how it works?

Question

when a rocket is launched it is propelled by the thrust given by the engine by pushing the air down wards. While it is comming back from space and entering atmosphere is it required to do the same ?

Answer 1

1) When launched, the rocket is propelled by the exhaust it shoots behind itself (it is not because it "pushes" against the air -- rokets do work in the vacuum of space). Newton's action-reaction principle.

2) Most things we send in space are left there in free-fall: they orbit the Earth at a speed such that it keeps them from falling down. When the time comes to return, the rocket engines are pointed forward and fired. Pushing its exhaust forward causes the rocket to slow down and fall towards Earth. Care is taken to have a proper angle to enter the atmosphere (too shallow and you bounce back into space like a rock skipping on water -- too deep an angle and you burn up like a meteor).

The speed of reentry is then reduced by air friction and lift energy from wings (the shuttle works that way) or by parachutes (the Russian capsules still use this principle).

One could land by using rocket engines pointing down and firing to slow down the falling rocket. Turns out to be the most expensive way to land AND requires a lot of coordination so that the speed reaches zero exactly at the same time as altitude reaches zero, without running out of fuel. Oops -- splat.

Answer 2

Rockets don't need air to work. It's like the recoil of a gun - the bullet goes one way and the gun tries to go the opposite way. In a rocket engine hot gasses come out of the engine in one direction and the rocket travels in the opposite one. For reentry from orbit, rocket engines are fired to slow the spacecraft down and it loses altitude. Once it's in the atmosphere it's slowed down by friction with the air. This generates a lot of heat - hence the thermal tiles on the shuttle.

Answer 3

Thrust opposes gravity on launch. However, when re-entering the earth's atmosphere, a rocket is traveling in a vector where the gravitational force accelerates the rocket. Thus no rocket thrust is needed to travel in a direction that uses a vector component of gravitational force.

Answer 4

A rocket does not push on the air. The escaping gas from the engine produces an opposite reaction that causes the rocket to move. It's like rolling a bowling ball while you're on skates. The ball goes one way, you go the opposite way. To come back down, you only need to slow down enough to get out of orbit and let gravity bring you back. It costs a lot lest fuel to return.

Answer 5

Yes and No. At a certain orbit, the rocket must maintain a certain velocity in order to maintain that orbit (the velocity depends upon the height of the orbit. The speed is slower at higher orbits and faster at lower orbits). Thus, to reenter the atmosphere, it simply has to change its heading to reenter the atmosphere. However, in order to maintain a lower orbit, it would have to increase its speed and thus, it would still require fuel to land (it cannot just point its nose downward, or the gravity would burn up the shielding).

Answer 6

As earths gravity pulls with a good acceleration no extra thrust is required.

Answer 7

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