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Why do electric rockets need to be in a vaccum?

  1. Aug 19, 2009 #1
    Yeah, why do they need to place plasma rockets in a vacuum? Temperatures? Air resistance from the particles? Is there any thrust at all in an atmosphere?
     
  2. jcsd
  3. Aug 19, 2009 #2

    MATLABdude

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    If you mean an ion drive, it is because most of them produce so very little thrust:
    http://en.wikipedia.org/wiki/Ion_drive

    The whole philosophy is that slow and steady wins the race. Whereas a normal chemical rocket will exhaust its fuel in a matter of a few minutes (if going at full throttle), the ion drive can keep on accelerating for a very long time (a little bit at a time).

    If you look at the Wikipedia link above, you'll see that the NSTAR (developed to power interplanetary probes, and used in the Deep Space 1 probe launched in 1998) has a thrust of 92 milliNewtons (mN):
    http://en.wikipedia.org/wiki/Deep_Space_1

    A letter-sized piece of 20 lb. paper (grammage is 75 g / m^2, letter is 8.5 x 11 inches) has a mass of 4.52 grams. Its weight (how much gravity pulls on it) is then 44.4 milliNewtons (using 9.81 N / g). The NSTAR then has a thrust equal to the weight of two pieces of paper!

    Clearly, you won't be going anywhere (in an atmosphere and/or subject to earth-ish gravity) on that much thrust. But do it in outer space, with minimal particles to run into, and working against microgravity, and do it for a few years? You can get up to quite a decent speed (Deep Space 1, though it accelerated for 2 years, was intended to stay in the solar system, rather than going where no man, woman, or probe has gone before).
    http://nmp.nasa.gov/ds1/arch/mrlog69.html
     
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