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Why can't a rocket continue to travel without fuel?

  1. Sep 20, 2015 #1
    By Newton's first law of motion, every object will continue to be in state of movement or rest, until acted upon by external force. Now, why can't a rocket continue to travel after it was initially thrusted (do not know if its right word) by fuel ? like a satellite that is set in orbit with some threshold velocity and continue to rotate in orbit afterwards.
  2. jcsd
  3. Sep 20, 2015 #2


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    It does. If there's no gravity or other external force acting on it, it accelerates as long as it's burning fuel and generating thrust. Once the fuel is exhausted, it stops accelerating and coasts at a constant speed.
  4. Sep 20, 2015 #3
    then why do we need fuel (other than initial fuel) to get to far away planets? Why cant we just propel our rocket in right direction and leave it to move at constant speed after it runs out of fuel??
  5. Sep 21, 2015 #4
    That is exactly what does happen, but in addition we also can use clever tricks to get spacecraft to accelerate beyond their initial speed by taking advantage of the gravity of planets that the spacecraft passes nearby to.
    Spacecraft do need to carry a small amount of additional fuel which is used to make gentle adjustments to trajectory if needed, but not major changes., once it's launched it's going where it is intended to unless it unfortunately encounters an unknown small asteroid.
    Last edited: Sep 21, 2015
  6. Sep 21, 2015 #5


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    The small amount of extra fuel that is carried is for fine trajectory adjustments, to help with slingshot maneuvers and orbit insertions, etc. Does that make sense?
  7. Sep 21, 2015 #6
    that does make sense..what I was wondering about is that why don't we send spacecrafts to remote planets, citing the cost of fuel???
  8. Sep 21, 2015 #7
    That reminds me of Rosetta.... ;-)
  9. Sep 21, 2015 #8
    New Horizons was a spectacularly successful mission to Pluto/Charon system, and we only have about 10% of the data collected so far.
    Fuel cost is not a particularly significant consideration, fuel cost would have probably been greater for the Curiosity explorer now on Mars.
    Fuel is not at the top of the list of costs for spacecraft projects, probably the cost of engineers and scientists to make it happen is greater.

    Now you mention Rosetta, the gravitational slingshot worked as planned, but apparently it accelerated more than it was expected to on it's second pass by of Earth.
    I don't know if anybody figured out yet what happened there.
    Last edited: Sep 21, 2015
  10. Sep 21, 2015 #9
    Sending a space craft on an interplanetary mission does cost a lot of fuel, but not so much for the journey, but for getting the craft off of earth. Longer, more complex missions generally require more complex, heavier space craft. Getting a pace craft into earth orbit costs around $10,000 per pound. If you want to make it a manned mission, then you also have to take with you all of the fuel and equipment needed to get off the destination planet and propel the craft back home.

    Take a mission to Mars for example.
    We've had several un-manned missions to mars, and they were plenty expensive. You know that a pretty big rocket and a lot of fuel were necessary to get the robots there. Now imagine you were trying to send a crew of people with all of the necessary protective gear, food, life support, etc. That's a lot bigger payload, and therefore, a lot more expensive. Now imagine you have to not only get them there, but after their mission, they'll still need a space craft with enough thrust and enough fuel to get them off the surface of Mars and push them on their way back home.

    If it takes a big expensive rocket to get a little robot to Mars, then it will take a big expensive rocket to get a crew from Mars back to earth. What kind of ungodly behemoth will it take to get that big expensive rocket to Mars?
  11. Sep 21, 2015 #10

    Vanadium 50

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    We do send spacecraft to remote planets. Who cites the cost of fuel?

    Before asking why something is true, it helps to know if it is true.
  12. Sep 21, 2015 #11


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    In addition, it isn't so much the cost of fuel as the volume and mass of fuel that are a problem. The reason New Horizons was a flyby of Pluto, rather than going into orbit around it was because it is traveling incredibly fast (in order to get to Pluto in a reasonable amount of time). To slow down from such a high speed would have required an enormous amount of fuel, which would have dramatically increased the size of the spacecraft. However, it then would have taken a much larger launch vehicle to launch the spacecraft, since the mass of the craft would be much higher. This compounding effect means that it is prohibitively expensive and difficult to add additional fuel onto a lot of vehicles, and that does limit where we can go currently. This is somewhat mitigated by high-efficiency space engines such as hall effect thrusters and ion engines, but fuel mass is definitely a huge concern in space exploration as it stands today.
  13. Sep 22, 2015 #12
    One factor is how long it would take to get there. If you just launch and coast, it takes way longer than if you have more fuel to keep blasting and accelerate. This is especially relevant if you want to send people somewhere.
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