Would a spaceship that used a planets gravity as a slingshot save on fuel by doing that?

Question

Or would it end up being the same amount of fuel used because fuel would need to be used to break away from the gravitational pull right?

Answer 1

Yes, gravity-assist trajectories save fuel.

The "slingshot effect" works because you're not dealing with a two-body system, but rather with what is effectively a restricted three-body system. (The restriction is that the spaceship has negligible mass.) The planet is in orbit around the sun; i.e., it's an accelerating object. The spaceship approaches the planet while generally going around the sun in the same direction as the planet is going, trying to pass the planet at small distance in superior conjunction to it. The planet's gravity pulls the spaceship toward it on the approach, and slows it down as it recedes, but since the planet has accelerated toward the sun in the meantime it is further from the receding spaceship than it was an equal amount of time before closest approach. Hence, the spaceship gets the benefit of a boost from the inverse square law and leaves with more kinetic energy than it arrived with. The cost is paid out of the planet's orbital energy - the planet drops imperceptibly closer to the sun.

Answer 2

Our robotic spacecraft use this technique all of the time to save on fuel. This is called a gravity assist. One of the most notable examples is the Voyager mission, which used each gas giant's gravity to boost it to the next planet in a rarely-available trajectory called "the grand tour." The 4 gas planets were roughly lined up so that each could be used to slingshot up to the next one, and eventually out of the solar system.

Cassini used a very long gravity assist where it flew by Venus for a gravity assist not once, but twice, then the Earth, then Jupiter in order to reach Saturn. New Horizons used Jupiter's gravity to cut the travel time to Pluto down from 15 years to about 11.

One mission, I forget the name at the moment, even used Jupiter's gravity to boost it into a polar orbit around the Sun, which would have been an impossible orbit using our rocket technology - no rocket has ever been built that is large enough to directly launch onto a polar orbit around the Sun.

The drawback to some of these trajectories is that they take longer than a direct ascent might be - for instance Cassini, which took 7 years to reach Saturn, and about 3 to reach Jupiter, while New Horizons reached Jupiter in under 1 year for its gravity assist en-route to Pluto.

Answer 3

Yes- This slingshot technique is used quite often to save fuel on returning expeditions to space.

No- The amount of fuel used to breakaway from the gravitational pull

would be less because you would calculate the benefit ahead of time. You would not use the technique unless fuel would be saved. However, I believe in some cases the additional velocity gained and time saved in flight could be a factor. Both of the above would depend on the mass of the objects, velocity and inertia.

Answer 4

Yes, the Voyager1 probe did this and is now the furthest man-made object from Earth. You would have to be careful though not to get too close as you could get pulled into the planet

Answer 5

It saves *vast* amounts of fuel, yes.  The momentum gained by the spacecraft is offset by an equivalent amount lost by the planet.  It would be virtually impossible to do some interplanetary trips any other way, with current technology - for example, Messenger to Mercury (you'd need a huge amount of fuel to avoid falling into the sun if you didn't do lots of flybys).

Answer 6

You're not actually using the "gravity" to slingshot the spacecraft, you are using the orbital speed of the planet to slingshot the craft.  If the planet was "stationary", then a craft would speed up as it swung toward the planet and then slow down as it got slung off, resulting in a net zero additional momentum.  but if the planet is moving very fast relative to the spacecraft, the craft rides the gravity well just like a wave and pulls out faster than it pulled in.  Just like a surfer.  It's not the "height" of the wave that is moving the surfer, because it would take just as much energy to climb the wave as it does to ride it back down.  It's the forward momentum of the wave that powers the surfer.  Same with planets.  

It's not your fault for thinking this.  People are always making it sound like they are tapping the "power of gravity" when they do this maneuver.  It's a huge misconception in physics that there's some sort of power in gravity that people can tap into.  Just look up "gravity airplane" on the internet and you will see people trying to scam you by working off your misconception of tapping the power of gravity.  You are using orbital momentum to speed you up at the expense of slowing down the planet a very slight amount.  

Another example of a misconception of tapping the power of gravity is using the tides to generate energy.  People often think this is somehow getting energy from the tidal gravitational force of the moon.  Well yes and no.  You are using the tidal force, but the actual energy comes from the orbital motion of the earth and the rotation of the moon.  When you are using tidal energy, you are actually slowing the earth down ever so slightly and using that kinetic energy to make power.  It's not gravitational energy.

Answer 7

It can actually give U a multiple of the max power that U can produce .We used that to get Apollo 13 back home.