A sodium bicarbonate rocket (sometimes called an Alka-Seltzer rocket) is a model rocket fashioned from a 35mm film canister and propelled by the pressure of a gas, often carbon dioxide, generated from the reaction of an acid with sodium bicarbonate. Sodium bicarbonate rockets are often used in science classes to demonstrate principles of chemistry and physics.
A sodium bicarbonate rocket is a model rocket fashioned from a film canister that is less than 34.5mm.
It's fashioned from a 35mm canister and 35mm is not less than 34.5mm. It's difficult because 34.5 is close to 35mm and perhaps the 0.5mm confused it even more because it also contains a 5.