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Flight in Space

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toyomotor View Drop Down
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    Posted: 28 Nov 2018 at 03:25
Within Earths atmosphere, flight is made possible by equal and opposite reactions. For example, if the gross weight of a Helicopter is 5000 pounds, the helicopter must produce in excess of 5000 pounds thrust in order to lift off the ground.

Similarly, with jet engines the same applies, and once in the air, the thrust is exerted against the other present gasses which make up the earth's atmosphere.

At least, that's how I think it works.

But, in space, we're told there is no breathable natural gasses, apart from nitrogen and helium, so what are the engines thrusting against?

When we see landing craft landing on the Moon and on Mars, we see dust being thrown up, so is this caused by the engines thrust against the Helium and Nitrogen?

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It's the Ireland that was born in me.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote franciscosan Quote  Post ReplyReply Direct Link To This Post Posted: 29 Nov 2018 at 00:26
Think of a mortar on a railway car, shooting the mortal expels m(v) in one direction which makes the railway car recoil in the other direction.  With a rocket in space, you are not 'pushing' against something,
you are jettisoning mass x velocity in one direction, causing the rocket to move forward at mass x velocity in the other.

I don't know what the propellant is, that is used by lunar landers or Mars landers, I don't think they would be helium or nitrogen, but I don't really know.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toyomotor Quote  Post ReplyReply Direct Link To This Post Posted: 29 Nov 2018 at 02:26
Yes, but once fired, the mortar would not increase mass or velocity, nor decrease it.

A space rocket takes off with a given amount of thrust, which obviously creates it's velocity, which, after a given amount of time becomes stable. The rocket, at various stages decreases mass due to jettisoning it's fuel tanks (which are by far the biggest components of the whole thing) but I don't understand how by decreasing it's mass, in space, the velocity is increased or decreased. It seems to me that velocity, once the tanks are dropped, would remain constant.

I thought for rockets to work in space they would need to be thrusting against an atmosphere of one kind or another.

Quote from Wiki
"These two forces are called action and reaction forces and are the subject of Newton's third law of motion. Formally stated, Newton's third law is: For every actionthere is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toyomotor Quote  Post ReplyReply Direct Link To This Post Posted: 29 Nov 2018 at 02:30
STOP PRESS

Should have looked further on the web.

The answer is that no air is required!

Quote from our old mate Wiki
Rockets and engines in space behave according to Isaac Newton's third law of motion: Every action produces an equal and opposite reaction. When a rocket shoots fuel out one end, this propels the rocket forward — no air is required. NASA says this principle is easy to observe on Earth.
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