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Adventures from the Technology Underground is Gurstelle’s lively and weirdly compelling report of his travels. In these pages we meet Frank Kosdon and others who draw the scrutiny of the FAA, ATF, and other federal agencies in their pursuit of high-power amateur rocketry, which they demonstrate to impressive—and sometimes explosive—effect at the annual LDRS gathering held in various remote and unpopulated areas (a necessary consideration since that acronym stands for Large Dangerous Rocket Ships). Here also are the underground technologists who turn up at the Burning Man festival in the Nevada high desert, including Lucy Hosking, “the engineer from Hell” and the creator of Satan’s Calliope, aka the World’s Loudest Thing, a pipe organ made from jet engines. Also at Burning Man is Austin “Dr. MegaVolt” Richard, who braves the arcing, sputtering, six-digit voltages of a giant Tesla coil in his protective metal suit. Add in a trip to see medieval-style catapults, air cannons, and supersized slingshots in action at the World Championship Punkin Chunkin competition in Sussex County, Delaware, and forays to the postapocalyptic enclaves of the flamethrower builders and the future-noir pits of the fighting robots, and you have proof positive that the age of invention is still going strong.
In the world of science and engineering, despite its buttoned-down image, there’s plenty of fun, humor, and sheer wonder to be found at the fringes. Adventures from the Technology Underground takes you there.
• Launch homemade high-power rockets.
• Catapult pumpkins the better part of a mile.
• Watch robot gladiators saw, flip, and pound one another into high-tech junk heaps.
• Dazzle the eye with electrical discharges measured in the hundreds of thousands of volts.
• Play with flamethrowers, potato guns, and other decidedly unsafe toys . . .
If this is your idea of fun, you’ll have a major good time on this wild ride through today’s Technology Underground.
From the Burning Man festival in Nevada’s high desert to the latest gathering of Large Dangerous Rocket Ship builders to Delaware’s annual Punkin Chunkin competition (a celebration of “science, radical self-expression, and beer”), you’ll meet the inspired, government-unregulated, and corporately unfettered men and women who operate at the furthest fringes of science, engineering, and wild-eyed arc welding, building the catapults, ultra-high-voltage electrical devices, incendiary artworks, fighting robots, and other machines that demonstrate what’s possible when physics meets human ingenuity.
From the Hardcover edition.
THE TECHNOLOGY OF
HIGH-POWER AMATEUR ROCKETRY
In the typical solid-fuel rocket, the rocket maker builds a fiberglass shell that houses the motor, the recovery system, and whatever sensors, cameras, or other payload is placed within.* But the bulk of the rocket's weight is contained in its powerful chemical engines. In and of themselves, rocket engines are marvelous things. Their most basic form goes back to first-millennium China, when crude black powder was stuffed into bamboo rockets and used to frighten the enemy's horses. A simple rocket engine is straightforward and easy to understand. There is chemical propellant packed inside a metal casing. The chemicals inside the motor burn, and as they do so, hot, expanding gas is produced. This gas rushes out the back of the motor through a nozzle and, as described in Isaac Newton's Third Law of Motion, the backward gush of the gas results in an equal and opposite forward thrust of the rocket body. Simple, yes. But hey, this is rocket science, and things get complicated quickly.
Small, commercially available model rocket motors consist of black-powder propellant pressed under tons of pressure into a hard, dense matrix called "grain." When the grain is ignited, the motor starts burning linearly, like a very fast-burning cigarette, from its back to its front. As it does so, it pushes hot gas out through a clay nozzle, and the rocket zips forward until the propellant is all burned up.
The world of high-power rocketry is different and much more complicated. Instead of using a simple black-powder chemical rocket motor, the experienced flyers most often use engines made out of "composite propellant"--a combination of an oxidizer chemical such as ammonium perchlorate (AP) and a synthetic rubber binder material to hold the oxidizer in a desired shape and provide fuel. In addition, the rocket engine maker may mix in plasticizers, catalysts, and crosslinkers, all of which can make the propellant burn stronger, longer, slower, or hotter, depending on the goals of the rocket designer. Composite motors are formed into various shapes with voids and holes precisely designed into the motor in order to shape the direction and velocity of the exiting gas. Such complex contours and figures are complicated to fabricate, requiring great quantities of heating, molding, curing, machining, and, above all, attention to detail.
