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B Laser thrusters

  1. May 13, 2017 #1
    I just had this thought while watching a video about the EM drive...basically using 2 lasers to bounce them off prisms inside an object to push the craft through space at a small but constant acceleration....could someone explain why this would not work?...please see attached image :)

  2. jcsd
  3. May 13, 2017 #2


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    Hi and welcome to PF
    Using a beam of EM waves is a possibility for obtaining thrust but it requires a vast amount of power. An on board laser isn't good value as you don't need such a well collimated beam and you can get more power out of other sources.
    PS you may as well fire the lasers straight out of the back. It would achieve the same amount of momentum transfer (Newton's Third Law and reaction) and wouldn't involve heating up the prism reflectors.
  4. May 13, 2017 #3
    yea I realise you wouldn't get much thrust, but these kind of devices never would, like a solar sail......plus the idea for using prisms would be to get a double bounce effect....but you would probably have to angle the second prisms, so you get more fwd push than sideways push...but anyway...i'm an animator, i just wanted to understand why this wouldn't work! :)
  5. May 13, 2017 #4


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    The drive can only deliver the same amount of momentum change as the momentum in the beam that's leaving out of the back. All the other 'bounces' cancel each other out. Shame but that's Newton for you.
    There are a lot of discussions about this topic on PF and elsewhere and sometimes the Physics isn't always well understood. But, as you are hardly likely to be investing all your money in such a system, it's not that desperate.
  6. May 31, 2017 #5
    I also just had a thought!
    What happens to the CO2 that the astronauts breath out? If this was collected, then perhaps it could be used to propel the craft?
    [I did say I had a thought, but not that I had thought it through:) ]
  7. May 31, 2017 #6


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    you should read up on space flight :wink:

    you would have found they use CO2 scrubbers to get back breathable oxygen/air
  8. May 31, 2017 #7
    Hi S,
    I did say I hadn't thought it through :smile:
    Thinking back, I used to service oxygen producing machines, which included CO2 scrubbers.
    I'm sure they recycle almost everything.
  9. May 31, 2017 #8

    Vanadium 50

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    But there's a more fundamental problem. How does collecting CO2 propel the craft? Do the astronauts blow it out the back? (Make a wish everyone!) You need to impart momentum somehow.
  10. May 31, 2017 #9
    Yes, anything blown or thrown "out the back", will push the craft forward, CO2, Computers etc.
    (So far I don't think wishing helps)
  11. May 31, 2017 #10


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    One of the bounces is cancelled by the recoil of the laser. With ideal refection it would be the same as simply pointing the laser back. But in reality it's worse, due to partial absorption at the reflector.
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