Please bear in mind that I'm no physicist, my knowledge of hydrodynamics is limited.

I've been thinking that when you push a floating object to the bottom of a swimming pool and then release it, it will get back to the surface with considerable force.

Could this be used, for instance, to impulse a very hydrodynamic rocket pulling it to the bottom of the sea in some kind of more-dense-than-water capsule and then release the rocket to launch it out of the water without using any fuel?

If there are any ambiguities to my question please let me know.

  • $\begingroup$ There's going to be a terminal velocity. The buoyancy pulling your missile up toward the surface will be pretty close to constant, while the drag pulling it down will be a function of its speed. The forces will be equal at some speed (a.k.a., "terminal velocity"), and your missile will never rise any faster. Therefore, there will be some depth, beyond which, starting deeper will not be of any help. For a toy ball in a swimming pool, I happen to know from experience that that depth is less than 1m. I can't say how much it would help for the missile to be "very hydrodynamic." $\endgroup$ Commented Feb 22, 2018 at 15:50

1 Answer 1


Considering the friction of the water, makes it questionable how high the rocket would get out of the water

But the energy required comes from lifting the capsule from the bottom of the sea for the next launch. This would be comparable to a car that doesn't use any fuel to coast down a hill after it first drove up.

  • $\begingroup$ Good point! But let's assume that the priority is in using less fuel in the rocket/accelerating the rocket further; we have enough resources to pull the capsule back up. The point is: if we impulse it this way and once out of the water the engines kick in, is this method more viable than just accelerating from the surface? (consuming less rocket fuel, more acceleration than accelerating from the surface) $\endgroup$ Commented Feb 22, 2018 at 15:05
  • $\begingroup$ If you just let go of the rocket under water I think it will soon enough reach terminal velocity and the extra speed will be very small compared to the speed you need your rocket to ultimately reach. $\endgroup$
    – Daniel
    Commented Feb 22, 2018 at 15:09

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