The most extreme rocket makers spend days on end experimenting with rocket designs and motor formulations. There are so many variables that the maker can adjust to affect the performance of the rocket. A quick list of their concerns includes the shape of the rocket body, fin design, the shape of the nozzle, the geometry of the motor's core, the combination of various chemicals that make up the propellant mixture, the rate of burn, and the ignition method. It takes a lot of scientific, mechanical, and seemingly alchemical knowledge to become a really good rocket maker. There is also an element of danger working with toxic and flammable chemicals such as ammonium perchlorate, potassium nitrate, and liquid oxygen.*
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What rocket makers care about most is the physics quantity called "total impulse." Total impulse is the product of the force acting on a rocket (the thrust) multiplied by the amount of time the thrust is applied. Expressed mathematically, it is:
Total Impulse = Average Thrust ´ Burn Time
An engine that applies a lot of thrust, for a long period of time, is a high-performance engine. To a rocket engine maker, the goal is lots and lots of impulse.
The size of a rocket motor and the amount of total impulse it produces are described by assigning the motor a letter of the alphabet. The smallest rocket motor is an A and is commonly sold in hobby stores without need for a permit. The B motor is twice as big as an A, and a C is twice as big as a B. Each increase in letter size denotes a doubling of the engine's rocket-lifting ability, or total impulse. The total impulse of an A-motor is about 2.5 newton-seconds (N-s), which is enough to lift a small rocket a few hundred feet. A B-motor provides 5 N-s, C-motors provide 10 N-s, and so on. The largest commercially produced rocket motor available to certified amateur flyers, the mammoth N motor, provides a muscular 41,000 N-s. Custom engines are available from a number of boutique rocket engine designers. Some of these go into the O and P range and even beyond. They are large and energetic enough to power a half-ton rocket to jet-fighter altitudes. (Using this scale, the NASA space shuttle's 8.3 million Newton-second booster rockets are about two letters beyond a Z-motor.)
Although there are many variations in the design and construction of homemade rocket engines, one of the clearest differentiating factors is the type of chemicals used to provide the energy and hence the impulse. The two most common general categories of chemicals are those involving variations of black powder and those that use composite propellant. Composite engines are, pound for pound, significantly more powerful than black-powder engines, that is, they have a higher specific impulse.
Every rocket engine, from black powder to solid fuel composite to liquid fuel to hybrid systems, works in similar fashion and is subject to the same basic physical laws: The propellant is ignited. It burns. Hot and expanding gases are produced and then stream out of a nozzle. Thrust is produced and the rocket and whatever is attached to it goes forward.*
The force produced by the gas issuing out of the nozzle is called "momentum thrust." Imagine that a rocket engine builder constructs an engine with a burn rate of 10 pounds of fuel per second. Now further assume that the builder's rocket engine handbook tells him that his choice of rocket fuels will result in the gases leaving the rocket nozzle at a velocity of around 3,000 feet per second.
The thrust produced is equal to the propellant burn rate multiplied by the exhaust velocity. So the momentum thrust is:
Momentum Thrust = Propellant Burn Rate ´ Exhaust Gas Velocity
So, in the example above,
Thrust = (10 lbs/sec) ´ (3,000 ft/sec)/(32.2 ft/sec2)*
Momentum Thrust = 932 pounds of force
So far, so good. But momentum thrust is only part of the reason rockets go up. The other reason is pressure thrust.
Inside a rocket engine, there are unbalanced forces at work. The rocket engine has an open end (the nozzle where the gases come out) and a closed end. During the burn time, the combustion of rocket engine chemicals results in a pressure buildup inside the engine. But since one end is closed and one end is open through the exit nozzle, there is a net force pushing against the closed end.
Posted March 22, 2011
No text was provided for this review.
Posted July 3, 2011
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