The father of modern rocketry Werhner von Braun once said, “It takes sixty-five thousand errors before you are qualified to make a rocket.” Well, he never made one from an effervescent tablet and a film canister.
The Alka-Rocket, as it’s known, is a mainstay of science classrooms and a stellar example of the Newtonian physics that guide larger rockets. In modern rockets, such as the Space X Falcon 9 or the Saturn V, the propulsion system is powered by a fast, exothermic reaction between a liquid fuel (usually hydrogen, methane or hydrazine) and oxidizer (usually oxygen or nitrogen tetroxide). That chemical reaction generates Newtonian physics.
For Newton fans—which includes pretty much every rocket scientist that’s ever lived—the laws to look for are numbers one and three. According to his first law, a rocket at rest will only move once the thrust produced by its engines exceeds the gravity keeping it in place. That can occur by way of Newton’s third law: For every action there is an equal and opposite reaction. The gas and flame and smoke that howls downward out of a rocket’s nozzle end exerts enough thrust against the surrounding air molecules to send the rocket upwards.
Overlooking a few critical differences (massive combustion, the possibility for disaster), Alka-Rockets work in much the same way. A chemical reaction between water and the sodium bicarbonate and citric acid in the tablet generates CO2 gas. When enclosed in a film canister, that gas builds to the point that, eventually, its forces pops the canister’s top and rushes out with such force that it sends the rocket toward the stars—or about 25 feet.
So the next time someone says, “This isn’t rocket science,” just break out your effervescent tablets and say, “But this is.”
How to Build the Best Alka-Rocket Ever in 6 Steps
- Index card (5 x 8 in.)
- Empty plastic film canister with a lid that snaps inside
- Markers, crayons or colored pencils
- Original effervescent formula tablets
- Tape measure
- Safety glasses
Assemble Your Materials — A rocket’s fuselage is referred to as the structural system, and it’s typically constructed from titanium or aluminum. Your structural system will be an index card—but doesn’t mean it shouldn’t be incredibly decorated. If inspiration is elusive, consider the pigeon. Around 400 B.C. craftsmen in the city of Tarentum (in today’s southern Italy) strung a wooden pigeon on a wire and propelled it through the air with a steam reaction. It stands as one of the first known examples of rocketry.
Build Your Rocket — Gravity is unforgiving, so keep your design simple and efficient. Roll the index card into an 8-inch-tall tube, and slide the film canister into it so that it opens at one end. Securely fasten your structural system (index card) around the propulsion system (film canister) and avoid adding anything else to your rocket. That way, you’ll maximize your chances of overcoming gravity’s effect of the rocket’s mass.
Add Some Stability — To engineers, air is just another liquid and it exerts drag on anything that moves through it. Drag is an aerodynamic friction generated between air molecules and the structural system and engineers both fight it and rely on it. Rockets have pointed nose cones to minimize drag but they also have fins that generate strategic resistance to keep crafts flying straight (imagine a rudder in three dimensions). Many Alka-Rocketeers often add nose cones and fins to their creations. But to achieve the maximum lift off—usually 20 to 30 feet—Bryan Palaszewski, Leader of Advanced Fuels at NASA Glenn Research Center, recommends leaving them off. “For maximum dramatic effect in height, you’ll want to take away any complexity from around the canister to limit the amount of weight that the energy will need to carry,” he says.
Prep the Propulsion System — Hold the rocket upside down and fill the canister one fourth with water. As with nearly all reactions, your Alka-Rocket reaction is heavily dependent on temperature. Warmer water will speed reaction time, since molecules move faster, creating more opportunities for the products of sodium bicarbonate and citric acid to react with water. Colder water leads to slower-moving molecules and a longer reaction time.
Mix Your Fuel — Just as full-sized rockets rely on reactions between fuel and oxidizer, so do Alka-Rockets. In this case the effervescent tablet is the fuel and water the oxidizer. Crushing the tablet (typically ½ will do) will speed the reaction by increasing the reactive surface area. Once you add the fuel to the canister, put the lid on quickly and step back. Important point: Your lid must be completely airtight for energy to build up. “Don’t rely on a canister you’ve used 100 times; the lid will be worn out and it won’t snap on as tight,” says Carl Nelson, a physicist and the chief scientist at an interactive children’s museum called the Imagination Station in Toledo, Ohio.
Lift-Off — Place the rocket on the ground, lid down. The pressure of the building CO2 will quickly overcome the resistance of the film canister’s top, popping it off. When it does, the gas escapes downward, generating thrust, which in turn overcomes the gravity holding the Alka-Rocket in place. Thanks Newton. “It all goes back to the strength and the seal of the canister,” Palaszewski says. “We’re trying to trap as much energy from the gas as possible, so the tighter the lid, the more pressure can build and the higher the Alka-Rocket may go.”
Landing — Though not a law, Newton is probably best known for saying, “What goes up, must come down.” Once the Mighty Pigeon takes flight, friction and gravity will weigh heavily on your rocket. As your fuel dwindles, Newton’s third law begins to swing the other direction, sending your spent Alka-Rocket back down to Earth.
Big Bonus Rocket — The thing about rockets is that basic principles scale up nicely. If you want a bigger whoosh, start with a two-liter water bottle, a cork, 4 effervescent tablets, warm water, some safety glasses and a mug—the use for which will become clear. Follow the same steps as before but place the water bottle upside down in the glass mug for stability. Then stand back! Important to note: a larger rocket doesn’t necessarily go higher. Since there’s a limit to the energy stored in the effervescent tablet, if the rocket is too large or heavy, the energy won’t be powerful enough to blast the rocket very high.