Static Fire and Launch Day: Where Do the Kilograms Go?

Twenty Seconds of Fire

1Every spring, a university's student rocket team holds an open house at its test site in the desert. This year, the Hillcrest Middle School space club made the trip. Club members Yusra Qureshi and Declan O'Rourke watched on a big screen from a safe building while the team ran a static fire: the team's small rocket engine was bolted to a heavy steel stand so that it could not fly, and it burned for 20 seconds.

2Team leader Mateus Albuquerque explained that the engine burns ethanol, a kind of alcohol, with liquid oxygen. Some early liquid-fueled rockets burned ethanol too. Each ethanol molecule (C₂H₆O) has 2 carbon atoms, 6 hydrogen atoms and 1 oxygen atom. Each oxygen molecule (O₂) has 2 oxygen atoms. When they react, the atoms are rearranged into carbon dioxide (CO₂) and water (H₂O), which rush out of the nozzle as hot gases: C₂H₆O + 3 O₂ → 2 CO₂ + 3 H₂O.

3For this test, the team set the engine to use ethanol and oxygen in amounts that can react completely, and the team's computer model assumes that they did. The two tanks were weighed just before and just after the burn (Table 1). Their vent valves stayed closed during the burn, so the only way out of each tank was the line to the engine. The computer model says that the burn produced 8.8 kg of carbon dioxide.

4The next Saturday, the space club held its own launch day on a soccer field. An adult safety officer launched each small model rocket with an electric launch controller while everyone stood well back. A model rocket motor holds a solid propellant packed inside a cardboard casing. The propellant contains an oxygen-rich ingredient, so it burns without needing oxygen from the air. Yusra weighed three motors before launch, and she weighed their empty casings after the rockets were found (Table 2).