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Q & A: Egg bottle rockets

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Most recent answer: 10/22/2007
Q:
What is the best way to shoot up a bottle rocket powered by air pressure, and at the same time keep an egg inside the rocket from cracking?
- Anonymous
A:
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.




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 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
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
bottle rocket.

Tom

(published on 10/22/2007)

Follow-Up #1: More egg bottle rockets

Q:
In my science class we are supposed to make bottle rockets. I have read your section on them , but need some help in finding out how to atach the parachute. The goal is to send a egg up with the bottle rocket and make it come back down...without breaking!
- Steve Lessaris (age 12)
Edison Middle School, Champaign, Illinios, US
A:
Hi --

Yes, a parachute will help quite a lot in ensuring a safe landing for a bottle rocket. I found the web site of a high-school teacher who gives this as an assignment -- attaching an egg to an elaborate rocket made from a two-liter soda bottle, optional fins, a parachute and a nose cone. I won't include a link here because I think there may be a problem or two with his explanations, but the apparatus description sounded pretty good.

The most difficult part of attaching parachute to one of these things is to get it to deploy when the rocket is coming down, and not when it is going up. When it is going up the parachute needs to be folded up tight so it will not slow down the rocket. The way to do this is to extend the rocket with a tube and add a nose cone. You make your bottle rocket longer by putting a sleeve on the end of the bottle opposite the neck. This sleeve is most easily made from another two-liter soda bottle, cut so the top and bottom are gone. This sleeve can be taped to the main bottle. A nose cone can be made of paper folded and taped in a cone shape. The parachute can be made from a lightweight plastic trash bag sheet tied with strings on the edges. Fold up the parachute like an accordion -- do not wrap the string around the plastic, and do not wrap the plastic around itself, folding back and forth instead. The string can be taped to the sleeve.

The main trick is to get the nose cone off and the parachute out when the rocket falls. The high-school teacher suggests putting a lump of clay in the tip of the nose cone so that it will fall more quickly through the air on the way down than the bottle (now empty) which will flutter and tumble on its way down. The high-school teacher said "inertia" separated the nose cone from the soda bottle, but it really is air resistance which does the job. The nose cone will then separate from the bottle and the parachute can deploy. You should attach the nose cone to the rest of the rocket with string so it does not fall far away. This string may be used to help pull the parachute out of the sleeve. The egg can be taped inside the sleeve along with some additional padding to help that last bump when it hits the ground.

Tom

(published on 10/22/2007)

Follow-up on this answer.