Alka-Seltzer® Rockets
Originally by Mike
Passerotti,
re-designed by Tony Alfrey for Nixon Elementary Noon Science
Introduction
Just to prove that there is
nothing new under the Sun, we built Alka Seltzer Rockets. Let's
see; a
Google search on Alka Seltzer Rockets returns some 59,000 hits as of
May 2006.
But wouldn't you know that I'd never made one before. I must have
been living in a cave when I was a kid.
Background
The paper rocket in
this activity is propelled according to the principle stated in Isaac
Newton's third law of motion: "For every action there is an opposite
and equal reaction." In this case, we can interpret the law to mean
that if there is a force applied between two things, each thing feels
the same sized force, but applied in opposite directions. In our
rocket, gas pressure builds inside the film canister due
to the mixing of Alka-Seltzer® and water and the gas applies a
force to both the canister and the lid. Eventually, enough
pressure builds to blow apart the canister and lid; the
canister and lid/gas/water are pushed apart in opposite directions.
Materials
1. Select a rocket pattern.
Rocket Pattern 1
Rocket Pattern 2
Rocket Pattern 3
Rocket Pattern 4
2. Empty 35 mm film canister
with lid that snaps inside (apparently made mostly for Fuji® film.
The
ones with the lids that snap on over the top (e.g. Kodak®) are not air-tight).
3. Felt-tipped markers, crayons, or colored pencils (if you're using
the plain Rocket Pattern 4).
4. Tape or glue
5. Scissors
6. Alka-Seltzer® tablets; the "original" kind.
7. Water (hot water works faster, but is it better?)
8. Metric tape measure or meter sticks
Construction Please see our
"Commentary" section below regarding your decision to add the nose
cone, body and fins. 1. The printed pattern has two parts: "Fins" and "Nose Cone/Body". Cut the fins out. Cut the nose cone and body out as one piece. |
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| Destruction This is an outdoor
activity. If
gusty winds are a problem, then put up a shield or wind block instead
of adding weight to the rocket (which will just reduce the
altitude, unless you add weight only
to the lid. Do you know why?). Launching near a wall where
a metric tape
has been hung or where meter sticks have been stacked may make it
easier to judge how high the rocket goes. Everyone
should stand away from loaded rockets when they are on the launch pad. Use enough water to fill the canister 1/3 to 1/2 full. Use about 1/4 of an Alka-Seltzer tablet. Add the water, drop in the tablet, stuff the lid on, and plant the rocket onto your launch site. It may take 30 to 45 seconds to build up enough pressure to launch with cold water, so a loaded rocket should not be approached prematurely. These rockets can shoot 5 meters or more into the air. No sharp objects should be placed on top of the nose cone or elsewhere on the rocket. You may also make a
launch pad
with
a block
of wood and a straight piece of wire. Drill a hole for the wire and
insert the wire straight up to guide the rocket at lift
off. But, to paraphrase Eli Wallach; "We don't need
no stinkin' launch pad". Commentary The reaction of the
parents that implimented this experiment for elementary school kids
think that the paper nose cone, body and fins are a superfluous
distraction from
the actual launching of the canisters. After one firing, the fins
and nose cone get sloppy, and the assembly is a little
tedious for 1st and 2nd
graders. Additionally, unless one has large, strong adult hands,
the paper parts get a little mangled in pushing the lid onto the
canister after water and Alka-Seltzer have been added. A nose
cone might make some sense from the standpoint of air resistance
and actually convincing oneself that a "rocket" has been made.
But a
more abbreviated decoration consisting of a few simple stickers,
perhaps using Avery mailing labels printed on the computer, might be
superior. While the more methodical amateur scientists will enjoy
carefully studying the effects of fins and nose cone, amount of water
in the canister, and a myriad of other experimental variables, we can
confirm that a big group of kids had a lot of fun launching their
rockets several times in the allotted 25 minutes by abandoning the
added complexity of fins and nose cone. A sheet of stickers that
can be used with a sheet of Avery #8160 address labels can be found here, which we used along
with various stars, dots and colored stripe
stickers purchased at a local "party supply" store. Some Science and
Other Thoughts Alka-Seltzer (look at the side of the box) is mainly
Sodium
Bicarbonate and Citric Acid. When these two chemicals are mixed
in a water solution, Carbon Dioxide gas and Sodium Citrate in
aqueous solution are
created in an exothermic
(generates heat) reaction (the reaction
is: H3C6H5O7(aq) + 3 NaHCO3(s) --> 3 CO2(g) + 3 H2O(l) +
NaC6H5O7(aq) ). The heat released in the reaction increases the
pressure of the CO2 gas that is liberated. The mechanical pressure required to snap the lid onto
the
cannister is the limiting factor that determines the altitude that can
be reached. This is because, regardless of water temperature,
amount of water or amount of CO2 gas, or chemical composition of the
reactants, the "lid sealing pressure" sets an upper limit on the CO2
gas pressure that can build up in the cannister. So
let's say that you decide to speed up the reaction by using hot
water. Unfortunately, the film
canisters are made of High Density Polyethylene (look at the recycling
label on the canister which says "HDPE") which is a
thermoplastic. This is another way of saying that the plastic
gets soft when you heat it. As the plastic gets softer, it
stretches more easily. This means that the gas pressure seal
between the lid and the container gets weaker when the plastic is
heated, thereby lowering the pressure at which the lid pops off.
So although you can speed up the rate at which CO2 gas is generated by
using hot water, the altitude will decrease, because the maximum
pressure attainable within the canister decreases. Thoughts About Two-Stage Rockets It is, in principle, possible to make a two-stage
rocket.
First let's assume that you can get two film canisters connected
togther, the lid of the second stage connected to the canister of the
first stage (more on that later). There is a delay between the
time that the rocket is loaded and when it
fires, so there is time to load a two-stage rocket. Finally,
there is the issue of timing the firing of the two stages; you
want the second stage to fire sometime after the
first. This may be enabled by simply loading the first stage
first, followed by the second stage. If the reactions proceed at
the same rate in each stage, the delay between stage firing is simply
the length of time it takes you to load the stages. With
practice, you can get this down to about a second. But the timing
is still very dependent on the reaction rates in the two stages and the
strength of the lid/canister connection,
requiring precise control of the amount of Alka-Seltzer, water and
uniformity of the canisters. Glue is not effective for fastening the HDPE lid of the
second stage
to the canister of the first stage. These are more effectively
connected with a small machine screw and nut (preferably nylon) and
sealed with a dab of silicone adhesive on both screw and nut. Larger Rockets While film canisters are convenient, a simple rubber
stopper may be
added to a closed container to which water and Alka Seltzer are
added. My favorite closed container is just a plastic water
bottle
with a rubber stopper. Larger bottles loaded with water and
Alka-Seltzer are not toys for smaller children; they are toys for
larger children, like the author. LARGER Rockets Go here to see cool things that an earlier generation used to build. I hope this link lasts for awhile: http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1322&start=16 Your parents (or now, maybe your grandparents) got so
used to seeing this stuff when they were kids that they thought it was
about as easy as driving their SUV to the market. |