# The Naked Scientists Forum

### Author Topic: How do shuttles and space craft change direction?  (Read 1720 times)

#### thedoc

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##### How do shuttles and space craft change direction?
« on: 02/08/2013 04:30:01 »

Hello

I was wondering, how do shuttles and space craft change direction? In movies I see thrusters, but what are they pushing against in the vacuum of space?

Naomi

What do you think?
« Last Edit: 02/08/2013 04:30:01 by _system »

#### CliffordK

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##### Re: How do shuttles and space craft change direction?
« Reply #1 on: 02/08/2013 05:24:08 »
Keep in mind that with our current technology, fuel in space is at a premium.  Directional and velocity changes are subtle, and great effort is made to use natural phenomena such as the moon's gravity to assist with direction changes.

As far as thrusters, it is is basic action/reaction, or Newton's third law of motion.  A rocket takes onboard fuel and fires it at great velocity behind the rocket, so even though the mass of the rocket exhaust is less than the rocket being moved, the high velocity of the rocket exhaust causes changes in the speed or direction of the rocket at a lower magnitude.

In theory, you could simulate this by sitting on a skateboard on level ground, and throwing objects behind you to propel yourself forward.

Another example would be the kick of a gun as it fires a bullet.  Even a gun shooting blanks with a reasonably large load should experience a kick.

#### starbuck1963

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##### Re: How do shuttles and space craft change direction?
« Reply #2 on: 02/10/2013 03:09:47 »
of course they move and maneuver. they wouldn't be much good if they couldn't. :)

The rocket exhaust does not push against anything, the rocket is pushing against its own exhaust.

The higher the velocity of the exhaust, the greater "push" it gives the rocket...generally speaking.

For example, an oxygen/hydrogen fueled rocket (like the space shuttle main engines) have a lot of thrust, very fast exhaust velocity and the thrust has mass and they are fairly efficient in a vacuum.
The kerosene/oxygen fueled F-1 engines of the Saturn-V moon rockets, weren't as efficient but the slower exhaust had more mass so they gave a better initial push to the rocket.  While the F-1's did not have an exhaust as fast and were not as fuel efficient as the shuttle engines, they gave  a stronger push at launch, very helpful in getting a heavy vehicle to start moving.

Compare that to ion engines which have super fast velocity, are super fuel efficient, but have a thrust measured in grams or ounces at best. getting even the lightest object off the ground is impossible for an ion engine but in space, with no gravity or atmospheric drag to fight against, ion engines can apply a small thrust for a long long time.  And since ion engines are hyper-efficient and can be used for years at a time, that slight thrust over that time can add up to tremendous velocities.

#### The Naked Scientists Forum

##### Re: How do shuttles and space craft change direction?
« Reply #2 on: 02/10/2013 03:09:47 »