Explain how rocket works ?
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Rockets work in much the same way. Exhaust gases coming out of the engine nozzle at high speed push the rocket forward. Most modern launchers, such as Europe's Ariane 5, are very complicated and weigh hundreds of tonnes at liftoff. Most of this weight is fuel, such as liquid hydrogen and liquid oxygen.
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A space rocket is a vehicle with a very powerful jet engine designed to carry people or equipment beyond Earth and out into space. If we define space as the region outside Earth's atmosphere, that means there's not enough oxygen to fuel the kind of conventional engine you'd find on a jet plane. So one way to look at a rocket is as a very special kind of jet-powered vehicle that carries its own oxygen supply. What else can we figure out about rockets straight away? They need great speed and a huge amount of energy to escape the pull of gravity and stop them tumbling back down to Earth like stones. Vast speed and energy mean rocket engines have to generate enormous forces. How enormous? In his famous 1962 speechchampioning efforts to go to the Moon, US President John F. Kennedy compared the power of a rocket to "10,000 automobiles with their accelerators on the floor." According to NASA's calculations, the Saturn V moon rocket "generated 34.5 million newtons (7.6 million pounds) of thrust at launch, creating more power than 85 Hoover Dams."
Forces
Rockets are great examples of how forces make things move. It's a common mistake to think that rockets move forward by "pushing back against the air"—and it's easy to see that this is a mistake when you remember that there's no air in space to push against. Space is literally that: empty space!
When it comes to forces, rockets perfectly demonstrate three important scientific rules called the laws of motion, which were developed about 300 years ago by English scientist Isaac Newton (1642–1727).
1. A space rocket obviously doesn't go anywhere unless you start its engine. As Newton said, still things (like rockets parked on launch pads) stay still unless forces act on them (and moving things keep moving at a steady speed unless a force acts to stop them).
2.Newton said that when a force acts on something, it makes it accelerate (go faster, change direction, or both). So when you fire up your rocket engine, that makes the force that accelerates the rocket into the sky.
3.Rockets move upward by firing hot exhaust gas downward, rather like jet planes—or blown-up balloons from which you let the (cold) air escape. This is an example of what's often called "action and reaction" (another name for Newton's third law of motion): the hot exhaust gas firing down (the action) creates an equal and opposite force (the reaction) that speeds the rocket up. The action is the force of the gas, the reaction's the force acting on the rocket—and the two forces are of equal size, but pointing in opposite directions, and acting on different things (which is why they don't cancel out).
Thrust and drag
The force that pushes a rocket upward is called thrust; it depends on the amount (mass) and speed of gas that the rocket fires and the way its exhaust nozzle is shaped to squirt out that gas in a high-pressure jet. When a rocket's engine develops enough power, the thrust force pushing it upward will be bigger than its own weight (the force of gravity) pulling it down, so the rocket will climb into the sky. As the rocket climbs, air resistance (drag) will try to pull it back too, fighting against the thrust. In an upward-climbing rocket, thrust has to fight both drag and weight. This is slightly different to an airplane, where thrust from the engines makes the plane fly forward, drag pulls the plane backward, and the forward motion of air over the wings generates lift, which overcomes the plane's weight. So a key difference between a rocket and a jet plane is that a rocket's engine lifts it directly upward into the sky, whereas a jet's engines simply speed the plane forward so its wings can generate lift. A plane's jet engines fire it forwards so its wings can lift it up; a rocket's engines lift it up directly.
Escape velocity
Rockets burn huge amounts of fuel very quickly to reach escape velocity of at least 25,000 mph (7 miles per second or 40,000 km/h), which is how fast something needs to go to break away from the pull of Earth's gravity. "Escape velocity" suggests a rocket must be going that fast at launch or it won't escape from Earth, but that's a little bit misleading, for several reasons. First, it would be more correct to refer to "escape speed,"
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