Using the ocean to propel objects into space.

Discussion in 'Alternative Theories' started by wellwisher, Mar 6, 2016.

1. wellwisherBannedBanned

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This is not an alternate theory, but more of a physics challenge question that will take ingenuity. I would have posted this in the physics section, but they seem to be afraid of my challenges. Anyone is welcome to participate.

To set the stage; if you take a beach ball and try to submerge the ball in the pool, there will be an upward force connected to buoyancy. The question becomes, can we use buoyancy to generate enough velocity to launch a missile, from deep down in the ocean, into space?

Before dwelling on the technical limitations, such as friction and materials, this exercise is only to access whether the ideal missile, in frictionless water and air, could gain enough speed to reach space, driven only by the forces of buoyancy.

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3. cosmictravelerBe kind to yourself always.Valued Senior Member

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I'm not a rocket scientist but wouldn't the water slow the object down enough so that it doesn't have enough "lift" to go very far above the water. As your beach ball doesn't go very far upwards when it breaks through the surface. A missile doesn't have any air packed away that could let it go far once it reached the surface and the bigger you make it for more air in it would slow it down once more.

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Really?

7. OphioliteValued Senior Member

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It would not take ingenuity, but laws of physics different from those that obtain in this universe.

8. exchemistValued Senior Member

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They are not afraid of your challenges, they are just annoyed by constantly mangled science.

To your challenge, though, let me try at least to get the process started. What you are effectively asking is whether an object could have a high enough velocity on leaving the surface of the sea to get to, say, the Karman line, which is 100km up. The kinetic energy needed for that will be a lot less than required to achieve low Earth orbit, so that is the minimum one would have to achieve.

All that kinetic energy would have to have been imparted by the force of buoyancy, by the time the projectile left the surface of the sea.

So let's try a bit of maths:

The gravitational potential gained by an object of mass m raised a distance d is its weight x the distance, that is: mgd. (This assumes g is constant throughout which is a bit wrong since it falls off with inverse square, so we'd really need to integrate with g as a function g(d) rather than a constant, but let's ignore that to start with.) The required kinetic energy will need to equal that. So 1/2mv² = mgd. Rearranging and cancelling m on both sides, v² = 2gd. So v = √(2gd) ~ √(2 x 10 x 100 x 1000) = 1.4 x 10³ m i.e. 1.4km/sec. (Just realised this is the standard equation of motion v² = 2aS, durrh. )

This figure agrees with one I saw in a Wiki article so let's use that at least to start with. (We can go back and do the integration later - if we can be arsed.)

If your object starts its acceleration from the deepest point in the oceans, the Mariana Trench, it will start its acceleration from about 11km deep, but let's call it 10km to keep the maths simple. Using the formula v²=2aS with a as the acceleration, we need 2 x 100 x 1000 = 2 x a x 10x 1000. So actually this is a lot easier than I first thought: the acceleration due to buoyancy has to be of the order of 10g. (Again this is neglecting water resistance, so it's a highly artificial scenario, but never mind - it's what you asked about.)

The question thus becomes what sort of object will develop a buoyancy acceleration of 10g when fully submerged. It also however has to withstand a pressure of 1000 bar without being crushed! So your beachball, sadly, would not do.

Anyone one else care to point out any errors or otherwise improve on this?

Last edited: Mar 6, 2016
9. DaveC426913Valued Senior Member

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This guy almost has it nailed.

10. birchValued Senior Member

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It would cause massive tidal waves for that level of acceleration.

11. originIn a democracy you deserve the leaders you elect.Valued Senior Member

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The forces on an object in the water is the buoyant force and in the opposite direction the gravitational force.
Since we are making assumptions like there is no drag we can make some other approximations and simplity the equations to this:
$F_b=\rho V_b g = m_b g$
$F_g = mg$
$F_t = F_b - F_g$

So the acceleration can be approximated to this:

$a_t = g \frac {m_b - m}{m}$
Where m is the mass of the object and $m_b$ is the mass of the water displaced.

So lets assume the object is 500 kg and that is 1/2 the mass of the water that is displaced.

Which comes out to an acceleration due to buoyancy is 9.8 m/sec^2. Of course the less dense the object the higher the buoyant force and the higher the acceleration.

Now of course the drag from the water is so huge that the terminal velocity will actually be tiny but...

Neglecting the effects of friction from water and from air this will give the result that the distance the object will fly into the air is exactly equal to the depth the object is released.

If the object is released 5 miles under water the object will fly in the air 5 miles.

By the way, the reason you are not allowed to post in the science section is because you post pseudoscience. When it is pointed why your ideas are wrong, you do not admit you were wrong you just start up another pseudoscience idea.

Last edited: Mar 7, 2016
12. trevor borocz johnsonRegistered Senior Member

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buoyancy is interesting. I can tell you something will sink in the ocean at no greater speed then 35 mph. your missile would probably sink. buoyancy means a balance between two resistances or forces. the beach ball drifts up in the water because water pressure squeezes it upward. It does this uniformly though as the ball goes up so it wouldn't gain any energy unless you applied energy in the first place.

13. exchemistValued Senior Member

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Vintage borocz from Trev.

14. trevor borocz johnsonRegistered Senior Member

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Wait I've got it! If you put a long pipe in the ocean, maybe a thousand feet deep from top to bottom, and sealed it air tight, placing the pipe perpendicular to the surface, then put the missile in the bottom of the pipe and opened the seal at the bottom, the water pressure would come shooting up the pipe and launch the missile.

15. KristofferGiant HyraxValued Senior Member

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Have a look at his next post, exchemist-san.

16. exchemistValued Senior Member

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Ah so. 1000 ft would be 300metres. I work out in plevious post 10g accerelation from depth of 10km. So to do in 300m, mean accerelation of 300g. It seem unrikery ingless of water in pipe can do enough, I think.

So, indeed it seem our Trev talk even more borocz now, Kristoffer-san.

17. OysteinRegistered Senior Member

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I got a better idea. All we need is a very large rubber band and a very large sling shot . . .