'It's All About Fire, Smoke, and Noise'
You know those little rockets made of wood and glue that you can stuff a motor in and launch from the field next door? These aren't them.
- By Preston Lerner
- Air & Space magazine, January 2004
(Page 2 of 5)
“Nothing’s coming out,” Braye says glumly as he tracks the Bobcat’s progress. “It’s all still together. Nothing’s coming out.”
Meanwhile, other rockets lift off. Whoosh: There goes a Performance Rocket packing a J350. Whoosh: Wave goodbye to a homebuilt flying on a honking M1850 Green Gorilla (so called because barium causes the flame to burn green). But Braye has eyes only for his Bobcat. “Now,” he whispers. “Now! Open now!”
Suddenly, finally, the parachute pops out. “Yes!” Braye shouts, pumping his fist. The rocket floats down under a black and orange canopy and lands in a plowed field. Braye lopes over to pick it up. By the time he gets back, his son and wife Penny are lounging in the shade. “You know how high it went?” Braye asks. He works the altimeter, which beeps in response. “You know what that means, Nick? Sixty-six-forty-one,” Braye crows. “Six thousand, six hundred, and forty-one feet! Isn’t that something?” Nicholas and Penny seem only mildly impressed, but Braye is too jazzed to notice. “In a half-hour,” he says, “I could have this thing cleaned up, stick another motor in it, and be ready to go again.”
In terms of fundamental architecture, rockets are simple devices. Load a combustible propellant in an enclosed chamber, light the fuse, and thrust is generated as the resulting gases expand, accelerating through the exhaust nozzle. The Chinese were launching rockets centuries ago with a mixture of saltpeter, charcoal, and sulfur. Black-powder propellant is still used in modern fireworks and small model rockets. But it’s an inefficient fuel, and its use stunted—literally—the growth of amateur rockets when the sport took off during the 1960s. Liquid oxygen and kerosene was a powerful alternative; after all, liquid oxygen and liquid hydrogen sent rockets to the moon. But LOX is a don’t-try-this-at-home product. And while rocketeers who wanted to push the propulsion envelope used various other fuels, all were either too dangerous, too complicated, or too expensive—often all three—for widespread use.
High-power rocketry is a product of the development of so-called composite motors, which feature a witches’ brew of ingredients. Known as AP motors because ammonium perchlorate serves as the oxidizer, most composites consist primarily of a plastic binding agent and a hard rubber fuel called HTPB, for hydroxyl terminated polybutadene, that is molded to fit in the motor case. Unlike their liquid-fueled cousins, solid-propellant motors don’t explode in a fireball in a launch mishap. Also, composite motors provide far more bang for the buck (and ounce) than black powder.
“When I got involved, I didn’t realize how many brownie points Tim would get for having a wife who flies rockets,” says Beth Sapp of West Tawakoni, Texas, whose husband and two sons are avid rocketeers. “But he’s created a monster. I became a high-power junkie. Now we have to fight over who’s going to get to use the next big load of AP.”
High-power motors are described with codes like “K550.” The number is the average thrust in newtons, the force required to impart an acceleration of one meter per second squared to a mass of one kilogram (one newton equals 0.2248 pound of force). The letter rates the total impulse, or overall power, of the motor. (Dividing the total impulse by the thrust provides the duration of the burn in seconds). Each successive letter denotes a motor that is twice as powerful as the one that precedes it—a B motor is twice as powerful as an A, and a C is twice as powerful as a B (and four times as powerful as an A). A small A motor produces less than one pound of average thrust. An I motor, a common mid-power choice, might generate 50 pounds; a popular high-power M, closer to 500. Although Qs and Rs have been flown elsewhere, the largest motor at this LDRS is a P, rated at 1,800 pounds of thrust. By comparison, the Redstone that took Alan Shepard 116 miles above Earth pumped out 78,000 pounds of thrust.
Back in the 1960s and ’70s, virtually all model rockets went up on black-powder motors no larger than D; there were only a handful of Es, Fs, and Gs. Sure, a few radical types clustered these motors together for additional pop. But the party line at the National Association of Rocketry was: Big is bad. So it was left to a few California renegades to challenge the status quo. One, Gary Rosenfield, later founded AeroTech Consumer Aerospace, which is now the world’s largest manufacturer of hobbyist rocket motors. Another, Chuck Piper, became the chief guru of the Rocket Ranch research and development facility in a canyon near Patterson, California.