Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Fred Neumann on 03/11/2011 07:30:03

Title: How can spacecraft achieve auto-propulsion in space?
Post by: Fred Neumann on 03/11/2011 07:30:03

Fred Neumann asked the Naked Scientists:
   
Hi Chris,

I listen with great pleasure to your various podcasts (http://www.thenakedscientists.com/HTML/podcasts/) - especially the naked scientist (http://www.thenakedscientists.com/HTML/podcasts/) and naked astronomy (http://www.thenakedscientists.com/astronomy).  I have a question that long puzzled me.

How can spacecraft achieve auto-propulsion in space?  Presumably, space is a near vacuum.  So how can a spacecraft generate "push" from a rocket or other engine?

On Earth, obviously, air provides a level of resistance which allows propulsion. But in a pure vacuum, how can thrust be generated?  Hope my question is clear enough - am struggling a little to make it crystal-clear.  I would be very grateful for your insights!

Thanks,  Fred

What do you think?

Title: How can spacecraft achieve auto-propulsion in space?
Post by: Gyzin on 03/11/2011 09:19:48

The rocket does not push against the air in the atmosphere to drive itself upward.

The motion of the rocket is analogous to the recoil (http://en.wikipedia.org/wiki/Recoil) of a cannon, with the rocket as the cannon and the high-speed jets of gas ejected from it acting like the cannonballs.

http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_third_law

Title: How can spacecraft achieve auto-propulsion in space?
Post by: Flyberius on 22/11/2011 16:17:11

As you can see in the rather substandard drawing of a combustion chamber I have made below, the rocket fuel is injected into the rockets combustion chamber and ignited.  The ignited fuel attempts to expand and exerts force in equally in all directions.  As there is a hole in one end of the chamber (the rocket nozzle or whatever) the forces are not equalized and the net effect is for the rocket to shoot off in the opposite direction to the escaping gases.

[diagram=653_0]

Here is a better explanation:

http://en.wikipedia.org/wiki/Rocket_engine#Principle_of_operation

And a better diagram:

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F2%2F2a%2FR%25C3%25BCckstoss1600.png%2F200px-R%25C3%25BCckstoss1600.png&hash=5e2bbc09453b649cf9fa3ba7100af109)