The authors try to get the highest possible gas pressure inside the
bottle. The system shown in the video implies a maximum
quantity of gas is pumped into or produced inside the bottle before it
takes off. To cope with the pressure the bottle is
reinforced, which means more work and more weight. What matters most is
not the pressure but the volume of the gas inside
the ejection nozzle. A way to increase the gas volume and decrease the
pressure is to use a rocket motor chamber where two liquids mix
or a
liquid dissolves a solid (hybrid motor). While pressure
enhances the yield of a rocket, it is not mandatory. The key to high
energy is the speed of the foam ejected, hence the volume of the gas.
So, for example water can be used inside the rocket with a high
concentration in sodium bicarbonate. Before it enters the ejection
nozzle it
flows between pebbles of citric acid. There the maximum amount of gas
is produced and the foam will eject at the maximum speed. Should those
same
pebbles mix completely with the liquid before the nozzle is opened, the
pressure would
become much higher than what the current bottles can withstand.
I wonder if some improvements can be attained by tuning the diameter of
the nozzle and the mass of the rocket structure. If the water bottle
rocket is launched in the vacuum of open space its nozzle diameter
doesn't matter. It will have a longer and slower acceleration using a
narrow nozzle but the end speed is the same (actually it is a little
slower because a bigger nozzle is heavier). Now Imagine the water
bottle rocket is launched on the Moon, where there is no atmosphere.
The aim is to get as high as possible. Then clearly it best uses quite
a big nozzle diameter, to get the water out as quickly as possible. The
faster the bottle empties, the higher it gets. To understand this,
imagine the opposite: a very narrow nozzle. Then the bottle will just
rise slowly, hover a few meters over the Moon surface during lengthy
seconds and fall to the ground. To fight the attraction of the Moon,
best spew all the power out at once. That's one reason why solid rocket
motors are often used as add-ons or first stages for rockets to orbit;
they produce a tremendous thrust in a very short time. Now the problem
of the water bottle is it travels in the Earth atmosphere. It has a low
density once empty, so it is strongly braked by the aerodynamic drag.
If it spews out all its water in a fraction of a second and becomes
lightweight as a soap bell, it will be braked and won't rise very high.
It will rise higher if it contains a weight, for example if it keeps
some water inside (or simply batteries or a piece of lead). Now,
once it reaches its maximum altitude, it still can gain a few more
meters by spewing out that water it kept behind. Of course such an
intermittent operation is not sound. But this makes you understand
spewing out all the water at once is not the best solution inside the
atmosphere. An optimal ejection rate can be found; strong at the
beginning then decreasing. This is probably obtained naturally by the
decrease of the gas pressure as the bottle rises. Anyway it can be
optimized and maybe motorized. A compromise can be found between the
nozzle diameter and the weight of the empty bottle rocket.
More stability can be acquired if the bottle empties from the bottom
on. That is if the liquid keeps towards the traveling front of the
bottle. This can be attained by using stapled reservoirs inside the
bottle or an inflating balloon. Probably quite difficult...