 tonne INSAT 3E has anoperational life time of 15 years. The satellite will provide telecommunication andTV transmission services for the Indian subcontinent.

Today various sophisticated satellites are being utilised in various fields such ascommunications, agriculture, flood forecast, identifying geological mineral resources,identifying underground water sources, monitoring air, water, sound & thermalpollutions. The various application centres of ISRO are located at Bangalore,Tiruvananthapur, Thumba, Ahmedabad, Lucknow, and many other cities. IndianSpace Programme has placed India in a prominent position at the internationallevel.

EXERCISE1. Name the principle used in launching of rockets.2. Explain the principle of rocket launching3. What is orbital velocity? Give the expression for orbital velocity of a body

circling round the earth, near its surface.4. Define escape velocity? Give the expression for escape velocity. What is the

value in the case of the earth?5. What are the factors that decide the value of escape velocity?6. Give the relation between escape velocity and orbital velocity.7. Describe with a diagram, the parts of a single stage rocket.8. A jet air craft can not be used to put satellites into orbits. Explain why.9. What is a multistage rocket? How is it advantageous?10. Explain briefly the launching of a satelite.11. What is a geostationery satellite? Explain.12. Write a short note on IRS satellites.13. Write a short note on INSAT series satellites.14. Give a brief account of Indian Space Programme.15. Write a note on uses of satellites.