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Propulsion in a gravityless vacuum?


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Posted
  • Location: Bangor, Northern Ireland (20m asl, near coast)
  • Weather Preferences: Any weather will do.
  • Location: Bangor, Northern Ireland (20m asl, near coast)

There's probably a straight forward answer to this, but last night I was pondering about space travel and the like.

 

My question is, how does one achieve propulsion or acceleration in a gravity free and vacuum environment, ie space?

 

When I think of propulsion I imagine something creating force by pushing against another item to propel itself. So how do space rockets actually propel themselves in a place were there's no gravity or air/gas (vacuum)?

 

Also, if you leave earth at 12,000mph and turn off your engines as you get away from the gravitational pull, would you not continue to travel at the same speed without propulsion due to no frictional force to stop you?

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Posted
  • Location: South Shields Tyne & Wear half mile from the coast.
  • Location: South Shields Tyne & Wear half mile from the coast.

G'day watcher I once pondered over this too. Its simple really Newtons third law "For every action there is an equal and opposite reaction". The rocket actually pushes against its own fuel!

 

http://www.straightdope.com/columns/read/1846/how-do-rockets-work-in-the-vacuum-of-space

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Posted
  • Location: Eastbourne, East Sussex (work in Mid Sussex)
  • Location: Eastbourne, East Sussex (work in Mid Sussex)

 

Newton's laws of mechanics do not require anything "to push against". That is a common misconception. One way to see how this works is to view the mechanism as one that obeys the principle that the center of mass does / must remain "in the same place" before and after some mass is moved from place to place. 
 
So if we start with a rocket ship in outer space, in a vacuum, and start throwing out particles in one direction (that is, fire up the booster rockets) the vehicle has to move in the opposite direction so that the center of mass remains fixed in the same place that it started. The more particles we throw out, and the faster we throw them, the further and quicker the vehicle must move in the opposite direction to keep the center of mass unchanged. 
 
This is a way of thinking about the propulsion process, and is an oversimplification, but it is basically true. It is one of the conservation laws, the conservation of mass. There are other conservation laws as well: energy, momentum, angular momentum that must also be obeyed too, but the conservation of the center of mass is sufficient for your inquiry

 

 

http://www.newton.dep.anl.gov/askasci/phy05/phy05086.htm

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Posted
  • Location: Bangor, Northern Ireland (20m asl, near coast)
  • Weather Preferences: Any weather will do.
  • Location: Bangor, Northern Ireland (20m asl, near coast)

That explains it quite easily. It's obviously a misconception on my part. Thanks both of you.

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