Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Launching satellites using electromagnet power

  1. Oct 5, 2018 #1
    Why can't we launch satellite by using electromagnet power?
    Last edited by a moderator: Oct 5, 2018
  2. jcsd
  3. Oct 5, 2018 #2


    User Avatar
    Homework Helper

    As with a rail gun.
    The first question would be: How long is that rail gun?

    Let's say that you only wanted to use the rail gun to get the rocket as far as a first stage booster would go - about Mach 12. The first problem you have is that Mach 12 is a big problem in dense air. So your gun would have to be miles high just to get the rocket into air thin enough for Mach 12 flight.
  4. Oct 5, 2018 #3
    The satellite needs to have a large speed to orbit the earth. Most of the accelerating to reach this speed has to be done at a high altitude where the atmosphere is very thin. A railgun that can accelerate something to orbital speed is possible, but the satellite would break up an never reach orbit because of the air resistance.
    Finally, We don't have any technology to accelerate a satellite that is far away with electromagnetic forces.

    If there was no atmosphere we could build a railgun at the earths surface, and launch a satellite to low earth obit with only a small included rocket, to make the orbit of the satellite circular when it's at the right altitude.
  5. Oct 5, 2018 #4
    Um, you could use such as a 'stage zero', using vacuum containment with a well-timed diaphragm at the muzzle, but then your launch emerges into 'dense' atmosphere at near-hypersonic speed.
    Thermal Issues ensue...
    Nearest analogy is probably the HARP project...
  6. Oct 5, 2018 #5


    User Avatar
    Science Advisor
    Gold Member

    The sudden large acceleration at lift-off could also damage equipment and definitely be lethal to any people on board. The same problem as with the cannon-fired spacecraft that appeared in 19th century science fiction if I remember correctly.
  7. Oct 5, 2018 #6
    As an addendum , do you always have to reach the standard escape velocity ?

    The standard equation is
    where v is the escape velocity and r is the distance (radius) from the centre of mass of the earth

    This works out ≅ 11 km s-1 at the earth's surface.

    Intuitively, if one ( or the vehicle ) could instantaneously achieve 11km s-1 at the surface , would it
    of necessity escape the pull of the earth's mass (gravity) if it did not undergo ANY further acceleration?

    Similarly, if one had a hypothetical rocket that had an unlimited supply -internallyproduced- of thrust ,
    then could that rocket continue into space / orbit without actually achieving the so-called ESCAPE VELOCITY?
    Last edited: Oct 5, 2018
  8. Oct 5, 2018 #7


    User Avatar
    Science Advisor

    One assumes that you are not counting the Earth's gravity as a "further acceleration". Then yes, a 11 km/sec velocity at the surface in any direction yields a trajectory that will escape to infinity, barring any other acceleration. Typically, that other acceleration will be present either by smacking head-on into the Earth's surface (if you aim downward) or air resistance (if you aim upward).
    See https://www.physicsforums.com/threa...on-vehicle-escape-earths-gravity-well.954278/
  9. Oct 5, 2018 #8

    Found this when Googling Professor Eric Laithwaite , inventor of the linear motor
    [ ICU , London ] when NASA Scientists (2) visited his subsequent work on
    electromagnetic rails at the University of Sussex ,Southern England
  10. Oct 5, 2018 #9
    Not that this changes the question much, but I thought I would mention that orbital velocity is less than escape velocity.

    Also, firing a gun (ok, chemical rather than rail gun, but a similar problem) to reach orbit is not entirely science fiction. While nobody has ever fired anything into orbit, a few attempts and some progress have been made over the years. See the Wikipedia article on space guns. Mach 9 is more than a third of the way there in terms of speed, but, of course, the engineering problems are not linear with speed. Still, I certainly wouldn’t want to say it’s impossible.
  11. Oct 6, 2018 #10
    It was just an idea i thought and thanks for your suggestions

    << Post edited by a Mentor to remove all caps and text speak >>
    Last edited by a moderator: Oct 6, 2018
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?