Tuesday, 24 March 2020

newtonian mechanics - What is the maximal height for a water rocket's flight?


A water rocket works like this: there is a circular slot of area A1 at the bottom centre of a cylinder of cross-sectional area A0 and height L that is filled with water to an initial height h0. This slot will fall away during launch. The water has pushed all of the air that was originally in the cylinder to the top Lh0 of the cylinder (I believe this is an isothermal compression: the compression was fast), so is at a higher pressure of Lh0LP0, where P0 is atmospheric pressure.


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To launch, the slot is instantaneously removed (leaving a hole of area A1 in the bottom of the cylinder), and water is pushed downwards, as the air pressure is higher inside the cylinder than outside, at a speed u(t). There is no sloshing of the water in the cylinder: the body of water remains cylindrical. Thus the air in the cylinder now takes up more volume (has expanded in an adiabatic expansion), but because of the upwards impulse imparted to the cylinder by the leaving water, the cylinder is now moving upwards with speed v(t). The rocket will reach a maximum height Hmax(h0), where h0 is the original height of the water. What h0 will give the maximum value of Hmax(h0) for fixed A0,A1,l?


Partial solution.


In the adiabatic expansion, let V(t) be the volume of the air in the rocket and P(t) the pressure. As the air is mostly diatomic (Nitrogen and Oxygen are),


P(t)V(T)1+5252=k

P(t)V(t)75=A0P0(Lh0)2L


dVdt=A1u(t)



The change of momentum per unit time of the water being spewed out the bottom is


ρδV(v(t)+u(t))




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