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Thrust sources

  1. Dec 26, 2008 #1
    I was reading about a few mission that sent spacecrafts very far from the Earth.
    Like the Pioneer 10, which went past the asteroid belt, Jupiter, and lost contact with Earth on 2003.
    I tried to search for, but didn't find what those kind of spacecrafts use to gain speed in the space.
    I know they use Radioisotopes thermoelectric generators to power up all experiments and computers on the spacecraft, but can you move a spacecraft using a kind of electric motor in space?
     
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  3. Dec 26, 2008 #2

    Janus

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    It followed a ballistic trajectory after being set on course by the third stage of the launch vehicle(A standard chemical rocket) . After it left Earth's vicinity, it just followed a path determined by its own inertia and gravity.
     
  4. Dec 26, 2008 #3

    tiny-tim

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    Hi Sakha! :smile:

    We can only power a spaceship by using conservation of momentum …

    basically, we throw something out the back, and the spaceship moves faster forward …

    so either the spaceship has to carry something with it (fuel), which it gradually gets rid of,

    or it has to collect something on the way (ion propulsion, invented decades ago but never used, or solar wind).

    However, there are some unpowered orbits which will take a spaceship to distant planets … the "slingshot" orbits, which Pioneer 10 etc used, where the spaceship gets very close to one planet, whose gravitational pull then "slingshots" it to the next planet. :smile:

    (the onboard generators only provide very weak power, and anyway would still require some sort of fuel to enable momentum to be chucked out the back :wink:)
     
  5. Dec 26, 2008 #4

    D H

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    To pick some nits:
    Spacecraft with ion propulsion engines have to carry their own propellant -- mass carried by the spacecraft and ejected from the spacecraft as a means of providing a propulsive force. Several spacecraft have used ion propulsion engines. These including Deep Space 1 and Dawn (NASA), SMART 1 (ESA), and Hayabusa (JAXA). The advantage of ion propulsion engines is that they have a much higher specific impulse (change in momentum per unit propellant mass) than does chemical propulsion. The disadvantage is that ion propulsion engines provide very low thrust (i.e., we don't know yet how to make a big ion thruster).

    Solar sails do not carry propellant. They don't use the solar wind, however. They use sunlight.
     
  6. Dec 26, 2008 #5

    Astronuc

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  7. Dec 27, 2008 #6

    tiny-tim

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    Hi D H! :smile:
    wow … didn't know that …

    I read an article years ago about how a spaceship could collect hydrogen from space (which isn't a complete vacuum, of course), ionise it, and chuck it out the back :rolleyes:
    hmm … so ion propulsion is useful over long journeys, but chemical propulsion is needed for "impulsive" manoeuvres such as getting off a planet? :smile:
     
  8. Dec 27, 2008 #7

    D H

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    You're talking about a Bussard ramjet, a darling of hard science fiction. There are many feasibility issues associated with Bussard ramjets. Science fiction authors can solve these issues with a simple wave of the hand.

    The Apollo missions to the Moon followed a more-or-less direct path from the Earth to the Moon, taking 3 days or so to get from low Earth orbit to lunar orbit. Compare this with SMART 1. SMART 1 was launched on September 27, 2003 by an Ariane 5 rocket and inserted into a 7,035×42,223 km orbit (perigee × apogee). SMART 1 then used its Hall effect thruster to spiral out from the Earth. Thirteen and a half months later, on November 11, 2004, SMART 1 passed through a keyhole at the Earth-Moon L1 point. Four days after that it passed its first perilune. It took several more months (February 28, 2005) to achieve its desired 2,200×4,600 km lunar orbit. Total time from initial Earth orbit to low(ish) lunar orbit: seventeen months.
     
  9. Dec 27, 2008 #8

    Astronuc

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    High Isp systems have low thrust, so basically are limited to low gravity (out in space). To move in high gravity fields, e.g. earth, one needs a lot of thrust so one needs high mass flow rate, which usually means chemical propulsion (e.g. LH2/LOX), although one could use a monopropellant like straight H2 (e.g. nuclear rocket).
     
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