A take on the film canister pop-rockets with a more reliably and impressive explosion!

WARNING: An impressive amount of pressure does build up in these rockets! Be patient after you place them right side up and MOVE AWAY. One of our rockets reached the third floor with a full tablet! If the rocket fails to launch after 5 minutes, carefully pick up and loosen the cap pointing the rear end at the ground.

These Alka-Seltzer Rockets were inspired by CSC Toys' Mighty Seltzer Powered Rockets. The calculations in the sketch are from an article on the calculation of sodium bicarbonate (NaHCO₃) in an Alka-Seltzer tablet (Click here for the article: Chen & Yaung. 2002. J. Chem. Ed.)

NASA also has some great background info and student handouts in their: Propulsion: Rockets Away activity or 3-2-1-POP! (for younger students)

Supplies

50 mL Centrifuge Tubes
Plastic Straws (larger McDonald's size) cut into 70mm lengths
3 mm foam sheet (from leftover packaging)
Plastic tube that will fit in snugly in the centrifuge tubes (I found a Mega-Block that fit perfectly and some leftover 1" PVC pipe from an outdoor fishpond)
Antacid tablets or citric acid + baking soda
Hot glue gun + glue
Silicon caulking (optional)
Hacksaw

Step 1: If Rockets Can't Launch, Is It Because They Have Thrust Issues?

Measure the inner diameter of your centrifuge tubes (~22 mm) and find an appropriate "plug". The key is to find something that fits snugly. The studs on an old Mega-Blok fit my tubes perfectly, and I used a hacksaw to cut the individual studs off. If you can find enough single Mega-Blok studs you could probably use them as a base and skip the next step. I also used pieces of 1" PVC pipe.
Glue the plugs into the caps using the hot-glue or silicon caulking.

Step 2: It's Not Rocket Science, or Is It?

Time to make the internal support system - the foam donut should fit inside the nose of the rocket, and the "X"-shaped platform will support the Alka-Seltzer tablet so it doesn't react with the water until you flip it over.

Cut the straws into 70 mm lengths.
Cut 20 mm circles and "X"-shaped platforms from the foam, and make a hole in each piece for the straw using a single-hole punch.
Hot-glue a donut on one end of the straw, and an X on the other.
Put a generous glob of hot-glue or silicon caulking on the donut and insert the straw into the centrifuge tubes. A chopstick/wooden dowel was helpful to insert the straw into the centrifuge tubes. You can use a second chopstick to push the straw off.
Hot glue the rocket fins onto the outside of your rocket.

Step 3: How Do You Organize a Rocket Launch? You Planet.

General Launch Instructions (see photo):

Fill with 10 mL of water. The gradations on the tube are a handy way to measure the volume of the water.
Insert the tablet so it sits on top of the foam platform. This delays the reaction from starting until you flip the rocket over,
Insert the plug making sure it is nice and snug.
Place the rocket right side up, MOVE AWAY, and BE PATIENT! If the rocket fails to launch after 5 minutes, carefully pick it up and loosen the plug, pointing the rear of the rocket at the ground.

The little insert that came with the rocket also had some great experiments to try:

hot water vs cold water,
different amounts of water,
different amounts of tablet.

Great conversations about independent, dependent, control/constant variables, and experimental design.

Other ideas:

Stoichiometry, Limiting & Excess Reagents: Press your own Alka-Seltzer tablets with different amounts of citric acid and baking soda (sodium bicarbonate) and work out the stoichiometric ratios. Test out some fizzy tablets recipes with gelatin as a binder and a garlic press, cornstarch bathbombs, or a DIY tablet press?
Gas Laws: Work out the volume of gas produced and the minimal pressure to launch the rocket?
Quantify it! Can we use a micro-controller/sensor to measure acceleration/height?