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Aerospace Feasability of an Ion Engine For Manned Missions?

  1. Mar 22, 2004 #1
    ION Drives are relatively new technologys, and have been used for small un-manned missions to comets and stuff, however, there incredibly weak (0.2N thrust from what i have read). Does this teachnology have the potential to accelerate something like a 5 tonne manned ship at 10m/s ?

    I have done the mechanics of it, it would need to be about 500 000 times more powerful than it currently is to pull off such a feat!

    Are the current drives crude designs? I dont know how NASA and such work you see! when i build something (say a website). I just build it to work, then pile everything else that needed, like design and usability on top of that! Do NASA work like this, thus the technology should become alot more powerful in the future ?

    Thanks!
     
  2. jcsd
  3. Mar 22, 2004 #2

    enigma

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    The determining factor for ion engines is mission duration.

    If you're going to the moon, the ion will never be the right choice. If you're trying to lift out of the Earth's atmosphere, it's physically impossible to use an ion.

    If you're going to Jupiter (for example), the ion may be the way to go.

    Ion engines burn continuously over a long period of time, while chemical rockets use their fuel in one big burst of energy.

    The benefit of ion engines are their low fuel usage.

    Specific Impulse, typically denoted Isp, is defined as:

    [tex]I_{sp} = \frac{T}{dm/dt * g_0}[/tex]

    T is the thrust
    dm/dt is the mass flow (or fuel use)
    g_0 is the gravity at Earth sea level.

    It is a parameter which defines how fuel efficient a rocket is.

    For a typical chemical rocket, you get an Isp between 200 and 450 depending on on type of fuel. You get huge thrust, but you also get huge fuel use.

    For ion engines, the typical Isp's run between 2000 and 10,000. That means that for the low thrust, they use next to no fuel.

    Summing it up, if the mission is going to last a long time anyway, then ion engines may be the way to go, since you need less fuel overall to get where you're going (which translates into more useful mass once you get there). For short hops, they're not worth the wait for human missions.
     
  4. Mar 22, 2004 #3
    hmmmmm, thanks fr the detail of the answer, but i wonder if i could extract a little more from you :P

    You say its physicsally impossible to use Ions to use this engine to break free from earths atmposphere, is this because earths atmosphere is full of stuff ? or because its so close to the earth that the pull of gravity is far too great for the engine to thrust against ?

    Could you not for instance increase its power ? so for instance, they use about 2500 Watts of electrical energy using current power sources (fission or a bigass battery!). Imaine then an anti-matter engine, gaining a thousand times the yield of fusion reactors, could this not enable them to up the power on the Ion engine a significant amount to get half descent accelerations?

    Im finding it hard to imagine that a device with 0.2N of thrust could get anywhere atall. There is so much gravitational interference all over our solar system. When you finally break free of earths gravity, then surely you have either the sun's or the other planets (perhaps) to contend with!

    Thanksfor your answer, it was informative :)
     
  5. Mar 22, 2004 #4

    wj

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    If one ion drive can't beat Earth's gravity, then a hundred thousand all strapped together wouldn't work either. They would work LESS efficently because of the extra weight binding them together. One gigantic rocket using the same technology still wouldn't work. You would need a lighter battery or a more efficent engine. I'm not sure a using fission would be more efficent, at least today. It is easy to make a fission reaction but harnessing one takes a fairly massive device. Research into more practical fission power plants is underway.

    Ion drives can sail around the solar system because the gravitational attraction of the planets and sun balance out at certain lines and points called Lagranges. Objects in these areas are pulled in oppisite directions resulting in a very small net accelaration. A spaceship can fly around in these Lagranges using almost no energy. It can even accelarate until it has enough momentum to fly by planets and pull gravity slingshots.

    http://www.physics.montana.edu/faculty/cornish/lagrange.html


    However I have been mistaken before.

    Hopefully someone with some experience or a fancy degree will fill in the holes for you :)
     
    Last edited: Mar 22, 2004
  6. Mar 22, 2004 #5

    enigma

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    It is because the thrust/weight ratio is so low for an ion engine. Typical rockets are designed with T/W of at least 1.3, so they get positive acceleration at liftoff. Ion engines (I don't have an exact number) produce thrusts less than 1N, but have a mass of several hundred kilos at least.

    Ion engines work by accelerating ions in a magnetic or electrical field. You are limited in the rate of fuel depletion by the nature of the ions. If you try to push more out, the ions will interfere with each other. If you increase the voltage, you'll get arcing inside the machine.

    Once summer rolls around, I hope to finish the article for www.physicspost.com which I've been meaning to write on this subject. It really is one of the coolest topics out there (coming from me... an A1 aerospace dork), but one that is really hard to "get" without pictures.

    When you're standing on the ground, you are getting accelerated downward at 9.81 m/s^2. It is that acceleration which keeps you chained to the ground even though the Earth is rotating out from underneath you. To be in orbit, you need to be going really fast... ~7.75KM/sec for low Earth orbit. The reason that the speed puts you in orbit is because the Earth curves away from you at exactly the same rate which you are falling toward it. If you were going slower, you'd fall in. Go faster, and you'd fall out.

    It's the 'go faster' which is why ion engines work when you're in space. When you're in microgravity, you're in freefall. Any force, of any size, in any direction changes your orbit. If you turn on an ion thruster and point it in the direction you're heading, you'll speed up, and your orbit will become higher altitude. The trick is that you keep pushing. Run a quick elementary physics calculation and see how fast a 100kg block will be going after 3 months with a .2N force with no friction. Yes, the Earth and the Sun change your path away from a straight line, but as long as you keep pushing forward, you will keep gaining velocity and keep going into higher energy (and higher altitude) orbits.
     
  7. Mar 25, 2004 #6

    drag

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    Greetings !
    Actually, what you need for manned spacecraft is just an engine a few tens
    of times larger. The total thrust during the journey will be sufficient
    for even the more massive spacecraft. The problem is power, such a system
    will require a massive power source and lots of fuel, or alternativly -
    lots of solar panels. Together with the much lower but still considrable
    fuel mass and thruster mass you have little gain compared to chemical
    propulsion unless it comes to long duration missions like a trip
    to Mars and beyond. Also, all that stuff will cost more than just chemical
    fuel tanks, though it'll cost less to launch it into orbit due to
    the lower mass (which you could also use for a larger payload with
    the same mass as a chemicly power spacecraft).

    That's the reason for the increased intrest in nuclear power sources
    for large manned spacecraft travelling beyond Earth's orbit and the Moon.
    Nuclear power is relativly effective in terms of fuel mass and power
    source mass, but you first need to launch it all through the atmposphere
    which presents a certain level of risk, then use it in space which
    is also a bit risky, and you need a non-neglagible mass of the radiation
    shield - or alternativly you could position the manned modules
    at some distance from the reactor (in space, there are no forces that
    constrain the structure design) and use your ussual radiation shielding -
    which is still supposed to be quite massive for manned interplanetary
    missions.

    Live long and prosper.
     
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