Colourful Diwali rockets that enthrall us, missiles that threaten world peace, and the rockets that lunch spacecraft have certain things in common. They all are basically tubes in which a fuel is ignited to produce hot gases that escape at high velocity and create an explosive thrust that propels them in the opposite direction. They all follow Sir Isaac Newton’s third law of motion, which states: “To every action, there is an equal and opposite reaction.”

 How does a rocket work is very similar to how an inflated balloon zigzags across a room when the air is let out from one end. Take an inflated balloon. Hold the opening tightly closed. Air inside the inflated is made up of gases that are pushing equally in all directions. Let go of the balloon and it will fly across the room whizzing past in a zigzag path. Why? As pressurized air from the balloon escapes through the mouth in one direction, the balloon is pushed in the other direction.

When we think of rockets, we rarely think of balloons. Instead, our attention is drawn to the giant vehicles that carry satellites into orbit and spacecraft to the moon and planets.

Nevertheless, the basic principle is the same. The one significant difference is the way the pressurized gas is produced. In a balloon you pump in the air under pressure. With space rockets, burning propellants that can be solid or liquid in form or a combination of the two produces the gas. In a rocket some fuel is burned with an oxidizer in a combustion chamber, creating hot gases under high pressure-analogous to the air in an inflated balloon. In a (propellant) tanks, plus payload or spacecraft-lift off from the lunch pad.

Rockets use chemicals that can expand manifold when ignited. Historic Chinese fire arrows, Tipu Sultan’s rockets, British-made Congreve rockets were rockets with solid fuels. Gunpowder, which, when ignited provides enough exhaust to propel the missile forward  was essentially used in those. Even the present day Diwali rockets are of these types. The thrust per weight provided by gunpowder is relatively small; but it was adequate for early missiles. Of course, it would be totally useless to lift payloads weighing tonnes.

Whether powered by liquids or solids, rocket systems are propelled by gas pressure resulting from fuel combustion. The force driving them forward is called ‘thrust’. In the rocket system- the liquid or solid booster plus payload-is to move upward against gravity the thrust must be greater than gravity’s downward pull.

In modern times semi-solid polymers have been developed as propellants. A polymer, as we know, is made by stringing identical molecules together to create larger molecules without changing the basic chemical composition. It was discovered that mixing a chemical oxidizer and fuel, often aluminium powder, with the polymer provide the needed oxygen produced a substance with a consistency similar to chappati dough. This substance could be moulded into forms ad baked into a rubbery solid material that burned furiously when ignited ad created large volumes of gases, producing much greater thrust per weight than gunpowder. Even now in solid rockets the propellant mixture is called the ‘grain,’ a word carried over from gunpowder days.

In other words, in solid rockets the propellant is solid cast. But if the solid cast propellant is ignited it may not burn at the same or desired rate. So one has to find a way to control the burning characteristics of a solid propellant. To meet this demand, a carefully designed hole through the propellant or a ‘perforation’ is formed when the propellant is cast into the rockets cavity. By varying the size and shape of the perforation it is possible to determine the rate and duration of combustion and thereby control the thrust.

The Polar Satellite Lunch Vehicle (PSLV), India’s third-generation launch vehicle, large enough to carry polar-orbiting earth resources satellites, is a four-stage launch vehicle, whereas the Geostationary Satellite Lunch Vehicle (GSLV), Indian launcher for geosynchronous satellites using a liquid oxygen/liquid hydrogen upper stage developed from Russian technology, is a three-stage vehicle. What are these stages?

We know that for sending a given payload farther and farther out into space a higher thrust applied for a longer duration is required. The principles of physics tell us that the longer the thrust is exerted, the faster a rocket would go. For example, provided its mass does not change, a vehicle starting from rest under a constant net thrust will be going 100 times faster after 100 seconds than after the first second.

When a rocket is launched its total mass does not remain the same; it decreases. As each kilogram of propellant is burned, the mass of the ascending vehicle becomes one kilogram less, and large rocket engines may burn hundreds of tonnes of propellant in seconds. As a result, the same thrust has much less mass to accelerate. The decrease in weight allows an increase in acceleration.

This principle is used in the concept of ‘staging’. Once the fuel is used up the tank, pumps, and the rocket casing are useless and only add weight to the vehicle, which slows down its acceleration. If somehow the useless parts could remove, as their utility is completed, than with less fuel one would be able to achieve higher acceleration of the payload.

A space launch vehicle usually consists of a number of such well-defined sections known as ‘stages’. After the first rocket stage accelerates the entire vehicle and finally burns out, its tanks and motors are discarded. By dropping the stages, which are no longer useful, the rocket lightens itself. The thrust of the successive stages is able to provide more acceleration than if the earlier stages were still attached. When a stage drops off, the rest of the rocket would be still traveling near the speed that the whole assembly reached at burn out time. This means that it needs less total fuel to reach a given velocity and or altitude. The same happens in the second and subsequent stages. The payload is finally all that remains, having been accelerated to the necessary velocity in successive stages.

Packaging the energy of a rocket vehicle into stages that can be discarded as they burn out has been the secret of launching into space. The number of stages may vary two to five or even more. There are two types of rocket staging, serial and parallel. In serial staging, the second stage, usually a smaller rocket engine is placed on top of a larger first engine. The first stage ignited at launch and buns through the powered ascent until its propellants are exhausted. The first stage engine is then extinguished; the second stage engine is ignited. The payload is carried atop the second stage separates from the first stage, and the second stage engine are ignited. The payload is carried a top the second stage into orbit. In parallel staging several small first stages are strapped onto to a central core rocket. At launch, all of the engines are ignited. When the propellants in the strap-on are discarded. The core engine continues burning and payload is carried atop the core rocket into orbit.

 If a balloon’s flight is erratic, and Diwali rockets do not soar straight into sky it is because neither of them has a guidance system, or exhaust control. A balloons erratic movement is the result of resistance that the balloon encounter as it flies through the air. Unlike airplanes and rocket, balloons are not shaped well for fast flight. Further, the material with which a balloon made is not even and hence when the balloon deflates it may not do so at all points at the same rate. In case of Diwali rockets though there are some controls such as fins, etc., the rate at which the fuel pellet fires or the nozzle through which exhaust comes out are not perfect, resulting in erratic movement. Rockets that are designed to make space flights are carefully designed with aerodynamic controls and stability.