By "bottle rockets" I'll assume you're working with rockets which
you fill up part way with water, and then pressurize the air with
a pump. Releasing the rocket allows the air pressure to force
water out the nozzle, and the reaction force (from Newton's Third Law)
pushes the rocket upwards. We have some answers
already on file on bottle rockets. this one and this one.
Your addition of an egg inside you would like to protect
makes the question even more interesting (and complicated).
If the egg is outside of the bottle but still attached somehow,
then a suggetion is to put a parachute on the rocket.
We have an answer on file
for this case.
What follows below assumes (perhaps wrongly) that the
egg is to go inside the bottle.
There are several issues to worry about with an egg in your bottle rocket:
1) Getting the egg in the bottle. Most eggs won't fit in most
bottlenecks, and the toy water rockets I had when I was young
had a very small nozzle on the back. If you can take the top
off, put the egg in and screw it back on with a good high-pressure
seal, then this part is solved.
2) Not putting so much pressure on the egg that it cracks when
pressurizing the air. This should be the very least of your worries,
for two reasons. Eggs mostly contain incompressible fluids (albumen
and yolk) and so these can press as hard on the eggshell as the
outside presses on it. Also, eggshells are nearly spherical, which
is the best shape for holding a uniform external pressure. See our answer on submarine windows
for an explanation
of why spherical windows work best for submarines. Some eggs may
have a tiny pocket of air or water vapor inside and this may weaken
them. I recommend using a fresh egg. Eggshells are porous -- they
let air in and out. If the inside dries out a little after a long
time in storage, they can get these pockets of air in them.
3) Keeping the egg from plugging up the nozzle. You'll have to
arrange it so the air and water can flow around the egg somehow
so the rocket can fly well.
4) THE MOST IMPORTANT ONE: keeping the egg from hitting the side
wall of the bottle too hard when it comes crashing back down to
earth. This is the sort of thing egg cartons are designed to protoect
against -- sudden shocks and jolts against hard surfaces. It may be
easiest to wrap your whole bottle rocket in styrofoam or something
similar but you may still need some additional padding or support
for the egg on the inside. Another strategy may be to fire your
bottle rocket so that it comes down on a soft trampoline or cushion
or something that will not stop it instantaneously. Firing it
over a swimming pool or lake may help. And the parachute idea is
good one for a smooth landing. You could get a metal egg-shaped
container and line it with felt or styrofoam so that the egg does
not rattle around inside the container and put that in your
(published on 10/22/2007)