Balsa, Plastic or Fiberglass? Fins for Launch Day

Choosing Fins for Twelve Rockets

1Every spring, the rocket club at Bramblewood Middle School holds a launch day on the school's soccer field. Adult volunteers handle the store-bought rocket motors and run the launch pad. This year, the club is building 12 new beginner rockets, and adviser Mr. Anders Kjeldsen asked members Yusra Karimi and Ezra Kowalczyk to choose a material for the fins, the small wings at the tail that keep a rocket flying straight.

2Natural materials come from plants, animals or rocks and are used much as they are found, after they are cut, dried or shaped. Synthetic materials are made in factories, where chemical reactions turn natural resources into new substances that do not occur in nature. Many synthetic materials start as crude oil or natural gas.

3The club looked at four fin choices. Balsa wood comes from a fast-growing tropical tree; the logs are sawn into thin sheets and dried. Basswood plywood is made of three thin sheets of basswood glued together. Polystyrene is a plastic. At a chemical plant, substances from crude oil or natural gas react to make styrene, and then many styrene molecules link into long chains. The plastic is melted and squeezed into molds shaped like three fins joined by a ring, called a fin can. G10 fiberglass is made of glass fibers, spun from melted sand and other minerals, soaked in epoxy. Epoxy is made from substances in crude oil, and its two liquid parts react and harden into a tough solid.

4Synthetic materials have changed many hobbies. Molded plastics let factories make many identical parts quickly and cheaply, which has helped make kits for model rockets and other hobbies affordable. But polystyrene is made from crude oil or natural gas, which cannot be replaced once they are used up, and broken plastic pieces left in a field can last for many years. Fiberglass is very strong, but sanding it makes fine dust that can irritate skin and lungs, so workers wear masks. The club's shock cords, which tie the nose cone to the rocket, are made of Kevlar, a strong synthetic fiber made from chemicals that come from crude oil. The same fiber is used in protective vests and helmets that help keep police officers and soldiers safe.

5To test the fins, the club built one test rocket body and fitted it with each fin set in turn. They dropped it tail first from a height of 2 m onto the field's hard-packed dirt, a harder landing than most rockets make under a parachute. They counted the drops until a fin cracked. They tested three sets of each material and averaged the results. The club's criteria: the three fins together must have a mass of 7 g or less, so the tail is not too heavy; they must survive at least 12 drops; and they must cost $4 or less per set.