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Could you send an object into space completely without fuel?

  1. May 13, 2015 #1
    Was pondering this today. Obviously you wouldn't be able to accelerate a 1-ton satellite to escape velocity, but what about small objects? 11km/s (well, even more, to overcome friction on the way) is very daunting, but if it could be managed you could easily send large amounts into space. And without the "a single mistake wrecks millions of dollars" to boot.

    EDIT: A quick v^2 = 2as says that if you could accelerate at 10g for 55m, you would reach escape velocity. That doesn't sound too bad.
     
    Last edited: May 13, 2015
  2. jcsd
  3. May 13, 2015 #2

    SteamKing

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    Been there, done that:

    http://en.wikipedia.org/wiki/Project_HARP

    This project used a couple of 16" naval rifles joined back to back to try to fire small projectiles into space to study re-entry effects on these projectiles. The rifles were located in Barbados and made a number of test firings before the program was shut down. Although no longer used, at least one of the rifles is still on the island.
     
  4. May 13, 2015 #3
    Wow, the guy who did those experiments was one hell of a sketchy dude.
     
  5. May 13, 2015 #4

    wabbit

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    Had a quick look, quite a story - complete with assassination by secret agents after selling his technology to Saddam Hussein, when does the movie come out ?; )
     
  6. May 13, 2015 #5

    DaveC426913

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    Keep in mind that, when you lob that unpowered payload into space, you'd better have something up there to catch it, or it's going to come right back down.

    A ballistic path is the same as an elliptical path and, unless you give it a boost, you cannot actually get it into an orbit that does not intersect the Earth.
     
  7. May 13, 2015 #6
    That's an interesting point, I didn't consider that. But, I would hazard a guess that the majority of a rocket's propellant is spent on overcoming gravity, and that the acceleration to get it from an elliptical to a circular orbit would require much less propellant.
     
  8. May 13, 2015 #7

    DaveC426913

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    Actually, the majority of propellant is spent gaining speed. Climbing to 200 miles altitude is a pittance compared to accelerating to 18,000mph.
     
  9. May 13, 2015 #8
    Well, that scratches that idea then! :D
     
  10. May 13, 2015 #9

    DaveC426913

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    Not entirely. Firing a cargo to orbital speed will certainly save you a huge amount of fuel. You are right that it wouldn't take too much to alter its ballistic trajectory into a stable orbit.
     
  11. May 13, 2015 #10

    ShayanJ

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    Is it possible to shoot such artilerry guns from an airplane?
    If it was, people could take the gun to a very high altitude and shoot it with some angle, not quite vertical, so that it has some angular velocity too which makes it possible to have a closed orbit.
     
  12. May 13, 2015 #11

    Nugatory

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    No movie, but it did inspire the novel "The Fist of God" by Frederick Forsyth.
     
  13. May 13, 2015 #12

    Nugatory

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    This is why anti-satellite weapons are launched from zoom-climbing fighter aircraft instead of the ground but we don't bother with aircraft boost for satellite launches. Anti-satellite weapons just need to get up there long enough to destroy their target, but satellites have to get up there and stay there.
     
  14. May 14, 2015 #13

    SteamKing

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    In a word, No, it would not be possible to carry one of these guns aloft and fire it, let alone two joined in tandem.

    Just one 16" naval gun weighs more than 100 long tons (100,000 kg) and is approximately 20 meters long. This photo gives an indication of how massive one of these guns is:

    33287.png
    Project HARP used two of these monsters in tandem. Should you even get such a massive assembly aloft, firing it from an aircraft would be interesting, to say the least.
     
  15. May 14, 2015 #14

    DaveC426913

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    It would shortly stop flying. And then shortly thereafter stop being an aircraft. :smile:
     
  16. May 14, 2015 #15

    Janus

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    Putting aside the problems with getting such a gun on a plane and firing it. This would not have the effect you desire. Any "orbit" the shell enters after leaving the gun will return it to the point where it was fired. You would have an orbit that would keep dipping back into the atmosphere back to the altitude from which it was fired. Passing through the atmosphere will cause it to lose speed with each pass, causing the upper part of the orbit to lower down into the atmosphere over time, which will result with it falling out of the sky.

    Secondly, firing it at any angle other than parallel to the ground just makes matters worse. The shell will still return ( or try to return) to the firing point, but now the lowest part of the orbit will be closer to the Earth than the firing point and it will spend even a longer time in the atmosphere. As you increase the angle towards vertical you get to a point where the trajectory will intersect the surface of the Earth before it returns to the firing point.
     
  17. May 14, 2015 #16
  18. May 14, 2015 #17

    SteamKing

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    Recently, the US Navy has been performing trials on a prototype electromagnetic rail gun:

    http://en.wikipedia.org/wiki/Railgun

    The test gun has been shown to throw a projectile at a velocity of 2400 m/s. Eventually, the Navy hopes to develop a gun capable of accelerating a projectile to 7000 m/s.

    At the equator, the earth's rotational speed is about 465 m/s. Orbital speed at the same location is 7900 m/s relative to a fixed reference. A rail gun capable of orbiting a payload would need to be capable of imparting a Δv of about 7500 m/s, in round numbers.

    http://en.wikipedia.org/wiki/Orbital_speed

    Here is a pic of the USN rail gun test:

    Railgun_usnavy_2008.jpg

    The flames are from the atmospheric heating taking place after the projectile has reached a velocity of 2400 m/s​
     
  19. May 15, 2015 #18

    mfb

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    sqrt(2*100m/s^2*55m)=105 m/s. You need a factor of 100 more speed, or a factor 10000 more energy. 1000g at 5.5 km would work.

    HARP only reached 3.6 km/s. Guns give massive accelerations, but reaching a high speed is difficult - exceeding the speed of sound with that concept is hard to impossible, and even light gases like hydrogen or helium have to be very hot to get a sound velocity in the range of 10 km/s. See also light gas gun.
     
  20. May 20, 2015 #19
    This concept is entirely possible, if the atmosphere didn't get in your way to stop your projectile from reaching space without completely disintegrating. If the atmosphere were magically removed, yes the objects you launch can certainly get into space. As everyone said before, it wouldn't complete an orbit as it would want to return to the same spot it was launched at.
     
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