# Can water pressure be used to impulse a rocket out of the water?

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.

• 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." Commented Feb 22, 2018 at 15:50