Launching a satellite

Launching a satellite or any other spacecraft is a very complex process that requires a large number of components to work together precisely. You must have seen the launch of a spacecraft on TV. What you have not seen perhaps are the numerous other activities that go into making a launch a success. These include meticulous and precise calculations by scientists and engineers, programming of the various stages of a launch and tracking of the spacecraft. Also involved in the process are ground stations that send and receive signals to the spacecraft. For example, ground stations send signals to communications satellites for relaying. They also receive information gathered by various satellites, and so on. In this section, we shall learn the basics of two components of a space programme—the launch vehicles and tracking.


Launch Vehicles

A rocket system used for carrying a satellite to its orbit, or other spacecraft to their destinations, is called a launch vehicle or a launcher. A rocket or a spacecraft has a certain purpose for which it is used. For example, the purpose of a rocket could be to launch a satellite, and the purpose of the satellite could be to perform scientific observations using the instruments on it. The weight of the part that constitutes the main purpose of the mission is called the payload. Thus, the weight of the satellite is the payload for the rocket, while the weight of its instruments is the payload for the satellite.

The launch vehicle for a satellite not only has to lift the payload, it has to lift its own weight too. Apart from this, it needs to achieve very high speeds to launch the satellite. For this, it is accelerated in phases by firing rockets at various stages of its flight. A launch vehicle fitted with a number of rockets to be fired at different times of the flight is called a multistage launch vehicle. By firing rockets at appropriate heights, the launch vehicle takes the satellite to the desired height and places it in orbit with the required speed. For example, for a low-earth orbit of 200 km, the orbital speed needed is about 8 km/s, and for a satellite in geostationary orbit (35,790 km), the orbital speed needed is about 3 km/s. Not all satellites are launched directly by rockets. A satellite can be put into orbit by a spacecraft such as the Space Shuttle.

As you might know, rockets move forward by ejecting gas in the backward direction. The rocket exerts a force on the gas to eject it in the backward direction. The gas exerts an equal force on the rocket in the forward direction (Newton's third law). This force accelerates the rocket, and is called the thrust. To produce the large acceleration required, the mass of the gas ejected per unit time has to be very large. This is achieved by burning special fuels in the rocket's engine. The fuel is burnt in a special chamber to produce a gas at high temperature. The volume of the chamber is such that the rapidly evolving gas at high temperature develops large pressure. The high-pressure gas is allowed to escape through a narrow nozzle at very high speed.


Rocket fuels

Special fuels are used to meet the requirements of rockets. A rocket fuel should have the following properties





(a) The fuel must bum very rapidly to produce a lot of gas in a short time.

(b) The fuel should produce gas at a very high temperature.

(c) The fuel should burn in a controlled fashion and should not explode on ignition.

(d) A small mass of the fuel should produce a large thrust.

When a jet engine burns fuels, it uses atmospheric oxygen. Since oxygen is not available in space, rockets need to carry their own oxidizer, which could be oxygen or a compound containing oxygen. The fuel and the oxidizer together form the propellant of the rocket. Rockets use either solid or liquid propellants.

Solid propellants The earliest rockets were built by the Chinese, and they used solid propellants. They were very much like the rockets we fire on festive occasions like Diwali. Some modern launch vehicles also use solid propellants. One common solid propellant is powdered aluminium as fuel and ammonium perchlorate or ammonium nitrate as the oxidizer. The launch vehicle for the Space Shuttle uses the largest solid-propellant engines ever built to provide additional thrust that adds to the thrust of the shuttle's main engines. They use aluminium and ammonium perchlorate propellant.

Solid-propellant rockets are relatively simple. The propellant mixture is packed around a hollow core. Once ignited, the fuel burnsat a rapid rate and stops burning only when it is exhausted. Once it starts, you cannot stop the combustion or control its rate. Advantages of solid propellants are: they can be easily stored and handled, they take less space, they are stable at ordinary temperature, and are relatively inexpensive. Also, the propellants can be stored in the rocket for a long period before launch.
Liquid propellants

There are several types of liquid propellants in use. Liquid fuels include a special grade of kerosene and liquefied hydrogen. In both cases, liquefied oxygen is used as the oxidizer. The main engines on the Space Shuttle use liquefied hydrogen and oxygen propellant. The Saturn V rockets used in the Apollo missions that took man to moon also used liquid propellants.

The liquid propellants are stored in large tanks. Pumps are required to inject them into the high-pressure combustion chamber. Valves are used to control the flow of the liquids. You can control the amount of thrust produced by regulating their flow. By starting and stopping the flow, you can start and stop the engine at will. Hydrogen is the best fuel available from the point of view of the amount of thrust produced per unit mass.





Although liquid propellants have many advantages, they pose some problems tpo. Oxygen and hydrogen get liquefied at -183°C and -253°C respectively. Propellants that require such low ■ temperatures are called cryogenic propellants (kyros in Greek means ice-cold). It is expensive to maintain such low temperatures for a long time. Thus, liquid fuels and oxidizers are pumped into the rocket's storage tanks only a short time before the launch. Their storage and liquefaction pose special problems. Since liquid propellants have lower densities than solid propellants for the same mass, they occupy more volume than solid propellants. Thus, for the same mass of fuels, the tanks for liquid propellants need to be larger than those for solid propellants. To maintain the low temperature, the tanks need to be specially insulated, which increases the weight of the rocket. Apart from this, liquid-propellant rockets, in general, are very complicated because of the nature of the fuel, the powerful pumps, valve systems, etc.



Table 11.2 Comparison of rocket propellants


Solid propellant
Liquid propellant

1.
Solid-propellant rockets are relatively simple.
Liquid-propellant rockets are complicated.

2.
Once started, no control over the combustion.
The combustion can be started, stopped and



regulated at will

3.
Easy to store and handle, and takes less
Difficult to store and handle. Large, insulated


space.
tanks are needed for storage.

4.
Stable at ordinary temperature.
Cryogenic propellants require low temperatures.

5.
Can be stored in the rocket much before
Cryogenic propellants cannot be put in the


launch.
rocket's tanks much before launch.

6.
Relatively inexpensive
Expensive





Tracking

After a satellite (or any other spacecraft) is launched, it needs to be constantly monitored by ground-based stations. This is called tracking the spacecraft. A ground controller tracking a satellite always knows its position in orbit. Because of the various forces which might be experienced by a satellite in space, its orbit may get disturbed. If any deviation is observed in the orbit, small rockets on the satellite are fired to set the orbit right. You can also track certain satellites using software available on the Internet.