- The Washington Times - Wednesday, July 20, 2005

Flying model rockets may seem like child’s play, but tell that to thousands of science teachers nationwide who lean heavily on the cardboard creations.

In May, more than 500 middle and high school students — whittled down from nearly 10,000 contestants — gathered in The Plains, Va., for the third annual Team America Rocketry Challenge.

Teachers nationwide incorporated those efforts into their own plans, letting the rockets’ razzle-dazzle sell students on the physics of flight.

Trip Barber, vice president of the National Association of Rocketry, says the basic physics lifting model rockets into the sky is identical to the forces applied to genuine rockets.

A rocket kit straight out of the pack might fly to heights around 1,500 feet, says Mr. Barber, whose nonprofit group promotes the educational aspects of the hobby.

Some rockets can fly much higher, particularly models created by more skilled rocket enthusiasts.

After 1,500 feet, though, “you can’t see it anymore,” he says.

The average model rocket weighs less than a pound, thanks to its balsa-wood fins, cardboard tubing and light plastic nose cone. The rockets fly straight skyward, in part because of the fins, which are placed strategically on the main tube and stabilize the flight, much like the feathers that keep an arrow’s path true.

Mary Roberts, technical services manager with Estes Rockets, says model rocketry is a legally defined term. A rocket must weigh no more than 16 ounces and contain no more than 4 ounces of propellant, Ms. Roberts says. Rocketeers can fly models with engines less than or equal to those figures, or they can fly multistage rockets whose combined thrusting material doesn’t outweigh those standards.

Should a model rocket overstep those bounds, the flier must notify the Federal Aviation Administration for permission to fly the rocket, she says.

Ms. Roberts says even though her company’s models use black powder solid, not liquid propellant, they still slice through the atmosphere just like the National Aeronautics and Space Administration’s rockets.

The engines themselves are categorized by a letter system in which the total impulse the craft provides doubles with every letter change. A “B” engine contains twice the impulse power of an “A” engine, for example, says Ms. Roberts, whose company has been making model rockets since 1958.

Model rockets start at less than $10, with engines running roughly the same for a pack of three.

An “A” engine produces up to 2 Newton-seconds of total impulse, she says, a measurement unit used for rocket speeds that analyzes how many pounds of force are exerted for one second.

Alexandria resident John Hochheimer, a member of the Northern Virginia Association of Rocketry, says a garden-variety model rocket can be improved by using sturdy carbon fiber tubing for the rocket’s main body. Most fall back on the standard balsa and plastic constructs, says Mr. Hochheimer, whose group has about 130 model-rocket enthusiasts.

Whether the rocket in question took weeks to build or just a few hours, the rocketeer wants to ensure that it floats back safely to earth after its voyage. The average rocket can be used over and again, assuming it isn’t damaged when it returns to the ground and the safety wadding is properly inserted.

“We’re building them to recover them,” Mr. Hochheimer says.

These rockets return to earth thanks to a collapsed parachute or collection of streamers tucked above the engine. A smaller number of rockets are built to either glide back to earth or spin like a helicopter to retard their fall.

The heat generated by the engine normally would tear right through standard recovery devices, so enthusiasts turn to wadding to form a wall between the parachute and the engine’s churning heat. Readily available wadding — tissue-paper-like material treated with a fire-retarding chemical — is the standard way rocketeers protect their parachutes.

The engines typically have a small charge built into them that releases a gust of hot gas to trigger whatever’s between the motor and nose cone to deploy the recovery system, he says.

Dave Akin, an associate professor of aerospace engineering with the University of Maryland, says newer rockets employ backup systems to make sure the delivery system works as needed.

More sophisticated models use computers to fire the ejection charge based on barometric readings, Mr. Akin says. “If the [air] pressure goes up and it hasn’t deployed yet, the backup system would trigger it,” he says. In some cases, a radio-controlled trigger can be activated by the rocketeer to eject the parachute, he adds.

Mr. Barber says the rockets’ payload sections also can carry miniaturized cameras so those on the ground can watch the flight from the rocket’s point of view.

“You can see the ground suddenly pull away,” Mr. Barber says.

Mr. Akin says the technology used by model-rocket companies such as Estes dates back “hundreds of years.”

Even the ignition system is a model of simplicity.

Rocketeers run an electrical current through a Nichrome wire, a high-resistance wire like the heating element in a toaster, which ignites the fuel, Mr. Akin says.

Some igniter elements are coated with a black material similar to what would be found on a match head.

“It’s easier to ignite, and it burns hotter for a very brief period of time,” he says. “Everything is a variant on electric resistance.”

In the past, model rocket fliers could reasonably assess how high their creations flew with basic trigonometry.

The flier would stand a set distance away from the launch pad, then measure the angle of flight at its peak using a clinometer and plug those figures into a mathematical formula to calculate the height reached.

Today, some fliers install tiny computers with pressure sensors that register how air pressure drops with altitude, data that can tell when the rocket reached its highest point.

Model-rocket fliers need to act quickly when taking their measurements. The devices may be a fraction of a real rocket’s size, but they can move at speeds quicker than the eye can catch.

“A typical rocket reaches 100 to 200 miles per hour,” Mr. Akin says. “Some of the high-power rockets can go supersonic” (faster than the speed of sound).


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