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.




2. Tape the body onto the film canister, roll the paper around the side, and tape the end down. The lid end (that's the open end) of the film canister goes down.



3. Roll the nose cone around into the shape of a cone and tape it together. Straighten the nose cone so that it points toward the center of the rocket and tape it to the sides.



4. Fold the fins so that the colored side is out. Tape or glue the fin halves together to form a complete circle.



5. Slide the fins over the body and tape in place.




6.  If you'd like to try a launch pad,
cut a 1 inch piece of straw and tape it to the body .




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.